PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...

SPECTROSCOPIC INVESTIGATIONS OF THE 

ELECTRONIC STRUCTURE OF Pr 1−x Ca x MnO 3 

SYSTEM 

A THESIS SUBMITTED TO THE 

HOMI BHABHA NATIONAL INSTITUTE 

FOR THE DEGREE OF 

DOCTOR OF PHILOSOPHY IN SCIENCE 

(PHYSICS) 

MAY, 2009 

By 

MANAS KUMAR DALAI 

INSTITUTE OF PHYSICS, 

BHUBANESWAR 751 005, 

INDIA

CERTIFICATE 

This is to certify that the thesis entitled Spectroscopic investigations of the 

electronic structure of Pr 1−x Ca x MnO 3 system, which is being submitted by 

Manas Kumar Dalai in partial fulfillment of the degree of Doctor of Philosophy 

in Science (Physics) of Homi Bhabha National Institute is a record of his own 

research work carried out by him. He has carried out his investigations for the last 

six years on the subject matter of the thesis under my supervision at Institute of 

Physics, Bhubaneswar, India. To the best of my knowledge, the matter embodied 

in the thesis has not been submitted for the award of any other degree. 

Signature of the Candidate 

Manas Kumar Dalai 

Institute of Physics 

Bhubaneswar 

Date : 

Signature of the supervisor 

Dr. B. R. Sekhar 

Associate Professor 

Institute of Physics 

Bhubaneswar 

ii

iii 

To My Parents

Acknowledgements 

I owe a debt of gratitude to many people who made this thesis possible due to their 

support, suggestions, and encouragements. 

First of all, it is a great pleasure for me to express my sincere gratitude and indebtedness 

to my thesis supervisor Prof. B. R. Sekhar for his invaluable guidance 

and constant encouragement to complete this work. His excellent training in handling 

the intricacies of various experiments and making me acquainted with the data 

analysis have been an inspiration for me to pursue my career as an experimental 

condensed matter physicist. I wish to thank him for giving me the complete freedom 

both academically and socially during the period of my work. 

I would like to thank my group members Prabir and Rupali for their untiring 

support during the experiments and for many interesting discussions. I would also 

like to thank our technical staff Mr. Chittaranjan Swain for his help during the 

experiments. My sincere thanks go to Prof. C. Martin (Laboratoire CRISMAT, 

France) for providing good quality samples for this thesis work. 

I would like to thank Dr. S. R. Barman, Dr. S. Banik, Dr. A. K. Shukla and Dr. 

R. Dhaka (UGC-DAE CSR, Indore) for their help and discussions for inverse photoemission 

measurements. I would also like to thank Prof. Stefan Schuppler, Dr. Peter 

Nagel and Dr. Michael Merz for their help and cooperation during the high resolution 

photoemission and x-ray absorption measurements at WERA beamline of ANKA 

synchrotron, Karlsruhe, Germany. I would like to thank Dr. Federica Bondino, Dr. 

Marco Zangrando, Dr. Elena Magnano and Dr. Bryan Doyle of ELETTRA synchrotron, 

Trieste for their help and technical discussions for XAS experiments. 

It is a great pleasure for me to thank Prof. N. L. Saini (Departimento di fisica, 

Universita di Roma, Rome, Italy) for his fruitfull discussion at different stages of 

my Ph. D. career. I also thank Prof. Anil Kumar and Prof K. K. Nanda of IISc 

Bangalore for discussion and suggestions during their short visit to IOP. 

I would like to thank all my teachers, who had taught me at various stages of 

my educational career, especially my M. Sc teachers Profs. N. Barik, L. P. Singh, L. 

iv

Maharana, P. Khare, D. K. Basa, K. Maharana, S. Mohapatra and D. Behera and 

B. Sc teacher Mr. Dipak Singh for their encouragement and excellent teaching. It 

is my great pleasure to thank Profs. A. Khare, A. M. Jayannavar, S. G. Mishra, D. 

P. Mohapatra, K. Kundu, P. V. Satyam, S. Varma, S. Mukherji, J. Maharana, G. V. 

Raviprasad, A. M. Srivastava, S. M. Bhattacharjee, A. Kumar, P. Agrawal, T. Som, 

G. Tripathy, S. K. Patra for their teaching, suggestions and encouragements. I thank 

our director Dr. Y. P. Viyogi and ex-director Dr. R. K. Choudhury for their support 

and encouragements in all respect. 

I would like to thank Mr. Santosh Choudhury, Mr. Anup Behera, Mrs. Ramarani 

Das, Mr. Arun Dash, Mr. Khirod Patra, and Dr. S. N. Sarangi for providing technical 

help when needed. I would also like to thank the library staff Rama, Rabaneswar, 

Duryodhan and Kissan for their cooperation at every stage. I thank Dipankar, Srijesh 

and Dhiren for their help and cooperation at every computer related problems. I wish 

to thank the hostel mess staff for their wonderful services. 

I am thankful to our registrar Mr. C. B. Mishra for his imense help and cooperation 

in all administrative problems. I also thank Mr. Sk. Kefaytulla and Rajesh 

Mohapatra for their administrative help. 

I take this oppurtunity to thank my M.Sc friends Rajesh, Tapas, Swayambhu, Hitu 

and Manmohan and village friend Suda for their constant support and encouragement 

in all stage of my life. A special thank to Ananta for his constructive criticism, support 

and company for both academic and social activities in my Ph.D life. 

It is my pleasure to thank Zashmir bhai, Satya bhai, Jhasa bhai, Satchi bhai, 

Bedanga bhai, Pal bhai, Kamal bhai, Pradeep bhai, Bishnu bhai, Birendra bhai, 

Sudhira bhauja and Raj babu for their encouragement and cooperation during my 

stay at IOP. I would like to thank Millind, Sinu, Boby, Chandra, Hara, Amulya, Ajay, 

Trilochan, Smruti, Chitrasen, Jatis, Ashutosh, Kuntala, Sadhana, Mamata, Ranjita, 

Binata, Jaya, Poulomi, Saumia, Pramita, Probodh, Jay, Dipak, Sumalay, Bharat, 

Anupam, Umanand, Dilip, Suchi, Srikumar, Sankha, Nabyendu, Somnath, Sandeep, 

Sourabh, Subrat, Sanjay, Raghavendra, Vanaraj, and Tanmoy for making my stay at 

the Institute a memorable one. I also thank Dr. V. R. R. Medicherla for his valuable 

discussions. 

I would like to give my special thanks to Dr. Arun Kumar Pati and Dr. Biswajit 

Pradhan for making my stay in the beautiful campus of IOP an exciting, memorable 

and pleasing one. 

v

I am extremely grateful to Mrs. Ranjana Sekhar for making my stay homely 

during the course of my work. 

Gratefully I would like to thank my younger sister Tuin and younger brother Tulia 

for their unlimited love, affection, encouragements, assistance and support at every 

stage of my life. I do not find proper words to express my sense of indebtedness to my 

wife Manasmita, whose unlimited love, affection, encouragements, and support have 

become a special inspiration to me during my Ph. D life. I also thank my brother 

in-law Munu for his love, affection and support. 

Finally, I wish to express my deep sense of gratitude to my parents and late grand 

parents for their immense patience, love, affection, encouragements and support. I am 

indebted to them for giving me the freedom to choose my career and for constantly 

providing me the moral and emotional support at every stage of my life. It is only 

because of their kind blessings that I could complete this work. 

Date: 22.05.2009 

(Manas Kumar Dalai) 

vi

List of Publications/Preprints 

[1] Valence band electronic structure of Pr 1−x Sr x MnO 3 from photoemission studies, 

P. Pal, M. K. Dalai, B. R. Sekhar, S. N. Jha, S. V. N. Bhaskara Rao, N. C. 

Das, C. Martin, and F. Studer; J. Phys. : Condens. Matter 17, 2993 

(2005). 

*[2] Electronic structure of Pr 0.67 Ca 0.33 MnO 3 near the Fermi level studied by ultraviolet 

photoelectron and x-ray absorption spectroscopy, M. K. Dalai, P. Pal, 

B. R. Sekhar, N. L. Saini, R. K. Singhal, K. B. Garg, B. Doyle, S. Nannarone, 

C. Martin, and F. Studer; Phys. Rev. B 74, 165119 (2006). 

[3] Near Fermi level electronic structure of Pr 1−x Sr x MnO 3 : Photoemission study, P. 

Pal, M. K. Dalai, R. Kundu, M. Chakraborty, B. R. Sekhar, and C. Martin; 

Phys. Rev. B 76, 195120 (2007). 

[4] Pseudogap behavior of phase-separated Sm 1−x Ca x MnO 3 : A comparative photoemission 

study with double exchange, P. Pal, M. K. Dalai, R. Kundu, B. R. 

Sekhar, and C. Martin; Phys. Rev. B 77, 184405 (2008). 

*[5] Electronic structure of Pr 1−x Ca x MnO 3 from photoemission and inverse photoemission 

spectroscopy, M. K. Dalai, P. Pal, R. Kundu, B. R. Sekhar, S. Banik, 

A. K. Shukla, S. R. Barman, and C. Martin; Submitted to Physica B. 

*[6] Spectroscopic investigations of the electron-doped Ca 0.86 Pr 0.14 MnO 3 , M. K. 

Dalai, P. Pal, R. Kundu, B. R. Sekhar, M. Mertz, P. Nagel, S. Schuppler, 

and C. Martin; Submitted to Phys. Rev. B. 

*[7] Core level electronic structure of Pr 1−x Ca x MnO 3 using x-ray photoelextron spectroscopy, 

M. K. Dalai, P. Pal, R. Kundu, B. R. Sekhar, and C. Martin; To 

be submitted. 

vii

[8] Electron spectroscopic investigations of the paramagnetic insulator to antiferromagnetic 

insulator transition in Pr 0.5 Sr 0.5 MnO 3 , P. Pal, M. K. Dalai, R. 

Kundu, B. R. Sekhar, S. Schuppler, M. Merz, P. Nagel, M. Channabasappa and 

A. Sundaresan; Submitted to Phys. Rev. B. 

(*) indicates papers on which this thesis is based. 

viii

Synopsis 

The perovskite type manganites having general formula R 1−x A x MnO 3 (where R is a 

trivalent rare earth element like La, Pr, Nd. and A is a divalent alkaline earth element 

like Sr, Ca, Ba.) have been one of the interesting compounds among the transition 

metal oxide systems due to their important physical property such as colossal magnetoresistance 

(CMR) [1, 2, 3, 4]. CMR is defined as the colossal decrease of resistance 

by the application of magnetic field. These materials are insulators at high temperatures 

and poor metals at low temperatures. Accompanying this insulator-metal 

transition is a magnetic transition from a high temperature paramagnetic phase to a 

low temperature ferromagnetic phase. These transitions occurs around the same transition 

temperature T C . Applying a magnetic field around T C will greatly reduce the 

resistivity, i.e negative magnetoresistance effect. These materials were first described 

in 1950 by Jonker and van Santen [5] in perovskite type of manganites. One year later 

(1951), Zener [6, 7] explained this unusual correlation between magnetism and transport 

properties by introducing a novel concept called ”Double exchange” mechanism 

(DE). The pioneering work of Zener was followed by Anderson and Hasegawa [8] in 

1955 and de Gennes [9] in 1960. Double exchange theory argues that the electron 

hoping is related to the relative spin orientations of the neighbouring sites. Although 

it qualitatively explains the connection between the insulator-metal and the magnetic 

transitions, it was noticed latter by A. J. Millis et. al. [10] that it fails to explain 

the quantitative change in resistivity and the very low T C . Thus it is revealed that 

the double-exchange mechanism is not enough to explain the CMR effect. Several 

mechanisms [11, 12, 13, 14], based on electron-phonon interactions (the polaronic 

effect), electronic phase separation, charge and/or orbital ordering etc. have been 

proposed to account for the discrepancy. Although a detailed picture is still unclear, 

there seems to be a growing consensus that one or more of these additional effects 

probably exists, with the various phenomena both competing and cooperating with 

each other. This leads to a very rich and exotic behaviour, as observed in these 

ix

manganites. This mixed valence manganites have been studied for more than five 

decades but are still considered modern materials because of their wide potential for 

technological application. 

Experimental electronic structure studies [15, 16, 17, 18] on these materials have 

contributed substantially to the understanding of their unusual behavior which could 

not be fully accomodated within the framework of the DE model. The temperature 

and doping dependent metal-insulator transitions in these materials are found to be 

closely related to the unique electronic structure derived from the Mn 3d and O 2p 

hybridized orbitals. Most of their electrical and magnetic properties originate from 

the Jahn-Teller distortion in the MnO 6 octahedra around the Mn 3+ ions. The strong 

interplay between spin and orbital ordering originating from the single occupancy of 

the doubly degenerate e g orbitals of Mn 3+ (t 3 2ge 1 g) ions make the phase diagram of these 

compounds rich with physics. Many of these compounds, for example L 1−x Ca x MnO 3 

(L = La, Pr) show strong charge and/or orbital ordered (CO/OO) insulating nature 

at low temperatures, especially when x equals commensurate value. 

Electron spectroscopy is a very powerful experimental tool for probing the electronic 

structure of these materials. For this thesis work, the electronic structure of 

Pr 1−x Ca x MnO 3 series have been investigated using ultra-violet photoelectron spectroscopy 

(UPS), X-ray photoelectron spectroscopy (XPS), Inverse photoemission spectroscopy 

(IPES) and X-ray absorption spectroscopy (XAS). Pr 1−x Ca x MnO 3 is one of 

the interesting among these materials due to their insulating phase at all temperatures 

and great variety of ordered phases, that are very sensitive to the cation/anion 

doping. For 0.3 ≤ x ≤ 0.8, a charge ordering of Mn 3+ and Mn 4+ was found and an 

antiferromagnetic (AF) ordering can be observed with neel temperature ranging from 

100 to 170 K. For x ≤ 0.25, a ferromagnetic insulating state (FMI) is observed and 

with no CO, whatever the temperature. In this region the metallic state is never realized 

even upon the application of magnetic field. But the charge ordered insulating 

state can be melted into ferromagnetic metallic state (FMM) upon application of a 

magnetic field [4]. Interestingly the x = 0.33 doping shows the coexistence of FM and 

AFM phases. Around x = 0.9 a metallic cluster glass domain [19] has been observed. 

Using valence band photoemission and O K edge x-ray absorption, we studied 

the temperature dependent finer changes in the near E F electronic structure of 

Pr 0.67 Ca 0.33 MnO 3 , which is regarded as a prototype for the electronic phase separation 

models in CMR systems. With decrease in temperature the O 2p contributions 

x

to the t 2g and e g spin-up states in the valence band were found to increase until T c . 

Below T c , the density of states with e g spin-up symmetry increased while those with 

t 2g symmetry decreased, possibly due to the change in the orbital degrees of freedom 

associated with the Mn 3dO 2p hybridization in the pseudo-CE-type charge or orbital 

ordering. These changes in the density of states could well be connected to the electronic 

phase separation reported earlier. By combining the UPS and XAS spectra, 

we derived a quantitative estimate of the charge transfer energy E CT (2.6±0.1 eV), 

which is large compared to the earlier reported values in other CMR systems. Such 

a large charge transfer energy was found to support the phase separation model. 

For a complete understanding of the changes in the occupied and unoccupied 

electronic states we performed a detailed study of the Pr 1−x Ca x MnO 3 system across 

their ferromagnetic - antiferromagnetic phase boundary using UPS and IPES. We 

used three compositions x = 0.2, 0.33 and 0.4. Our photoemission studies showed 

that the pseudogap formation in these compositions occur over an energy scale of 

0.48±0.02 eV. Here again, we have estimated the charge transfer energy in these 

compositions to be of the order of ∼ 2.8±0.2 eV. We have also undertaken a detailed 

study of the core level electronic structure of Pr 1−x Ca x MnO 3 ((x = 0.2, 0.33, 0.4 and 

0.84) using X-ray photoelectron spectroscopy. These studies have been performed at 

the Mn 2p, Ca 2p and Pr 4d levels. We have made systematic measurments of the 

changes in the binding energy positions of these levels as a function of the charge 

carrier concentration. We also have analyzed the line shapes of these core levels. 

To a large extent the traditional models employing the charge-spin coupling, have 

been able to explain the CMR in many of the hole doped compositions. But, much 

less is known about the electron-doped versions of these materials in which the charge, 

orbital and spin ordering add more complexity to the ferromagnetic (FM) double exchange 

and the superexchange interactions. These Mn(IV) rich compositions exhibit 

marked differences in the electronic and magnetic properties from their Mn(III) rich 

counterparts. It has been shown that the substitution of Ca by a trivalent cation 

in the G-type antiferromagnet, CaMnO 3 leads to the formation of a FM component 

with a maximum for optimal electron doping. Depending on the nature of the trivalent 

cation, the x opt was found to lie between 0.135 and 0.16. This FM component is 

correlated to the distortion of the MnO 6 octahedra and thereby the e g band width. 

With low filling of the narrow e g band, these systems have strong electron-electron 

interactions and consequent charge localizations. Using high resolution photoelectron 

xi

spectroscopy and O K edge x-ray absorption spectroscopy we have studied the temperature 

dependence of the near Fermi level electronic structure in the electron doped 

CMR, Ca 0.86 Pr 0.14 MnO 3 . We found that the temperature dependent changes in the 

electronic structure is consistent with the conductivity behavior of the electron doped 

systems in general. As the temperature is lowered from 293 K a feature due to the e g 

states starts appearing near the E F . This increase in the DOS is a signature of the 

semi-metallic behavior of the sample in the range 110 - 300 K. Surprisingly, though 

the resistivity measurement on this sample showed an insulating behavior below its 

transition temperature T c (110 K), the intensity of this feature further increases below 

the T c . Further, we have found an increase in the e g band width (W) also at 

low temperatures. It was also interesting to note that the increase in the near E F 

DOS in this compound is apparently different in nature from that associated with the 

pseudogap formation in the other systems like Pr 1−x Sr x MnO 3 . We have discussed our 

results from the point of view of phase sepation models considering the temperature 

dependent structural changes and consequent localization of the electrons in a narrow 

e g band. 

xii

Bibliography 

[1] R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, and K. Samwer; Phys. Rev. 

Lett. 71, 2331 (1993). 

[2] Y. Tokura, A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido and 

N. Furukawa; J. Phys. Soc. Jpn. 63, 3931 (1994). 

[3] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, and Y. Tokura; 

Phys. Rev.B 51, 14103 (1995). 

[4] Y. Tomioka, A. Asamitsu, Y. Moritomo, and Y. Tokura, J. Phys. Soc. Jpn 64, 

3626 (1995). 

[5] G. H. Jonker and J. H. Van Santen Physica 16, 337 (1950). 

[6] C. Zener, Phys. Rev. 81, 440 (1951). 


[8] P. W. Anderson and H. Hasegawa, Phys. Rev. 100, 675 (1955). 

[9] P. G. de Gennes, Phys. Rev. 118, 141 (1960). 

[10] A. J. Millis, B. I. Shraiman, and P. B. Lttlewood, Phys. Rev. Lett. 74, 5144 

(1995). 

[11] A. J. Millis, Nature 392, 147 (1998). 

[12] A. Moreo, S. Yunoki, and E. Dagatto, Science 283 2034 (1999). 

[13] A. Moreo, M. Mayr, A. Feiguin, S. Yunoki, and E. Dagotto, Phys. Rev. Lett. 84 

5568 (2000). 

xiii

[14] H. Kuwahara, Y. Tomioka, A. Asamitsu, Y. Moritomo, Y. Tokura; Science 270, 

961 (1995). 

[15] T. Saitoh, A. E. Bocquet, T. Mizokawa, H. Namatame, A. Fujimori, M. Abbate, 

Y. Takeda, and M. Takano; Phys. Rev. B 51, 13942 (1995). 

[16] D. D. Sarma, N. Shanthi, S. R. Krishnakumar, T. Saitoh, T. Mizokawa, A. 

Sekiyama, K. Kobayashi, A. Fujimori, E. Weschke, R. Meier, G. Kaindl, Y. 

Takeda, and M. Takano; Phys. Rev. B 53, 6873 (1996). 

[17] J.-H. Park, C. T. Chen, S-W. Cheong, W. Bao, G. Meigs, V. Chakarian, and Y. 

U. Idzerda; Phys. Rev. Lett. 76, 4215 (1996). 

[18] A. Chainani, H. Kumigashira, T. Takahashi, Y. Tomioka, H. Kuwahara, and Y. 

Tokura; Phys. Rev. B 56, R15513 (1997). 

[19] C. Martin, A. Maignan, M. Hervieu, and B. Raveau, Phys. Rev. B 60 12191 

(1999). 

xiv

Contents 

CERTIFICATE 

Acknowledgements 

List of Publications/Preprints 

Synopsis 

ii 

iv 

vii 

ix 

1 Introduction 1 

1.1 An overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

1.2 Colossal magnetoresistance . . . . . . . . . . . . . . . . . . . . . 3 

1.3 Crystal structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

1.4 The basic electronic structure . . . . . . . . . . . . . . . . . . . . 5 

1.5 Double exchange mechanism . . . . . . . . . . . . . . . . . . . . 7 

1.6 Jahn-Teller effect in CMR manganites . . . . . . . . . . . . . . 8 

1.7 Phase separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

1.8 Spin, Charge and Orbital ordering in manganites . . . . . . . 11 

1.9 Pseudogap behavior in manganites . . . . . . . . . . . . . . . . 13 

1.10 Basic properties, phase diagram and CMR effect in Pr 1−x Ca x MnO 3 

system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

2 Experimental Techniques 23 

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 

2.2 Photoemission Spectroscopy . . . . . . . . . . . . . . . . . . . . . 24 

2.2.1 Basic principle . . . . . . . . . . . . . . . . . . . . . . . . . . 24 

2.2.2 Theoretical interpretation of photoemission . . . . . . . . . . . 25 

2.2.3 The electron escape depth . . . . . . . . . . . . . . . . . . . . 29 

2.2.4 Experimental set-up . . . . . . . . . . . . . . . . . . . . . . . 30 

xv

2.3 Inverse Photoemission Spectroscopy . . . . . . . . . . . . . . . . 38 

2.3.1 Historical background . . . . . . . . . . . . . . . . . . . . . . 38 


2.3.3 Basic interpretation of inverse photoemission . . . . . . . . . . 39 

2.3.4 Experimental Set-up . . . . . . . . . . . . . . . . . . . . . . . 41 

2.4 X-ray Absorption Spectroscopy . . . . . . . . . . . . . . . . . . . 45 


2.4.2 Detection techniques . . . . . . . . . . . . . . . . . . . . . . . 47 

2.5 Synchrotron Radiation . . . . . . . . . . . . . . . . . . . . . . . . 49 

3 Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 59 

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 

3.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 

3.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . 62 

3.3.1 The UPS study of Pr 0.67 Ca 0.33 MnO 3 . . . . . . . . . . . . . . 62 

3.3.2 The XAS study of Pr 0.67 Ca 0.33 MnO 3 . . . . . . . . . . . . . . 67 

3.3.3 The Charge Transfer Energy (E CT ) . . . . . . . . . . . . . . . 69 

3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 

4 Electronic Structure of Pr 1−x Ca x MnO 3 77 

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 

4.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 


4.3.1 The UPS study of Pr 1−x Ca x MnO 3 , x = 0.2, 0.33 and 0.4 . . . 80 

4.3.2 The IPES study of Pr 1−x Ca x MnO 3 , x = 0.2, 0.33, 0.4 and 1 . 84 

4.3.3 The core levels of Pr 1−x Ca x MnO 3 , x = 0.2, 0.33, 0.4, and 0.84 87 

4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 

5 Electronic Structure of Ca 0.86 Pr 0.14 MnO 3 96 

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 

5.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 


5.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 

6 Summary and Conclusions 108 

xvi

List of Figures 

1.1 Temperature dependence of the resistivity of La 1−x Sr x MnO 3 (x = 

0.175) under various magnetic fields [2]. . . . . . . . . . . . . . . . . . 4 

1.2 Schematic view of the cubic perovskite structure. . . . . . . . . . . . 5 

1.3 Phase diagram of temperature versus tolerance factor for the system 

R 0.7 A 0.3 MnO 3 , where R is a trivalent rare earth ion and A is a divalent 

alkaline earth ion [12]. . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

1.4 A schematic of the electronic structure of Mn 3+ (a) and Mn 4+ (b). . . 7 

1.5 Schematic features of the double exchange mechanism. . . . . . . . . 8 

1.6 Schematic view of the Jahn-Teller effect for the d 4 ion. . . . . . . . . 9 

1.7 Schematic representation of a macroscopic phase separated state (a), 

as well as possible charge inhomogeneous states stabilized by the longrange 

Coulomb interaction (spherical droplets in (b), stripes in (c)) 

[20]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

1.8 Different types of spin arrangements. . . . . . . . . . . . . . . . . . . 11 

1.9 A Schematic picture of antiferromagnetic CE-type structure with the 

charge and e g orbital ordering. . . . . . . . . . . . . . . . . . . . . . . 12 

1.10 A schematic picture of the antiferromagnetic pseudo-CE-type structure 

in the case of Pr 1−x Ca x MnO 3 (x = 0.4) [38]. . . . . . . . . . . . . . . 13 

xvii

1.11 (a) DOS of the one-orbital model on a 10×10 cluster at J H = ∞ and 

temperature T = 1/130 (hopping t = 1). The four lines from the top 

correspond to densities 0.90, 0.92, 0.94, and 0.97. The inset has results 

at 〈n〉 = 0.86, a marginally stable density at T = 0. (b) DOS of the 

two-orbital model on a 20-site chain, working at 〈n〉 = 0.7, J H = 8, and 

λ = 1.5. Starting from the top at ω − µ = 0, the three lines represent 

temperatures 1/5, 1/10 and 1/20 respectively. Both (a) and (b) are 

taken from Ref. [40]. (c) DOS using a 20-site chain of the one-orbital 

model at T = 1/175, J H = 8, 〈n〉 = 0.87, and at a chemical potential 

such that the system is phase separated in the absence of disorder. W 

regulates the strength of the disorder, as explained in Moreo et. al. 

[41] from where this figure was taken. . . . . . . . . . . . . . . . . . . 14 

1.12 Schematic explanation of pseudogap formation at low electron density 

[40]. In (a) A typical Monte Carlo configuration of t 2g -spin is sketched. 

In (b) the corresponding e g density is shown, with electrons mostly 

located in the FM regions. In (c), the effective potential felt by the 

electrons is presented. A populated cluster band is formed (thick line). 

In (d) the resulting DOS is shown. . . . . . . . . . . . . . . . . . . . 15 

1.13 Phase diagram of Pr 1−x Ca x MnO 3 . Figure taken from ref. [44] . . . . 17 

1.14 (a) Temperature dependence of the resistivity of Pr 1−x Ca x MnO 3 with 

x = 0.3 at various magnetic fields. The inset is the phase diagram 

in the temperature magnetic field plane. The hatched area indicates 

the hysteretic region [45]. (b) The temperature dependence of the 

resistivity of Pr 1−x Ca x MnO 3 (x = 0.5, 0.4, 0.35 and 0.3) at magnetic 

fields of 0, 6 and 12 T [44]. . . . . . . . . . . . . . . . . . . . . . . . . 18 

2.1 Principle of a modern photoemission experiment. The photon source is 

either UV light or X-ray, which hit the sample surface under an angle 

ψ with respect to the surface normal. The kinetic energy E kin of the 

photoelectrons can be analysed by use of electrostatic analysers. . . . 24 

xviii

2.2 Schematic view of the energetics of the photoemission process [3], which 

gives the relation between the energy levels in a solid and the electron 

energy distribution produced by photons of energy hν. The electron 

energy distribution which can be measured by the analyser as a function 

of kinetic energy (E kin ) is more conveniently expressed in terms of 

the binding energy E B when one refers to the density of states inside 

the solid. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 

2.3 ”Universal curve” for electron inelastic mean free path λ. . . . . . . . 30 

2.4 Photoemission spectrometer at Institute of Physics, Bhubaneswar. . . 31 

2.5 Schematic diagram of the VUV source. . . . . . . . . . . . . . . . . . 32 

2.6 Dimensional outline of the VUV source HIS 13. The viewport can be 

used for inspection of the discharge and for adjustment of the light 

spot position on the sample. The dimensions are in mm. . . . . . . . 32 

2.7 Schematic of the X-ray source in operation . . . . . . . . . . . . . . . 33 

2.8 Schematic diagram of the EA 125 hemispherical analyser. . . . . . . . 34 

2.9 The principle of operation of the concentric hemispherical analyser, 

schematic diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 

2.10 Schematic diagram of inverse photoemission spectroscopy (IPES). . . 39 

2.11 (a) IPES spectrometer at UGC-DAE consortium for scientific research, 

Indore, (b) External view of photon detector attached to the IPES 

chamber, and (c) Inside view of the chamber showing a rotatable (left) 

and fixed (right) electron gun and the sample holder . . . . . . . . . . 42 

2.12 A schematic diagram of the electron gun [30]. . . . . . . . . . . . . . 43 

2.13 Schematic cross section of the photon detector showing the window 

(1), O-ring (2), cylindrical cap (3), detector tube (4), teflon spacer (5), 

anode (6), double sided DN40 CF flanges (7, 10), gas inlet and outlet 

(8, 9), barrel connector (11), teflon sleeve (12), DN40 CF flange (13) 

and SHV connector (14). . . . . . . . . . . . . . . . . . . . . . . . . . 44 

2.14 Schematic diagram of x-ray absorption spectroscopy . . . . . . . . . . 46 

2.15 The x-ray loses its intensity via interactions with material. The absorption 

coefficient decreases smoothly with higher energy, except for 

special photon energies. When the photon energy reaches a critical 

value for a core electron transition, the absorption coefficient increases 

abruptly. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 

xix

2.16 Schematic of the Si-photodiode. . . . . . . . . . . . . . . . . . . . . . 49 

2.17 Schematic of the synchrotron radiation. . . . . . . . . . . . . . . . . . 50 

2.18 Schematic representation of an insertion device. . . . . . . . . . . . . 51 

2.19 Optical layout of the WERA soft x-ray beam line, ANKA. . . . . . . 52 

2.20 Optical layout of the BEAR beam line. . . . . . . . . . . . . . . . . . 53 

2.21 Optical layout of the BACH beam line. . . . . . . . . . . . . . . . . . 53 

2.22 The variable included angle monochromator : the combined movement 

of PM1 (plane mirror) and one of the four spherical gratings SG1(-4) 

leads to the monochromatization of the light keeping both the entrance 

and exit slit fixed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 

3.1 Valence band photoemission spectra of Pr 0.67 Ca 0.33 MnO 3 taken using 

He I photon energy (21.2 eV) at 77 (solid line), 110 (dotted), 150 

(dashed), 220 (dot-dashed), and 300 K (double-dot dashed). All the 

spectra have been normalized and shifted along y-axis by a constant 

for clarity. The subbands around -1.2 (e g↑ ), -3.5 (t 2g↑ ), and -5.6 eV (e g↑ 

+ t 2g↑ ) are marked as A, B, and C respectively. . . . . . . . . . . . . 63 

3.2 High-resolution photoemission spectra of the near-E F region of the 

valence band of Pr 0.67 Ca 0.33 MnO 3 . In (a) the spectrum taken below T c 

(red) is compared with the normal state (300 K) spectrum (blue). The 

difference spectrum (green) obtained by subtracting the spectra at 300 

K from 77 K and multiplied by 5 is also shown in the panel. Similarly, 

in (b), (c), and (d) the near-E F spectra taken at 110, 150, and 220 K 

are compared with that taken at 300 K. The feature in the difference 

spectra corresponds to the tail feature A (e g↑ ) in Fig. 3.1. . . . . . . . 64 

3.3 Temperature dependence of the area of the three valence band features 

obtained from fitting the spectra with Lorentzian line shapes using a 

χ 2 iterative program. We have used an integral background, which was 

kept the same for all the spectra. The energy positions and FWHMs 

were determined by finding the best fit common to all the spectra by the 

iterative program. The final fit for all spectra at different temperatures 

were obtained with the same energy positions and FWHMs. (a), (b), 

and (c) correspond to the features A, B, and C positioned at -1.19, 

-3.49, and -5.63 eV with FWHMs 2.56, 1.93, and 1.37 eV respectively. 65 

xx

3.4 O K edge x-ray absorption spectra of Pr 0.67 Ca 0.33 MnO 3 taken at 300 

(solid line), 150 (dashed line), and 95 K (dotted). The pre-edge feature 

centered around 529.5 eV (marked by a box) is where most interest lies. 

Since this feature consists of two peaks (a main line and a shoulder on 

the low-energy side), this part of the spectrum was fitted with two 

components of Lorentzian line shapes. Results of the curve fit are 

given in Table 3.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 

3.5 (a) Combined spectra from valence band photoemission (occupied energy 

states) and pre-peak in O K edge x-ray absorption spectra (unoccupied 

energy states) of Pr 0.67 Ca 0.33 MnO 3 taken at room temperature 

(above T c ). The three features corresponding to the subbands in the 

valence region are marked A, B, and C.The two peaks of the fitted O 

K edge pre-peak are marked A ′ and B ′ . We used integral background 

for both sides of E F . Details of the fitting are mentioned in the text. 

(b) Combined spectra of Pr 0.67 Ca 0.33 MnO 3 below T c . The valence band 

spectrum was taken at 77 K and O K edge XAS was taken at 95 K. . 70 

3.6 Schematic diagram of the near-E F energy levels of Pr 0.67 Ca 0.33 MnO 3 

in the occupied and unoccupied parts. The diagram is not drawn to 

scale. Some of the e g↑ states are occupied (at the Mn 3+ sites) and some 

are unoccupied (at the Mn 4+ sites). Hence the last occupied and first 

unoccupied states are marked with dotted lines. . . . . . . . . . . . . 71 

4.1 Phase diagram of the Pr 1−x Ca x MnO 3 system. In this study we have 

used the x = 0.2, x = 0.33 and x = 0.4 compositions. (FMI = ferromagnetic 

insulating, CO-AFMI = charge ordered antiferromagnetic 

insulating, CG = cluster glass, T N = Neel Temperature, T C = Critical 

Temperature, T CO = charge ordering temperature). . . . . . . . . . . 79 

4.2 The angle integrated valence band photoemission spectra of the Pr 1−x Ca x MnO 3 

(x = 0.0, 0.2, x = 0.33, x = 0.4 and x = 1.0) samples taken at room 

temperature and 77K using He I photons (21.2 eV). . . . . . . . . . . 81 

xxi

4.3 The near E F region of the valence band spectra (panel(a): 300 K, 

panel(b): 77 K) of the three samples plotted against that of CaMnO 3 . 

The difference spectra (lower panel) is obtained by subtracting the 

spectra of CaMnO 3 from those of the three Pr 1−x Ca x MnO 3 compositions 

and multiplied by 5. The difference spectra corresponds to the 

density of e g states. The red and blue peaks are A’ and A” respectively. 

The green line shows the spectral background used in the fitting. . . . 83 

4.4 The IPES spectra of the Pr 1−x Ca x MnO 3 samples (x = 0.2, 0.33 and 

0.4) and the CaMnO 3 sample taken at room temperature. The spectra 

represents the unoccupied electron states of the samples. . . . . . . . 85 

4.5 Panel(a) shows the IPES spectra of the near E F unoccupied states. 

Panel(b) shows the difference spectra obtained by subtracting the spectra 

corresponding to the x = 0.2 from those of other compositions. . . 86 

4.6 The Mn 2p core-level XPS (Mg Kα) spectra of Pr 1−x Ca x MnO 3 taken 

at room temperature (a) and 77 K (b) . . . . . . . . . . . . . . . . . . 88 

4.7 The Ca 2p core-level XPS (Mg Kα) spectra of Pr 1−x Ca x MnO 3 taken 

at room temperature (a) and 77 K (b). . . . . . . . . . . . . . . . . . 89 

4.8 The Pr 4d core-level XPS (Mg Kα) spectra of Pr 1−x Ca x MnO 3 taken 

at room temperature (a) and 77 K (b). . . . . . . . . . . . . . . . . . 90 

5.1 The angle integrated valence band photoemission spectrum of Ca 0.86 Pr 0.14 MnO 3 

taken using 124 eV photons. The spectra taken at different temperatures 

(30 K, 70 K, 110 K, 220 K and 293 K) are normalized for their 

intensities and shifted along Y-axis by a constant for clarity of presentation. 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 

5.2 The high resolution spectra of the near E F region of the valence band 

of Ca 0.86 Pr 0.14 MnO 3 for 30 K, 70 K, 110 K, 220 K and 300 K. This 

feature refers to the e g band. . . . . . . . . . . . . . . . . . . . . . . . 100 

5.3 The O K-edge x-ray absorption spectra of the Ca 0.86 Pr 0.14 MnO 3 taken 

at 30 K, 80 K, 150 K, 200 K and 300 K. The pre-edge feature appears 

between 527 - 533 eV is mostly due to the strong hybridization between 

Mn 3d and O 2p orbitals, where most interest lies. Origin of other 

features are given in the text. . . . . . . . . . . . . . . . . . . . . . . 101 

5.4 The pre-edge peak of the O K -edge XAS spectra of Ca 0.86 Pr 0.14 MnO 3 

for temperatures 30 K, 80 K, 150 K, 200 K and 300 K. . . . . . . . . 102 

xxii

Chapter 1 

Introduction 

1

Introduction 2 

1.1 An overview 

The perovskite type manganites having general formula R 1−x A x MnO 3 (where R is a 

trivalent rare earth element like La, Pr, Nd. and A is a divalent alkaline earth element 

like Sr, Ca, Ba.) have been one of the interesting compounds among the transition 

metal oxide systems due to their intriguing physical property such as colossal magnetoresistance 

(CMR) [1, 2, 3]. CMR is defined as the colossal decrease of resistance by 

the application of magnetic field. These materials are insulators at high temperatures 

and poor metals at low temperatures. Accompanying this insulator-metal transition 

is a magnetic transition from a high temperature paramagnetic phase to a low temperature 

ferromagnetic phase. These transitions occurs around the same transition 

temperature T C . Applying a magnetic field around T C will greatly reduce the resistivity, 

i.e negative magnetoresistance effect. These materials were first described in 

1950 by Jonker and van Santen [4] in perovskite type manganites. 

One year later (1951), Zener [5, 6] explained this unusual correlation between 

magnetism and transport properties by introducing a novel concept, so-called ”Double 

exchange” mechanism (DE). The pioneering work of Zener was followed by Anderson 

and Hasegawa [7] in 1955 and de Gennes [8] in 1960. Double exchange theory 

argues that the electron hopping is related to the relative spin orientations of the 

neighbouring sites. Although it qualitatively explains the connection between the 

insulator metal and the magnetic transitions, it was noticed latter by A. J. Millis et. 

al. [9] that it fails to explain the quantitative change in resistivity and the very low 

T C . Thus it is believed that double-exchange mechanism is not enough to explain 

the CMR effect. Several mechanisms, such as electron-phonon interactions (the polaronic 

effect), electronic phase separation, charge and/or orbital ordering etc. have 

been proposed to account for the discrepancy. Although the detailed picture is still 

unclear, there seems to be a growing consensus that one or more of these additional 

effects probably exists, with the various phenomena both competing and cooperating 

with each other. This can lead to very rich and exotic behaviour, as observed in the 

manganites. This mixed valence manganites have been studied for more than five 

decades but are still considered modern materials because of their wide potential for 

technological applications. Potential applications of the CMR effect in these manganites 

include magnetic sensors, magnetoresistive read heads, magnetoresistive random 

access memory (MRAM) etc.


The temperature and doping dependent metal-insulator transitions in these materials 

are found to be closely related to the unique electronic structure derived from 

the Mn 3d and O 2p hybridized orbitals of MnO 6 octahedra. The 3d orbitals of Mn 

in the MnO 6 octahedra, which are split by the crystal field into t 2g and e g states, 

are further split by the Jahn-Teller distortion. With charge carrier doping, many of 

them show well defined charge ordering (CO) and/or orbital ordering (OO) at low 

temperatures, especially when x equals a commensurate value (e.g x = 0.5). The CO 

state where the Mn 3+ (t 3 2ge 1 g) and Mn 4+ (t 3 2g) ions arranged like in a checkerboard was 

found to bear a strong influence on the one electron band width (W) of the e g band 

and the transfer interaction of the e g holes (electrons). 

The work presented in this thesis consists of the investigations of the electronic 

structure of Pr 1−x Ca x MnO 3 using different spectroscopic techniques as photoemission 

spectroscopy, inverse photoemision spectroscopy and x-ray absorption spectroscopy 

etc. The thesis is structured as follows : Following this overview, the Chapter 1 gives 

a general idea about the physical properties (structural, electronic and magnetic) and 

the mechanisms associated with the CMR manganites. Chapter 2 describes the principles 

and operations of the experimental techniques used for this thesis work. Chapter 

3 presents the electronic structure of Pr 0.67 Ca 0.33 MnO 3 at different temperatures 

using ultra-violet photoelectron and x-ray absorption spectroscopy. The electronic 

structure of the Pr 1−x Ca x MnO 3 series at different temperatures using ultra-violet 

photoemission and inverse photoemission spectroscopy are discussed in Chapter 4. 

In Chapter 5, the electronic structure of Ca 0.86 Pr 0.14 MnO 3 at different temperatures 

using high resolution photoemission and x-ray absorption spectroscopy are discussed. 

Chapter 6 gives the summary and conclusions of this thesis. 

1.2 Colossal magnetoresistance 

Colossal magnetoresistance is a property of some materials, which is the gigantic 

decrease of resistance by the application of magnetic field. The magnetoresistance 

can be defined as, 

MR = 

R(H) − R(0) 

, (1.1) 

R(0) 

where R(H) and R(0) are the resistances with and without magnetic field H respectively. 

Expressing the results as a percentage (i.e multiplying by an additional factor 

100), it has been shown by Jin et. al. [10] that the MR value as large as -100,000


Figure 1.1: Temperature dependence of the resistivity of La 1−x Sr x MnO 3 (x = 0.175) 

under various magnetic fields [2]. 

% near 77 K. Which corresponds to thousand-fold magnetoresistance. The temperature 

dependent resistivity of La 1−x Sr x MnO 3 (x = 0.175) single crystal under several 

magnetic fields are shown in the Fig. 1.1. 

1.3 Crystal structure 

The CMR samples used in this thesis work are of perovskite type (ABO 3 ) structure. 

The general formula for perovskite type manganites is R 1−x A x MnO 3 , where R is the 

trivalent rare earth element such as La, Pr, Nd, and Sm etc. and A is the divalent 

alkaline earth ion such as Sr, Ca, and Ba etc. An ideal cubic perovskite structure is 

shown in the Fig. 1.2, which contains a MnO 6 octahedron, with one Mn ion at the 

center and oxygen ions at each of the six corners. The trivalent or divalent ions (R 

or A) are situated at the corners of the cube. The stability of the cubic perovskite 

structure depends strongly on the size of the cations (R, A or Mn). If there is a 

size mismatch occurs between the cations, then the cubic perovskite structure gets


Figure 1.2: Schematic view of the cubic perovskite structure. 

distorted. This size mismatch is governed by a factor called tolerance factor (t), and 

is defined by Goldschmidt [11] as, 

t = (r A + r O ) 

√ 

2(rMn + r O ) , (1.2) 

where r A , r Mn and r O are the ionic radii of the A-site(R or A), Mn- and O- atoms 

respectively. For cubic perovskite structure t = 1, where the Mn-O-Mn bond angles 

are relatively straight. Due to the ionic size mismatch the Mn-O-Mn bond angles 

deviate from 180 o and lead to distorted perovskite structure. When t decreases (0.96 < 

t < 1), the cubic structure transforms to rhombohedral and then to the orthorhombic 

structure (t < 0.96). The properties of manganites strongly depend on the tolerance 

factor. In 1995 Hwang et. al. [12] has reported the structure-property relationship 

as a function of tolerance factor for 30 % dopant concentration with a variety of 

rare earth and alkaline earth ions, shown in the Fig. 1.3. It shows the presence 

of three regimes: a paramagnetic insulator at high temperature, a low-temperature 

ferromagnetic metal at large tolerance factor and a low-temperature ferromagnetic 

insulator at small tolerance factor. 

1.4 The basic electronic structure 

In manganites the electronically active orbitals are the Mn 3d orbitals. In the cubic 

lattice, the five-fold degenerate 3d-orbitals of an isolated Mn atom or ion are split into


Figure 1.3: Phase diagram of temperature versus tolerance factor for the system 

R 0.7 A 0.3 MnO 3 , where R is a trivalent rare earth ion and A is a divalent alkaline earth 

ion [12]. 

a triple degenerate (lower energy levels) d xy , d yz , and d zx orbitals, called t 2g states 

and a double degenerate (higher energy levels) d x 2 −y 2 and d 3z 2 −r 2 orbitals, called e g 

states [13]. Energy separation between the t 2g states and e g states is 10Dq due to the 

crystal field splitting of MnO 6 octahedra. The schematic of the electronic structure 

from ionic point of view are shown in the Fig. 1.4. Since the e g orbitals point towards 

the six O atoms, they are expected to have a larger hybridization with the O 2p 

states (p x , p y , and p z ) and hence are more delocalized than the t 2g states. So the e g 

state electrons play an important role of conduction electrons in this type of crystals. 

In Mn 3+ ions three electrons occupy the t 2g states and one electron occupies the e g 

states. The strong Hund’s coupling (J H ) in these systems favors the alignment of 

the e g electron spins with the core-like t 2g spins. When the manganites are doped 

with holes through chemical substitution, the Mn 4+ ions are formed. In Mn 4+ ions, 

there are only three electrons occupying the t 2g orbitals. When the excess electron is 

present in case of Mn 3+ ion, a further distortion known as the Jahn-teller distortion 

may lower the crystal symmetry and break the degeneracy of the e g states. This can 

be energetically favorable as the excess electron will fill the lower energy orbital while 

the upper energy orbital remains empty.


Figure 1.4: A schematic of the electronic structure of Mn 3+ (a) and Mn 4+ (b). 

1.5 Double exchange mechanism 

The starting point for understanding the properties of manganites is the so-called 

”double” exchange (DE) mechanism and was first proposed by C. Zener [5, 6] in 

1951, which says the simultaneous hopping of e g elctrons from Mn 3+ sites to O 2− 

sites and then from O 2− to the empty e g orbitals of Mn 4+ sites. This transfer of e g 

electron from Mn 3+ to Mn 4+ by DE is the basic mechanism of electrical conduction 

in manganites. In 1955, the DE theory was expanded by P. W. Anderson, and H. 

Hasegawa [7], where the hopping probability (t ij ) of the e g electron from Mn 3+ to 

neighbouring Mn 4+ is, 

t ij = t cos( θ ij 

), (1.3) 

2 

where θ ij is the angle between the Mn spins in the case of strong Hund’s coupling. 

The concept of double exchange mechanism is illustrated in Fig. 1.5. For parallel spin 

configuration (θ ij = 0) the hopping probability is maximum and for antiparallel spin 

(θ ij = 180 o ), the hopping probability is zero, that means the hopping cancels. Thus it 

predicts maximum conductivity at ferromagnetic state where as insulating behaviour 

at antiferromagnetic state. This provides the qualitative explanation for the close 

connection between the insulator-metal transition and the magnetic transition. It 

also explains why the external magnetic field will increase the conductivity since θ ij 

will be reduced as the external magnetic field polarizes the core (t 2g ) spins of the 

Mn ions. Using mean-field approximations, P. -G. degennes [8] in 1960 suggested 

that the interpolation between the antiferromagnetic state of the undoped limit and 

the ferromagnetic state at finite hole density, where the DE mechanism works, occurs


Figure 1.5: Schematic features of the double exchange mechanism. 

through a ”canted” state, similar as the state produced by a magnetic field acting over 

an antiferromagnetic state. Although DE mechanism gives an intuitive explanation 

for the coupling between the spin and charge degrees of freedom, it fails to predict 

the correct magnitude of the change in resistivity. For paramagnetic case (T > T C ), 

θ ij = 90 o , and so the hopping probability will be reduced to cos ( 90o ) = 0.7 times 

2 

that in the ferromagnetic case. While experimentally it is seen that the conductivity 

decreases across the ferro - para transition may be many orders of magnitude. Which 

says that some other mechanisms beyond DE are necessary to explain the conductivity 

change across the transition. 

1.6 Jahn-Teller effect in CMR manganites 

A. J. Millis et. al. [9] in 1995 pointed out that, the DE model alone is not enough to 

explain the CMR effect in manganites, where he argued that DE produces wrong T C 

by a large factor and the resistivity that grows with reducing temperature (insulating 

behaviour) even below T C . By taking these reasons Millis et. al. [9] concluded that 

the model based only on a large J H is not adequate for the manganites and claimed 

that the necessity of Jahn-Teller phonons to explain the change in curvature of the 

resistivity close to T C . The e g electrons on a given MnO 6 octahedron are strongly 

coupled to the distortions of the O 6 octahedron. In the pseudocubic materials such 

as La 1−x Ca x MnO 3 the strong coupling is due in part to the Jahn-Teller effect, the


Figure 1.6: Schematic view of the Jahn-Teller effect for the d 4 ion. 

essence of which is that a local distortion in which some Mn-O bonds get shorter and 

other gets longer breaks the local cubic symmetry and therefore splits the degeneracy 

of the e g level on that site (Fig. 1.6). If only one electron is present then it will in 

the absence of hybridization reside in the lower level and gain energy. For example, 

a distortion in which the Mn-O bond get longer in the ±z direction and get shorter 

in the ±x,y directions raise the energy of the d x 2 −y2 orbital and lowers the energy of 

the d 3z 2 −r 2 orbital (Q 3 mode). Millis et. al. [14, 15, 17, 18] argued that the physics of 

manganites is dominated by the interplay between a strong electron-phonon coupling 

and the large Hund’s coupling effect that optimizes the electronic kinetic energy by the 

generation of a FM-phase. The large value of electron-phonon coupling in manganites 

is clear in the regime below x = 0.2, where a static JT distortion plays a key role in the 

physics of the material and a dynamical JT effect may persists at higher hole densities 

[16], without leading to long-range order but producing important fluctuations that 

localize electrons by splitting the degenerate e g levels at a given MnO 6 octahedron. 

Millis et. al [14] argued that the ratio λ eff = E JT 

t eff 

(coupling strength) dominates 

the physics of the problem. Here E JT is the static trapping energy at a given octahedron, 

and t eff is an effective hopping that is temperature dependent following the 

standard DE discussion. In this context it was conjectured that when the temperature 

is larger than T C the effective coupling λ eff could be above the critical value that 

leads to the insulating behavior due to electron localization, while it becomes smaller


Figure 1.7: Schematic representation of a macroscopic phase separated state (a), as 

well as possible charge inhomogeneous states stabilized by the long-range Coulomb 

interaction (spherical droplets in (b), stripes in (c)) [20]. 

than the critical value below T C , thus inducing metallic behavior. The calculations 

were carried out using classical phonons and t 2g spins. The results of Millis et. al. 

[14] for T C and the resistivity at a fixed density n = 1 when plotted as a function of 

λ eff had formal similarities with experimental results (which are produced as a function 

of density). In particular, if λ eff is tuned to be very close to the metal-insulator 

transition, the resistivity naturally strongly depends on even small external magnetic 

fields. 

1.7 Phase separation 

Another possibly complementary route to understand the CMR effect in manganites 

is the phase separation. It has been recently predicted from computational studies 

of realistic models that an electronic phase separation can occur in manganites in a 

certain range of doping [19]. In particular at low doping level and at low temperature, 

a phase separation between hole-poor antiferromagnetic (AFM) regions and hole-rich 

ferromagnetic (FM) regions is energetically more favourable than the homogeneous 

canted AFM phase. The energy of charge carrier is minimal for FM ordering. With 

a density insufficient for establishing the FM ordering in the entire sample, the carriers 

concentrate into droplets or stripes which become FM inside insulating AFM 

matrix [20](Fig. 1.7). The fluctuations length scales range from nm to µm. This 

point towards the conduction in metallic phase and the CMR effect is argued as a


Figure 1.8: Different types of spin arrangements. 

result of percolation [21, 22, 23]. On increasing the doping level the FM regions become 

connected to each other and a low electrical resistance is achieved above the 

percolation threshold. There are many reports about the coexistence of FM clusters 

in the CO AFM insulating phase of La 1−x Ca x MnO 3 using different experimental 

techniques [23, 24, 25, 26, 27]. And very recently the neutron diffraction and the 

inelastic neutron scattering studies of Pr 1−x Ca x MnO 3 have shown the coexistence of 

ferromagnetic fluctuations in antiferromagnetic domains [28, 29, 30, 31]. 

1.8 Spin, Charge and Orbital ordering in manganites 

The unusual properties of manganites challenge the current topic of research in the 

field of strongly correlated electrons system due to their strong spin, charge and 

orbital degrees of freedom. The strong interplay between spin, charge and orbital 

ordering originating from the single occupancy of the doubly degenerate e g orbitals 

of Mn 3+ (t 3 2g e1 g ) ions make the phase diagram of these compounds rich with physics. 

With charge carrier doping, some of them for example, La 1−x Ca x Mon 3 system show 

strong charge and/or orbital ordered (CO/OO) insulating nature at low temperatures, 

especially when x equals a commensurate value. The CO state where the Mn 3+ (t 3 2ge 1 g) 

and Mn 4+ (t 3 2g ) are arranged like a checkerboard influences strongly the one electron 

bandwidth of the e g band [32] and the transfer interaction of the e g holes (electrons). 

There are various types of ordering occurs across the phase diagram of manganites, 

which are described below. 

The lattice distortion in manganites is due to the J-T interaction acting on the 

Mn 3+ ion, which lifts the degenerate e g orbitals in the cubic electronic field. The J-T


Figure 1.9: A Schematic picture of antiferromagnetic CE-type structure with the 

charge and e g orbital ordering. 

distorted Mn 3+ O 6 in LaMnO 3 form an orthorhombic structure (space group P bnm ) 

where the e g charge occupies alternating 3d 3x 2 −r 2 and 3d 3y 2 −r2 orbitals. The magnetic 

structure of LaMnO 3 was found to be of A-type antiferromagnetic, where all Mn 3+ 

moments lying in c-plane are aligned ferromagnetically and the the FM layer stacks 

antiferromagnetically along the c-axis by the weaker superexchange interaction along 

this axis. The A-type AFM ordering exists below 140 K [33]. Different types of spin 

arrangements are shown in the Fig. 1.8. Upon substituting divalent cations such 

as Ca, Sr, Pb etc. to the parent compound like LaMnO 3 , the magnetic structure 

changes from A-type AFM to the FM with the rotation of the spin direction from 

the b-axis in the c-plane towards the c-axis associated with the metallic conduction, 

which was interpreted successfully in terms of DE mechanism at an early stage of the 

investigation. Further increase of doping, the charge ordered (CO) state near x = 0.5 

is observed. For example in La 1−x Ca x MnO 3 (x = 0.5), the ordering becomes C-E type 

(Fig. 1.9), where the Mn 3+ and Mn 4+ line up forming alternate stripes in the C plane 

and stack also along the c-axis. This phenomenon in manganites was first observed 

by Wollan and Koehler [34] in the neutron powder diffraction of La 0.5 Ca 0.5 MnO 3 . 

Later these charge-ordering phases are observed in the low band width compound 

Pr 1−x Ca x MnO 3 with a broad doping region 0.3 < x < 0.75 [28, 35, 36, 37, 38]. 

The CO state for Pr 0.6 Ca 0.4 MnO 3 is called pseudo-CE type ordering [38] is shown


Figure 1.10: A schematic picture of the antiferromagnetic pseudo-CE-type structure 

in the case of Pr 1−x Ca x MnO 3 (x = 0.4) [38]. 

in Fig. 1.10, where the arrangement of spin is canted along the c-axis while the 

CE-type ordering is kept within the orthorhombic ab-plane. For the ground state 

of R 1−x A x MnO 3 , i.e AMnO 3 (x = 1), the magnetic structure is called G-type AFM 

[34, 39] ordering in which the spins on all Mn 4+ sites are antiparallel to their nearest 

neighbours. 

1.9 Pseudogap behavior in manganites 

The conducting properties of the materials depend on the finer changes associated 

with the density of states (DOS) near the Fermi level. If there are no allowed states 

at the Fermi level a gap forms by making the system insulating and finite density of 

states makes the system metallic. A Pseudogap exists in the mixed-phase regimes of 

manganites, which suggests that the prominent depletion of spectral weight (density 

of states) at the chemical potential, but not exactly zero (few states associated with 

it). This feature is similar to that extensively discussed in high T c superconductor. 

The calculations in the pseudogap context in manganites was carried out by Moreo 

et. al. [40, 41] using both one- and two-orbital models (Fig. 1.11). The density of 

states of the one-orbital model on 2D cluster varying the electronic density slightly 

below 〈n〉 = 1.0 is shown in the Fig. 1.11(a). At zero temperature, this density 

regime is unstable due to phase separation, but at the temperature of simulation 

those densities still correspond to stable states, but with a dynamical mixture of


Figure 1.11: (a) DOS of the one-orbital model on a 10×10 cluster at J H = ∞ and 

temperature T = 1/130 (hopping t = 1). The four lines from the top correspond to 

densities 0.90, 0.92, 0.94, and 0.97. The inset has results at 〈n〉 = 0.86, a marginally 

stable density at T = 0. (b) DOS of the two-orbital model on a 20-site chain, working 

at 〈n〉 = 0.7, J H = 8, and λ = 1.5. Starting from the top at ω − µ = 0, the three 

lines represent temperatures 1/5, 1/10 and 1/20 respectively. Both (a) and (b) are 

taken from Ref. [40]. (c) DOS using a 20-site chain of the one-orbital model at 

T = 1/175, J H = 8, 〈n〉 = 0.87, and at a chemical potential such that the system is 

phase separated in the absence of disorder. W regulates the strength of the disorder, 

as explained in Moreo et. al. [41] from where this figure was taken.


Figure 1.12: Schematic explanation of pseudogap formation at low electron density 

[40]. In (a) A typical Monte Carlo configuration of t 2g -spin is sketched. In (b) the 

corresponding e g density is shown, with electrons mostly located in the FM regions. 

In (c), the effective potential felt by the electrons is presented. A populated cluster 

band is formed (thick line). In (d) the resulting DOS is shown. 

AFM and FM features. A clear minimum in the density of states at the chemical 

potential can be observed. Very similar results also appear in 1D simulations [40]. 

Fig. 1.11(b) shows the results for two-orbitals and a large electron-phonon coupling, 

at a fixed density and changing temperature. Clearly a pseudogap develop in the 

system as a precursor of the phase separation that is reached as the temperature is 

further reduced. Similar results have been obtained in other parts of parameter space, 

as long as the system is near unstable phase separated regimes. A pseudogap appears 

also in cases where disorder is added to the system. Fig. 1.11(c) shows the results 

for the case where a random on-site energy is added to the one orbital model. 

Moreo et. al. [40] explained this phenomenon tentatively for the case of ”without 

disorder”. A schematic of explanation is shown in the Fig. 1.12. In Fig. 1.12(a) a 

typical mixed phase FM-AFM state is sketched. In the FM regions, the e g electrons 

improve their kinetic energy, and thus they prefer to be located in those regions as 

shown in Fig. 1.12(b). The FM domains act as effective attractive potentials for 

electrons, as shown in Fig. 1.12(c). When other electrons are added FM clusters 

are created and new occupied levels appear below the chemical potential, creating 

a pseudogap [Fig. 1.12(d)]. These results are compatible with the photoemission


experiments by Dessau et. al. [42] for bilayer manganites (La 2−2x Sr 1+2x Mn 2 O 7 , x = 

0.4). In this experiment it was observed that the low-temperature ferromagnetic 

state was very different from a prototypical metal. Its resistivity is unusually high, 

the width of ARPES features are anomalously broad, and they don’t sharpen as they 

approach the Fermi momentum. In addition, the centroid of the near E F peak never 

approach close than approximately 0.65 eV to the Fermi level. This implies that 

even in the expected metallic regime, the density of states at the Fermi level is very 

small. Dessau et. al. [42] refers to these results as the formation of a pseudogap. This 

effect is present both in ferromagnetic and paramagnetic regimes. Scanning tunneling 

microscopy data of single crystals and thin films of hole doped manganites by Biswas 

et. al. [43] showed a rapid variation in the density of states for the temperatures 

near the Curie temperature, such that below T c a finite density of states is observed 

at the Fermi level while above T c a hard gap opens up. This result suggests that 

the presence of a gap or pseudogap is not just a feature of bilayers, but it appears in 

other manganites as well. 

1.10 Basic properties, phase diagram and CMR 

effect in Pr 1−x Ca x MnO 3 system 

The Pr 1−x Ca x MnO 3 is a low band width manganite. The band width W is smaller 

than in other manganites like La 1−x Sr x MnO 3 and La 1−x Ca x MnO 3 system. In the lowbandwidth 

compounds, a charge-ordered (CO) state is stabilized in the vicinity of x = 

0.5, while manganite with large W (La 1−x Sr x MnO 3 as example) present a metallic 

phase at this concentration. The Pr 1−x Ca x MnO 3 presents a stable CO state in a 

broad density region between x = 0.3 and 0.75, as showed by Jirak et. al [38]. Part of 

the phase diagram is shown in the Fig. 1.13 [44]. A ferromagnetic insulating (FMI) 

state exists in the range from x = 0.15 to 0.30. For x≥0.30, an antiferromagnetic CO 

state is stabilized. Neutron diffraction studies by Jirak et. al. [38] showed that at 

all densities between 0.30 and 0.75, the arrangement of charge/spin/orbital order of 

this state is similar to the CE-state (Fig. 1.9). However, certainly the hole density is 

changing with x, and as consequence the CE-state can not be perfect at all densities 

but electrons have to be added or removed from the structure. Jirak et. al. [38] 

discussed a ”pseudo”-CE-type structure for x = 0.4 that has the proper density. 

The effect of magnetic fields on the CO-state of Pr 1−x Ca x MnO 3 is remarkable. The


Figure 1.13: Phase diagram of Pr 1−x Ca x MnO 3 . Figure taken from ref. [44] 

resistivity vs. temperature graph of Pr 1−x Ca x MnO 3 (x = 0.3) under various magnetic 

fields [45] is shown in the Fig. 1.14(a). At low temperatures, changes in resistivity 

by several orders of magnitude can be observed. The stabilization of metallic state is 

realized upon the application of magnetic field. This state is ferromagnetic according 

to magnetization measurements, and thus it is curious to observe that a state not 

present at zero field in the phase diagram, is nevertheless stabilized at finite fields. 

The magnetic field effects on the other compositions of Pr 1−x CaxMnO 3 (x = 0.35, 

0.4 and 0.5) are also shown in the Fig. 1.14(b). The shape of the curves in Fig. 1.14 

present a large magnetoresistance effect, and a possible origin based on percolation 

between the CO- and FM-phase. Tomioka et. al. [46] showed that, as x grows away 

from x = 0.3, larger fields are needed to destabilize the charge-ordered state at low 

temperatures (e.g., 27 T at x = 0.50 compared with 4 T at x = 0.30). 

This thesis is based on the study of electronic structure of Pr 1−x Ca x MnO 3 series 

using different spectroscopic techniques.


Figure 1.14: (a) Temperature dependence of the resistivity of Pr 1−x Ca x MnO 3 with 

x = 0.3 at various magnetic fields. The inset is the phase diagram in the temperature 

magnetic field plane. The hatched area indicates the hysteretic region [45]. (b) The 

temperature dependence of the resistivity of Pr 1−x Ca x MnO 3 (x = 0.5, 0.4, 0.35 and 

0.3) at magnetic fields of 0, 6 and 12 T [44].

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Chapter 2 

Experimental Techniques 

23

Experimental Techniques 24 

2.1 Introduction 

Spectroscopic methods are some of the most powerful techniques for studying the 

electronic structure of solids. This chapter is intended to describe the details of the 

different spectroscopic techniques which have been used in this thesis work. 

2.2 Photoemission Spectroscopy 

2.2.1 Basic principle 

The principle of photoemission spectroscopy is based on the principle of photoelectric 

effect. The photoelectric effect was first discovered by the German Physicist Heinrich 

Hertz [1] in 1887 and latter explained by Albert Einstein [2] in 1905 as a manifestation 

of the quantum nature of light. 

Figure 2.1: Principle of a modern photoemission experiment. The photon source is 

either UV light or X-ray, which hit the sample surface under an angle ψ with respect 

to the surface normal. The kinetic energy E kin of the photoelectrons can be analysed 

by use of electrostatic analysers. 

When light is incident on a sample, electron can absorb the photon and escape 

from the sample with a maximum kinetic energy as shown in the Fig. 2.1. If a photon 

having energy hν is absorbed by the electron of binding energy E B , then the kinetic 

energy E kin of the electron is given by, 

E kin = hν − E B − φ, (2.1) 

where φ is the work function of the material. The resulting photoelectron spectra 

is a plot of the number of emitted electrons versus their kinetic energy, which 

gives the information about the electronic structure of the material. Fig. 2.2 shows


Figure 2.2: Schematic view of the energetics of the photoemission process [3], which 

gives the relation between the energy levels in a solid and the electron energy distribution 

produced by photons of energy hν. The electron energy distribution which 

can be measured by the analyser as a function of kinetic energy (E kin ) is more conveniently 

expressed in terms of the binding energy E B when one refers to the density 

of states inside the solid. 

schematically how the energy level diagram and the energy distribution of photoemitted 

electrons relate to each other. 

2.2.2 Theoretical interpretation of photoemission 

There have been many important theoretical studies to describe and analyse the spectral 

features in photoemission process [3, 4, 5, 6]. The most general and widely applied 

theoretical description of the photoemission spectrum is based on using Fermi’s golden 

rule as a result of perturbation theory in first order. The photo current produced in 

the photoemission spectroscopy experiment is due to the excitation of electrons from 

the initial states i with wave function ψ i to the final states f with wave function ψ f 

by the photon field having vector potential A. According to the Fermi’s Golden Rule,


the transition probability can be written as, 

w ∝ 2π¯h |〈ψ f|r|ψ i 〉| 2 δ(E f − E i − hν) = 2π¯h m fiδ(E f − E i − hν) (2.2) 

where m fi is the square of the transition matrix element. To discuss about the 

transition matrix element, one has to take certain assumptions of the wave functions 

contained in it. In the simplest approximation one can take a one electron view for 

the initial and final state wave functions. The final state is in addition with a free 

electron having kinetic energy E kin . The initial state wave function is then written as 

a product of the orbital φ k from which the electron is excited and the wave function 

of the remaining electrons ψi,R(N k −1), assuming that system under consideration has 

N electrons, where the index k indicates that the electron k has been left out and R 

refers to the remaining. Now one can write, 

ψ i (N) = Cφ i,k ψ k i,R(N − 1) (2.3) 

where C is the operator that antisymmetrizes the wave function properly. 

Similarly the final state can be written as a product of the wave function of the 

photoemitted electron φ f,Ekin and that of the remaining N −1 electrons ψf,R k (N −1), 

ψ f (N) = Cφ f,Ekin ψf,R k (N − 1) (2.4) 

So the transition matrix element can be written as, 

〈ψ f |r|ψ i 〉 = 〈φ f,Ekin |r|φ i,k 〉〈ψf,R(N k − 1)|ψi,R(N k − 1)〉 (2.5) 

Thus the matrix element is the product of one electon matrix element and the 

(N − 1) electron overlap integral. Let us assume that the remaining orbitals (called 

the passive orbitals) are the same in the final state as they were in the initial state, 

meaning ψf,R(N k − 1) = ψi,R(N k − 1), which renders the overlap integral unity and 

the transition matrix element is just the one-electron matrix element. Under this 

assumption [7] the PES experiment measures the negative Hartree-Fock orbital energy 

of the orbital k meaning, 

which is sometimes called Koopman’s binding energy. 

E B,k ≃ − ǫ k , (2.6)


Let us now assume that the final state with N − 1 electrons has s excited states 

with the wave function ψf,s k and energy E s (N − 1). 

The transition matrix element can be calculated by summing over all the possible 

excited final states yielding, 

〈ψ f |r|ψ i 〉 = 〈φ f,Ekin |r|φ i,k 〉 ∑ s 

c s (2.7) 

with 

c s = 〈ψf,s k (N − 1)|ψk i,R (N − 1)〉 (2.8) 

Here |c s | 2 is the probability that removal of electron from orbital φ k of the N 

electron ground state leaves the system in the excited state s of the N − 1 electron 

system. For strongly correlated systems many of the c s ’s are nonzero. In terms of PE 

spectrum this means that for s = k one has the so-called main line and for the other 

non-zero c s additional so-called satellite lines occur. If the correlations are weak, then 

ψ k f,s(N − 1)≃ψ k i,R(N − 1) (2.9) 

meaning that c s 2 ≃ 1 for s = k and c s 2 ≃ 0 for s ≠ k, i.e one has only one peak. 

Now the photocurrent I detected in a PE experiment as, 

I∝ ∑ |〈φ f,Ekin |r|φ i,k 〉| 2∑ 

f,i,k 

s 

|c s | 2 δ(E f,kin + E s (N − 1) − E 0 (N) − hν, (2.10) 

where E 0 (N) is the ground state energy of the N-electron system. The photo 

current thus consists of the lines created by photoionizing the various orbitals k where 

each line can be accompanied by satellites according to the number of excited states 

s created in the photoexcitation of that particular orbital k. 

The above discussion is very convenient for atoms and molecules [4]. However, 

it is not always easy to apply for solids. For solids, a slightly different formalism 

has been developed [3, 8, 9] and hence the above equation can be rewritten in the 

following form 

I∝ ∑ f,i,k|〈φ f,Ekin |r|φ i,k 〉| 2 A(k, E) (2.11) 

where A(k, E) is the so-called spectral function for the wave number k and energy 

E. The spectral function can be related to the single particle Green’s function by


A(k, E) = π −1 Im{G(k, E)} (2.12) 

Here A(k, E) describes the probability of removing (below the E F ) or adding 

(above the E F ) an electron with the energy E and the wave vector k from (to) the 

N-electron system (Interacting). 

For a non interacting system with the one electron energy Ek, 

0 

G 0 (K, E) = 

where ǫ is very small 

and 

1 

(2.13) 

E − Ek 0 − iǫ, 

A 0 (k, E) = δ(E − E 0 k) (2.14) 

which means the spectral function is a δ-function at E = Ek 0 . This result is 

identical to that of equation (2.6) as in a noninteracting system the binding energy 

is by definition the Koopman’s binding energy. 

In an interacting electron system the electron energy gets renormalized by the 

so-called self energy Σ(k, E) = Re{Σ(k, E)} + iIm{Σ(k, E)}, yielding 

and 

G(K, E) = 

1 

E − E 0 k − Σ(k, E) (2.15) 

A(k, E) = 1 π 

Im{Σ(k, E)} 

[E − E 0 k − Re{Σ(k, E)}]2 + [Im{Σ(k, E)}] 2 (2.16) 

If we assume that the spectrum of the interacting system is not too different 

from that of the noninteracting system (Im{Σ(k, E)}), then the poles in the Green’s 

function of the interacting one occur at 

with the condition 

E 1 k = Re{E1 k } + iIm{E1 k } (2.17) 

E 1 k − E0 k − Σ(k, E1 k ) = 0 (2.18) 

This allows to separate the Greens function into two parts, a pole part and a 

non-singular term G inc , i.e,


and 

G(k, E) = 

Z k 

E − (Re{E 1 k } + iIm{E1 k }) + G inc (2.19) 

A(k, E) = 1 π 

Z k Im{E 1 k } 

(E − Re{E 1 k })2 + (Im{E 1 k })2 + A inc (2.20) 

where Z k is a number smaller than one. The first term in (2.19) is similar to 

the result for the noninteracting system, and one can therefore say that it describes 

a slightly modified electron (with renormalized mass) which is generally called a 

quasiparticle. The first term in (2.19) is called the coherent part of the Green’s 

function, it gives rise to the coherent part of the spectrum. And the second term in 

(2.19) is called the incoherent part of the Green’s function, which gives rise to the 

incoherent part of the spectrum. In a very approximate way one can identify the 

coherent part of the spectrum with the main line and the incoherent one with the 

satellites in equation (2.20) [3]. 

2.2.3 The electron escape depth 

In photoemission spectroscopy, when electrons are impinged on, or emitted from a 

sample surface, the fundamental phenomenon occurs is inelastic scattering of electrons. 

The escape depth of electron from the sample surface is depend on the inelastic 

mean free path (IMFP) λ, which is the average distance between the scattering 

events. λ is small for low energy electrons in solids and hence gives rise to surface 

sensitivity. So ultra - high vacuum (UHV) condition is necessary to study the solid 

surfaces [3, 10, 11]. Because a monolayer is adsorbed on the surface of the sample 

in about 2.5 Langmuirs (1L = 10 −6 torr.s). At a pressure of 10 −9 torr it therefore 

takes about 1000 seconds, or less than an hour, until a coverage of one monolayer is 

achieved. Thus in order to get spectra of clean surfaces, vacuum of even better than 

10 −10 torr is mandatory. The IMFP of an electron in a solid is a strong function of 

its kinetic energy [3, 12, 13], and the relationship termed as the ”universal curve” as 

shown in Fig. 2.3. 

The electron kinetic energy in the range of interest between about 10 and 2000 

eV. The escape depth is only on the order of a few Å. From the universal curve, one 

can see the IMFP has a minimum value of 5 to 10 Å for the kinetic energy range


Figure 2.3: ”Universal curve” for electron inelastic mean free path λ. 

of 20 to 200 eV. And hence the surface sensitivity of photoemission spectroscopy is 

highest in this energy range. 

2.2.4 Experimental set-up 

The OMICRON photoemission system at Institute of Physics consists of two connected 

UHV chambers, a sample preparation chamber and an analysis chamber. The 

preparation chamber is equipped with a precision manipulator, sample heater, evaporator 

and a diamond file. The analysis chamber is made up of µ metal to protect low 

energy electrons from earth’s magnetic field and stray magnetic fields. It is equipped 

with a 125 mm hemispherical analyser [10] for angle integrated photoemission and a 

movable 65 mm hemispherical analyser mounted on a 2-axis goniometer is also there 

for angle resolved photoemission. Two photon sources [10, 14] (ultra-violet and x-ray) 

are used for photoexcitations. The analysis chamber is also equipped with a 4-axis 

sample manipulator-cum cryostat which can go down to 20 K. Facility for performing 

low energy electron diffraction (LEED) is also available in the analysis chamber. The 

external view of the spectrometer is shown in the Fig. 2.4. Some of the important 

components of the spectrometer are described below.


Figure 2.4: Photoemission spectrometer at Institute of Physics, Bhubaneswar.


Figure 2.5: Schematic diagram of the VUV source. 

Figure 2.6: Dimensional outline of the VUV source HIS 13. The viewport can be 

used for inspection of the discharge and for adjustment of the light spot position on 

the sample. The dimensions are in mm. 

Vacuum ultra-violet photon source (VUV source HIS 13) 

The high intensity VUV Source [14] HIS 13 can be operated with various discharge 

gases, such as helium, neon, argon, krypton, xenon or hydrogen. But in our laboratory 

we use only helium and neon gases. The operation of the lamp is based on the principle 

of a cold cathode capillary discharge [15]. If an increasing potential is applied between 

the ends of an insulating tube filled with gas at a pressure of typically 1 mbar, a


point will be reached when spontaneous breakthrough occurs leading to a continuous 

discharge. The ignition potential is about an order of magnitude higher than the 

operating potential necessary to maintain a continuous discharge. The schematic of 

the VUV source is shown in the Fig. 2.5. There is a windowless direct-sight connection 

between the discharge area and the target. The discharge current is electronically 

stabilised. The lamp is water cooled in order to allow for high discharge currents 

(up to 300 mA) and to reduce electrode degradation resulting in prolonged service 

intervals. The dimensional outline of the VUV source is shown in the Fig. 2.6 

X-ray source (DAR 400) 

The X-ray source we used is DAR 400 from OMICRON designed specifically for X-ray 

photoelectron spectroscopy [10]. It has twin anodes that allow either Mg Kα (1253.6 

eV) or Al Kα (1486.6 eV) radiation to be selected. The schematic of the X-ray source 

is shown in Fig. 2.7. 

Figure 2.7: Schematic of the X-ray source in operation 

Electrons are extracted from a heated filament to bombard the selected surface of 

an anode at high positive potential. The focus ring and the shape of the nose cone 

ensure that the electrons hit the anode in the right area. By switching the filament


used, the second face of the anode can be excited. The anode is water cooled to 

prevent the aluminium or magnesium surfaces from evaporating. 

X-rays generated in the surface of the anode pass through a thin aluminium window 

to the sample under analysis. The aluminium window forms a partial vacuum 

barrier between the source and the sample region. 

The hemispherical electron energy analyser (EA 125) 

The EA 125 energy analyser based on a 125 mm mean radius electrostatic hemispherical 

deflection analyser composed of two concentric hemispheres [10, 16]. The inner 

and outer spheres are biased positive and negative with respect to the pass energy of 

the analyser. The analyser disperses electrons according to the energy across the exit 

plane (between the two hemispheres) and focuses them in the angular dimension from 

the entrance to the exit plane. Slits which are variable are located at the entrance 

and exit of the analyser. A schematic of the EA 125 analyser is shown in Fig. 2.8. 

Figure 2.8: Schematic diagram of the EA 125 hemispherical analyser. 

The principle of operation of concentric hemispherical analyser is shown schematically 

in Fig. 2.9. It consists of two concentric hemispheres of radii R 1 (inner hemisphere) 

and R 2 (outer hemisphere) with mean radius R 0 mounted with a common 

centre O. A potential V is applied between the two hemispheres so that the outer is


negative and the inner positive with respect to V 0 which is the median equipotential 

surface between the hemispheres. 

Figure 2.9: The principle of operation of the concentric hemispherical analyser, 

schematic diagram. 

If E p is the kinetic energy of an electron travelling in an orbit of radius R 0 , then 

the relationship between E p and V 0 is given by the expression, 

eV 0 = E p · 

( 

R2 

− R ) 

1 

R 1 R 2 

(2.21) 

The voltages on the inner and outer hemispheres are V 1 and V 2 respectively and 

these are given by, 

V 1 = E p · 

V 2 = E p · 

(a) Modes of analyser operation 

( 

2 R ) 

0 

− 1 

R 1 

( 

2 R ) 

0 

− 1 

R 2 

(2.22) 

(2.23) 

The kinetic energy can be scanned either by varying the retardation ratio whilst 

holding the analyser pass energy constant known as constant analyzer energy (CAE)


mode or by varying the the pass energy E p whilst holding the retardation ratio constant 

known as the constant retard ratio (CRR) mode [10]. 

i. Constant retard ratio (CRR) mode 

In the CRR mode electrons entering the analyser system from the sample are 

retarded by the lens stack by a constant proportion of their kinetic energy so that 

the ratio of electron kinetic energy to analyser pass energy is kept constant during a 

spectrum. The retard ratio k is defined as, 

K = E k − φ a 

E p 

∼ E k 

E p 

(2.24) 

where φ a is the analyser work function. Throughout the scan range the pass energy 

of the analyser is continuously varied to maintain a constant retard ratio. Sensitivity 

and resolution are proportional to the pass energy and therefore the kinetic energy 

in this mode. In this mode the analysed sample area and the emission angle remain 

almost constant throughout the whole kinetic energy range. 

ii. Constant analyser energy (CAE) mode 

In the CAE mode, the analyser pass energy is held constant, and the retarding 

voltage is changed thus scanning the kinetic energy of detected electrons. The resolution 

obtained in CAE is constant throughout the whole kinetic energy range. The 

sensitivity, however is inversely proportional to the kinetic energy and at low kinetic 

energies is improved over that of CRR. In the CAE mode, the analysed sample area 

and the emission angle may vary slightly with the kinetic energy, but this is dependent 

on the lens design. 

(b) Analyser resolution 

The analyser is a band pass energy filter for electrons at a specific energy E p 

and has a finite energy resolution ∆E which is dependent on the chosen mode of 

operation and specific operating conditions. The energy resolution of the analyser is 

given approximately by,


∆E = E p 

( d 

2R 0 

+ α 2 ) 

(2.25) 

where d is the slit width, R 0 is the mean radius of the hemispheres and α is the 

half angle of electrons entering the analyser at the entrance slit. 

(c) The electrostatic input lens 

The input lens collects the electrons from the source and focuses them onto the 

entrance aperture of the analyser whilst simultaneously adjusting their kinetic energy 

to match the pass energy of the analyser. The lens is also designed to define the analysis 

area and angular acceptance of electrons which pass through the hemispherical 

analyser. The lens design employs double lens concept where two lenses are stacked 

one above the other. 

The first lens selects the analysis area (spot size) and angular acceptance. This is 

an Einzel lens, i.e it does not change the energy of the electrons and therefore has a 

constant magnification throughout the entire energy range. This lens can be operated 

in three discrete magnification modes: high, medium and low. In high magnification 

mode, the focal plane is nearer to the sample and the lens accept a wide angle of 

electron beams from a small region. In low magnification mode, the focal plane is 

farther from the sample and the lens accept only a small angle of beams but from a 

larger area. The medium magnification is in-between the two. 

The second lens retards or accelerates the electrons to match the pass energy of 

the analyser and uses the zoom lens function to ensure that the focal point remains 

on the analyser entrance aperture. The magnification of this lens varies with retard 

ratio as a result of the law of Helmholtz-Lagrange. 

The analysis area is defined by the combination of the selected analyser entrance 

aperture and the magnification of the entire lens. The magnification of the entire lens 

is a product of the magnifications of the two discrete lenses. 

(d) The detector 

A single channel electron multiplier (Channeltron) is placed across the exit plane 

of the analyser. The channeltron amplifies the current of a single electron by a factor 

of about 10 8 . The small current pulse present at the output of the channeltron is


passed through a vacuum feedthrough and then directly into the preamplifier. And 

then the signal is passed onto a pulse counter for processing and production of an 

electron energy spectrum. 

2.3 Inverse Photoemission Spectroscopy 

2.3.1 Historical background 

The inverse photoemission spectroscopy (IPES) [17, 18, 19] is a powerful technique 

to study the unoccupied electronic states of solids. The IPES experiment can be 

carried out in two different photon energy regimes. One is at higher energy range 

(detected photons are in x-ray range) called Bremsstrahlung isochromat spectroscopy 

(BIS) and the other one is at low energy region (detected photons are in the ultraviolet 

range) which is called UV-IPES or simply IPES. The history of IPES technique 

was developed in 1915 by Duane and Hunt [20] when an inverse relationship is observed 

between the short wave-length cutoff of emitted x-rays and incident electron 

energy when a solid is bombarded with electrons. Then pursued by Ohlin [21] and 

Nijboer [22] in 1940s and was resumed in the 1950s by Ulmer and Vernickel [23]. The 

IPES work in the ultra-violet region is of relatively recent origin. The experimental 

work was started by Dose [24] and the theoretical work by Pendry [25, 26]. The 

experimental advances since then have been rapid. 


When a beam of electrons is incident on a solid surface, after entering the system, 

the electrons decay either radiatively or non-radiatively to states at lower energy. If 

the decay is radiative the emitted photon can be detected, and gives the information 

about the unoccupied states of the system. So this is complementary to direct photoemission 

spectroscopy (PES) [17, 18, 19, 27]. A schematic of the inverse photoemission 

spectroscopy is shown in the Fig. 2.10 

The energy of the photons (hν) emitted when a beam of electrons having kinetic 

energy E k is incident on a solid surface and then relax to a lower energy unoccupied 

state (E f ), is given by the conservation of energy as, 

E k = hν + E f − φ, (2.26)


Figure 2.10: Schematic diagram of inverse photoemission spectroscopy (IPES). 

where φ is the work function of the material. 

There are two modes [18, 19, 27] used for this measurement . One is isochromat 

mode and the other one is spectrograph mode. In isochromat mode, the incident 

electron energy is ramped and the emitted photons are detected at a fixed energy. 

In the spectrograph mode, the energy of the incident electron remains fixed and the 

emitted photons are detected over a range of photon energies. 

2.3.3 Basic interpretation of inverse photoemission 

The Inverse photoemission process can be described [28] by a comparison with photoemission. 

Both photoemission and inverse photoemission involve the interaction 

between photons and electrons. Now the interaction Hamiltonian associated with the 

electromagnetic field can be written as,


H ′ = 

e (A.p + p.A) (2.27) 

2mc 

where A is the vector potential of the electromagnetic radiation. For photoemission 

the field A can be treated as a classical time-dependent perturbation, and 

results will be the same as a proper quantum treatment had been made. For inverse 

photoemission, however, this is no longer true. Since photons are created it becomes 

necessary to quantize the electromagnetic field. In such a treatment the classical 

vector potential is replaced by the field operator A(x, t) defined by [29] 

A(x, t) = 

1 ∑ ∑ 2π¯hc 

(V p ) 1/2 q α ω [a q,α(t)ˆε (α) e iq·x + a † q,α(t)ˆε (α) e −iq·x ], (2.28) 

where ˆε α is the linear polarization vector, a real unit vector whose direction depends 

on the photon propagation direction q. The two operators a † q,α and a q,α either 

create or destroy a photon in the state q, α respectively. V p is the normalization 

volume for the photon. For inverse photoemission the initial state consists of an electron 

in a continuum state |i〉≡ψ i (r) with no photons and the final state consists of 

an electron in a bound state |f〉≡ψ f (r) and a photon with wave vector q. Then the 

transition rate is given by the above two equations using the first order perturbation 

theory, 

R = 

2π¯h 

c22π¯h 

ω 

where ρ p is the photon density of states 

and dΩ is the solid angle of emission. Then 

1 

|〈f|ˆε.p|i〉| 2 ρ p (2.29) 

V p 

V p 

ω 2 

ρ p = 

(2π) 3 ¯hc 3dΩ 

(2.30) 

R = 1 e 2 ω 

2π ¯hc m 2 c 2 |〈f|ˆε.p|i〉|2 dΩ, (2.31) 

where e2 

¯hc = α≈ 1 

137 

To obtain a cross section, we devide by the incident electron flux 

j e = ¯hk 1 

(2.32) 

m V e 

We further assume that the continuum electron state is normalized to a box of 

volume V e . Then


( ) dσ 

= α ω 1 

dΩ 2π mc 

IPES 2 ¯hk |〈f|ˆε.p|i〉|2 (2.33) 

It is interesting to compare this cross section with that of the photoemission. 

In photoemission the final states are those of electrons, The density of states factor 

becomes 

ρ e = V e mk 

8π 3 ¯h 2 (2.34) 

and the incident flux is the photon flux 

Then 

j p = 

ω 1 

(2.35) 

8π¯hc V p 

( ) dσ 

= α k 1 

dΩ 2π m ¯hω |〈i|ˆε.p|f〉|2 (2.36) 

PES 

To relate the cross sections of the two processes (PES and IPES), let us suppose 

the matrix element in PES is same as that of the IPES. Then the ratio of two cross 

sections reflects only the different phase space available for photon creation or electron 

creation, and is given by 

or 

r = 

( ) dσ 

/ 

dΩ 

IPES 

( ) dσ 

= ω2 

dΩ c 

PES 

2 k = q2 

(2.37) 

2 k 2 

r = 

( 

λe 

λ p 

) 2 

(2.38) 

where λ e and λ p are the electron and photon wavelengths respectively. 

And the equation (2.38) says the ratio of the inverse photoemission cross section to 

that of the phtoemission is the ratio of the square of the wave length of electron to that 

of photon. At an energy of 10 eV, characteristics of the vacuum ultraviolet region, the 

ratio r∼10 −5 . Therefore the signal levels in IPES is much lower comparable to PES. 

At higher energies ∼1000 eV, characteristics of the x-ray range, we have r∼10 −3 . 

2.3.4 Experimental Set-up 

The external view of IPES spectrometer at UGC-DAE consortium for scientific research, 

Indore, is shown in the Fig. 2.11.


Figure 2.11: (a) IPES spectrometer at UGC-DAE consortium for scientific research, 

Indore, (b) External view of photon detector attached to the IPES chamber, and (c) 

Inside view of the chamber showing a rotatable (left) and fixed (right) electron gun 

and the sample holder


The experimental UHV chamber is made of µ metal which provides effective shielding 

for the low energy electrons from earth’s magnetic field and stray magnetic fields. 

A photon detector capable of detecting fixed energy photons with well defined narrow 

bandpass and an electron gun producing nearly monoenergetic low energy electrons 

with well defined angular spread are equipped in the UHV chamber. A sample manipulator 

is mounted vertically which allow to move the sample both along axial and 

azimuthal direction. 

The low energy electron gun (e-gun) 

The e-gun used for the experiment is of Stoffel Johnson type [30] is shown schematically 

in Fig. 2.12. This is chosen because it has less space-charge effect and high 

beam current. 

Figure 2.12: A schematic diagram of the electron gun [30]. 

In this type of e-gun the minimum beam size obtained is 1.1 mm with full angular 

convergence of 5 ◦ to 7 ◦ . It consists of a filament in a mounting block followed by 

three element refocusing lens. All the lens elements are with cylindrical cross-section. 

In the design (Fig.) L = D = 16mm, P = 1.3D and Q = 2.5D. Ratio of voltages 

applied on the cathode and the subsequent electrostatic lens elements is V C : V A : 

V F : V O = −1 : +5 : −0.9 : 0. The Kinetic energy of emitting electrons is governed by 

V C . The whole surface of the lenses is gold plated to obtain a uniform work function. 

BaO dispenser cathode is used as the electron emitter. The e-gun operates in the 

range of 5 − 40 eV.


The photon detector 

Gas filled band-pass Geiger-Muller (GM) type counters are used for photon detection 

in IPES because of their high efficiency, low cost and simple design [31, 32, 33, 34, 

35, 36, 37, 38]. In our IPES experiment the acetone/CaF 2 detector [39] was used. 

The design of the photon detector is based on the following operating principle 

: the high energy cut-off of the acetone/CaF 2 detector is due to the CaF 2 window 

that does not transmit photons with energy > 10.2, while the threshold for the 

photoionization of acetone at 9.7 eV sets the low energy cut-off [40]. This determines 

the band pass function and results [33] in a mean photon detection energy of 9.9 

eV with a FWHM of 0.4 eV. Thus, 9.9±0.2 eV photons can enter the detector to 

photoionize acetone. 

Figure 2.13: Schematic cross section of the photon detector showing the window (1), 

O-ring (2), cylindrical cap (3), detector tube (4), teflon spacer (5), anode (6), double 

sided DN40 CF flanges (7, 10), gas inlet and outlet (8, 9), barrel connector (11), 

teflon sleeve (12), DN40 CF flange (13) and SHV connector (14). 

The detector assembly [41] (2.13) has a modular design consisting of a 250 mm 

long stainless steel cylindrical tube with inner diameter 17.8 mm and outer diameter 

21.2 mm (Fig), welded to a double-sided DN40 CF flange (7) with a through hole equal 

to the inner diameter of the tube (4). Another double-sided DN40 CF flange (10) 

with a similar through hole has two additional 6 mm diameter through holes drilled 

in the radial direction. Two 6 mm diameter of stainless steel tubes of length 25 mm


are welded to these radial holes, and on the other end, DN16 CF flanges (8, 9) are 

welded to these tubes. These are used as gas inlet (8) and outlet (9). The anode (6) 

is attached to a SHV feedthrough (14) mounted on another DB40 CF flange (13) by 

a barrel connector (11). The three DN40 CF flanges (7, 10, 13) are mounted together 

on a linear translator. The anode is a 1.6 mm diameter electropolished stainless steel 

wire and is held by two perforated teflon spacers (5) in the detector tube. High voltage 

is applied to the anode. A 25 mm diameter, 2 mm thick CaF 2 window (1) sits on a 

viton O-ring (2) and is tightly pressed by a stainless steel cap (3) at the entrance of 

the detector. This isolates the detector from the UHV chamber. Use of viton O-ring 

ensures easy exchange of windows at the testing stage of the detector. The count rate 

increases with higher solid angle of detection. So the detector is brought as close as 

possible to the sample using the linear translator. 

2.4 X-ray Absorption Spectroscopy 

X-ray absorption spectroscopy (XAS) is another technique to study the unoccupied 

electronic states as well as the geometrical arrangements of any materials around an 

atom [42, 43]. But in this thesis XAS technique is used only to study the electronic 

structure of the materials. 


When a beam of monochromatic x-ray enters a solid, either it will be scattered by 

the electrons or absorbed and excite the core electrons. In the absorption process the 

core electron is excited to an empty state [21, 45]. The schematic of XAS is shown in 

Fig. 2.14. 

If the energy of the x-ray is much larger than the binding energy of the core state 

the excited electron behaves like a free electron in the solid. If however the x-ray 

energy is just enough to excite a core electron, it will occupy a lowest available empty 

state. 

When a monochromatic x-ray beam of intensity I 0 goes through a material of 

thickness d, its intensity I will get reduced as, 

I = I 0 e −ρµd (2.39)


Figure 2.14: Schematic diagram of x-ray absorption spectroscopy 

where µ is the absorption coefficient of the material, which depends on the types 

of atoms and density ρ of the material. The absorption coefficient decreases smoothly 

with higher energy, except for certain photon energies where the absorption increases 

drastically, and gives rise to an absorption edge. Each such edge occurs when the 

energy of the incident photons is just sufficient to excite a core electron to an unoccupied 

state or into the continuum state (leave the atom). After each absorption edge 

the absorption coefficient continues to decrease with increasing photon energy (Fig. 

2.15). 

There are two distinguishable parts in x-ray absorption process : The low energy, 

near edge x-ray absorption fine structure (NEXAFS) [46, 47] or x-ray absorption 

near edge structure (XANES) region and the extended x-ray absorption fine structure 

(EXAFS) [48, 49], where the ejected electrons have high kinetic energy with single 

scattering. In this thesis work, only the XANES (or NEXAFS) region has been taken 

to study the unoccupied states.


Figure 2.15: The x-ray loses its intensity via interactions with material. The absorption 

coefficient decreases smoothly with higher energy, except for special photon 

energies. When the photon energy reaches a critical value for a core electron transition, 

the absorption coefficient increases abruptly. 

2.4.2 Detection techniques 

The normal way to measure the x-ray absorption is in the transmission mode [21]. The 

intensity of the x-ray is measured before and after the sample and the percentage of 

transmitted x-rays is determined. Such transmission mode of experiment is standard 

for hard x-rays. But for soft x-rays they are difficult to perform because of the strong 

interaction of soft x-rays with matter and requires a very thin sample of 0.1 µm to 

obtain a detectable signal. An alternative to the tansmission mode experiment is the 

”Yield mode”, where the decay product of the core hole (created in x-ray absorption) 

will be measured. The decay of the core hole gives rise to an avalanche of electrons, 

photons and ions escaping from the sample surface. There are four types of detection 

methods [21] in yield mode XAS measurement such as Auger electron yield (or partial 

yield), fluorescence yield, ion yield and total electron yield. 

In Auger electron yield method, the electrons of a certain energy range (i.e electrons 

of a specific Auger decay channel of the core hole) are detected. It is found 

that the mean free path of a 500 eV Auger electron is of the order of 20 Å. Hence


the number of Auger electrons emitted is equal to the number of core holes which 

are created in the first 20 Å from the surface. The Auger electron yield method is a 

measurement of only 20 Å of material, hence the Auger electron yield is rather surface 

sensitive. In fluorescence yield XAS mode, the fluorescence decay of the core hole is 

measured instead of Auger decay electrons. The intensity of the fluorescence yield 

is small compared to Auger electron yield mode for the elements of atomic number 

Z ≤ 50. The main advantage of fluorescence yield is that the created photon has a 

mean free path of the same order of magnitude as the x-ray (∼ 1000 Å) and hence it 

probes the bulk. In ion yield mode, only the surface ions created by Auger decay are 

measured. So it is highly surface sensitive and is used for special surface study. The 

most applied detection technique for soft XAS is the total electron yield. In total 

electron yield mode, all escaping electrons are counted regardless of their energy. The 

count rate is very large in this case. This thesis contains the XAS spectra measured 

in total electron yield and fluorescence yield mode. In TEY mode, the electrons are 

detected by the hemispherical analyser, which is discussed in the last section. In the 

FY mode the radiative photons are detected by the Si-photodiode. 

Si - photodiode 

Silicon photodiode detectors are widely used over a broad wavelength range extending 

from the x-ray to the visible regions. The detector used for the XAS measurement 

involved in this thesis are AXUV and SXUV photodiodes [50, 51]. The photodiodes 

are fabricated by an ULSI (Ultra large Scale Integrated circuit) compatible process. 

Fig. 2.16 shows the cross section structure of the AXUV and SXUV photodiodes. 

The AXUV photodiode has a thin nitrided SiO 2 surface layer (approximately 7 nm 

thick) and 100 % internal quantum efficiency. The SiO 2 layer is known to have a 

fixed positive charge which repels minority carrier holes generated at the surface by 

strongly absorbing extreme ultra-violet (EUV) radiation. The SXUV photodiode has 

metal silicide surface layer which replaces the oxide layer on the AXUV photodiode 

and improves the radiation hardness. However, the absence of the oxide layer causes 

significant surface recombination in the SXUV photo diodes and reduces the internal 

quantum efficiency below 100 % value, that is characteristics of AXUV photodiodes. 

When the silicon photodiode is exposed to photons with energy greater than 1.12 

eV (wavelength less than 1100 nm), electron-hole pairs (carriers) are created. These 

photogenerated carriers are separated by the p-n junction electric field and a current


Figure 2.16: Schematic of the Si-photodiode. 

proportional to the number of electron hole pairs created flows through an external 

circuit. Several unique properties of the AXUV photodiodes have resulted in previously 

unattained stability and 100 % carrier collection efficiencies giving rise to near 

theoretical quantum efficiencies. The first property is the absence of a surface dead 

region i.e no photogenerated carrier recombination occurs in the doped n-type region 

and at the silicon-silicon dioxide interface. As the absorption depths for the majority 

of UV/EUV photons are less than 1 micrometer in silicon, the absence of a dead region 

yields complete collection of photogenerated carriers by an external circuit resulting 

into 100 % collection efficiency. The second unique property of the AXUV diodes is 

their extremely thin (4 to 8 nm), radiation-hard silicon dioxide junction passivating, 

protective entrance window. Owing to their 100 % collection efficiency and the thin 

entrance window, the quantum efficiency of the AXUV diodes can be approximately 

predicted in most of the XUV region by the theoretical expression E ph /3.65, where 

E ph is the photon energy in electron volts. 

2.5 Synchrotron Radiation 

The unique properties of synchrotron radiation are its continuous spectrum, high 

flux and brightness, and high coherence, which make it an indispensable tool in the


exploration of matter. Synchrotron radiation are produced when charge particles 

Figure 2.17: Schematic of the synchrotron radiation. 

moving with relativistic velocity are deflected in a magnetic field. The synchrotron 

radiation had been developed by taking the effect of Heinrich Hetz [1] as the electrons 

fly around the curves through the magnetic field emit electromagnetic radiation. In 

SR facilities the accelerated electrons fly in a closed orbit inside a stainless steel 

tube, which consists of large electromagnets at regular intervals to guide the electrons 

around the curves and keep them focussed in the center of the tube called the storage 

ring. The electrons are generated in an injection system consisting of an electron 

source and an accelerator, either a synchrotron or a linear accelerator. When the 

electrons are accelerated to the operating energy of the storage ring, they are injected 

into the storage ring. The bending magnets are used to deflect the electron beam. 

In order to compensate the energy loss of the electrons during the SR emission, the 

electrons are accelerated each time as they pass through the RF cavity installed inside 

the storage ring. A schematic of SR light source is shown in the Fig. 2.17. 

In the developed synchrotrons the intensity of the emitted radiation is enhanced 

by insertion devices (undulator or wiggler). The insertion devices are periodic arrays 

of permanent magnets. When electrons traverse in the periodic array of magnets are 

forced to undergo oscillations and radiate (Fig. 2.18 shows the schematic representation).The 

static magnetic field is alternating along the length of the insertion devices 

with a wave length λ 0 . The radiation produced by insertion devices is very intense and 

is guided through beam lines for various experiments. An important dimensionless 

parameter K which characterizes the nature of electron motion is defined as,


Figure 2.18: Schematic representation of an insertion device. 

K = eB 0λ 0 

(2.40) 

2πmc 

where e is the electronic charge, B 0 is the magnetic field, m is the electron rest 

mass and c is the speed of light. 

For undulator K > 1, the oscillation amplitude is bigger and the radiation contributions 

from each field period sum up independently leading to a broad energy spectrum. 

Insertion devices can provide several orders of magnitude higher flux than a simple 

bending magnet and are in high demand at synchrotron radiation facilities. For an 

undulator with N periods, the brightness can be up to N 2 more than a bending 

magnet. 

The experiments presented in this thesis were performed at different synchrotrons 

using monochromators of various type. The HRPES and XAS experiments were 

performed at the WERA soft x-ray beamline of ANKA synchrotron, Germany. At 

BEAR and BACH beamlines of ELETTRA synchrotron, Italy, the XAS experiments 

were performed. 

At WERA soft x-ray beamline of ANKA synchrotron, a spherical grating monochromator 

(DRAGON type) [21, 52] was used. The layout of the WERA soft x-ray beamline 

is shown in the Fig. 2.19. It consists of a pair of mirrors 1 (horizontal) and


Figure 2.19: Optical layout of the WERA soft x-ray beam line, ANKA. 

2 (vertical) to focus the incoming beam into the entrance slit of the grating box 

and also a pair of bendable focusing mirrors after the exit slit for optimum focus at 

sample position. Both entrance and exit slits are movable. The energy resolution 

∆E/E < 10 −4 . The photon energy range is 100 - 1500 eV. The experimental facilities 

associated with the WERA soft x-ray beamline are PES, PEEM, NEXAFS and 

SXMCD. 

The BEAR beamline at ELETTRA is dedicated to the study of optical, electronic 

and magnetic properties of materials under UHV condition. The lay out of the BEAR 

beam line is shown in the Fig. 2.20. The beamline optics is based on a couple of 

parabolic mirrors. The radiation is collimated by the first parabolic mirror (P1). The 

parallel beam produced by P1 reaches the monochromator stage based on the plane 

grating/plane mirror scheme according with the Naletto-Tondelo scheme [53]. There 

are three gratings (G-NIM, G-1200 and G1800) associated with the monochromator. 

Three gratings cover 5 - 1600 eV photon energy range. The G-NIM (1200 1/mm) 

works at near normal incidence to cover the 5 - 45 eV region. The G1200 (1200 1/mm) 

and G1800 (1800 1/mm) work in the 45 - 1600 eV energy range. The monochromatic 

radiation is focused onto the exit slit by a second parabolic mirror (P2). The elliptical 

re-focusing mirror (REFO) brings the beam at the sample position. The experimental


Figure 2.20: Optical layout of the BEAR beam line. 

facilities available at BEAR beamline are ARPES, XAS, photoelectron diffraction and 

specular and diffuse reflectivity. 

The BACH beam line is designed to deliver a photon beam with high intensity 

and brilliance in the soft x-rays energy range 35 -1600 eV with full control of the 

polarization of the light. The goal of this beam line is to perform the electron and 

photon spectroscopy experiment with polarization dependence. The optical layout of 

the BACH beam line [54, 55] is shown in the Fig. 2.21. 

Figure 2.21: Optical layout of the BACH beam line.


Figure 2.22: The variable included angle monochromator : the combined movement 

of PM1 (plane mirror) and one of the four spherical gratings SG1(-4) leads to the 

monochromatization of the light keeping both the entrance and exit slit fixed. 

The radiation source is based on two APPLE-II elliptical undulators that are 

used alternatively to optimize the flux. The photon dispersion system is based on the 

Padmore ”variable angle spherical grating monochromator (VASGM)” scheme and it 

includes four different interchangeable spherical gratings (Fig. 2.22). The first three 

gratings works in the energy ranges 35 - 200 eV, 200 - 500 eV and 500 - 1600 eV 

respectively. The fourth grating operates in the 400 - 1600 eV range providing higher 

flux allowing fluorescence and x-ray scattering experiments. Two refocusing sections, 

based on plane elliptical mirrors in Kirkpatrick - Baez configuration provide nearly 

aberration free spots on the samples. A third branch line after the monochromator will 

host a microscope based on Fresnel zone plate focusing. The following experimental 

facilities are associated with the BACH beam line ; XPS, UPS, XAS, XMCD, XMLD, 

SXES and RIXS.

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Chapter 3 

Electronic Structure of 

Pr 0.67 Ca 0.33 MnO 3 

59

Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 60 


Recently, a lot of attention have been focused on the charge-ordered compositions of 

Pr 1−x Ca x MnO 3 due to their importance as possible prototypes for the electronic phase 

separation (PS) models [1, 2, 3, 4, 5, 6, 7, 8, 9] proposed to explain the phenomenon 

of colossal magnetoresistance (CMR). The PS models are qualitatively different from 

the double-exchange model [10, 11, 12] or those based on strong Jahn-Teller polarons 

[13, 14]. According to the PS model the ground state of CMR materials is comprised 

of coexisting nanosize clusters of metallic ferromagnetic and insulating antiferromagnetic 

nature [6]. The insulator-metal transition in this scenario is through current 

percolation. Though there have been a number of experimental studies showing the 

existence of phase separation, their size varies from nano- to mesoscopic scales [8, 9]. 

Radaelli et al. have shown the origin of mesoscopic phase separation to be the intergranular 

strain [9] rather than the electronic nature as in PS models. Nanosized 

stripes of a ferromagnetic phase [15] were reported in Pr 0.67 Ca 0.33 MnO 3 . This compound 

also attracted much attention earlier due to the existence of a nearly degenerate 

ferromagnetic metallic state and a charge-ordered antiferromagnetic insulating state 

with a field-induced phase transition possible between them [16]. With slight variations 

in the Ca doping the Pr 1−x Ca x MnO 3 system turns ferromagnetic (x=0.2) or 

antiferromagnetic (x=0.4) at low temperatures [17, 15]. The composition x=0.33 

shows a coexistence of ferromagnetic and antiferromagnetic phases [18]. This makes 

the charge- or orbital-ordered Pr 0.67 Ca 0.33 MnO 3 a prototype for the PS scenario. 

One of the requisites for the existence of an electronic phase separation in manganites 

is a strong-coupling interaction affecting the hopping of the itinerant e g electrons. 

The PS is expected to occur [5] when J H /t ≫ 1, where J H is the Hund’s coupling 

contribution between the localized t 2g and the e g electrons and t is the hopping amplitude 

of the e g electrons. The PS also favors a strong electron-phonon coupling, 

like the influence of a strong Jahn-Teller (JT) polaron arising from the Q 2 and Q 3 

JT modes. Essentially, one expects a strong localization of charge carriers to accompany 

the electronic separation of phases. Apart from charge, the orbital degrees of 

freedom also play an important role in this scenario [5]. The itinerant electron hopping 

term is strongly influenced by the symmetry of the orbitals (d x 2 −y 2 and d 3z 2 −r 2) 

hybridized with the O 2p orbitals of the MnO 6 octahedra. In comparison with the 

most popular CMR material La 1−x Sr x MnO 3 , the charge-ordered Pr system has an 

inherently reduced e g bandwidth W due to the smaller ionic radius of Pr, which also


enhances the tendency for carrier localization. All these charge and orbital interactions 

are reflected in the near-E F electronic structure of these materials and could 

be probed using electron spectroscopic techniques. Valence band photoemission and 

O K x-ray absorption are two such proven tools sensitive to the changes in the lowenergy 

states crucial to these interactions. Although, there are many reports on 

the near-E F electronic structure of other CMR compounds using these techniques 

[19, 20, 21, 22, 23, 24, 25, 26, 27, 28] only a few studies have been reported on their 

charge-ordered compositions [8, 29]. One of the key energy terms these spectroscopies 

could show earlier was the charge transfer energy E CT which is intimately related to 

the electron-electron and electron-lattice interactions [20]. 

In this study we have used ultraviolet photoelectron spectroscopy and x-ray absorption 

spectroscopy (XAS) in order to probe the electronic structure of the occupied 

and unoccupied states on a well-characterized, high-quality single crystal of 

Pr 0.67 Ca 0.33 MnO 3 . Another part of this single crystal had earlier been used for a detailed 

neutron scattering experiment that indicated the possible existence of a phase 

separation of ferromagnetic and antiferromagnetic stripes [15]. In the present study 

we have analyzed the temperature-dependent changes in the near-E F electron energy 

states from the perspective of a phase separation. 

3.2 Experimental 

The single-crystal sample of Pr 0.67 Ca 0.33 MnO 3 was grown by the floating zone method 

in a mirror furnace. The compositional homogeneity of the crystal was confirmed using 

energy-dispersive spectroscopic analysis. Magnetization and transport measurements 

on this crystal showed the transition temperatures T c and T N to be 100 and 110 

K, respectively. Details of the sample preparation, magnetization, and transport measurements 

and structural studies are published elsewhere [15, 30]. Angle-integrated 

ultraviolet photoemission measurements were performed using an Omicron µ-metal 

UHV system equipped with a high-intensity vacuum-ultraviolet source (HIS 13) and 

a hemispherical electron energy analyzer (EA 125 HR). At the He I (21.2 eV) line, 

the photon flux was of the order of 10 16 photons/s/sr with a beam spot of 2.5 mm 

diameter. Fermi energies for all measurements were calibrated using a freshly evaporated 

Ag film on a sample holder. The total energy resolution, estimated from the


width of the Fermi edge, was about 80 meV. The single-crystalline samples were repeatedly 

scraped using a diamond file inside the chamber with a base vacuum of ∼ 

1.0 × 10 −10 mbar. Scraping was repeated until negligible intensity was found for 

the bump around 9.5 eV, which is a signature of surface contamination [19]. For 

the temperature-dependent measurements, the sample was cooled by pumping liquid 

nitrogen through the sample manipulator fitted with a cryostat. Sample temperatures 

were measured using a silicon diode sensor touching the bottom of the sample 

holder. XAS measurements were performed using the BEAR [31] and BACH [32] 

beamlines associated with ELETTRA at Trieste, Italy. At the BEAR beamline we 

used monochromatized radiation from a bending magnet in order to record the O K 

edge spectra at room temperature and 150 K in fluorescence detection mode on a 

freshly scraped surface of the single crystal. The energy resolution was around 0.2 

eV in the case of these two spectra. The O K edge spectra at 95 K was recorded at 

the BACH beamline, using the total electron yield mode. Before the measurements, 

the sample surface was scraped inside the UHV chamber (∼ 1.0 × 10 −10 mbar) using 

a diamond file. At this beamline we used radiation from an undulator, monochromatized 

using a spherical grating. The resolution at the O Kedge for this measurement 

was better than 0.1 eV. 

3.3 Results and Discussion 

3.3.1 The UPS study of Pr 0.67 Ca 0.33 MnO 3 

The angle-integrated valence band photoemission spectra of Pr 0.67 Ca 0.33 MnO 3 taken 

at different temperatures below and above T c are shown in Fig. 3.1. Intensities of 

all the spectra were normalized and shifted along the y axis by a constant for clarity. 

The spectra, dominated by the states due to the Mn 3d-O 2p hybridized orbitals, look 

similar to those reported earlier on the La 1−x Sr x MnO 3 system [19, 20, 21, 22, 23]. The 

origin of the two prominent features, one at ∼ -3.5 eV (marked B) and another at 

∼ -5.6 eV (marked C) below E F , are now well known from earlier experiments and 

band structure calculations [23, 24, 33] on similar systems. While the feature at -3.5 

eV is mainly due to the t 2g↑ states of the MnO 6 octahedra, the -5.6 eV subband 

has contributions from both t 2g and e g states. More important contributors to the 

properties of these systems are the states nearer to E F , which appear as a tail at 

∼ -1.2 eV (marked A) from the chemical potential. The intensity of this feature


C 

B 

Intensity (arb. units) 

A 

-10 -8 -6 -4 -2 0 

Energy Relative to E F 

(eV) 

Figure 3.1: Valence band photoemission spectra of Pr 0.67 Ca 0.33 MnO 3 taken using He 

I photon energy (21.2 eV) at 77 (solid line), 110 (dotted), 150 (dashed), 220 (dotdashed), 

and 300 K (double-dot dashed). All the spectra have been normalized and 

shifted along y-axis by a constant for clarity. The subbands around -1.2 (e g↑ ), -3.5 

(t 2g↑ ), and -5.6 eV (e g↑ + t 2g↑ ) are marked as A, B, and C respectively.


300 K 

77 K 

5 X Difference 

(a) 

300 K 

110 K 


(b) 


-1 -0.5 0 0.5 1 1.5 

300 K 

220 K 


(d) 

-1 -0.5 0 0.5 1 1.5 

300 K 

150 K 


(c) 

-1 -0.5 0 0.5 1 1.5 

Binding Energy (eV) 

-1 -0.5 0 0.5 1 1.5 

Figure 3.2: High-resolution photoemission spectra of the near-E F region of the valence 

band of Pr 0.67 Ca 0.33 MnO 3 . In (a) the spectrum taken below T c (red) is compared with 

the normal state (300 K) spectrum (blue). The difference spectrum (green) obtained 

by subtracting the spectra at 300 K from 77 K and multiplied by 5 is also shown in 

the panel. Similarly, in (b), (c), and (d) the near-E F spectra taken at 110, 150, and 

220 K are compared with that taken at 300 K. The feature in the difference spectra 

corresponds to the tail feature A (e g↑ ) in Fig. 3.1. 

is quite small compared to B and C and is a signature of the insulating nature 

of this material. Earlier photoemission experiments on La 1−x Sr x MnO 3 also have 

shown that the intensity of this tail feature A is quite small [8, 20, 21, 22, 23]. 

The presence of the feature A is clear from Fig. 3.2, where we have shown the 

near-E F region of the valence band spectra taken with a higher resolution. Here, 

the spectra taken at different temperatures have been compared with those at room 

temperature. The figure also shows the difference spectra obtained by subtracting 

the room-temperature spectra from spectra taken at low temperatures. The feature 

in the difference spectra corresponds to A, which originates from the e g↑ states [22]. 

In order to estimate the relative changes in all the three features (A, B, and C) 

due to temperature, we have carefully fitted the whole valence band spectrum with


0.014 

(a) 

0.012 

Normalized Intensity 

0.01 

0.6 

0.55 

0.5 

0.45 

(b) 

(c) 

0.4 

100 150 200 250 300 

Temperature (K) 

Figure 3.3: Temperature dependence of the area of the three valence band features 

obtained from fitting the spectra with Lorentzian line shapes using a χ 2 iterative 

program. We have used an integral background, which was kept the same for all the 

spectra. The energy positions and FWHMs were determined by finding the best fit 

common to all the spectra by the iterative program. The final fit for all spectra at 

different temperatures were obtained with the same energy positions and FWHMs. 

(a), (b), and (c) correspond to the features A, B, and C positioned at -1.19, -3.49, 

and -5.63 eV with FWHMs 2.56, 1.93, and 1.37 eV respectively.


three components corresponding to the three subbands contributing to this region. 

We used a χ 2 iterative program for fitting the different spectra with Lorentzian line 

shapes for A, B, and C. Except for the area of the peaks, the energy positions, 

widths, background, and all other parameters of all the peaks were kept the same for 

different temperatures. The positions and full widths at half maximum (FWHMs) 

of the three peaks are given in the caption of Fig. 3.3. The fitted spectra for 77 

K (below T c ) and 300 K (above T c ) are shown on the left sides of Figs. 3.5(a) and 

3.5(b) below. One can see from Figs. 3.3(a) - (c) and that the area of all the peaks 

keeps increasing as we go down in temperature T c , but below this temperature the 

intensities of both peaks C and B decrease. On the other hand, the intensity of A 

does not show any decrease across the transition [Fig. 3.3(a)]. 

Pr 0.67 Ca 0.33 MnO 3 shows a transition to a charge-ordered state below 220 K (T co ) 

[30, 37, 38]. Neutron diffraction studies [15] on another part of this single crystal 

have shown that this charge ordering turns into an antiferromagnetically structured 

pseudo-CE type charge or orbital ordering below 110 K. Also, the coexistence of 

ferromagnetic and antiferromagnetic phases below this transition temperature has 

been shown on this single crystal. In the pseudo-CE-type ordering the Mn 3d z 2 −r 2 

orbitals, where the e g state is occupied, are aligned with the O 2p orbitals. Such an 

alignment can increase the hybridization between Mn 3d z 2 −r2 and O 2p. Furthermore, 

a simultaneous decrease in the hybridization strength could also be expected for the 

core-like in-plane t 2g and O 2p states. The results of the curve fitting (Fig. 3) of 

our valence band spectra reflects these changes in hybridization with temperature. 

The charge ordering following the decrease in temperature causes an increase in the 

O 2p contribution to both the t 2g and e g spin-up subbands in the valence region and 

hence the intensities of A, B, and C go up. As mentioned earlier, the pseudo-CE-type 

charge or orbital ordering below 110 K, results not only in a stronger hybridization of 

O 2p with the e gz 2 −r 2 ↑ states compared to that with the t 2g states but a simultaneous 

decrease in the latter also. This is reflected in the increase in intensity of A and 

decrease in the intensities of both C and B below 110 K. Though in Pr 0.67 Ca 0.33 MnO 3 

there is no temperature-dependent insulator-metal transition, the change in intensities 

of these features across T c appears similar to the shifting of spectral weight found in 

the La 1−x Sr x MnO 3 system [8].


Normalized Absorption (arb. units) 

530 540 550 560 

Photon Energy (eV) 

Figure 3.4: O K edge x-ray absorption spectra of Pr 0.67 Ca 0.33 MnO 3 taken at 300 

(solid line), 150 (dashed line), and 95 K (dotted). The pre-edge feature centered 

around 529.5 eV (marked by a box) is where most interest lies. Since this feature 

consists of two peaks (a main line and a shoulder on the low-energy side), this part 

of the spectrum was fitted with two components of Lorentzian line shapes. Results 

of the curve fit are given in Table 3.1. 

3.3.2 The XAS study of Pr 0.67 Ca 0.33 MnO 3 

In a reasonable approximation, the near-E F region of the O K x-ray absorption 

spectra could well represent the density of unoccupied states in many of the transition 

metal oxide compounds [20, 34]. In order to probe the electronic structure of the 

unoccupied states we have performed XAS on our Pr 0.67 Ca 0.33 MnO 3 single crystal. 

The O K edge XAS spectra taken at room temperature and 150 and 95 K (below 

T c ), shown in Fig. 3.4, have been normalized in intensity all along the region starting 

from 550 eV. The pre-edge feature in the O K spectra (centered around 529.5 eV) is 

due to the strong hybridization between Mn 3d and O 2p orbitals. The broad feature 

around 536.5 eV is due to the bands from hybridized Pr 5d and Ca 3d orbitals, while 

the structure above 540 eV is due to states like Mn 4sp and Pr 6sp, etc. These 

states are known to contribute least to the near-E F electronic structure of these


Table 3.1: χ 2 iterative fitting parameters for the pre-edge peak in O K XAS. The 

intensities of B ′ and A ′ at low temperature are normalized with respect to the total 

intensity of the pre-peak at room temperature. We have used an integral background, 

which was kept the same for all the spectra. The energy positions and FWHMs were 

determined by finding the best fit common to the three spectra by a χ 2 iterative 

program. The final fit for all the spectra from different temperatures were obtained 

with the same energy positions and FWHMs. 


O K edge x-ray absorption spectra 

B ′ A ′ 

Position FWHM Normalized Position FWHM Normalized 

95 (eV) (eV) area (eV) (eV) area 

2.11 1.52 0.72 1.48 0.97 0.29 

150 2.11 1.52 0.72 1.48 0.97 0.26 

300 2.12 1.52 0.73 1.47 0.97 0.26 

transition metal-oxide compounds. The assignments of the features mentioned above 

are consistent with the band structure calculations on similar systems [34, 35]. 

The pre-edge feature in the O K edge spectra carries a substantial amount of 

physics involved in the properties of these materials. It has earlier been shown that 

the pre-peak in the O K edge spectra of different CMR materials consists of two lines, 

one of which appears as a shoulder on the low-energy side of the other [21, 25, 27, 28]. 

The intensity of this shoulder was found to increase as the material goes across the 

insulator-metal transition [27, 28]. Since our Pr 0.67 Ca 0.33 MnO 3 sample is an insulator 

at all temperatures the pre-peak in our O K edge spectra (Fig. 3.4) does not show any 

splitting, though the presence of the shoulder is visible as an asymmetry on the lowenergy 

side of this peak. This shoulder feature is due to the first available unoccupied 

states and in XAS corresponds to the addition of an electron to the e g↑ state of the 

crystal-field-split MnO 6 octahedra. The main feature in the pre-peak arises from the 

t 2g↓ states [21]. For a quantitative estimate of the temperature-dependent changes in 

the intensities of the two features we have fitted the pre-peak using two components of 

Lorentzian line shapes. Here also the data fitting was done using χ 2 iterative method, 

keeping the energy positions and widths the same for all temperatures. Results of the 

data fit are shown in Table 3.1. Fitted spectra for temperatures above (300 K) and 

below Tc (95 K) are shown on the right side of Figs 3.5(a) and 3.5(b). Though very 

small, the intensity of the feature A ′ shows a slight increase below T c . The best fit


gives a 0.6±0.1 eV difference between the energy positions of A ′ and B ′ . Following a 

different assignment of origin to the pre-peak features Dessau et al. have interpreted 

this energy difference as a measure of the Jahn-Teller distortion (4E JT ) [26]. 

3.3.3 The Charge Transfer Energy (E CT ) 

Having shown the temperature-dependent changes in the UPS and XAS results in 

Figs. 3.2 and 3.4, we present a combined picture of them in order to derive some insights 

into the charge transfer energy involved in the properties of the Pr 0.67 Ca 0.33 MnO 3 

sample. The combined spectra of the valence band from photoemission and the preedge 

region of the O K edge XAS, presented in Figs. 3.5(a) and 3.5(b) give the density 

of both occupied and unoccupied states above and below the Curie temperature (T c ). 

A schematic of these energy levels is shown in Fig. 3.6. The hole or electron doping 

(value of x) causes the symmetry of the last occupied and first unoccupied bands to 

be e gz 2 −r 2 ↑. In Fig. 3.5 these states are marked by A and A ′ . In order to scale the left 

and right sides of E F , the O K edge spectra have been shifted such that the energy 

of the first unoccupied state (1.4 eV from E F ) found from inverse photoemission [36] 

coincides with that of A ′ . We have used integral backgrounds for both sides of E F 

and, as mentioned earlier, all the fitting parameters were kept the same including 

the energy positions for the high and low temperatures. The important parameter 

which can be derived from this combined spectrum is the charge transfer energy E CT , 

which is the energy required for an e g electron to hop between the Mn 3+ (t 3 2ge 1 g) and 

Mn 4+ (t 3 2g) sites. In our spectra (Fig. 3.5) E CT is the energy difference between A 

and A ′ which corresponds to the last occupied and first unoccupied states. The value 

of E CT is 2.6±0.1 eV from our spectra, which is higher than the value (1.5±0.4 eV) 

determined indirectly by Park et al. for the La 1−x Ca x MnO 3 system [20]. The larger 

value of E CT shows the existence of a stronger charge localization in the charge- or 

orbital-ordered Pr 0.67 Ca 0.33 MnO 3 compound. 

The value of E CT indicating a strong charge localization, found in our Pr 0.67 Ca 0.33 

MnO 3 has significant implications for the models proposed to explain the CMR effect 

in manganites. Charge localization can result from strong-coupling interactions 

of the e g electron with the core-like t 2g or the Q 2 and Q 3 Jahn-Teller modes. Both 

these interactions influence the electron hopping term (t). The former interaction, 

ferromagnetic in nature, is the Hund’s coupling J H . A large value of J H favors 

the electronic separation of phases. A strong electron-lattice coupling results from


E CT 

(a) 


C 

B 

A’ 

B’ 

A 

-5 0 5 10 


(eV) 

(b) 

B 

E CT 

B’ 


C 

A’ 

A 

-5 0 5 10 


(eV) 

Figure 3.5: (a) Combined spectra from valence band photoemission (occupied energy 

states) and pre-peak in O K edge x-ray absorption spectra (unoccupied energy states) 

of Pr 0.67 Ca 0.33 MnO 3 taken at room temperature (above T c ). The three features corresponding 

to the subbands in the valence region are marked A, B, and C.The two 

peaks of the fitted O K edge pre-peak are marked A ′ and B ′ . We used integral background 

for both sides of E F . Details of the fitting are mentioned in the text. (b) 

Combined spectra of Pr 0.67 Ca 0.33 MnO 3 below T c . The valence band spectrum was 

taken at 77 K and O K edge XAS was taken at 95 K.


2 2 

e g x − y 

J ex 

2 2 

e g z − r 

e g x 2 − y 2 

t 2g 

e z 2 2 

g − r 

2 2 

e g z − r 

E F 

t 2g 

Figure 3.6: Schematic diagram of the near-E F energy levels of Pr 0.67 Ca 0.33 MnO 3 in 

the occupied and unoccupied parts. The diagram is not drawn to scale. Some of the 

e g↑ states are occupied (at the Mn 3+ sites) and some are unoccupied (at the Mn 4+ 

sites). Hence the last occupied and first unoccupied states are marked with dotted 

lines. 

the cooperative Jahn-Teller distortions of the MnO 6 octahedra. These distortions, 

particularly the Q 2 and Q 3 vibrational modes of the oxygen ions are stronger in 

the case of Pr-containing manganites compared to La-containing ones. Park et al. 

[20] have reported a smaller charge transfer energy (E CT = 1.5 eV) in the case of 

La 1−x Ca x MnO 3 and have attributed it to small polarons (Anderson localization) induced 

from the ionic size difference between Mn 3+ and Mn 4+ . A small-polaronic 

model may not be able to explain the high value of E CT found in Pr 0.67 Ca 0.33 MnO 3 . 

On the other hand, a strong charge localization is expected in the case of the PS 

model in which the ground state is described as a mixture of ferromagnetic and antiferromagnetic 

regions. It is also possible that this large charge localization is due to 

the Zener polaron proposed by Aladine et al [39]. In the Zener polaron picture, the 

Mn ions of adjacent MnO 6 octahedra form a dimer with variations in the Mn-O-Mn 

bond angle. The pseudo-CE-type charge ordering in Pr 1−x Ca x MnO 3 favors such regular 

distortions in the MnO 6 octahedra. Further studies using electron spectroscopic 

techniques on samples in the vicinity of the charge-ordered compositions may be able 

to differentiate between these possible driving mechanisms behind the strong charge


localization. 

3.4 Conclusions 

Using valence band photoemission and O K edge x-ray absorption, we have probed 

the electronic structure of Pr 0.67 Ca 0.33 MnO 3 , which is regarded as a prototype for the 

electronic phase separation models in CMR systems. With decrease in temperature 

the O 2p contributions to the t 2g and e g spin-up states in the valence band were 

found to increase until T c . Below T c , the density of states with e g spin-up symmetry 

increased while those with t 2g symmetry decreased, possibly due to the change in 

the orbital degrees of freedom associated with the Mn 3d - O 2p hybridization in the 

pseudo-CE-type charge or orbital ordering. These changes in the density of states 

could well be connected to the electronic phase separation reported earlier. Our 

quantitative estimate of the charge transfer energy (E CT ) is 2.6±0.1 eV, which is 

large compared to the earlier reported values in other CMR systems. Such a large 

charge transfer energy may support the phase separation model.

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Chapter 4 


Pr 1−x Ca x MnO 3 

77

Electronic Structure of Pr 1−x Ca x MnO 3 78 


Mixed phase manganite systems with coexisting ferromagnetic (FM) and antiferromagnetic 

(AFM) domains have been attracting considerable attention recently due to 

their importance in understanding the role of spin, charge, and orbital ordering in the 

phenomena of colossal magneto resistance (CMR). Theoretical models, particularly 

those [1, 2, 3] based on electronic phase separation (PS) exploits this character of the 

CMR compounds in explaining the insulator-metal (IM) transitions found in these 

systems. The electronic PS scenario is phenomenologically different from the double 

exchange (DE) mechanism [4, 5, 6] or those based on strong Jahn-Teller polarons 

[7, 8]. In this scenario, the IM transition is driven by current percolation through 

FM metallic domains embedded in an AFM insulating background [2]. Though, there 

have been many structural studies [9, 10] showing the co-existence of such magnetic 

domains, the reported sizes of those vary from nano - to mesoscopic scales [11, 12]. In 

manganites, the electronic PS is expected to occur in systems in which the itinerant 

e g electrons have strong Hund’s coupling interaction with the localized t 2g electrons 

of the MnO 6 octahedra in their structure. 

Some of the charge ordered (CO) compositions of the Pr 1−x Ca x MnO 3 system are 

considered to be the prototypes for the PS models owing to their inherently reduced 

e g bandwidth W. Neutron diffraction studies have shown the co-existence of nanosized 

FM and AFM stripes [10, 12] in Pr 0.67 Ca 0.33 MnO 3 . It was also found that a fieldinduced 

transition possible between their nearly degenerate FM metallic and chargeordered 

AFM insulating phases [12, 13]. With slight variations in the Ca doping 

the Pr 1−x Ca x MnO 3 system turns ferromagnetic (x = 0.2) or antiferromagnetic (x = 

0.4) at low temperatures [10, 14]. The composition x = 0.33 shows a co-existence of 

ferromagnetic and antiferromagnetic phases [15]. The energy scales involved in the 

IM transitions in this FM-AFM system are of importance for the understanding of 

the physics behind the phenomena of CMR. 

In this chapter we have compared the binding energy positions of some of the near 

Fermi level (E F ) occupied and unoccupied states, crucial to the electrical and magnetic 

transitions of the Pr 1−x Ca x MnO 3 system using the complementary techniques 

of direct and inverse photoelectron spectroscopies. Electron spectroscopic techniques 

have been quite successful in probing the near E F low energy states involved in the 

charge and orbital interactions of CMR materials. Although, there are many reports 

on the electronic structure of other CMR compounds using some of these techniques


Figure 4.1: Phase diagram of the Pr 1−x Ca x MnO 3 system. In this study we have used 

the x = 0.2, x = 0.33 and x = 0.4 compositions. (FMI = ferromagnetic insulating, 

CO-AFMI = charge ordered antiferromagnetic insulating, CG = cluster glass, T N = 

Neel Temperature, T C = Critical Temperature, T CO = charge ordering temperature). 

[16, 17, 18, 19, 20, 21, 22] only a few studies have dealt with the energy scales of the 

electron interactions in charge ordered compositions [23, 24]. One of the key terms, 

these spectroscopies could show earlier, was the charge transfer energy E CT which is 

intimately related to the electron-electron and electron-lattice interactions [23, 24]. 

Here we have analyzed the temperature dependent changes shown by three compositions 

across the charge ordered FM-AFM phase boundary of the Pr 1−x Ca x MnO 3 

system using ultraviolet photoelectron spectroscopy (UPS) and inverse photoelectron 

spectroscopy (IPES). Also we have studied different core levels of Pr 1−x Ca x MnO 3 

using x-ray photoelectron spectroscopy (XPS). 


Single crystals of Pr 1−x Ca x MnO 3 with x = 0.2, 0.33 and 0.4 were grown by the floating 

zone method in a mirror furnace. The compositional homogeneity of the crystals 

had been confirmed using energy dispersive spectroscopic (EDS) analysis. The 

crystals were characterized by using magnetization, electrical transport and neutron 

diffraction measurements. Details of the sample preparation and characterization


measurements are published elsewhere [10, 25]. There are also three polycrystalline 

samples (PrMnO 3 , CaMnO 3 and Pr 0.16 Ca 0.14 MnO 3 ) used for different studies. Which 

were prepared by the standard solid state reaction method. The details of the sample 

preparation is published elsewhere [26]. Consolidated results of these studies are 

presented in the phase diagram shown in Fig. 4.1 [27]. Angle integrated ultraviolet 

photoemission measurements were performed by using an Omicron mu-metal ultra 

high vacuum system equipped with a high intensity vacuum ultraviolet source (HIS 

13) and a hemispherical electron energy analyzer (EA 125 HR). At the He I (21.2 

eV) line, the photon flux was of the order of 10 16 photons/sec/steradian with a beam 

spot of 2.5 mm diameter. The Fermi energies (E F ) for all measurements were calibrated 

by using a freshly evaporated Ag film on a sample holder. The total energy 

resolution, estimated from the width of the Fermi edge, was about 80 meV. The XPS 

measurements were performed using a twin anode Mg/Al x-ray source (DAR 400) in 

the same chamber as of UPS. In our measurements we have used the Mg Kα line with 

photon energy 1253.6 eV and the resolution was about 1.0 eV. The samples were repeatedly 

scraped by using a diamond file inside the chamber under a base vacuum of 

∼ 1.0 x 10 −10 mbar. The negligible intensity found for the ∼ 9.5 eV bump [16] in the 

UPS spectra ensures the cleanliness of our sample surface. For the temperature dependent 

measurements, the samples were cooled by pumping liquid nitrogen through 

the sample manipulator fitted with a cryostat. The sample temperatures were measured 

by using a silicon diode sensor touching the bottom of the sample holder. The 

IPES measurements were performed by using an electrostatically focused electron gun 

of Stoffel Johnson design and an acetone gas filled band pass Geiger Muller counter 

with a CaF 2 window [28, 29]. These measurements were carried out in the isochromat 

mode in which the photon energy is fixed at 9.9 eV and kinetic energy of the incident 

electrons is varied in 0.05 eV steps. Overall resolution of the IPES measurements 

were estimated [29] to be approximately 0.55 eV. 


4.3.1 The UPS study of Pr 1−x Ca x MnO 3 , x = 0.2, 0.33 and 0.4 

Neutron diffraction studies [27] on Pr 1−x Ca x MnO 3 have shown that the compositions 

with x = 0.2 and x = 0.4 are FM and AFM respectively (Fig. 4.1) while the x = 0.33 

has co-existing FM and AFM phases [14, 15]. The ground state of x = 0.33 has been


identified to be an inhomogeneous state below T CO . The x = 0.4 is above the critical 

doping (x c ) found in the Pr 1−x Ca x MnO 3 system and is homogeneous in nature. 

(a) 300 K 

C 

B 

(b) 77 K 

C 

B 


A 

1.0 

0.4 

0.33 

Intenisty (arb. units) 

A 

1.0 

0.4 

0.33 

0.2 

0.2 

0.0 

0.0 

10 

8 

6 

4 

2 

0 

10 

8 

6 

4 

2 

0 



Figure 4.2: The angle integrated valence band photoemission spectra of the 

Pr 1−x Ca x MnO 3 (x = 0.0, 0.2, x = 0.33, x = 0.4 and x = 1.0) samples taken at 

room temperature and 77K using He I photons (21.2 eV). 

Figure 4.2 shows the angle integrated valence band photoemission spectra of the 

Pr 1−x Ca x MnO 3 (x = 0.2, x = 0.33 and x = 0.4) samples taken at room temperature 

and 77K. The figure also shows the spectra of the end compositions of this system (x 

= 0.0 and x = 1.0). It may be noted from the phase diagram that the compositions 

with x = 0.2, x = 0.33 and x = 0.4 are PM insulators at 300K and FM/AFM insulators 

at 77K. Intensities of all the spectra shown in both the panels were normalized 

and shifted along the ordinate axis by a constant value for clarity. All the features 

seen in the spectra are dominated by the states due to the Mn 3d - O 2p hybridized 

orbitals. The two prominent features, one at ∼ 3.5 eV (marked B) and another at ∼ 

5.6 eV (marked C), are primarily due to the O 2p nonbonding states and the Mn 3d 

- O 2p bonding states [30]. The feature closest to the E F , which appear as a tail at


∼ 1 eV (marked A) is due to the e g states of the MnO 6 octahedra [18, 24, 31, 32]. 

The intensity of this feature is low due to the insulating nature of all our samples. 

Presence of the feature A is clear from Fig. 4.3(a) and 4.3(b) where we have shown 

the photoemission spectra from the near E F region of the x = 0.2, x = 0.33 and x = 0.4 

samples along with that of CaMnO 3 , taken at temperatures above and below the T CO . 

The lower panels of the figures show the difference spectra obtained by subtracting the 

spectra of CaMnO 3 from those of the different Pr 1−x Ca x MnO 3 compositions. Since, 

CaMnO 3 does not have any electrons in its e g levels, the difference spectra should 

correspond to the density of the e g electrons in the Pr 1−x Ca x MnO 3 samples. The 

shape of the difference spectra shows that it consists of two features, similar to those 

reported by Ebata et al. [33]. We have fitted all the difference spectra corresponding 

to the three compositions at two different temperatures with a mixture of Lorentzian 

and Gaussian line shapes using a χ 2 iterative programme. We used a fixed width 

for all the fitted lines marked A’ for both the temperatures in order to see the finer 

changes associated with the A”. The energy positions of the A’ and A” and the 

width of the A” were determined by the iterative programme. Table 4.2 shows the 

results of the fitting. At room temperature, the changes across the three compositions 

are negligible except that the sample with x = 0.33 has a larger width for its near 

E F feature marked A”. At 77K, both A’ and A” have been found to be shifted 

slightly towards the E F . Further, at this temperature also, the x = 0.33 shows a 

wider A”. The width of the near E F feature is coupled to the conductivity of these 

samples. Earlier experiments using valence band photoemission have shown that 

a finite number of states get transferred from A” to A’ when some CMR systems 

go across the temperature - composition phase diagram [31, 33, 34] resulting in the 

formation of pseudogaps at the E F . The energy scales involved in such shifts of DOS 

are of importance to the theoretical models, particularly for those based on electronic 

phase separation. Formation of pseudogaps are a generic feature of mixed phase 

CMR systems [31, 35] with FM electron rich phases embedded in an insulating AFM 

matrix. The feature A’ in our spectra could possibly be ascribed to the e g electrons 

in the AFM phases while the A” to those in the FM phases. The difference in the 

binding energy positions of A’ and A” (0.47±0.01 eV) gives an approximate measure 

of the energy difference between the e g electronic states in these two phases. In other 

words, this gives the energy scale over which the pseudogap forms. The increased 

width of the feature A” of x = 0.33 at temperatures above and below the T CO may


(a) 

300 K 300 K 300 K 


A’ 

A’ 

A’ 

A’’ A’’ A’’ 

0.2 - CaMnO3 0.33 - CaMnO3 0.4 - CaMnO3 

1.5 1 0.5 0 1.5 1 0.5 0 

1.5 

1 

0.5 

0 


(b) 

77 K 77 K 

77 K 


A’ 

A’ 

A’ 

A’’ 

A’’ 

A’’ 

0.2 - CaMnO3 0.33 - CaMNO3 0.4 - CaMnO3 

1.5 

1 

0.5 

0 

1.5 

1 

0.5 

0 

1.5 

1 

0.5 

0 


Figure 4.3: The near E F region of the valence band spectra (panel(a): 300 K, 

panel(b): 77 K) of the three samples plotted against that of CaMnO 3 . The difference 

spectra (lower panel) is obtained by subtracting the spectra of CaMnO 3 from 

those of the three Pr 1−x Ca x MnO 3 compositions and multiplied by 5. The difference 

spectra corresponds to the density of e g states. The red and blue peaks are A’ and 

A” respectively. The green line shows the spectral background used in the fitting.


Table 4.2: Results of the fitting of the difference spectra using a χ 2 iterative programme. 

We have used a mixture of Lorentzian and Gaussian line shapes for fitting 

the various difference spectra corresponding to the three compositions at 300K and 

77K. In order to see the finer changes in the near E F feature A”, we used a fixed 

width for all the fitted lines marked A’. The energy positions of the A’ and A” and the 

width of the A” were determined by the iterative programme. At both the temperatures 

the sample with x = 0.33 shows a larger full width at half maximum (FWHM) 

for its A”. At 77K, both A’ and A” have been found to be shifted slightly towards 

the E F . 


300 

77 

x 

A’ A” 

Position (eV) FWHM (eV) Position (eV) FWHM (eV) 

0.2 0.97 0.5 0.49 0.5 

0.33 0.96 0.5 0.49 0.65 

0.4 0.96 0.5 0.49 0.59 

0.2 0.92 0.5 0.45 0.49 

0.33 0.93 0.5 0.45 0.53 

0.4 0.92 0.5 0.46 0.5 

indicate the formation of FM clusters in the AFM insulating background. From the 

fitting of the difference spectra we feel that the increase of the width of A” is largely 

due to the formation of a tail towards the E F , which makes it difficult to estimate 

the energy difference between A’ and A” accurately. 

4.3.2 The IPES study of Pr 1−x Ca x MnO 3 , x = 0.2, 0.33, 0.4 

and 1 

Fig. 4.4 shows the IPES spectra of the Pr 1−x Ca x MnO 3 samples (x = 0.2, 0.33 and 

0.4) and the CaMnO 3 sample taken at room temperature. The spectra have been 

normalized for their intensities and shifted along the vertical axis by a constant for 

clarity. The spectra shows two broad features; one between 5 to 12 eV (marked Q) 

and another at around 2.4 eV (marked P). For the x = 0.2, 0.33, and 0.4, the feature 

appearing at higher energy is mostly due to Pr 5d and Ca 3d orbitals, whereas for 

the CaMnO 3 it is due only to the Ca 3d, 4s orbitals. The feature P is due to the 

hybridized Mn 3d - O 2p states. These assignments of the features are consistent 

with band structure calculations and other studies on similar systems [24, 36, 37]. 

Similar energy positions were reported earlier for La 1−x Sr x MnO 3 by Chainani et al.


Q 

x=1 

P 

x=0.4 


x=0.33 

x=0.2 

0 2.5 5 7.5 10 12.5 15 

Energy Relative to Fermi Energy (eV) 

Figure 4.4: The IPES spectra of the Pr 1−x Ca x MnO 3 samples (x = 0.2, 0.33 and 0.4) 

and the CaMnO 3 sample taken at room temperature. The spectra represents the 

unoccupied electron states of the samples. 

[38], though another study [39] showed the feature P to be more closer to the E F . 

Again, in the IPES spectra also, the feature appearing near to E F is important to 

properties of these compounds. In Fig. 4.5 we have shown the spectra of the near E F 

region containing this feature along with the difference spectra (Fig. 4.5(b)). One can 

see clearly that the intensity of P increases with the Ca concentration x due to the 

increase of unoccupied states in the Mn 3d orbitals with hole doping. From a linear 

extrapolation of the rising band edge in the spectra shown in the Fig. 4.5(a) we have 

estimated the band gap to be ∼ 0.7 eV for the three compositions (0.2, 0.33, and 0.4). 

At this temperature, the value of the gap does not seem to vary with the charge carrier 

concentration, whereas the parent compound (Ca x MnO 3 ) shows a smaller value (∼ 

0.67 eV). In case of La 1−x Sr x MnO 3 , Chainani et al. [38] has shown the value of the 

band gap to be ∼ 0.6 eV. The larger values shown by our Pr 1−x Ca x MnO 3 samples 

could be attributed to their insulating nature. In an alternate approach, one can also 

estimate the band gap from the difference spectra shown in the Fig. 4.5(b). This 

gives a some what larger value; i.e ∼ 0.86 eV for the x = 0.4 and 0.33 and ∼ 0.82 for


(a) 

(b) 


x=1 

x=0.4 

1 - 0.2 

0.4 - 0.2 

x=0.33 

x=0.2 

0.33 - 0.2 

-1 0 1 2 3 4 

0 1 2 3 4 

Energy Relative to Fermi Energy (eV) 

Figure 4.5: Panel(a) shows the IPES spectra of the near E F unoccupied states. 

Panel(b) shows the difference spectra obtained by subtracting the spectra corresponding 

to the x = 0.2 from those of other compositions. 

the Ca x MnO 3 . 

Band structure calculations on similar CMR systems [12, 32] have unambiguously 

shown that both the highest occupied and lowest unoccupied states in these systems 

are of Mn 3d e g spin-up symmetry. As mentioned earlier, within this frame work the 

features marked A’ and A” in Fig. 4.3 and the feature P in Fig. 4.4 correspond to the 

e g spin-up states. Combining the results presented in Table I and these two figures 

one can estimate the energy required by an e g electron to hop from a Mn 3+ site to the 

neighboring Mn 4+ site. If we consider the centroids of the A’ in Fig. 4.3 and the P in 

Fig. 4.4 we arrive at a value of ∼ 3.3±0.2 eV, which is large compared to the earlier 

reported value [24]. With the A” of Fig. 4.3 and P of Fig. 4.4 we get ∼ 2.8±0.2 

eV which is close to the earlier report (2.6±0.1 eV). Due to the large widths and 

long tails of the A” and P towards the E F we expect that this value of the energy be 

close to its upper bound. Energy required for the transfer of e g electrons between the 

Mn sites again is an important parameter for the phase separation models to work in


these systems. 

4.3.3 The core levels of Pr 1−x Ca x MnO 3 , x = 0.2, 0.33, 0.4, 

and 0.84 

The Mn2p core level spectra of Pr 1−x Ca x MnO 3 (x = 0.2, 0.33, 0.4, and 0.84) taken 

at room temperature as well as at 77 K are shown in Fig. 4.6. All the spectra were 

normalized and shifted along the Y axis by a constant for clarity. The two prominent 

peaks at around ∼ 641.5 and ∼ 653.5 eV are due to the spin orbit split 2p 3/2 and 2p 1/2 . 

These spectra look similar to those reported earlier on the similar system [17, 33]. In 

Fig. 4.6, the Mn 2p peak positions shift towards the higher binding energy for x = 

0.4 and 0.84 at room temperature (a) as well as at 77 K (b). But the binding energy 

shift is higher incase of room temperature spectra as compared to that of 77 K. The 

Mn LMV Auger peak is observed on the higher binding energy side of 2p 1/2 peak 

for all four concentrations at room temperature, whereas at 77 K this Auger peak is 

absent for x = 0.2, and 0.3. This Mn LMV was earlier observed in other hole doped 

manganites [17]. 

The Ca 2p spectra of Pr 1−x Ca x MnO 3 (x = 0.2, 0.33, 0.4 and 0.84) both at room 

temperature as well as 77 K are shown in the Fig. 4.7. The Ca 2p 3/2 and Ca 2p 1/2 , 

due to spin orbit splitting, are clearly distinguishable at around ∼ 345.5 and ∼ 349 

eV respectively. It is clear from the Fig. 4.7 (a) that, at room temperature the peak 

positions shift towards the lower binding energy when we move from x = 0.2 to x = 

0.84. Also the same kind of shift is observed for 77 K spectra (Fig. 4.7 (b)) with a 

lesser magnitude. 

The Pr 4d spectra of Pr 1−x Ca x MnO 3 (x = 0.2, 0.33 and 0.4) taken at room 

temperature as well as 77 K are shown in Fig. 4.8. The Pr 4d peaks come around 

116 eV binding energy. The peak positions shift towards the lower binding energy 

at room temperature (Fig. 4.8 (a)) when we move from x = 0.2 to 0.4. Here also 

the similar kind of shift is observed for 77 K spectra (Fig. 4.8 (b)) with a lesser 

magnitude. 

As mentioned above the binding energy shifts with respect to the value of x have 

been observed at the Ca 2p, Pr 4d and Mn 2p core levels. Incase of Mn 2p level, the 

binding energy shift is to the higher energy side while the other two levels shift to lower 

energy. The magnitude of the binding energy shifts are found to be higher at room 

temperature compared to 77 K. The shift to higher binding energy observed in Mn


Mn LMV 

Temp = 300 K 

2p 3/2 


0.84 

0.4 

0.33 

Mn 2p 

0.2 

670 

660 

2p 1/2 

640 

650 

640 

Binding energy (eV) 

(a) 

Mn LMV 

Temp = 77 K 

2p 2p 1/2 3/2 


0.84 

0.4 

0.33 

Mn 2p 

0.2 

670 

660 

650 


(b) 

Figure 4.6: The Mn 2p core-level XPS (Mg Kα) spectra of Pr 1−x Ca x MnO 3 taken at 

room temperature (a) and 77 K (b) .


Ca 2p 

Temp = 300 K 

2p 3/2 

2p 

1/2 


0.84 

0.4 

0.33 

0.2 

352 

348 344 


(a) 

340 

Ca 2p 

2p 1/2 

2p 3/2 

Temp = 77 K 


0.84 

0.4 

0.33 

0.2 

352 

348 344 


(b) 

340 

Figure 4.7: The Ca 2p core-level XPS (Mg Kα) spectra of Pr 1−x Ca x MnO 3 taken at 

room temperature (a) and 77 K (b).


Pr 4d 

Temp = 300 K 

Intensity (arb. unis) 

0.4 

0.3 

0.2 

130 

120 110 


(a) 

100 

Pr 4d 

Temp = 77 K 


0.4 

0.33 

0.2 

130 

120 110 


(b) 

100 

Figure 4.8: The Pr 4d core-level XPS (Mg Kα) spectra of Pr 1−x Ca x MnO 3 taken at 

room temperature (a) and 77 K (b).


2p could be due to the increase in the effective Mn valence state from Mn 3+ towards 

Mn 4+ . The binding energy shift incase of Ca 2p and Pr 4d core levels could possibly 

due to the shift in the chemical potential [33, 40]. The low temperature data shows 

a smaller value for this chemical potential shift. Such a suppression of the chemical 

potential shift have been observed earlier in other transition metal oxides [41, 42] is 

mostly due to the formation of stripes. According to the phase separation model, a 

pinning of the chemical potential is consequence of the mixed phase nature of these 

materials. The numerical studies [1, 3] have indeed shown the pinning of chemical 

potential in the composition range, where the phase separation is realized. These 

results also support the model of phase separation in the Pr 1−x Ca x MnO 3 system. 


Using UV photoelectron spectroscopy and inverse photoelectron spectroscopy we have 

deduced some of the important energy scales in the Pr 1−x Ca x MnO 3 system. Our 

photoemission studies across the FM-AFM phase boundary show that the pseudogap 

formation in these compounds occur over an energy scale of 0.47±0.01 eV. From a 

combined analysis of the photoemission and inverse photoemission spectra we also 

have found the energy required for the transfer of Mn 3d e g electrons between the adjacent 

Mn sites to be of the order of ∼ 2.8±0.2 eV. Though an accurate measurement 

of these energies are not possible due to the inherent limitations of the compounds 

and the technique, our results give an upper bound of them. Further, we have discussed 

our results from a possible phase separation scenario. From the core level 

study of Pr 1−x Ca x MnO 3 , we observed a suppression of chemical potential shift for 77 

K spectra compared to that of room temperature. These results support the model 

of phase separation in Pr 1−x Ca x MnO 3 system.

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Chapter 5 


Ca 0.86 Pr 0.14 MnO 3 

96

Electronic Structure of Ca 0.86 Pr 0.14 MnO 3 97 


The strong interplay among charge, spin, lattice and orbital degrees of freedom of electrons 

form the basis of the phenomena of colossal magnetoresistance (CMR) shown 

by manganites. To a large extent the traditional models [1, 2, 3] employing the 

charge-spin coupling, have been able to explain the CMR in many of the hole doped 

compositions (R 1−x A x MnO 3 with x



The polycrystalline sample of Ca 0.86 Pr 0.14 MnO 3 was prepared by standard solid state 

reactions. Details of the sample preparation technique is published elsewhere [4]. Purity, 

homogeneity and composition of the sample were checked by electron diffraction 

and energy dispersive spectroscopic analysis. The electrical and magnetic properties 

of this composition has also been published earlier [4, 15]. This sample shows 

a metallic behavior down to ∼ 200K and then the resistivity slightly increases till 

the T c (110 K). Below the T C the behavior is insulating and the resistivity goes up 

by many orders of magnitude. Generally, the conductivity behavior of this sample is 

similar to that of the other electron doped systems [4, 8, 15]. 

High resolution photoemission and x - ray absorption spectroscopy measurements 

were performed by using the light from the soft x - ray beam line WERA of ANKA 

synchrotron, Karlsruhe, Germany. We used the monochromatized radiation from 

an undulator. The HRPES measurments were carried out using the photon energy 

124 eV. The photoelectrons were collected using a Scienta SES 2002 electron energy 

analyzer. The energy resolution was 25 meV in case of the narrow scan (-2 - 2.5 eV) 

and it was 125 meV incase of wide scan (-2 - 15 eV) of the valence band. The O K XAS 

spectra has been taken in the fluorescence detection mode and the energy resolution 

was set to 150 meV. The sample surface was cut using a diamond knife under a base 

vacuum of ∼ 1.0 x 10 −10 mbar before taking the data for each temperature. In case of 

the photoemission measurements, scraping was repeated until negligible intensity was 

found for the bump around 9 - 10 eV, which is a signature of surface contamination 

[16]. For the temperature dependent measurements, the samples were cooled by 

pumping liquid helium through the sample manipulator fitted with a cryostat. The 

sample temperatures were measured by using a silicon diode sensor touching the 

bottom of the sample holder. 


Figure 5.1 shows the high resolution photoemission spectra of the Ca 0.86 Pr 0.14 MnO 3 . 

Varius spectra, taken at 30 K, 70 K, 110 K, 220 K and 293 K show the temperature 

dependent changes in the valence band electronic structure. Intensities of all the 

spectra were normalized and shifted along the ordinate axis by a constant value for 

the clarity of presentation. All the features seen in the spectra are dominated by the


B 

D 

C 


300 K 

220 K 

110 K 

A 

70 K 

30 K 

12 

10 

8 

6 

4 

2 

0 


Figure 5.1: The angle integrated valence band photoemission spectrum of 

Ca 0.86 Pr 0.14 MnO 3 taken using 124 eV photons. The spectra taken at different temperatures 

(30 K, 70 K, 110 K, 220 K and 293 K) are normalized for their intensities 

and shifted along Y-axis by a constant for clarity of presentation.



A 

30 K 

70 K 

110 K 

220 K 

300 K 

1.2 

1 

0.8 

0.6 

0.4 

0.2 

0 

-0.2 


Figure 5.2: The high resolution spectra of the near E F region of the valence band of 

Ca 0.86 Pr 0.14 MnO 3 for 30 K, 70 K, 110 K, 220 K and 300 K. This feature refers to the 

e g band. 

states due to the Mn 3d - O 2p hybridization. The broad feature appearing at ∼ 6 eV 

(marked D) is due to the bonding states of this hybridization while its non-bonding 

states appear as a shoulder at ∼ 3.2 eV (marked C). The peak at ∼ 2 eV (marked B) 

is due to the Mn 3d t 2g states of the MnO 6 octahedra [17]. The Mn 3d e g↑ states of 

the octahedra appear around ∼ 0.3 eV (marked A) in the spectra. It should be noted 

that the intensity of the peak B decreases with decrease of temperature while that of 

the feature A increases. These spectral weight shifts are characteristic of many CMR 

systems. The e g states lying close to the E F are important for the conductivity of this 

material. A high resolution spectra of the near E F region is presented in figure 5.2. 

As the temperature is lowered from 293 K the feature A starts appearing. Further, 

below the transition temperature T c (110 K), the intensity of A steeply increases and 

goes through a maximum. The 30 K spectrum again shows a smaller intensity for 

this feature compared to the one at 70 k. Interestingly, below the T c , the leading 

edges of the different spectra move towards the E F as we go down in temperature.


Mn 3d + O 2p 

Pr 5d, Ca 3d 

Mn 4sp, Pr 6sp 


300 K 

200 K 

150 K 

80 K 

30 K 

530 540 550 560 


Figure 5.3: The O K-edge x-ray absorption spectra of the Ca 0.86 Pr 0.14 MnO 3 taken at 

30 K, 80 K, 150 K, 200 K and 300 K. The pre-edge feature appears between 527 - 

533 eV is mostly due to the strong hybridization between Mn 3d and O 2p orbitals, 

where most interest lies. Origin of other features are given in the text. 

This shifting of the valence band edges is a clear indication of increase in the width 

(W) of the e g band below the T c . 

In order to see the corresponding changes in the near E F unoccupied states we have 

taken the O K edge x-ray absorption spectra (XAS) of the Ca 0.86 Pr 0.14 MnO 3 sample 

at different temperatures (Fig. 5.3). Here, all the spectra have been normalized to 

the incoming intensity and absolute cross sections and are also corrected for selfabsorption 

and saturation effects. The spectra shows three broad features. The one 

between 527 eV to 533 eV, called the pre-edge feature in the O K edge spectra, is 

mostly due to the strong hybridization between Mn 3d and O 2p orbitals. The feature 

around 536.5 eV is due to the bands from hybridized Pr 5d and Ca 3d orbitals, while



A’ 

B’ 

C’ 

30 K 

80 K 

150 K 

200 K 

300 K 

527 528 529 530 531 532 533 534 


Figure 5.4: The pre-edge peak of the O K -edge XAS spectra of Ca 0.86 Pr 0.14 MnO 3 for 

temperatures 30 K, 80 K, 150 K, 200 K and 300 K. 

the structure above 540 eV originates from the states like Mn 4sp and Pr 6sp, etc. 

These assignments of the features are consistent with the earlier experiments and 

band structure calculations on similar systems [21, 22, 23]. The pre-edge region of 

the O K XAS has earlier been shown to represent the near E F unoccupied states 

in these kind of compounds [21, 22, 23]. Figure 5.4 shows the pre-edge region of 

the XAS spectra of taken at different temperatures. The feature extending from 

∼ 527 to 531 eV consists of two lines. The first one is due to the O 2p orbitals 

hybridized with the Mn 3d e g↑ (marked A’) while the second arises from the O 2p - 

Mn 3d t 2g↓ hybridization (marked B’). The weak feature centered around ∼ 531.5 eV 

corresponds to the e g↓ (marked C’) states [23, 24, 25, 26, 27]. All the features show 

some temperature dependence. As the temperature is lowered from 300 K to 30 K 

the intensity of A’ decreases while that of B’ increases. This again shows a spectral 

weight shift complementary to the one observed in the photoemission spectra (Fig.


5.1). Here, the spectral weight get shifted from near E F positions to higher energy 

as we go down in temperature. One can also see a shift in the energy position of the 

feature originating from the e g↓ states. 

Structural studies [19] have shown that the Ca 0.86 Pr 0.14 MnO 3 has an orthorhombic 

perovskite structure at 293 K. Below 110 K its structure changes to elongated pseudotetragonal 

with four short a-b plane and two long out-of-plane Mn-O-Mn bonds. 

This structural change can lead to an increase in the in-plane Mn 3d - O 2p hybridization 

strength and a consequent increase in the W. The broadening of feature A below 

T c in the photoemission spectra (Fig. 2) is due to this increase in W. The parent 

compound of our sample, CaMnO 3 has an empty e g band. In the Ca 0.86 Pr 0.14 MnO 3 , 

the e g band could well be considered as rarely occupied. The spin-spin interaction of 

the e g electrons in this narrow band can lead to a localization of them resulting in 

the formation of FM clusters. Earlier Martin et. al [20] have shown the existence of 

such FM clusters embedded in an AFM matrix in electron-doped compositions. The 

near E F electronic spectra of such phase separated systems are known to display a 

pseudogap behavior [18]. The spectral weight shifts observed in the photoemission 

and O K XAS spectra of Ca 0.86 Pr 0.14 MnO 3 corroborate this behavior. The enhanced 

shift of electronic states from B to A observed in the photoemission spectra taken below 

the T c could be due to the structural changes across the transition. The changes 

in the topology of the MnO 6 octahedra below the T c mentioned earlier, might result 

in the shift of electrons from the t 2g levels to the near E F positions because of 

the increased strength in the in-plane Mn 3d -O 2p hybridization. The unoccupied 

states represented in pre-edge region of the O K XAS spectra, on the other hand, 

shows complementary shifts of states from the near E F to higher energy positions 

with decrease of temperature. 


Using high resolution photoelectron spectroscopy and O K edge XAS, we have studied 

the temperature dependent changes in the near E F electronic structure of the electron 

doped CMR, Ca 0.86 Pr 0.14 MnO 3 . High resolution photoemission study show changes 

in the e g electron band width and a spectral weight shift across the transition which 

could be ascribed to the structural changes and consequent changes in the Mn 3d - O 

2p hybridization strengths. On the other hand, a complementary shifts of states from


the near E F to higher energy positions were observed with decrease of temperature 

in the unoccupied states presented in the pre-edge region of O K XAS spectra.

Bibliography 




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105


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(2005).

Chapter 6 

Summary and Conclusions 

108

Summary and Conclusions 109 

In this thesis the electronic structure of Pr 1−x Ca x MnO 3 series have been investigated 

using photoemission, inverse photoemission and x-ray absorption spectroscopy. 

Using photoemission (UPS and XPS), we have studied the valence band and core levels 

of Pr 1−x Ca x MnO 3 . The unoccupied states of Pr 1−x Ca x MnO 3 have been studied 

by inverse photoemission spectroscopy and x-ray absorption spectroscopy. 

A brief overview of the structural, transport and magnetic properties associated 

with the CMR manganites have been described in the introduction of this thesis. 

Details of the experimental techniques used to study the electronic structure of 

Pr 1−x Ca x MnO 3 have been described in the chapter 2. 

Using valence band photoemission and O K edge x-ray absorption, we studied 

the temperature dependent finer changes in the near E F electronic structure of 

Pr 0.67 Ca 0.33 MnO 3 , which is regarded as a prototype for the electronic phase separation 

models in CMR systems. With decrease in temperature the O 2p contributions 

to the t 2g and e g spin-up states in the valence band were found to increase until T c . 

Below T c , the density of states with e g spin-up symmetry increased while those with 

t 2g symmetry decreased, possibly due to the change in the orbital degrees of freedom 

associated with the Mn 3dO 2p hybridization in the pseudo-CE-type charge or orbital 

ordering. These changes in the density of states could well be connected to the electronic 

phase separation reported earlier. By combining the UPS and XAS spectra, 

we derived a quantitative estimate of the charge transfer energy E CT (2.6±0.1 eV), 

which is large compared to the earlier reported values in other CMR systems. Such 

a large charge transfer energy was found to support the phase separation model. 

For a complete understanding of the changes in the occupied and unoccupied 

electronic states we performed a detailed study of the Pr 1−x Ca x MnO 3 system across 

their ferromagnetic - antiferromagnetic phase boundary using UPS and IPES. We 

used three compositions x = 0.2, 0.33 and 0.4. Our photoemission studies showed 

that the pseudogap formation in these compositions occur over an energy scale of 

0.48±0.02 eV. Here again, we have estimated the charge transfer energy in these 

compositions to be of the order of ∼ 2.8±0.2 eV. The core level electronic structure 

of Pr 1−x Ca x MnO 3 (x = 0.2, 0.33, 0.4 and 0.84) also have been studied using X-ray 

photoelectron spectroscopy. These studies have been performed at the Mn 2p, Ca 2p 

and Pr 4d levels. From these core level studies we observed a suppression of chemical 

potential shift for 77 K spectra compared to that of room temperature. These results 

support the model of phase separation in Pr 1−x Ca x MnO 3 system.

Summary and Conclusions 110 

Using high resolution photoelectron spectroscopy and O K edge x-ray absorption 

spectroscopy we have studied the temperature dependence of the near Fermi level 

electronic structure in the electron doped CMR, Ca 0.86 Pr 0.14 MnO 3 . We found that 

the temperature dependent changes in the electronic structure is consistent with the 

conductivity behavior of the electron doped systems in general. As the temperature 

is lowered from 293 K a feature due to the e g↑ states starts appearing near the E F . 

The intensity of this feature further increases below the T c with a shift of spectral 

weight from higher binding energy to the near E F states. Further, we have found an 

increase in the e g↑ band width (W) also at low temperatures. On the other hand, 

a complementary shifts of states from the near E F to higher energy positions were 

observed with decrease of temperature in the unoccupied states presented in the preedge 

region of O K edge x-ray absorption spectra. We have discussed our results from 

the point of view of phase separation models considering the temperature dependent 

structural changes and consequent localization of the electrons in a narrow e g band.

PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...

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