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Scientific Report 2007-2009
Condensed matter physics and biophysics
C6. Sub-Terahertz and Infrared studies of strongly correlated oxides
In the last two years we have studied the sub-THz,
infrared and optical spectra of different oxide families,
characterized by strong electron-electron and electronphonon
interaction. Those studies were aimed at further
investigating the exotic properties of these materials,
which range from high-T c superconductivity to the
formation of charge density waves, from the appearance
of pseudogaps at remarkably high T to Mott transitions.
In the cuprates we have measured the reflectivity of
a number of single crystals belonging to the BSLCO
family, in order to understand the mechanism of the
insulator-to-metal transition. We have found that this
this occurs when the Drude quasi-particle peak turns
into a band at finite frequencies, which opens a gap in
-1 -1
σ
1
(ω) (Ω cm )
1500
1000
500
1000
0
(b)
500
0
(c)
400
200
(a)
0
(d)
400
Bi 2
Sr 2-x
La x
CuO 6
x = 0.7 p ~ _ 0.12
T = 300 K
20 K
x = 0.8 p ~ _ 0.10
T = 300 K
10 K
x = 0.9 p ~ _ 0.07
x = 1.0 p ~ _ 0.03
the density of states of just a few tens of meV [1]. This
band has been identified as polaronic in nature, namely
due to the charges undergoing weak localization due to
a moderate electron-phonon coupling. This result confirms
the importance of this interaction in the cuprates,
at variance with several theoretical approaches where it
is neglected.
In the newly discovered Fe-As pnictides, both the infrared
and Raman phonon spectra of SmFeAsO have
been determined [2]. Also the competition between superconductivity
and spin-density wave at low doping has
been approached by detecting infrared spectra vs. doping.
An extensive study of manganites with commensurate
charge order has been performed in the sub-Terahertz
spectral region [3] by using the coherent synchrotron radiation
emitted by the storage ring BESSY-2 when working
in the so-called α-mode. We have thus first revealed
the collective modes of a charge density wave (CDW)
in these materials. The collapse of the CDW induced
by pressure has been observed by illuminating a single
crystal of Nd 0.5 Sr 0.5 MnO 3 , pressurized in a diamond
anvil cell, with the far-infrared radiation of SPRING-8
(Japan).
We have also studied the Magnéli phases V n O 2n+1 , to
understand the role played by strong electron-electron
correlations in their transport and optical properties. In
particular, the whole phase diagram of V 2 O 3 has been
probed in the infrared, and the opening of a pseudogap
has been observed above 425 K. This loss of coherence
has been explained by calculations taking into account
the lattice expansion in a strongly correlated system [4].
References
1. S. Lupi et al., Phys. Rev. Lett. 102, 206409 (2009).
2. C. Marini et al., EPL 84, 67013 (2008).
3. A. Nucara et al., Phys. Rev. Lett. 101, 066407 (2008).
4. L. Baldassarre et al., Phys. Rev. B 77, 113107 (2008).
200
0
(e)
400
200
Drude term
FIR band
MIR band
CT band
Y 1-x
Ca x
Ba 2
Cu 3
O 6
x = 0.03 p ~ _ 0.015
Authors
P. Calvani, S. Lupi, P. Maselli, A. Nucara, C. Mirri, D.
Nicoletti, F. Vitucci
http://www.phys.uniroma1.it/gr/irs/
0 3 4 5 6
10 2 2 3 4 5 6
10 3 2 3 4 5 6
ω (cm -1 )
10 4
Figure 1: The metal-to-insulator transition induced in a superconductor
by decreasing doping (from top to bottom) [1],
as observed in the real part of the optical conductivity at
two temperatures. The transition occurs through the change
of the Drude peak (violet symbols) into a polaronic band
peaked at finite frequency (blue symbols). A mid-infrared
band, nearly insensitive to the transition is also detected
(grey symbols). It is probably of magnetic origin.
Sapienza Università di Roma 59 Dipartimento di Fisica