Natalia Nikolaevna KOVALEVA - Max Planck Institute for Solid State ...

4. Current research activity
2001– present. Max-Planck Institute for Solid State Physics, Stuttgart – in collaboration with Prof. C. Bernhard and
Prof. B. Keimer.
Optical spectral weight (SW) redistribution caused by magnetic interactions in Mott-Hubbard insulators, such as
LaMnO 3 , YTiO 3 , and LaTiO 3 ; Charge dynamics in iron-based superconductor LaFeAsO 1-x F x ; C-axis and in-plane optical
conductivity of YBa 2 Cu 3 O 6..9 in a wide spectral range (FIR, mid-IR, UV-VIS); IR study of optical conductivity
in Na x CoO 2 ; C-axis lattice dynamics in single-, bi-, and trilayer Bi-cuprate HTSCs; Assignment of the IR and Raman-active
phonons in YTiO 3 , TiOCl and NaCoO 2 , calculation of the eigenvector components, dynamic effective
charges, and etc. [1-16].
Methods employed: Polarized angle-resolved optical probes in a wide spectral range (FIR, mid-IR, UV-VIS) and
temperature range (10-400 K) such as ellipsometry, reflectance and transmittance spectroscopy are used. Theoretical
group analysis and lattice dynamical calculations are applied in the interpretation of IR and Raman phonon spectra.
5. Scientific research experience and references
1999 – 2000. University College London, Department of Physics and Astronomy, CMMP – in collaboration with
Phof. A.M. Stoneham.
Modeling of polaron-related features in systems of complex oxides exhibiting “colossal magnetoresistance” (CMR)
effect in the framework of shell model approximation and Mott-Littleton approach [17-19].
Methods employed: Determination of shell model parameters from lattice equilibrium conditions and consistency
with experimental optical phonon frequencies and high- and zero- frequency dielectric constants. Defect calculations
in the Mott-Littleton approach using the shell model parameters. Calculation of key electronic and ionic polarization
energies associated with localized electronic charge carriers. Calculation of optical charge-transfer transitions in a
Born-Hyber cycle.
1996 – 1998. ISSP RAS – in collaboration with Dr. A.V. Bazhenov ISSP RAS; Dr. P.J.M. van Bentum, High-field
Magnet Laboratory, University of Nijmegen, The Netherlands; Prof. S-W. Cheong, Rutgers Univeristy, Piscataway,
New Jersey, USA; Prof. N.-C. Yeh, California Institute of Technology, Pasadena, USA.
Expeperimental IR-study of optical conductivity in correlated 3d-electron systems of doped perovskite manganites of
Re 1-x A x MnO3. IR reflectivity of La 0.7 Ca 0.3 MnO 3 single crystal was investigated over a broad range of temperatures
(78-340 K), magnetic fields (0-16 T), and frequencies (20-10000 cm –1 ). A strong reduction of the effective dc resistivity
extrapolated from IR optical conductivity is found with increasing external magnetic field in the vicinity of the
Curie point – the first observation of the “optical CMR” effect at IR frequencies. The temperature and magnetic field
dependencies of the optical conductivity are characterized by the interplay between the Drude-like and polaron features,
indicating the importance of electron-phonon coupling and double exchange mechanism in the description of
the CMR effect [22-25].
Methods employed: IR reflectance and transmittance spectroscopy in a broad range of temperatures (10-340 K),
magnetic fields (0-16 T), and frequencies (20-10000 cm –1 ).
1983 – 1996. ISSP RAS, Ph. D. (1989) – in collaboration with Prof. S.I. Bredikhin and Prof. Yu.A. Ossipyan;
Studies of electronic subsystem and mechanism of interaction between electronic and ionic subsystems in superionic
conductors. Investigations of optical properties and ac-conductivity of RbAg 4 I 5 superionic conductor, associated with
intrinsic crystal defects. Investigation of relaxation and steady-state processes at the electronic conductor-superionic
conductor boundary. The mechanisms of the low-temperature photoluminescence and color center formation due to
additive coloring and ionic implantation were studied. The exposure-induced absorption under light irradiation into
the impurity excitation region was discovered and investigated. The new phenomena observed are associated with
specific features of electronic excitation process due to the presence of the mobile ionic subsystem [26-39].
Methods employed: Low-temperature photoluminescence, UV-VIS optical absorption, space-resolved optical spectroscopy,
dc/ac ionic and electronic conductivity measurements.
1981 – 1983. MIPT & ISSP RAS, M. Sc. (1983) – in collaboration with Dr. Sc. K.P. Meletov and Prof. E.F. Sheka;
Studies of deep impurity centers in molecular crystals (naphthalene) by optical methods. Investigations of the anomalies
of phonon spectra of impurity centers by using temperature modulation technique of low-temperature luminescence
spectra [40].
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