• PDF / 262,214 Bytes
• 7 Pages / 612 x 792 pts (letter) Page_size
• 40 Downloads / 230 Views

DOWNLOAD

REPORT

---

AND LIQUIDS

Permittivity of Complex Oxide Crystals with Allowance for Spatial Dispersion Z. P. Mastropas* and E. N. Myasnikov Pedagogical Institute, Southern Federal University, ul. Bol’shaya Sadovaya 33, RostovonDon, 344082 Russia *email: [email protected] Received June 31, 2012

Abstract—The temporal and spatial dispersions of the permittivity of complex oxide crystals, which have numerous branches of dipole active oscillations, is considered. Formulas for calculating the susceptibility spectra of complex oxides in the terahertz region with allowance for both types of dispersion are obtained in the resonance approximation of photon–phonon interaction using quantum Green’s functions. The calcu lation results are used to discuss the wellknown experimental data. DOI: 10.1134/S1063776113010214

INTRODUCTION The improvement of experimental techniques for studying the crystal lattice motions in the region of phonon frequencies and theoretical methods for the description of these motions allowed researches to solve earlier unsolvable problems in many fields of the physics of crystals. In particular, these problems include the problems of the dynamics of crystal restructuring (phase structural transformations), the softmode dynamics in secondorder phase transitions (PT2) of the displacement type, and the mechanisms of softmode stabilization during these transitions. The methods of investigations in the optics of infrared frequencies were found to be most effective, although the wavelengths here are significantly larger than the unit cell sizes in crystals. Xray diffraction analysis cannot be effective in solving such problems, since the phonon frequencies are well below the Xray radiation frequencies. Therefore, Xray diffraction studies give a series of instant photographs of a structure. The most comprehensive information on lattice motion can now be obtained from phonon spectra. For example, the light reflection spectra recorded in [1] and processed with a relationship between the spectra and a permittivity allowed the author to state that the softmode stabilization in SrTiO3 is caused by the anharmonicity of lattice vibrations. However, sim ilar investigations in [2] led to the conclusion that the softmode stabilization in SrTiO3 is caused by the high anisotropy of oxygen ion deformability in this crystal. In [3], we calculated the thermodynamic potential in a system of interacting phonons and electrons and showed that the mechanism of stabilization is based on a redistribution of the electron density in the unit cell of a complex oxide crystal with ferroelectric PT2. This mechanism of phase transition in SrTiO3 is similar to the anisotropy of oxygen ion deformability. The rela

tionship analogous to that used in [3] was applied in [4] but in a factorized form for noninteracting modes. These discrepancies are related to the insufficient accuracy of processing the reflection and absorption spectra of electromagnetic waves in a crystal. For example, Cowley [1] even used models of noninteract ing modes fo