E.G. Maksimov

Electronic states and optical spectra of HTSC with electron-phonon coupling

Abstract It is shown that due to the strong electron-phonon interaction the usual Fermi-liquid description of the electron system is violated in the normal phase of HTSC metals ( T > T c ). As a result it is impossible to use the simple formulas of the Drude type for the calculation of FIR properties of the HTSC materials. The many-body calculation of the reflectivity gives a good agreement with the FIR measurements for the single-domain crystal. The processing of the obtained results by using the “extended Drude formula” gives the effective scattering rate 1/τ ∗ ≈βħω where β is about 1 up to 2000 cm -1 (and even for higher frequencies taking into account the interband transitions).

THERMAL PAIR-BREAKING IN SUPERCONDUCTORS WITH STRONG ELECTRON-PHONON INTERACTION

Abstract The influence of inelastic scattering in superconductors with a strong electron- phonon interaction was considered. The pair-breaking scattering leads to smearing of singularities in the electron density of states and to the appearance of states inside a “gap”. Normal electrons in such superconductors appear due to the filling of the states instead of the excitations across the gap. This leads to the temperature dependencies of some thermodynamic and kinetic characterics to be more close to the phenomenological Gorter-Casimir two-fluid model rather then BCS theory.

Phonon assisted tunneling in Josephson junctions

Abstract The expression for additional subgap current in the presence of electron–phonon interaction is derived. We show that the phonon assisted tunneling leads to the appearance of peaks on current-voltage characteristics at the Josephson frequencies corresponding to the Raman-active phonons. The relation of the obtained results to the experimental observation are discussed.

Nonadiabatic effects in optical phonon self-energy

Abstract The nonadiabatic renormalization of optical phonon frequencies and linewidths is studied. The phonon self-energy is calculated in the one-loop approximation including electron-phonon interaction and impurity scattering. The wave vector dispersion and the temperature dependence of the phonon self-energy are calculated for two model Eliashberg spectral functions. It is shown that a strong electron-phonon interaction diminishes the nonadiabatic renormalization of a phonon frequency and shifts the maximum of the linewidth to the values of qv F higher than it follows for Landau damping of noninteracting electrons. The vertex corrections are studied for a system with isotropic impurity scattering. It is shown that they can give in some cases a serious qualitative contribution to the nonadiabatic renormalzation of phonon frequencies and linewidths.

Relationship between the pyroelectric coefficient and the lattice dynamics of the pyroelectrics

Abstract The experimental results for a temperature dependence of the pyroelectric coefficient are represented for twenty different crystals in a wide temperature range. Current theories are used for an analytical fitting of this result. It is shown that in the ferroelectrics the pyroelectric coefficient γσ is defined mainly by optical phonon modes in comparison with the linear pyroelectrics in which γσ follows the T3 law due to acoustic phonon. The possibility of an appearance of the linear temperature dependence of γσ ≈ T suggested by Born is discussed briefly. Our experimental results show that such behavior indeed exists but only in very imperfect crystals and at very low temperature.

Electromagnetic response of superconductors and optical sum rule

Abstract The interrelation between the condensation energy and the optical sum rules have been investigated. It has been shown that the so called ‘partial’ sum rule violation is related mainly to a temperature dependence of the relaxation rate rather than to the appearance of superconductivity itself. Moreover, we demonstrate that the experimental data on the temperature dependence of the optical sum rule can be explained rather well by accounting for the strong electron–phonon interaction.

Comparative analysis of the optical spectra of YBa2Cu3O7

Abstract New experimental data on the temperature dependence of the optical reflectivity of YBa 2 Cu 3 O 7 films are studied in the framework of strong electron–phonon interaction. It is shown that the normal state reflectivity in the far- and mid-infrared region can be explained using an Eliashberg function α 2 (ω)F(ω) typical for this system with a constant of coupling λ=2 . The superconducting state data are compared to the theoretical calculations using the usual strong-coupling approach. This leads to the conclusion that some other mechanism, supplementary to electron–phonon interaction, exists in this system.

Microscopical calculations of lattice dynamics of ionic crystals and its dependence on the pressure

Abstract The generalized Gordon-Kim method is used to calculate static and dynamic properties of the ionic crystal ScBO3. The equilibrium volume and the crystalline structure are obtained in a good agreement with experimental data. The Raman active phonon modes are calculated by the “frozen phonon” method. The absolute values of these frequencies and its pressure dependence are in good agreement with the experimental data also.

The investigation of Eliashberg equations for superconductors with strong electron-phonon interaction

Abstract The superconductivity state in systems with strong electron-phonon interacting is studied. Eliashberg equation is transformed in the form, in which nonresonant terms which has energetical dependence similar to that in BSC theory and resonance terms (caused by specific of retarded electron-phonon interaction) are extracted. It is shown, that resonance terms increase with the constant of electron-phonon interaction λ and for large λ ⪢ 1 they mostly determine behavior both of Tc and of the gap Δ0. This leads to the behavior of the gap as Δ 0 ≅ λ . Using spectral density of electron-phonon interaction α2(ω)F(ω) which corresponds HTSC compounds we have studied the behavior of density of states of electrons Ns(ω) and have shown the essential difference from those in BCS theory mainly for temperatures near Tc. Corresponding behavior Ns(ω) leads to the differences in behavior of kinetic and thermodynamic characteristics of superconductors from BCS theory near Tc, that was demonstrated for the number of superconducting electrons and the London-limit penetration depth.

On microscopical derivation of Lorentz-Lorenz formula

Abstract Lorentz-Lorenz formula may be derived microscopically by the analytical inversion of the dielectric matrix (DM) in the limit of tightly bound electrons. Exchange and correlation effects are to be taken into account and atomic polarisability is to be defined correctly in terms of the atomic susceptibility instead of the polarization operator. A wrong definition was the origin of usual contradictions between microscopical calculations and classical formula. The inversion of the DM diminishes dielectric constant in comparison with the diagonal element of DM but this procedure takes into account only a part of the local field effects caused by rapidly oscillating fields. Another part is connected with exchange interaction and increases the diagonal element of DM in comparison with dielectric constant without any local field effects, hence all local field effects do increase the dielectric constant.