A. E. Karakozov

The terahertz and infrared spectra of the complex dynamic conductivity, as well as the temperature dependences of the density of a superconducting condensate and the electronic specific heat of superconducting Ba(Fe_{1 − x} Co _x )₂As₂ compounds, have been analyzed within a Bardeen-Cooper-Schrieffer-like model of a multiband superconductor with strong coupling. It has been shown that the superconducting state of these compounds is determined by three (one electronic and two hole) weakly interacting condensates. The order parameters of the condensates are: Δ₁ ≈ 15 cm⁻¹, Δ₂ ≈ 21 cm⁻¹, and Δ₃ ≈ 30–35 cm⁻¹. The results significantly refine the existing notions on the structure of the superconducting state of Co-doped BaFe₂As₂ multiband compounds.

A generalized α model for computing the superconducting parameters of real two-band superconductors is proposed based on an analysis of the properties of two-band equations in the theory of supercondu

The properties of a superconductor featuring strong electron-phonon and electron-impurity interactions have been studied in a single-band approximation. It is shown that the elastic scattering of electrons on static impurities decreases the electron-phonon interaction constant in proportion to the ratio of the electron-impurity relaxation rate and the seeding band width. The optical spectral weight (integral of the real part of the optical conductivity) in various energy intervals of a metal in the normal and superconducting states has been calculated as a function of the temperature and the superconducting gap. In the region of large (compared to the phonon) energies, the dependence of the spectral weight on the superconducting gap is weak, while the dependence on the temperature is completely determined by the corresponding dependence of the relaxation rate due to the elastic electron-phonon scattering far from the Fermi surface. It is shown that a difference in behavior of the spectral weight between the normal and superconducting states at lower energies is determined by so-called Holstein’s shift of the feature in the optical conductivity spectrum (rather than by the gap width, as it is commonly believed) and sharply decreases upon the introduction of impurities.

Complex conductivity spectra of superconducting Nb are calculated from the first principles in the frequency region around the energy gap and compared to the experimental results. The row experimental data obtained on thin films can be precisely described by these calculations.