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Microscopic and phenomenological theory of unusual metallic systems in the normal and superconducting states

The aim of the joint project was to study systems with a strong electron-phonon interaction in the normal and superconducting states, and systems showing structural phase transitions. Detailed investigations were made to study the electron-phonon interaction and related properties for a number of simple and transition metals. For these studies the density functional theory and a band structure approach were used.

It was shown that the band structure approach together with Eliashberg's equations describes surprisingly well properties such as the electrical resistivity, the thermal conductivity, and the electron-phonon coupling constant even for non-trivial materials such as transition metals with considerably strong exchange correlation interaction.

It was shown using many-body perturbation theory and Landau's theory of Fermi-liquids that the accuracy of the developed methods is the same as the accuracy for the application of Landau's theory. The accuracy is controlled by the smallness of average phonon frequencies in comparison with characteristic electron energies. The investigations of the problem of high-temperature superconductivity were continued. It was shown that their many-body correlation effects together with a strong electron-phonon interaction and impurity scattering can explain successfully the microwave and infrared properties of these systems. The generalized Kim-Gordon approach was applied to study the phonon frequencies of ionic crystals and their dependence on the pressure with a good success. The ferroelectric instability in some perovskite crystals was studied, and good agreement with experimental data was obtained.

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