Sidorov S. Fluctuations of paraconductivity and spectrum of elementary excitations in the two-dimensional systems.

The aim of the thesis is the establishment of nature wide resistive area in the temperature dependence of electrical resistivity, which precedes the transition to the superconducting state for a number quasi two-dimensional high-temperature superconductors. The object of research are the magnesium diboride, cuprate metaloxides Bi2223, Y123, LaSrCuO and pnictide FeTeSe. We investigated the transport and spectroscopic characteristics of these materials in the resistive state. This was done through different experimental methods: contact spectroscopy, study of the effect Nernst, study of the Seebeck coefficient, study of the dependence of the resistance in a wide temperature range. Experimentally by the example of magnesium diboride shows the relationship between excess current and the order parameter in the characteristics of Andreev contact. On the basis of detailed research of the phonon spectra of magnesium diboride exact found the situation of the energy mode, which determines the processes of electron-phone interaction that resolves the contradiction between calculations and data Raman-spectroscopy. For example, bismuth metaloxide and pnictide were showed the possibility of application of the Seebeck effect for determination of symmetry of the order parameter of metal oxides. For a number of cuprate superconductors discovered a link between the value of the energy gap and the value of the resistive region observed at temperatures above the critical temperature of superconducting transition. The experiments on the effect of Andreev reflection in the contacts of the superconductor - normal metal is established that for bismuth metal oxide Bi2223 in a wide temperature range, above the critical temperature, there is excessive current, which indicates the fluctuation origin of the local paires in the area of the pseudogap.