Structural and electroresistive properties of layered compounds based on the 1-2-3 HTSC system and transition metal dichalcogenides under extreme external influences (review)

The problem of the influence of extreme external influences (high pressure, sharp temperature drops, structural relaxation, and strong magnetic fields) on various mechanisms of electric transport of HTSC compounds Re1Ba2Cu3O7-δ (Re = Y or another rare-earth ion) and dichalcogenides of transition metals are considered. The features of the crystal structure and the effect of structural defects of various morphologies on the electrical conductivity of these compounds in the normal, pseudogap, and superconducting states are discussed. A review of the experimental data obtained in the study of the effect of high hydrostatic pressure and other extreme effects on various mechanisms of electric transport of Re1Ba2Cu3O7-δ compounds of various compositions and transition metal dichalcogenides of various technological backgrounds is carried out. Various theoretical models devoted to the effect of high pressure on the electrical conductivity of HTSC compounds of the 1-2-3 system and transition metal dichalcogenides are discussed, and a comprehensive comparative analysis of their magnetoresistive characteristics under extreme external influences is performed. In particular, it was shown, that the relatively weak effect of pressure on the Tc value of optimally doped samples can be explained within the framework of a model assuming the presence of a Van Hove singularity in the spectrum of charge carriers which is characteristic of strongly coupled lattices. This is confirmed by the observation similar features of the behavior of the baric derivatives dTc/dP depending on the change composition in NbSe2 single crystals, which also belong to systems of two-dimensional lattices and have a similar anisotropy parameter. Nevertheless, it is still possible to formulate a number of questions that have not yet found a final experimental and theoretical solution. Namely, what is the role of the crystal lattice and structural defects and, in particular, twinning planes? What is the reason for the broadening of the resistive transitions of HTSC compounds into the superconducting state under pressure, and what is the relationship between this broadening and charge transfer and the nature of the redistribution of the vacancy subsystem? What is the role of phase separation in the implementation of different modes of longitudinal and transverse transport? Obviously, more research, both experimental and theoretical, is needed to answer these questions.