CONDUCTIVITY ANISOTROPY AND METAL-INSULATOR TRANSITION OF Y1-zPrzBa2Cu3O7–δ SINGLE CRYSTALS WITH VARIOUS PRASEODYMIUM CONTENT (review)

Abstract

The paper analyzes the results of studies of the influence of praseodymium on the anisotropy of the electrical resistance ρ_с(Т)/ρ_ab(Т) of high-temperature superconductors Y_1-zPr_zBa₂Cu₃O_7–δ. The influence of Pr impurities on the anisotropy of conductivity in this system is important both for clarifying the nature of high-temperature superconductivity (HTSC) and for increasing its critical parameters. The temperature dependences ρ_ab(Т) and ρ_с(Т) in the normal state have linear sections at high temperatures. An increase in the praseodymium content, z, leads to a semiconductor shape of the ρ_с(Т) curves, but the ρ_ab(Т) curves retain a linear section longer. A downward deviation from the linear course with decreasing temperature indicates the appearance of excess conductivity. The critical temperatures, T_c, along and across the basal plane are different, which is associated with the Friedel transition in the 1-2-3 system, which consists in the suppression of transverse superconductivity by the growth of ring Josephson vortices in a layered superconductor. This is possible when the conducting subsystem breaks down into several regions with different T_c. The breakup occurs during structural disordering due to doping of HTSC cuprates with an inovalent impurity or changing the oxygen content, and leads to the emergence of electroneutral regions in the system – metallic and dielectric. In this case, the conductivity of the sample is jumpy with a variable jump length, and the resistance anisotropy is well described by the universal "law 1/2". Praseodymium-induced clustering of the sample can initiate a metal-dielectric transition of the "Anderson" type. The curves ρ_ab(Т) and ρ_с(Т) can be approximated over the entire temperature range, taking into account the metallic, semiconducting, and fluctuation conductivity. This allows us to determine the temperature dependences of the anisotropy of the charge carrier scattering mechanisms at different z. An increase in z causes a metal-dielectric transition, which always precedes the superconducting transition, and a decrease in the transition temperature. The difference in the conduction mechanisms along and across the layers emphasizes the difference between HTSC cuprates and Fermi-liquid metals.