THEORETICAL INVESTIGATION OF HIGH-TEMPERATURE SUPERCONDUCTIVITY

A brief history of the development of the concepts of superconductivity since its discovery and to this day is given. The early phenomenological and microscopic theories of superconductivity of metals and alloys are discussed. The properties of nonmetallic superconductors that contain transition elements and intermetallic compounds having an A-15 structure are analyzed. These superconductors are in wide technological use now. The properties of the compounds of palladium and thorium with hydrogen and deuterium, as well as of compounds with heavy fermions are presented. The theoretical attempts to explain the specific features of nonmetallic superconductors are reviewed. The peculiarities of the new high-temperature superconductors and the different models of their theoretical description based on the concepts of a nonphonon superconductivity mechanism are discussed. A new bisoliton model of high-temperature superconductivity based on a system of nonlinear equations is introduced. An exact solution to these equations is characterized by a single wave function for the whole crystal that describes a condensate of bisolitons - pairs of quasiparticles surrounded by local deformations - that move as a single whole with constant velocity. The conditions under which the condensate is stable are investigated. The bisoliton model enables us to explain the following points without employing nonphonon pairing mechanisms: (1) the small correlation radius of paired particles as compared with BCS theory; (2) a very weak isotopic effect; (3) a nonmonotonic dependence of Tc on the current carrier concentration; (4) different values of the energy gap in the spectrum of quasiparticle states determined from tunelling and spectroscopic measurements; (5) a weak dependence of Tc on the concentration of magnetic impurities for concentrations lower than the critical one, and an abrupt destruction of superconductivity for concentrations higher than the critical one; (6) a nonmonotonic dependence of Tc on the external magnetic field strength. © 1990.