A new approach is proposed for analysis of the excess conductivity σ′(T) arising below a characteristic temperature T*⪢Tc in YBa2Cu3O7−y (YBCO) films with different oxygen concentrations. The approach is based on the idea that σ′(T) is formed as a result of the formation at T⩽T* of local pairs (tightly bound bosons) obeying Bose–Einstein statistics in the temperature interval Tm<T<T*. At temperatures Tcmf<T<Tm the pairs obey BCS theory (Tcmf is the critical temperature separating the phase transition region from the region of critical fluctuations). Thus in Y123 systems a transition from Bose–Einstein condensation to condensation of the BCS type occurs with decreasing temperature. An equation in which the dynamics of formation of the tightly bound bosons is taken into account is proposed which gives a good description of the temperature dependence σ′(T) and in which the parameter Δ*, identified with the pseudogap in high-Tc superconductors, is contained in explicit form. The temperature dependence Δ*(T) is obtained for all the films studied.

No abstract

No abstract

A considerable part of the theoretical and experimental works reflecting the current status of research on high-temperature superconductivity and the unusual phenomenon of a pseudogap in high-temperature superconductors (HTSCs) is reviewed. The concept of local pairs in systems with low and intermediate charge-carrier density, which can include HTSCs, is examined. The experimental part is primarily based on the study of excess and fluctuation conductivity (FC) in YBa2Cu3O7−y (TBCO) and Y1−xPrxBa2Cu3O7−y (YPrBCO) thin epitaxial films. A new approach to analyzing FC and the pseudogap in such high-temperature systems is proposed and checked experimentally. The approach is based on the idea that excess conductivity σ′(T) forms in HTSCs at temperatures substantially above critical temperature Tc as a result of the formation of pair states in the form of noninteracting strongly bound bosons, demonstrating with decreasing temperature a transition from a regime with localized pairs conforming to the Bose–Einstein condensation theory into a regime with fluctuation Cooper pairs which conform to the BCS theory.

No abstract

The temperature dependence of the current-voltage characteristics of high-quality thin films of tin from 7 to 50 µm thick are investigated in the absence of an external magnetic field. For the first time, we have experimentally observed phase slip centres (PSCs) and phase slip lines (PSLs) on the same superconducting tin film with known parameters in the temperature intervals corresponding to the mechanisms of their formation and existence. We have shown that the states of a wide film with increasing transport current appear in the following order: the superconducting state for current less than critical; the resistive vortex state for current more than critical, but less than maximum current for the uniform flux flow (instability current); the critical state due to the onset of instability of the steady pattern of viscous motion of the vortices; a vortex-free resistive state with PSLs for current more than instability current, but less than the upper critical current; and the normal state at a current higher than the upper critical current.

The temperature dependences of the enhanced critical current in wide and thin Sn films exposed to the microwave field have been investigated experimentally and analyzed. It was found that the microwave field stabilizes the current state of a wide film with respect to the entry of Abrikosov vortices. The stabilizing effect of irradiation increases with frequency. Using similarity between the effects of microwave enhancement of superconductivity observed for homogeneous (narrow films) and inhomogeneous (wide films) distributions of the superconducting current over the film width, we have succeeded in partial extension of the Eliashberg theory to the case of wide films.

No abstract