Fluctuation conductivity and pseudogap in YBCO high-temperature superconductors (Review)

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.

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