ANOMALOUS TEMPERATURE-DEPENDENCE OF HCL IN LAYERED OXIDE SUPERCONDUCTORS

To study theoretically the structure of a flux line in copper oxide superconductors, we use a model in which their crystals are composed of arrays of superconducting and normal layers, and superconductivity in the normal layers is induced by a proximity effect. The superconducting order parameter in the normal layers is much enhanced at low temperatures. As a result of this enhancement, the lower critical field Hc1 is drastically enhanced at low temperatures. The anomalous enhancement of Hc1 at low temperatures has been observed in several oxide superconductors. The lower critical field perpendicular to the layers is expressed as Hc1=(G{cyrillic}(T)/2K{cyrillic}s)logK{cyrillic}s[Ø0/{2πξs(T)λs(T)}], K{cyrillic}s, ξs(T) and λs(T) being the parameters defined on the superconducting layers. The quantity G{cyrillic}(T) regulates the enhancement. © 1990.