Critical Fields and Characteristic Lengths in Copper Oxide Superconductors

Abstract

There have been a number of studies of the critical fields and critical currents in the two copper oxide superconducting families La2−xSrxCuO4 and YBa2Cu3O7−δ which show unexpected and unusual properties. Resistive measurements of the superconducting transition in zero and finite field often show long tails extending to low temperatures, and the critical field curves constructed from such data often show unusual upward curvature, especially near Tc. Initially most of the critical field studies were done on polycrystalline sintered samples. With such samples there is always a lingering question as to whether the measured properties are intrinsic to the material or are due to poor connections between particles or to the averaging of strongly anisotropic behavior over many crystallite orientations. With the advent of measurements on single crystal samples in several laboratories it is now possible to compare measurements on polycrystals and single crystals to determine the intrinsic behavior and to assess the degree of sample dependence which cannot be attributed to particle connections or to anisotropy. In this review we characterize the typical behavior of polycrystalline samples and compare it to results on single crystals. We find that much of the behavior seen in polycrystals also appears in single crystals and that among single crystal samples there is significant sample dependence. Nevertheless, there is sufficient agreement on the upper critical field values to define approximately the slope at Tc, from which the coherence length can be derived. We combine this information with new measurements of the lower critical field at low temperature to derive the anisotropic magnetic penetration depth. Finally, we compare the derived characteristic length with relevant unit cell dimensions and with the scale of the microstructure to infer information about the dimensionality of the superconductivity and the pinning forces in YBa2Cu3O7−δ.