Anomalous doping dependence of fluctuation-induced diamagnetism in Y1−xCaxBa2Cu3Oy superconductors

Abstract

Superconduting quantum interference device magnetization measurements in oriented powders of Y1−xCaxBa2Cu3Oy, with x ranging from 0 to 0.2, for y≈6.1 and 6.97, are performed in order to study the doping dependence of the fluctuating diamagnetism above the superconducting transition temperature Tc. While for optimally doped compounds the diamagnetic susceptibility and the magnetization curves −Mfl(T=const) vs H are rather well justified on the basis of an anisotropic Ginzburg-Landau (GL) functional, in underdoped and overdoped regimes an anomalous diamagnetism is observed, with a large enhancement with respect to the GL scenario. Furthermore, the shape of magnetization curves differs strongly from the one derived in that scheme. The anomalies are discussed in terms of phase fluctuations of the order parameter in a layered system of vortices and with the assumption of charge inhomogeneities inducing local, non-percolating, superconducting regions with Tc(loc) higher than the resistive transition temperature Tc. The susceptibility displays an activated temperature behavior, a marked characteristic of the vortex-antivortex description, while the history-dependent magnetization, with relaxation after zero-field cooling, is consistent with the hypothesis of superconducting droplets in the normal state. Thus the theoretical picture consistently accounts for most experimental findings.

Source-reported materials — not catalogue approval

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