Anomalous precursor diamagnetism at low reduced magnetic fields and the role of TC inhomogeneities in the superconductors Pb55In45 and underdoped La1.9Sr0.1CuO4

Abstract

The magnetic field dependence of the magnetization was measured above the superconducting transition in a high-TC underdoped cuprate (La1.9Sr0.1CuO4) and in a low-TC alloy (Pb55In45). Near the superconducting transition [typically for (T−TC)∕TC≲5×10−2] and under low applied magnetic field amplitudes [typically for H∕HC2(0)≲10−2, where HC2(0) is the corresponding upper critical field extrapolated to T=0K] the magnetization of both samples presents a diamagnetic contribution much larger than the one predicted by the Gaussian Ginzburg-Landau (GGL) approach for superconducting fluctuations. These anomalies have been already observed in cuprate compounds by various groups and attributed to intrinsic effects associated with the nature of these high-TC superconductors itself. However, we will see here that our results in both high- and low-TC superconductors may be explained quantitatively, and consistently with the GGL behavior observed at higher fields, by just taking into account the presence in the samples of a uniform distribution of TC inhomogeneities. These TC inhomogeneities, which may be in turn associated with chemical inhomogeneities, were estimated from independent measurements of the temperature dependence of the field-cooled magnetic susceptibility under low applied magnetic fields. These conclusions are further confirmed by some additional low field magnetization measurements in a Pb92In8 alloy and in pure Pb. The results summarized here also fully confirm the intrinsic character of our previous measurements of the fluctuation induced diamagnetism in La1.9Sr0.1CuO4 under moderate magnetic field amplitudes [up to H∕HC2(0)∼0.2] as well as the corresponding suggestions that the precursor Cooper pairs are not affected by the presence of a pseudogap in the normal state in this underdoped cuprate.

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