We first present detailed measurements of the rounding behavior around the superconducting transition temperature, T c , of the in-plane electrical conductivity, magnetoconductivity and magnetization, including the low and moderate magnetic field regimes, in a high-quality single crystal and a thin film of the prototypical optimally-doped YBa 2 Cu 3 O 7-δ (OPT Y-123), in which the inhomogeneity effects are minimized. Then, we present a comparison of these experimental data with the phenomenological Ginzburg-Landau (GL) approach that takes into account the unavoidable contribution of the fluctuating pairs, the only theoretical scenario that at present allows analysis of these roundings at the quantitative level. These analyses demonstrate that the measured rounding effects around T c may be explained quantitatively and consistently in terms of the GL scenario, even up to the rounding onset temperatures if the quantum localization, associated with the shrinkage of the superconducting wave function, is taken into account. The implications of our results on the pseudogap physics of optimally-doped cuprates are also discussed.

The new results summarized here, including a brief comparison with the paraconductivity, further suggest that the anomalous precursor (above T c ) diamagnetism recently observed in the underdoped La 1.9 Sr 0.1 CuO 4 superconductor could be attributed to the presence, in addition to the conventional superconducting pair fluctuations, of T c -inhomogeneities with long characteristic lengths associated with chemical disorder .

The magnetization around the superconducting transition was measured in YBa 2 Cu 3 O 7−δ with magnetic impurities in the CuO 2 layers (Cu substituted by Zn or Ni) or between them (Y substituted by Gd or Pr). While some of these impurities have an important effect on the superconducting transition temperature (T c ), the precursor diamagnetism observed above T c is not appreciably affected. This result contrasts with recent observations in a conventional BCS superconductor (La), in which the precursor diamagnetism was found to increase several orders of magnitude with the addition of a small amount (a few atomic per cent) of Gd or Pr magnetic impurities.

Keywords: superconductivity quenching under thermal smallness, optimizing the thermal behavior of practical superconductors.Abstract. We address here the superconductivity quenching under an external magnetic field of amplitudes up to 1 T and in the so-called "thermal smallness" condition, when the microbridge width becomes smaller than the thermal diffusion length of both the own superconductor and its refrigerant (the substrate, in the case of thin films), which breaks their thermal dimensional scaling. Our results further support that when the current perturbations have characteristic times in the millisecond range the quenching is due to thermal instabilities associated with regular (nonsingular) flux-flow, and they also suggest how to optimize the refrigeration of practical superconductors. a juanmanuel.doval@usc.es, b jesusj.maza@usc.es, c carolina.torron@usc.es, d antonio.veira@usc.es, e manuel.tello@ehu.es, f felix.vidal@usc.es (main corresponding author).

The interplay between fluctuating Cooper pairs and magnetic impurities in conventional BCS low-Tc superconductors has been studied through measurements of the magnetic field dependence of the fluctuation diamagnetism (FD) above Tc in lanthanum with praseodymium impurities. These measurements provide a crucial confirmation of our previous observation [Europhys. Lett. 73 587 (2006)] that in the dilute impurity regime the FD increases almost linearly with the concentration of magnetic impurities. This striking effect is attributed to a variation due to the presence of the fluctuating Cooper pairs of the coupling between magnetic impurities. To describe these results at a phenomenological level, we propose a Gaussian Ginzburg-Landau model for the FD which includes an indirect contribution proportional to both the impurities concentration and the Cooper pairs density. Our approach is able to explain simultaneously the FD increase due to magnetic impurities and its decrease with the application of large magnetic fields.

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In highly anisotropic cuprate superconductors it is generally accepted that the reversible magnetization vector, ��⃗ , is essentially perpendicular to the superconducting CuO 2 layers in a wide range of crystal orientations with respect to the applied magnetic field, � �⃗ . In a recent work [J. Mosqueira et al., Phys. Rev. B 84, 134504 (2011)] it is shown, however, that the dependence of ��⃗ on the � �⃗ orientation in the reversible mixed state of a high-quality Tl-based cuprate presents a notable deviation from this behavior. Here we extend these measurements to the fluctuation region above T c , in order to check whether the above mentioned effect is also present.

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