The confrontation between percolation processes and superconducting fluctuations to account for the observed enhanced in-plane electrical conductivity above but near T-c in cuprates is revisited. This dilemma is currently an open and debated question, whose solution would contribute to the phenomenological understanding of the emergence of superconductivity in these compounds. The cuprates studied here, La1.85Sr0.15CuO4, Bi2Sr2CaCu2O8+delta, and Tl2Ba2Ca2Cu3O10, have a different number of superconducting CuO2 (ab)-layers per unit-cell length and different Josephson coupling between them, and are optimally-doped to minimize T-c-inhomogeneities. The excellent chemical and structural quality of these optimally-doped samples also contribute to minimize the effect of extrinsic T-c -inhomogeneities, a crucial aspect when analyzing the possible presence of intrinsic percolative processes. Our analyses also cover the so-called high reduced-temperature region, up to the resistivity rounding onset epsilon(onset). By using the simplest form of the effective-medium theory, we show that possible emergent percolation processes alone cannot account for the measured enhanced conductivity. In contrast, these measurements can be quantitatively explained using the Gaussian-Ginzburg-Landau (GGL) approach for the effect of superconducting fluctuations in layered superconductors, extended to epsilon(onset) by including a total energy cutoff, which takes into account the limits imposed by the Heisenberg uncertainty principle to the shrinkage of the superconducting wavefunction. Our present analysis confirms the adequacy of this cutoff, which was introduced heuristically, and that the effective periodicity length is controlled by the relative Josephson coupling between superconducting layers, two long standing debated aspects of the GGL approaches for multilayered superconductors. These conclusions are reinforced by analyzing, as an example, one of the recent works that allegedly discards the superconducting fluctuations scenario while supporting a percolative scenario for the enhanced conductivity above T-c in cuprates.

The confrontation between the superconducting fluctuations and percolation effects as the origin of the in-plane paraconductivity in cuprate superconductors was earlier addressed at a quantitative level in the case of the optimally doped YBa2Cu3O7−δ (YBCO) compound. Using in-plane resistivity data from a high-quality YBCO thin film, we will extend these analyses to high reduced temperatures, in the case of the Gaussian-Ginzburg–Landau (GGL) approach for the conventional superconducting fluctuations, by considering the total energy cutoff. These data will also be analysed in terms of the mean field-approach of the effective-medium theory, to probe if emergent percolative effects may account for the resistivity rounding above $$T_{c}$$ . Our analyses confirm earlier conclusions: the measured paraconductivity cannot be explained in terms of emergent percolation processes, but it may be accounted for in terms of the GGL approach. These results also call into question alternative scenarios, including a recent proposal derived from emergent percolative effects.

Contrary to the starting assumption of Grbic et al. [Phys. Rev. B 83, 144508 (2011)], here we will argue that a 16-T magnetic field is not enough to quench all superconducting fluctuations above T-c in YBa2Cu3O7-delta. We conclude that through their measurements of microwave absorption these authors actually determine the AC fluctuation magnetoconductivity at 16 T, instead of the zero-field AC paraconductivity as they contend. So the temperature proposed by Grbi ' c et al. for the onset of the superconducting fluctuations, T', will correspond to the one at which the finite-field effects at 16 T become measurable in their experiments and the actual fluctuation onset will be located well above T'. These conclusions, which also concern influential recent publications on that issue, are confirmed by analyzing some of the Grbic et al. data on the grounds of the Gaussian Ginzburg-Landau approach for the finite-field (or Prange) fluctuation regime.

The electrical conductivity induced near the superconducting transition by thermal fluctuations was measured in different granular aluminum films. The seemingly anomalous behavior at high reduced temperatures and magnetic fields is explained by taking into account a total-energy cutoff in the superconducting fluctuation spectrum in both the direct (Aslamazov-Larkin) and the indirect (anomalous Maki-Thompson) contributions to the fluctuation effects. The analysis allowed a reliable determination of the coherence length amplitudes, which resulted to be much larger ($20--48$ nm) than the grains size ($5--10$ nm). This suggests that the grains are strongly Josephson-coupled, while the ${T}_{c}$ value is still as high as twice the bulk value. These results could contribute to identifying the mechanisms enhancing ${T}_{c}$ in these materials.

We first present detailed measurements of the rounding behavior around the superconducting transition temperature, Tc, 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 YBa2Cu3O7-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 Tc 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.

Measurements of the temperature dependence of the in-plane magnetic penetration depth, Delta(ab)(T) = lambda(ab)(0), are presented in the isovalently substituted iron pnictide BaFe2(As1-xPx)(2) near optimal doping (T-c similar to 28K). The data were obtained directly from the shielding magnetic susceptibility of thin single crystals (similar to 20 pm thickness along the crystals c-axis) under magnetic fields parallel to the ab layers. Complications associated to flux penetration were avoided by using fields in the 10(-4) T range, which ensured that the samples were well inside the Meissner region. At low temperatures (T < 0.25T(c)) our data confirm the linear temperature dependence of lambda(ab)(T) observed by using other procedures, and that is consistent with a nodal superconducting order parameter. By using values for lambda(ab)(0) in the literature we obtained the temperature dependence up to T-c of the superfluid density, n(s)(T) proportional to 1/lambda(2)(ab)(T). It is found that samples with slightly different Tc values present a significantly different qualitative behavior of ns(T). (C) 2016 Elsevier B.V. All rights reserved.

We report some of our experimental results about the transition to the normal state of high-temperature superconductors subjected to high current densities and, simultaneously, under external magnetic fields up to 1 T. Our data analysis, which is based on a recently published instability model, show that the quenching may be explained in terms of thermal instabilities due to self-heating favored by the nonlinear nature of the current-voltage characteristics of high-T c superconductors (HTS). In fact, we predict the density current J* at which the samples quench, i.e., jump abrupt to the normal state, with an accuracy around 1%. Beyond its interest from a fundamental point of view, this thermal model opens a practical way to estimate the quenching point thus avoiding the damaging or even burning up of devices in high power applications of HTS.

Due to the paraantiferromagnetic transition of oxygen at 45 K, the contamination with this element may deeply affect measurements performed with high-resolution magnetometers around this temperature. We have studied this spurious effect by measuring with a commercial SQUID-based magnetometer (Quantum Design, model MPMS-XL) the magnetization of granular La1.85Sr0.15CuO4 (LSCO) superconductors by using different sample holders. Our results demonstrate the crucial role played by the O-2 physisorption in the extended surface of the LSCO grains, an effect that may be deeply mitigated by encapsulating the grains into an epoxy resin (EPO-TEK 301). This proceduremay be also useful for high-precision magnetization measurements at low temperatures in other granular or porous materials.

Due to the paraantiferromagnetic transition of oxygen at 45 K, the contamination with this element may deeply affect measurements performed with high-resolution magnetometers around this temperature. We have studied this spurious effect by measuring with a commercial SQUID-based magnetometer (Quantum Design, model MPMS-XL) the magnetization of granular La1.85Sr0.15CuO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> (LSCO) superconductors by using different sample holders. Our results demonstrate the crucial role played by the O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> physisorption in the extended surface of the LSCO grains, an effect that may be deeply mitigated by encapsulating the grains into an epoxy resin (EPO-TEK 301). This procedure may be also useful for high-precision magnetization measurements at low temperatures in other granular or porous materials.

The nature of superconducting fluctuation effects in the isovalently substituted iron pnictide BaFe2(As1-xPx)(2) (x approximate to 0.35) is probed through measurements of the magnetization and magnetoconductivity around the superconducting transition. The results, obtained with magnetic fields up to 9 T applied in the two main crystal directions, are consistent with anisotropic Ginzburg-Landau (GL) approaches for finite applied magnetic fields. The analysis allowed us to determine with accuracy the out-of-plane, xi(c) (0), and in-plane, xi(ab) (0), GL coherence lengths. Significant differences are found between the xi(c) (0) values resulting from electrical transport and magnetization data. According to recent theoretical approaches, these differences could be interpreted in terms of the multiband nature of this material. The analysis of data in the low-field region around the transition temperature also suggests that phase fluctuations, although possibly relevant in other Fe-based superconductors, may play a negligible role in this compound.

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.

We study the diamagnetism induced by thermal fluctuations above the superconducting transition of the iron pnictide Ba(Fe 1 − x ?> Nix)2As2 with different doping levels. The measurements are performed with magnetic fields up to 7 T applied in the two main crystal directions. These data provide double information: first, they confirm at a quantitative level the applicability to these materials of a 3D-anisotropic Ginzburg–Landau approach valid in the finite field regime. Then, they allow us to determine the doping-level dependence of the in-plane coherence length and of the superconducting anisotropy factor, γ. Our results provide a stringent confirmation of the large increase of γ with the doping level, as recently proposed from magnetoresistivity measurements. The implications of the applicability of the model used to a multiband superconductor are discussed.

The superconducting fluctuations well inside the normal state of Fe-based superconductors were experimentally studied through the in-plane paraconductivity in several high-quality optimally-doped BaFe_2-xNi_xAs_2 crystals. These measurements were performed in magnetic fields with amplitudes up to 14 T, and different orientations relative to the crystals c axis (0, 53 and 90 degrees). The results allowed a stringent check of the applicability of a recently proposed Ginzburg-Landau approach for the fluctuation electrical conductivity of 3D anisotropic materials in presence of finite applied magnetic fields.

The influence of surface defects, in particular of a-axis grains, on the transition to the normal state induced by high current densities in YBa2Cu3O7−δ (YBCO) thin films and in a commercial 2G-coated conductor is investigated. For that purpose, the surface of the samples is observed by scanning electron microscopy and isothermal current-voltage curves are measured at different temperatures with pulsed currents up to the quenching value I*. The results show that the ratio of I* to the critical current is large if a-axis grains are not visible at the surface of the YBCO films, while it is much lower if the surface includes a-axis grains as this is the case for the coated conductor. The connection between the transition onset and the vortex dynamics, as well as the role of the a-axis grains in this process are discussed. The relation between the I* values obtained from thermal calculations and those resulting from vortex dynamics considerations is also discussed, as well as the possible consequences suggested by this work for the different applications of the coated conductors.

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.

In this paper, we present a theoretical discussion of the non-ohmic regime of the electrical conductivity of a type-II planar superconductor near its superconducting transition, focusing mainly in the region between the phase-coherence and the pair-condensation critical temperatures, T phase < T < T cond. For that purpose, we extend down to those temperatures existing calculations for the so-called Aslamazov-Larkin conductivity in presence of finite electric fields, and the resulting exponent α of the fluctuation voltage-current characteristics E∼j α . These results are then discussed by considering two possible scenarios for the phase diagram of the superconducting fluctuations in the underdoped high-temperature superconductors. Our results suggest that the strong phase fluctuations scenario (i.e., large T cond−T phase) leads to a plateau in the T-dependence of the exponent α near T phase, and that the T-region over which this plateau extends becomes wider when the applied electric field is increased. These features are much smaller in the conventional fluctuations scenario (i.e., small T cond−T phase).

The temperature dependence of the in-plane magnetic penetration depth lambda(ab) of Ba(Fe1-xNix)(2)As-2 single crystals is determined directly from the shielding magnetic susceptibility, measured in the Meissner region with the field parallel to the ab layers. The doping levels studied cover the underdoped, optimally doped and overdoped regimes. At temperatures below 0.5T(c) a well-defined power-law behavior lambda(ab)(T) - lambda(ab) (0) = AT(n) (with n approximate to 2.5) is observed. At lower temperatures (T < 0.3T(c)) the data are still consistent with n = 2 and A alpha T-c(-3), as predicted by the strong pair-breaking scenario proposed by Gordon et al (2010 Phys. Rev. B 81, 180501(R)). The temperature dependence of the superfluid density rho(s) alpha lambda(-2)(ab) presents a marked positive curvature just below T-c, which is a sign of two-gap superconductivity. The analysis of rho(s) (T) in terms of a two-gap model allowed estimation of parameters like the in-band and inter-band couplings, the relative weight of each band, and their dependence on the doping level. A comparison with rho(s) (T) data obtained by using other techniques in compounds with a similar composition is also presented.