We report the studies of detailed magnetic relaxation and isothermal magnetization measurements in the vortex state of the 112-type iron-pnictide Ca0.82La0.18Fe0.96Ni0.04As2 superconductor with Tc∼ 22 K. In the isothermal M(H), a well defined second magnetization peak (SMP) feature is observed in the entire temperature range below T c for measurements with H∥c -axis. However, for H∥ab -planes, the SMP feature is suppressed at low temperatures, which might be due to 2D Josephson vortices forming at low temperatures and high magnetic fields in such an anisotropic system. A rigorous analysis considering the magnetic relaxation data for H∥c -axis suggests an elastic to plastic pinning crossover across H p , which also seems accompanied with a possible phase transition in vortex lattice near Hp . Moreover, point disorder and surface defects are likely to be the dominant sources of pinning, which contribute to the δl -type of pinning in the sample. A high J c , in access of 105 A cm−2 observed could potentially make this material technologically important.

Detailed measurements of the in-plane resistivity were performed in a high-quality Ba( Fe_1-xCo_x ) _2As_2 ( x=0.065 ) single crystal, in magnetic fields up to 9 T and with different orientations θ relative to the crystal c axis. A significant ρ (T)_H,θ rounding is observed just above the superconducting critical temperature T_c due to Cooper pairs created by superconducting fluctuations. These data are analyzed in terms of a generalization of the Aslamazov-Larkin approach, that extends its applicability to high reduced-temperatures and magnetic fields. This method allows us to carry out a criterion-independent determination of the angular dependence of the upper critical field, H_c2(θ ) . In spite of the relatively small anisotropy of this compound, it is found that H_c2(θ ) presents a significant deviation from the single-band 3D anisotropic Ginzburg-Landau (3D-aGL) approach, particularly for large θ (typically above ∼ 60^o ). These results are interpreted in terms of the multiband nature of these materials, in contrast with other proposals for similar H_c2(θ ) anomalies. Our results are also consistent with an effective anisotropy factor almost temperature independent near T_c , a result that differs from the ones obtained by using a single-band model.

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.

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.

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 investigate thermal fluctuations in terms of diamagnetism and magnetotransport in superconducting NaFe1−xCo x As single crystals with different doping levels. Results show that in the case of optimal doped and lightly overdoped (x = 0.03, 0.05) crystals the analysis in the critical as well as in the Gaussian fluctuation regions is consistent with the Ginzburg-Landau 3D fluctuation theory. However, in the case of strongly overdoped samples (x ≥ 0.07) the Ullah-Dorsey scaling of the fluctuation induced magnetoconductivity in the critical region confirms that thermal fluctuations exhibit a 3D anisotropic nature only in a narrow temperature region around T c (H). This is consistent with the fact that in these samples the fluctuation effects in the Gaussian region above Tc may be described by the Lawrence-Doniach approach. Our results indicate that the anisotropy of these materials increases significantly with the doping level.

Isothermal magnetic field dependence of magnetization and magnetic relaxation measurements were performed for the H parallel to c axis on a single crystal of Ba(Fe-0.935 Co-0.065)(2)As-2 pnictide superconductor having T-c = 21.7 K. The second magnetization peak (SMP) for each isothermal M(H) was observed in a wide temperature range from Tc to the lowest temperature of measurement (2 K). The magnetic field dependence of relaxation rate R(H), showed a peak (H-spt) between H-on (onset of SMP in M(H)) and H-p (peak field of SMP in M(H)), which is likely to be related to a vortex-lattice structural phase transition, as suggested in the literature for a similar sample. In addition, the magnetic relaxation measured for magnetic fields near H-spt showed some noise, which might be the signature of the structural phase transition of the vortex lattice. Analysis of the magnetic relaxation data using Maley's criterion and the collective pinning theory suggested that the SMP in the sample was due to the collective (elastic) to plastic creep crossover, which was also accompanied by a rhombic to square vortex lattice phase transition. Analysis of the pinning force density suggested a single dominating pinning mechanism in the sample, which did not showing the usual delta l and delta T-c nature of pinning. The critical current density (J(c)), estimated using the Bean critical state model, was found to be 5 x 10(5) A cm(-2) at 2 K in the zero magnetic field limit. Surprisingly, the maximum of the pinning force density was not responsible for the maximum value of the critical current density in the sample.

We report on isochamp magneto-resistivity and ac susceptibility curves obtained in a high-quality single crystal of the isovalent optimally doped pnictide BaFe2(As0.68P0.32)(2) with superconducting temperature T-c = 27.8 K for H||c-axis. Plots of the logarithmic derivative of the resistivity curves allowed the identification of a vortex-glass (VG) phase and to obtain the values of the critical glass temperature T-g, the temperature T* marking the transition to the liquid phase and of the critical exponent s. The presence of the VG phase is confirmed by detailed measurements of the third harmonic signal of the ac magnetic susceptibility. The modified VG model was successfully applied to the data allowing the obtention of the temperature independent VG activation energy U-b. The activation energy U-0 obtained from the Arrhenius plots in the flux-flow region are compared with U-b and with U-0 obtained from flux-creep measurements on a M(H) isothermal in the same sample. A phase diagram of the studied sample is constructed showing the T-g glass line, the T* line representing a transition (melting) to the liquid phase, the mean field temperature T-c(H) line and the H-p line obtained from the peaks in isothermal critical current, J(c)(H) curves, which are explained in terms of a softening of the vortex lattice. The glass line was fitted by a theory presented in the literature which considers the effect of disorder.

Fluctuation magnetoconductivity and magnetization above the superconducting transition temperature (${T}_{c}$) are measured on the recently discovered 112 family of iron-based superconductors (IBS), ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}y}{\mathrm{Ni}}_{y}{\mathrm{As}}_{2}$, which presents an extra As-As chain spacer-layer. The analysis in terms of a generalization of the Lawrence-Doniach approach to finite applied magnetic fields indicates that these compounds are among the most anisotropic IBS ($\ensuremath{\gamma}$ up to $\ensuremath{\sim}30$) and provides compelling evidence of a quasi-two-dimensional behavior for doping levels near the optimal one.

We present numerical simulations of the width and shape of the resistance versus temperature transition, i.e., R(T), under zero external magnetic field, in a high-temperature superconducting film with a regular pattern of local variations of critical temperatures, induced, e.g., by means of micro- or nanofunctionalization. To obtain a realistic R(T), we also take into consideration the widening of the transition due to Gaussian and vortex-antivortex fluctuations, plus the effects of the intrinsic inhomogeneity due to the nonstoichiomety of the dopant ions. We focus mainly on square-lattice patterns and show that they may be used to engineer R(T) transitions broader or narrower than the ones of the parent nonstructured film.

We present numerical simulations of the width and shape of the resistance versus temperature transition, i.e., R(T), under zero external magnetic field, in a high-temperature superconducting film with a regular pattern of local variations of critical temperatures, induced, e.g., by means of micro- or nanofunctionalization. To obtain a realistic R(T), we also take into consideration the widening of the transition due to Gaussian and vortex-antivortex fluctuations, plus the effects of the intrinsic inhomogeneity due to the nonstoichiomety of the dopant ions. We focus mainly on square-lattice patterns and show that they may be used to engineer R(T) transitions broader or narrower than the ones of the parent nonstructured film.

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.

We measure magnetization as a function of temperature, magnetic field and time in a BaFe$_2$(As$_{0.68}$P$_{0.32}$)$_2$ single crystal with $T_c$ = 27.6 K. The fish-tail observed on many $M$($H$) curves are used to construct isofield $M$($T$) curves which show an anomalous peak at some temperature $Tt$ suggesting a possible phase transition in the irreversible regime. A vortex dynamics study performed along the peaks evidences a minimum in the relaxation rate occurring at the same position $Tt$ of the minimum value of $M$ in these peaks. A vortex dynamics study performed on $M$($H$) curves show two distinct minimum in the relaxation rate: a first minimum ($H1$) for a lower field correlating with $Tt$ and a second ($H2$) correlating with the peak in the fish-tail. A phase diagram is constructed and the line corresponding to $Tt$ and $H1$ points obey an expression developed in the literature for a structural rhombic to square lattice phase transition, further supporting this view.

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 measure magnetization as a function of temperature, magnetic field, and time in a BaFe2(As0.68P0.32)2 single crystal with Tc = 27.6 K. The fish tail observed in many M(H) curves is used to construct isofield M(T) curves which show an anomalous peak at some temperature Tt, suggesting a possible phase transition in the irreversible regime. A vortex dynamics study performed along the peaks evidences a minimum in the relaxation rate occurring at the same position Tt of the minimum value of M in these peaks. A vortex dynamics study performed on M(H) curves shows two distinct minima in the relaxation rate: a first minimum (H1) for a lower field correlated with Tt and a second (H2) correlated with the peak in the fish tail. A phase diagram is constructed, and the line corresponding to the Tt and H1 points obeys an expression developed in the literature for a structural rhombic to square lattice phase transition, further supporting this view.