Multiband effects on the upper critical field angular dependence of 122-family iron pnictide superconductors.

Detailed measurements of the in-plane resistivity were performed in a high-quality Ba([Formula: see text])[Formula: see text] ([Formula: see text]) single crystal, in magnetic fields up to 9 T and with different orientations [Formula: see text] relative to the crystal c axis. A significant [Formula: see text] rounding is observed just above the superconducting critical temperature [Formula: see text] 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, [Formula: see text]. In spite of the relatively small anisotropy of this compound, it is found that [Formula: see text] presents a significant deviation from the single-band 3D anisotropic Ginzburg-Landau (3D-aGL) approach, particularly for large [Formula: see text] (typically above [Formula: see text]). These results are interpreted in terms of the multiband nature of these materials, in contrast with other proposals for similar [Formula: see text] anomalies. Our results are also consistent with an effective anisotropy factor almost temperature independent near [Formula: see text], a result that differs from the ones obtained by using a single-band model.

Anisotropy dependence of the fluctuation spectroscopy in the critical and gaussian regimes in superconducting NaFe1-xCo x As single crystals.

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.

Fluctuation induced diamagnetism in the zero magnetic field limit in a low temperature superconducting alloy.

By using a Pb-18 at. % In alloy, the fluctuation induced diamagnetism was measured in the zero magnetic field limit, never observed until now in a low-T(C) superconductor. This allows us to disentangle the dynamic and the nonlocal electrodynamic effects from the short-wavelength fluctuation effects. The latter may be explained on the grounds of the Gaussian-Ginzburg-Landau approach by introducing a total energy cutoff in the fluctuation spectrum, which strongly suggests the existence of a well-defined temperature in the normal state above which all fluctuating modes vanish. This conclusion may also have implications when describing the superconducting state formation of the high-T(C) cuprates.

First observation for a cuprate superconductor of fluctuation-induced diamagnetism well inside the finite-magnetic-field regime

For the first time for a cuprate superconductor, measurements performed above T(c) in high quality grain aligned La1.9Sr0.1CuO4 samples have allowed the observation of the thermal fluctuation induced diamagnetism well inside the finite-magnetic-field fluctuation regime. These results may be explained in terms of the Gaussian Ginzburg-Landau approach for layered superconductors, but only if the finite field contributions are estimated by taking off the short-wavelength fluctuations.