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Rey R.I.,University of Santiago de Compostela | Ramos-Alvarez A.,University of Santiago de Compostela | Carballeira C.,University of Santiago de Compostela | Mosqueira J.,University of Santiago de Compostela | And 5 more authors.
Superconductor Science and Technology | Year: 2014

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-xNixAs2 crystals. These measurements were performed in magnetic fields with amplitudes up to 14 T, and different orientations relative to the c-axis of the crystals (, 53, and 90). The results allowed a stringent check of the applicability of a recently proposed Ginzburg-Landau approach for the fluctuating electrical conductivity of three-dimensional (3D) anisotropic materials in the presence of finite applied magnetic fields. © 2014 IOP Publishing Ltd. Source


Chagas L.H.,Federal University of Juiz de fora | Chagas L.H.,Instituto Nacional Of Metrologia Qualidade E Tecnologia | De Carvalho G.S.G.,Federal University of Juiz de fora | Do Carmo W.R.,Federal University of Juiz de fora | And 6 more authors.
Materials Research Bulletin | Year: 2015

Layered double hydroxides (LDHs) with Mg/Co/Al and Ni/Co/Al were synthesized for the first time by the urea hydrolysis method. The experimental conditions promoted aluminum rich and crystalline materials. The formation of LDHs was investigated by powder X-ray diffraction (XRD), chemical analysis, solid state nuclear magnetic resonance with magic angle spinning (27Al-MAS-NMR), simultaneous thermogravimetric/differential thermal analysis (TGA/DTA), FTIR spectroscopy, scanning electron microscopy (SEM), and N2 adsorption-desorption experiments. A single phase corresponding to LDH could be obtained in all the investigated compositions. Thermal calcination of these LDHs at 500 °C resulted in the formation of solid solutions in which Al3+ was dissolved. All the calcined materials have rock-salt like structures and high surface areas. © 2014 Elsevier Ltd. All rights reserved. Source


Rey R.I.,University of Santiago de Compostela | Ramos-Alvarez A.,University of Santiago de Compostela | Mosqueira J.,University of Santiago de Compostela | Salem-Sugui Jr S.,Federal University of Rio de Janeiro | And 5 more authors.
Superconductor Science and Technology | Year: 2014

The temperature dependence of the in-plane magnetic penetration depth λab of Ba(Fe1-xNix)2As 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 0.5Tc a well-defined power-law behavior λab (T) - λab (0) = ATn (with n ≈ 2.5) is observed. At lower temperatures (T < 0.3Tc) the data are still consistent with n = 2 and , 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 presents a marked positive curvature just below Tc, which is a sign of two-gap superconductivity. The analysis of ρs (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 ρs (T) data obtained by using other techniques in compounds with a similar composition is also presented. © 2014 IOP Publishing Ltd. Source


Rey R.I.,University of Santiago de Compostela | Carballeira C.,University of Santiago de Compostela | Mosqueira J.,University of Santiago de Compostela | Salem-Sugui S.,Federal University of Rio de Janeiro | And 5 more authors.
Superconductor Science and Technology | Year: 2013

The superconducting fluctuations well inside the normal state of Fe-based superconductors were studied through measurements of the in-plane paraconductivity and magnetoconductivity in high quality BaFe 2-xNixAs2 crystals with doping levels from the optimal level (x = 0.10) up to highly overdoped (x = 0.20). These measurements, performed in magnetic fields up to 9 T perpendicular to the ab (Fe) layers, allowed a reliable check of the applicability to iron-based superconductors of Ginzburg-Landau approaches for 3D anisotropic compounds, even at high reduced temperatures and magnetic fields. Our results also allowed us to gain valuable insight into the dependence on the doping level of some central superconducting parameters (coherence lengths and anisotropy factor). © 2013 IOP Publishing Ltd. Source


Salem-Sugui Jr. S.,Federal University of Rio de Janeiro | Alvarenga A.D.,Instituto Nacional Of Metrologia Qualidade E Tecnologia | Rey R.I.,University of Santiago de Compostela | Mosqueira J.,University of Santiago de Compostela | And 2 more authors.
Superconductor Science and Technology | Year: 2013

We study fluctuation conductivity in a single crystal of BaFe 1.9Ni0.1As2 superconductor (Tc = 20 K) as a function of temperature and applied magnetic field. Magneto-conductivity curves, Δσ versus T, were analyzed in terms of -1/ (dln(Δσ)/dT) versus T plots, which allow us to study different fluctuation regimes and to estimate exponent values and temperature widths of each regime. The analysis of magneto-conductivity curves evidences the existence of only two fluctuation regimes, a possible critical one (of glass-like type) going from the irreversible temperature to above Tc(H), followed by Aslamazov-Larkin fluctuations in the Gaussian regime. © 2013 IOP Publishing Ltd. Source

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