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Excess fluctuation conductivity of BaFe1.9Co0.1As2 single crystal was studied by measurements of temperature and magnetic field dependence of resistance. The mean field critical temperature Tc (H) is determined by using the criterion of the dR/dT as a function of temperature, where a unique sharp peak appears in the dR/dT curves. By analyzing the excess fluctuation conductivity and the resistance experimental data in the framework of Aslamazov and Larkin theory, different fluctuation regimes such as the three-dimensional (3D), the two-dimensional (2D), the one-dimensional (1D), and short-wave fluctuation (SWF) were identified. In the mean field region, it was also found that the analyses of the resistance experimental data based on the Aslamazov-Larkin theory describes the real measured resistance data very well down to about 0.5 Rn at different regions, which supported the excess fluctuation conductivity results. Based on the lowest Landau level method, both the three-dimensional (3D) and two-dimensional (2D) scaling behaviors were observed in the fluctuation conductivity at magnetic field larger than 5 T and different temperature interval around the mean-field critical temperature Tc. [ABSTRACT FROM AUTHOR]

The temperature and magnetic field dependences of the critical current density in silicone oil-doped MgB polycrystalline samples have been investigated by magnetic measurements. The upper critical magnetic field anisotropy, $\gamma =H_{\mathrm {c}2}^{\bot \mathrm {c}}/H_{\mathrm {c}2}^{\vert \vert \mathrm {c}}$ dependence of the critical current density, J( B), were analyzed within the percolation model. The calculated critical current densities based on percolation theory are in agreement with the experimental data. It was found that the anisotropy, γ, and the percolation threshold parameter, p, show different trends in their temperature dependence, where γ increases, but p decreases with increasing temperature. It is suggested that the anisotropy is responsible for the reduction of the critical current density in high magnetic field. The relationship between the anisotropy and the volume pinning force is investigated. It was found that the position of the maximum of the volume pinning force is shifted to lower reduced magnetic field by decreasing the anisotropy and increasing the percolation threshold p. [ABSTRACT FROM AUTHOR]

Polycrystalline samples of RECaMBaCuO with RE=Nd, Y and M=Pr, Th (with 0.0≤ x≤0.10), superconductors were prepared by the standard solid-state method. Resistivity was measured as a function of temperature and doping concentration x. Excess conductivity was analyzed using the modified Lawrence-Doniach (LD) expressions. The fluctuation regions, crossover temperatures, coherence lengths, and effective layer thickness were obtained and the values were compared for both samples. For both samples, it was found that with increasing doping, the crossover temperatures were reduced, while the coherence length decreased. The upper critical field and critical current density were increased with increasing doping concentration due to the introduction of disorder and the enhancement of flux pinning by charge neutral doping. Furthermore, the coherence lengths of the Nd-based samples are larger than that for the Y-based samples by a factor 2. It was found that the value of critical current density in Nd(CaPr)-123 is higher than Y(CaTh)-123, from which it is suggested that CaPr doping is more effective than CaTh doping. [ABSTRACT FROM AUTHOR]

The vortex pinning mechanisms of Ba0.72K0.28Fe2As2 single crystal have been studied systematically as a function of temperature and magnetic field. The temperature dependence of the critical current density, Jc(T), was analysed within the collective pinning model at different magnetic fields. It was found that both the δl pinning mechanism, i.e., pinning associated with charge-carrier mean free path fluctuation, and the δTc pinning mechanism, which is associated with spatial fluctuations of the transition temperature, coexist in the Ba0.72K0.28Fe2As2 single crystal in fields smaller than 4 T. Their contributions are strongly temperature and magnetic field dependent. At lower temperature and B ≤ 4 T, the δl pinning is the dominant mechanism, and its contributions decrease with increasing temperature. At temperatures close to the critical temperature, however, there is evidence for δTc pinning. At magnetic fields larger than 4 T, the δl pinning mechanism is the only effect. [ABSTRACT FROM AUTHOR]

Underdoped BaFe1.9Co0.1As2 single crystal was studied by angular dependence of magneto-transport at fields up to 13 T over a wide range of temperature. Our results show that pinning potential, Uo, decreases slightly for θ ≤ 45° and remains constant for θ ≥ 45°, while the upper critical field, Hc2, and the irreversibility field, Hirr, increase with θ. According to anisotropic Ginzburg-Landau theory, the anisotropy was determined by scaling the resistivity under different magnetic fields below the superconducting critical temperature, Tc. Anisotropy, Γ, in the underdoped crystal is found to be temperature dependent and decreases from 2.1 to 1.8 for as T is reduced from 17 to 12.5 K. [ABSTRACT FROM AUTHOR]

High pressure-high temperature electrical resistivity study on composition-controlled Nd0.9Ca0.1Ba2Cu3O7-δ high Tc superconductor (HTSC) is carried out by a four-probe technique using Bridgman anvils. A simple heating coil arrangement is used for heating the samples. Electrical resistivity behavior under pressure (up to a maximum of 8 GPa) at various temperatures (up to a maximum of 523 K) were studied and reported in this paper. Simulation of energy dispersive X-ray diffraction confirms substitution of calcium at the Nd site of Nd-123. Variation of the electrical resistivity under pressure is compared with that of the structural changes and the bulk modulus was determined. [ABSTRACT FROM AUTHOR]

High pressure-high temperature electrical resistivity study on composition-controlled Nd0.9Ca0.1Ba2Cu3O7-δ high Tc superconductor (HTSC) is carried out by a four-probe technique using Bridgman anvils. A simple heating coil arrangement is used for heating the samples. Electrical resistivity behavior under pressure (up to a maximum of 8 GPa) at various temperatures (up to a maximum of 523 K) were studied and reported in this paper. Simulation of energy dispersive X-ray diffraction confirms substitution of calcium at the Nd site of Nd-123. Variation of the electrical resistivity under pressure is compared with that of the structural changes and the bulk modulus was determined. [ABSTRACT FROM AUTHOR]

High pressure electrical resistivity studies were carried out on the high temperature superconductor Nd[sub 1-x]Ca[sub x]Ba[sub 2]Cu[sub 3]O[sub 7-δ] for various calcium concentrations x=0.00, 0.03, 0.06 and 0.10 obtained by the solid-state reaction method. The electrical resistivity study was performed using the four-probe technique with a Bridgman opposed anvil device. All four samples show an initial drastic fall in electrical resistivity up to a pressure of around 3 GPa that remains almost constant up to 8 GPa. [ABSTRACT FROM AUTHOR]

Tremendous efforts towards improvement in the critical current density 'Jc' of iron based superconductors (FeSCs), especially at relatively low temperatures and magnetic fields, have been made so far through different methods, resulting in real progress. Jc at high temperatures in high fields still needs to be further improved, however, in order to meet the requirements of practical applications. Here, we demonstrate a simple approach to achieve this. Hydrostatic pressure can significantly enhance Jc in NaFe0.97Co0.03As single crystals by at least tenfold at low field and more than a hundredfold at high fields. Significant enhancement in the in-field performance of NaFe0.97Co0.03As single crystal in terms of pinning force density (Fp) is found at high pressures. At high fields, the Fp is over 20 and 80 times higher than under ambient pressure at12 K and 14 K, respectively, at P = 1 GPa. We believe that the Co-doped NaFeAs compounds are very exciting and deserve to be more intensively investigated. Finally, it is worthwhile to say that by using hydrostatic pressure, we can achieve more milestones in terms of high Jc values in tapes, wires or films of other Fe-based superconductors. [ABSTRACT FROM AUTHOR]

Pressure is well known to significantly raise the superconducting transition temperature, Tc, in both iron pnictides and cuprate based superconductors. Little work has been done, however, on how pressure can affect the flux pinning and critical current density in the Fe-based superconductors. Here, we propose to use hydrostatic pressure to significantly enhance flux pinning and Tc in polycrystalline pnictide bulks. We have chosen Sr4V2O6Fe2As2 polycrystalline samples as a case study. We demonstrate that the hydrostatic pressure up to 1.2 GPa can not only significantly increase Tc from 15 K (underdoped) to 22 K, but also significantly enhance the irreversibility field, Hirr, by a factor of 4 at 7 K, as well as the critical current density, Jc, by up to 30 times at both low and high fields. It was found that pressure can induce more point defects, which are mainly responsible for the Jc enhancement. Our findings provide an effective method to significantly enhance Tc, Jc, Hirr, and the upper critical field, Hc2, for other families of Fe-based superconductors in the forms of wires/tapes, films, and single crystal and polycrystalline bulks. [ABSTRACT FROM AUTHOR]