Narrow Results

The Hall effect in epitaxial YBa₂Cu₃O_7-x films in perpendicularly applied magnetic fields B from 1 to 6 T reveals a sign reversal of the Hall coefficient for B ≤ 3 T. We analyze our experimental data by using the full quantitative expressions based on the renormalized superconducting fluctuation model. We propose an approach how the vortex pinning can be considered in the Hall coefficient, based on the observation that flux pinning mainly affects the longitudinal conductivity in the low-temperature part of the superconducting transition.

Three MTG Ag-Y_1.8Ba_2.4Cu_3.4O_7-x samples were fabricated to study the effect of doping Ag on magnetization. Magnetizations measured at various temperatures and fields show two peaks for all samples: the first peak in low field and second peak in middle field. We examined the effect of doping Ag and temperature on these peaks and discussed the origin of the peaks.

The measurements of lower-critical-field H_c1(T), upper-critical-field H_c2(T) and the bulk pinning force F_p(H) were carried out for a superconductor MgCNi₃ sample with T_c=7.5K prepared under high pressure. The sample has a good extensibility. We conclude that the flux pinning at lower fields results from the intra-granular structural defects, such as dislocations, and the enhancement of bulk pinning force near the irreversibility field H_irr is due to the softening of the flux lattice.

The effect of YBCO nanoparticles added into MgB₂ on T_c, J_c, and flux pinning was studied for MgB₂(YBCO)_x with x=0, 5, 10, 15 wt%. Phase analysis shows that none of elements are doped into the MgB₂ lattice in the samples with YBCO addition. For the samples with YBCO addition, the J_c-H characteristics behave poorly in comparison with the pure sample. Our experimental results show that the nanoscale size of addition dosen't comprise the only condition for its effectiveness as pinning centers.

Gd is successfully embedded into YBa₂Cu₃O_7-δ by the solid state reaction. Rietveld refinements show that Gd replaces Cu in the unit cell. By magnetization and resistivity measurements, the effects of Gd-doping in YBCO on the critical current density J_c and the critical temperature T_c in YBa₂Cu₃O_7-δ(YBCO) have been investigated. J_c varies with the applied field and the content of Gd and has a maximum at x = 0.03. The characteristic behavior of J_c with varying the Gd content can be explained in terms of the competition between two different effects caused by Gd-doping in YBCO: the inhomogeneity of grains distribution in nano-scale and the variation of superconductivity.

Ag-sheathed (Bi_1.6Pb_0.4)Sr₂Ca₂Cu₃O₁₀(Bi–2223) high temperature superconductor tapes with 0.1 wt% nano Cr₂O₃ were fabricated using the powder-in-tube method. The effects of intermediate rolling and sintering temperature on the transport critical current density (J_c) under self-field and applied magnetic field were studied. The X-ray patterns showed a dominant Bi–2223 phase (~80%). A decrease in J_c in applied magnetic field, parallel or perpendicular to the sample wide surface was observed for all tapes. Intermediate rolling improved the self-field J_c(T=77 K) of the tapes by 25%. Both J_c versus applied field curves and normalized pinning force density F_p/F_pmax versus applied field curves showed that the pinning strength was significantly enhanced with thermo-mechanical treatments. SEM micrographs showed improvements in the grain alignment of tapes with intermediate rolling. Enhancement in J_c was observed in the nano Cr₂O₃ added tapes.

Pinning force data, $F_{\mathrm{\text{p}}}$, of a variety of Fe-based high-$T_c$ superconductors (11-, 111-, 122- and 1111-type) were analyzed by means of a scaling approach based on own experimental data and an extensive collection of literature data. The literature data were mostly replotted, but also converted from critical current measurements together with data for the irreversibility line when available from the same authors. Using the scaling approaches of Dew-Hughes [J. Appl. Phys.44, 1360 (1973)] and Kramer [Philos. Mag.30, 293 (1974)], we determined the scaling behavior and the best fits to the theory. The data of most experiments analyzed show a good scaling behavior at high temperatures when plotting the normalized pinning force $F_{\mathrm{\text{p}}}/F_{\mathrm{\text{p}},\mathrm{\text{max}}}$ versus the irreversibility field, $H_{\mathrm{\text{irr}}}$. The resulting peak positions, $h_0$, were found at $\approx 0.3$ for the 11-type materials, at $\approx 0.48$ for the 111-type materials, between 0.32 and 0.5 for the 1111-type materials and between 0.25 and 0.71 for the 122-type materials. Compared to the typical results of Bi₂Sr₂CaCu₂O${}_{8+\delta }$$(h_0\approx 0.22)$ and YBa₂Cu₃O${}_{7-\delta }$$(h_0\approx 0.33)$, most of the 122 and 1111 samples investigated show peak values higher than 0.4, which is similar to the data obtained on the light-rare earth 123-type HTSC like NdBa₂Cu₃O_y. This high peak position ensures a good performance of the materials in high applied magnetic fields and is, therefore, a very promising result concerning the possible applications of the Fe-based high-$T_c$ superconductors.

Fe(Se/Te) family compounds are very important among iron-based superconductors. To explore the modification of their physical properties, Li-doped FeSe${}_{0.5}$Te${}_{0.5}$ polycrystalline samples were prepared by using sintering method with metal Li as the dopant. X-ray diffraction (XRD) and Raman scattering indicated that the lattice parameter c and Raman peaks change systematically with increasing Li content, suggesting Li atoms have entered the lattice. Magnetization and electrical transport measurements showed the type-II superconductor behavior. The upper critical field increased with increasing Li doping concentration, while the coherence length decreased with increasing dopant concentration. Li-doping generally improved the superconductivity, including the uniformity of superconducting contents. The thermally activated flux-flow (TAFF) activation energy also increased with increasing dopant concentration. The critical current densities were measured and the flux pinning mechanism and its change with doping were analyzed.

Magnetic Co ion doped SmFeAsO polycrystal was synthesized via solid-state reaction. Resistivity, SEM and magnetic hysteresis loops (MHLs) were measured to investigate magnetic properties of the sample. Critical current densities as well as the flux pinning forces densities were estimated from MHLs. This paper reports for the first time the research of superconducting MHLs as well as magnetic relaxation properties of SmFe_0.9Co_0.1AsO. Results suggest that: (i) A tail effect in the resistivity measurement together with the rapid decrease in critical current densities at low fields shows the evidence for granularity of the sample; (ii) The asymmetry of the MHLs may be caused by the Bean–Livingstone (BL) surface pinning or granular nature, and none of theoretical models are suitable to the scaling behaviors of flux pinning forces densities; (iii) The anomalous tendency of the temperature dependence of magnetic relaxation rate as well as the effective pinning energy were observed, which may be attributed to the competition between the bulk pinning and the BL surface pinning.

The intrinsic magnetization relaxation rate R(T)≡dM(t, T)/dlnt and the pinning potential U₀(T) have been determined for single crystalline and melt-textured YBa₂Cu₃O₇ (Y1:2:3) high temperature superconductors of different sizes in their critical state. In contrast to previous reports, R(T) is a monotonically decreasing function and U₀(T), deduced from R(T), a monotonically increasing function of temperature. The unusual temperature dependence of U₀ is qualitatively explained in terms of a collective flux pinning. The U₀-distribution model previously proposed for HTS’s is not consistent with certain experimental observations.