[en] The effect of Pr addition on the superconducting properties of the (Bi, Pb)-2212 system in the bulk polycrystalline form was studied. The Pr content was varied from x = 0.0 (pure) to 0.5 on a general stoichiometry of Bi_1.7Pb_0.4Sr_2.0Ca_1.1Cu_2.1Pr_xO_y. Phase analysis by XRD, microstructural examination by an SEM equipped with an energy dispersive x-ray spectrometer (EDS), density measurements and superconductivity characterizations were done to evaluate the relative performance of the samples. The critical current density (J_C) and superconductivity transition temperature (T_C) of Pr-added samples were found to be higher than that of the pure sample, i.e. without Pr added. A maximum J_C of 678.8 A cm^-2 at 64 K was measured for the sample with Pr = 0.2, which is more than seven times higher than that of the pure sample (J_C = 87.5 A cm^-2). Also the T_C of the sample with Pr = 0.2 was the highest (88.0 K) compared to all other samples. Microstructural examination showed distinct variations in the grain morphology of the samples containing Pr, and the porosity increases as the Pr content increases. Consequently, there is a significant reduction in the density of the samples as the Pr content increases. A Sr-rich oxide containing Pb(Bi) and Cu with square rectangular grains with rounded edges was observed in the microstructure of Pr-added samples whose composition has been analysed through EDS. But such a secondary phase could not be distinguished from XRD analysis, probably due to the peak overlapping with (Bi, Pb)-2212. (rapid communication)

[en] Bi_1.8Pb_0.35Sr_2.0Ca_2.1Cu_3.1Nd_xO_y (x = 0.00, 0.010, 0.020 and 0.030) superconductors were prepared in bulk form. Phase analysis by XRD, microstructural examination by SEM, superconductivity measurements at 77 K, etc were carried out to evaluate the relative performance of the samples. Both the self-field J_C and J_C in the presence of an applied magnetic field of the Nd-added samples are found to be much better than those of the undoped one. Also the peak value of the bulk pinning force density, F_pmax, of Nd-added samples get shifted towards higher magnetic fields, indicative of enhanced flux pinning strength in these samples. For the sample with Nd 0.030, small fractions of (Bi, Pb)-2212 were found even after the last stage of sintering. Accordingly the microstructure of this sample shows considerable difference from the other samples, wherein the natural layered growth mechanism of (Bi, Pb)-2223 is disrupted by grain growth and coarsening in random directions. It is likely that Nd ions are substituting at the Ca site, and the point defects thus created act as flux pinners, enhancing the self-field J_C as well as the J_C-B characteristics of the Nd-added samples

[en] The changes in phase evolution, structural, superconducting and flux pinning properties of (Bi, Pb)-2212 superconductor due to the addition of the rare earth Dy have been studied. Bulk polycrystalline samples were used for the study in which Dy content was varied from x = 0.0 to 0.5 on a general stoichiometry of Bi_1.7Pb_0.4Sr₂Ca_1.1Cu_2.1Dy_xO_y. It was found that the Dy atoms enter into the structure, replacing Sr and Ca with significant changes in the lattice parameters, microstructure, hole concentration, normal state resistivity and flux pinning strength of the system. Consequently, the critical temperature (T_C), critical current density (J_C) and the field dependence of J_C (J_C-B characteristics) of the Dy added samples were found to be enhanced considerably for optimum doping levels. A maximum T_C-onset of 93.4 K (for x = 0.4) and a maximum J_C of 692 A cm^-2 at 64 K (for x = 0.2) were observed for doped samples, as against 75.8 K and 107 A cm^-2, respectively, for the pure sample. The enhancement in the superconducting properties, particularly in the J_C-B characteristics, due to Dy addition seems to have great technological significance

[en] The effects of Gd addition on the phase evolution and superconducting properties of (Bi, Pb)-2212 prepared in the bulk polycrystalline form were studied. The Gd content in the samples was varied from 0.0 to 0.5 on a general stoichiometry of Bi_1.7Pb_0.4Sr_2.0Ca_1.1Cu_2.1Gd_xO_y. Phase analysis by means of x-ray diffraction, microstructural examination by scanning electron microscopy and superconducting property studies were carried out to evaluate the relative performances of the samples. It is observed that Gd addition enhances the transition temperature (T_C) and critical current density (J_C) of the system. Moreover no secondary phase containing Gd ions or any other cations was observed after the final stage of heat treatment. Microstructural examination shows clear and distinct morphologies for the Gd-added samples, wherein the grain growth is suppressed by the addition of Gd and the edges of the grains become more and more rounded. As a result, there is an increase in the porosity of the Gd-added samples, leading to a reduction in the sintered density for these samples

[en] The flux pinning properties of (Bi, Pb)-2212 superconductors modified by the addition of rare earth (RE) elements La, Gd and Yb with varying concentrations are studied. Samples were prepared by a conventional solid state route with initial stoichiometry of Bi_1.7Pb_0.4Sr₂Ca_1.1Cu_2.1RE_xO_y (where RE = La, Gd, Yb and x 0.0 (pure), 0.1, 0.2, 0.3, 0.4). The samples were characterized by powder x-ray diffraction, scanning electron microscopy, R-T measurements and superconductivity measurements at 64 K. It is found that the addition of these elements considerably enhances the flux pinning strength of the system, and there is also an increase of critical temperature (T_C) and critical current density (J_C). These values become maximum at optimum concentrations (x = 0.2-0.3), and at higher levels of addition, these values decrease. The results are explained based on the replacement of Sr²⁺ and/or Ca²⁺ by RE³⁺ ions and the associated distortion of the lattice and change in carrier concentration of the system. La and Yb are found to be more effective than Gd in obtaining better flux pinning properties

[en] The effect of multistage cold uniaxial pressing on Bi-2223 bulk superconductors with varying fractions of Bi-2223 in the precursor powder has been studied. The results show that the final density, grain orientation, J_C and J_C-B characteristics depend greatly on the fraction of Bi-2223 contained in the precursor and increase with increasing Bi-2223 fraction. In a three-stage repeated press-sinter schedule, density and J_C (77 K) values as high as 5.76 g cm^-3 and 3110 A cm^-2 respectively could be achieved with 80% Bi-2223 precursor, whereas the values are only 5.31 g cm^-3 and 1860 A cm^-2 for 22% Bi-2223 precursor under identical conditions. The likely reasons for the wide variation of microstructural and superconducting properties are discussed

[en] We report superconducting properties of tungsten meander structures fabricated using the focussed ion beam (FIB) induced technique. Three meander structures with individual line widths of ∼70, ∼300 and ∼450 nm were fabricated for evaluation and comparison of the superconducting properties. The resistance-temperature characteristics of the meanders were measured and analysed down to a temperature of 100 mK. The superconducting properties such as critical temperature (T _C) and upper-critical field (H _C2) of these wires are in comparison to the reported values of FIB deposited tungsten available in literature. While the normal state resistance increases sharply as the width of the wire decreases, the superconducting transition temperature registered a slight decrease. Significant amount of residual resistance (3.8% of normal state value at 100 mK) was observed for the sample with the lowest width (70 nm). The residual resistance trails as function of temperature was analysed invoking theoretical models governing the phase slip induced dissipations in superconducting nanowires. The results indicated signature of phase slips as the width of the wire decreases: thermally activated phase slips dominant near to the T _C and quantum phase slip (QPS) near to T _C as well as much below to the T _C. The magneto-resistance isotherms indicated quantum phase transitions (QPT); typical of a superconductor-to-insulator transition (SIT) driven by magnetic field. The SIT transition which originates from the intrinsic disorder present in the sample can be tuned by an external parameter such as magnetic field, and can be modelled by standard theories of QPT for quasi 2D or (2 + 1) D XY models. The successful fabrication of meander structures of W using FIB and the demonstration of superconductivity suggest that FIB deposited W can be exploited for many of the technological applications of superconducting nanowires such as superconducting nanowire single photon detectors, bolometers, transition edge sensors and even for quantum current standard employing the QPS phenomenon. (paper)

[en] The iron-pnictide Sm_1−xCa_xFeAsO_1−2xF_2x superconductor was prepared and the combined effect of electron and hole doping was studied in detail. It is observed that the binary doping using CaF₂ improves the microstructure tremendously with a preferred orientation of the (00l) planes. Moreover, a maximum T_C of 53.8 K and a transport J_C of 880 A/cm² (12 K), which is double to that of the F-doped sample, are achieved. The dopant CaF₂ seems to be a potential candidate for solving the grain-connectivity concerns in iron-pnictides paving the way towards conductor development

[en] Milling of 2D flakes is a simple method to fabricate nanomaterial of any desired shape and size. Inherently milling process can introduce the impurity or disorder which might show exotic quantum transport phenomenon when studied at the low temperature. Here we report temperature dependent weak antilocalization (WAL) effects in the sculpted nanowires of topological insulator in the presence of perpendicular magnetic field. The quadratic and linear magnetoconductivity (MC) curves at low temperature (>2 K) indicate the bulk contribution in the transport. A cusp feature in magnetoconductivity curves (positive magnetoresistance) at ultra low (<1 K) temperature and at magnetic field (<1 T) represent the WAL indicating the transport through surface states. The MC curves are discussed by using the 2D Hikami–Larkin–Nagaoka theory. The cross-over/interplay nature of positive and negative magnetoresistance observed in the MR curve at ultra-low temperature. Our results indicate that transport through topological surface states (TSS) in sculpted nanowires of Bi₂Te₃ can be achieved at mK range and linear MR observed at ∼2 K could be the coexistence of electron transport through TSS and contribution from the bulk band. (paper)