In the present paper we report the effect of graphene oxide (GO) doping on the structural and superconducting properties of MgB 2 . Bulk polycrystalline samples have been synthesized via a solid state reaction route with compositions MgB 2 + x wt% of GO (x = 0, 1, 2, 3, 5, 7 and 10) by sintering at ∼850 • C in a reducing atmosphere of Ar/H 2 (9:1). The x-ray diffraction results confirm the formation of the MgB 2 phase in all samples, together with traces of a MgO impurity phase. The XRD data results also show substitution of carbon for boron, but in the present case the actual amount of carbon substituting for boron is very small as compared to other carbon sources. A substantial improvement in the critical current density, J c (H), has been observed in the entire magnetic field range (0-8 T) for samples x = 1, 2 and 3 as compared to the undoped sample. In addition to J c (H), marginal improvements in the upper critical field (H c2 ) and the irreversibility field (H irr ) have been observed for the doped samples x = 1, 2 and 3 with respect to pristine MgB 2 . Furthermore, a curious result of the present investigation is that there is no change in the superconducting transition temperature (T c ) up to a doping level of 10 wt%. The possible mechanisms of flux pinning and correlations between the observed superconducting properties and structural characteristics of the samples have been described and discussed in this paper.

The discovery of Weyl semimetals (WSM) has brought forth the condensed matter realization of Weyl fermions, which were previously theorized as low energy excitations in high energy particle physics. Recently, transition metal mono-pnictides are under intense investigation for understanding properties of inversion-symmetry broken Weyl semimetals. Non-trivial Berry phase and chirality are important markers for characterizing topological aspects of Weyl semimetals. Most recently, theoretical calculations predict strong influence of the position of Weyl nodes with respect to Fermi surface and weak disorder that can drive WSMs into chirally symmetric Dirac semimetals. Using magneto-transport measurements in single crystals of WSM NbP, we observe an exceptionally large magnetoresistance at low temperature, which is non-saturating and linear at high fields. The origin of linear transverse magnetoresistance is assigned to charge carrier mobility fluctuations. Negative longitudinal magnetoresistance is not seen, suggesting lack of well-defined chiral anomaly in NbP. Unambiguous Shubnikov-de Haas oscillations are observed at low temperatures that are correlated to a trivial Berry phase corresponding to Fermi surface extrema at 30.5 Tesla. Our results are important towards identifying topological characteristics of Weyl semimetals and their experimental manifestations in the presence of weak disorder.

a b s t r a c tIn the present work, we have studied the effect of doping Sb and Si at the Se-site of FeSe 0.9 on the superconducting properties, such as transition temperature (T c ), upper critical field (H c2 ) and irreversibility field (H irr ). The polycrystalline samples have been synthesized via two step solid state reaction route with nominal compositions Fe[Se 1Àx (Sb/Si) x ] 0.9 (x = 0.0, 0.05, 0.10, 0.15 and 0.20). The X-ray diffraction resultsshow the presence of tetragonal a-FeSe phase with the P4/nmm space group symmetry in all the samples.The highest superconducting onset temperatures, T onset c $ 9:42 K and 9:20 K, respectively, for Si and Sb doped samples have been found for x = 0.05. The temperature dependence of H c2 (T) and H irr (T) have been calculated from the magnetoresistance data using the criteria of 90% and 10% of normal state resistivity (q n ) values, respectively. The values of H c2 (0) estimated from Werthamer-Helfand-Hohenberg (WHH) and Ginzburg-Landau (GL) theories are found to follow the same trends and maximum H c2 (0) is found for the composition x = 0.10 for both the Si and Sb doped samples. The irreversibility field, H irr and activation energy, U 0 have also been calculated to study the vortex motion behavior of the samples. A clear cut correlation between H irr and U 0 has been found.

In the present work, we have studied the effect of Fe composition on the superconducting properties, such as transition temperature (T c), upper critical field (H c2), and irreversibility field (H irr) of FeSe 1=2 Te 1=2. The polycrystalline samples have been prepared via solid state reaction route with nominal compositions Fe x Se 1=2 Te 1=2 (x ¼ 0.95, 1.00, 1.05 and 1.10). The x-ray diffraction results show the presence of tetragonal a-FeSe phase with the p4=nmm space group symmetry in all the samples. The zero resistance temperatures, T zero c , measured in zero magnetic field, have been found to be 10.0, 12.4, 12.3, and 11.7 K for x ¼ 0.95, 1.00, 1.05, and 1.10, respectively. The temperature dependence of H c2 (T) and H irr (T) have been calculated from the magnetoresistance data using the criteria of 90% and 10% of normal state resistivity (q n) values, respectively. The values of H c2 (0) are 121.3 T, 142.8 T, 82.7 T, and 79.3 T for x ¼ 0.95, 1.00, 1.05, and 1.10, respectively. The possible reasons for the variation of superconducting properties with Fe composition (x) have been described and discussed in this paper.

We study magneto-transport properties in single crystals of TaSb 2, which is a topological semimetal.In the presence of magnetic field, the electrical resistivity shows onset of insulating behaviour followed by a plateau at low temperature. Such resistivity saturation is generally assigned to topological surface states but we find that aspects of extremely large magneto resistance and resistivity plateau are well accounted by classical Kohler's scaling. In addition, magneto-resistance in TaSb 2 shows nonsaturating field dependence. Evidence for anomalous Chiral transport is provided with observation of negative longitudinal magneto-resistance. Shubnikov-de Haas oscillation data reveal two dominating frequencies, 201 T and 455 T. At low temperature, the field dependence of Hall resistivity shows nonlinear behaviour that indicates the presence of two types of charge carriers in consonance with reported electronic band structure. Analysis of Hall resistivity implies extremely high electron mobility.