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The high field magnetization and magnetotransport measurements are carried out to determine the critical superconducting parameters of MgB2-xCx system. The synthesized samples are pure phase and the lattice parameter evaluation is carried out using the Rietveld refinement. The R-T(H) measurements are done up to a field of 140 kOe. The upper critical field values, Hc2, are obtained from these data based on the criterion of 90% of normal resistivity, i.e., Hc2=H at which ρ=90%ρN, where ρN is the normal resistivity, i.e., resistivity of about 40K in our case. The Werthamer–Helfand–Hohenberg prediction of Hc2(0) underestimates the critical field value even below the field up to which measurement is carried out. After this model, the Ginzburg–Landau theory is applied to the R-T(H) data which not only calculate the Hc2(0) value but also determine the dependence of Hc2 on temperature in the low temperature high field region. The estimated Hc2(0)=157.2 kOe for pure MgB2 is profoundly enhanced to 297.5 kOe for the x=0.15 sample in MgB2-xCx series. Magnetization measurements are done up to 120 kOe at different temperatures and the other parameters such as irreversibility field Hirr and critical current density Jc(H) are also calculated. The nano carbon doping results in substantial enhancement of critical parameters such as Hc2, Hirr, and Jc(H) in comparison to the pure MgB2 sample. [ABSTRACT FROM AUTHOR]

One of the most important properties of very recently reported FeSe based superconductors is the robustness of their superconductivity under applied magnetic field. The synthesis and control of superconductivity in FeSe based compounds is rather a difficult task. Synthesis and physical property characterization for optimized superconductivity of FeSe1/2Te1/2 at 13 K is reported here. The compound crystallized in a tetragonal structure with lattice parameters a=3.8015(2) and c =6.0280(4) Å. Magnetization measurements indicated bulk superconductivity with lower critical field (Hc1) of around 180 Oe. By applying Ginzburg-Landau theory, the Hc2(0) value is estimated to be ∼1840 kOe for the 90% of resistive transition. A heat capacity measurement revealed bulk superconductivity by a hump at Tc near 13 K and an expected decrease in the same was observed under an applied magnetic field. [ABSTRACT FROM AUTHOR]

Abstract: This paper deals with the determination of critical properties of MgB2 along with the impact of carbon substitution on critical parameters. The change in lattice parameters and decrease of transition temperature, Tc confirms the successful substitution by carbon at boron site. The magneto transport measurements up to 140kOe are carried out to determine upper critical field (H c2). The upper critical field values, H c2 are obtained from ρ–T(H) data based upon the criterion of 90% of normal resistivity. The Ginzburg Landau theory (GL equation) is applied to the ρ–T(H) data which accounts for the temperature dependence behavior of Hc 2 in the low temperature high field region along with the determination of Hc (0) value. The Hc (0) value of about 300kOe is obtained for the carbon doped sample while the same is just near to 160kOe for the pure MgB2 sample. [ABSTRACT FROM AUTHOR]