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 FeSe 1/2 Te 1/2 at 13 K is reported here. The compound crystallized in a tetragonal structure with lattice parameters a = 3.8015͑2͒ and c = 6.0280͑4͒ Å. Magnetization measurements indicated bulk superconductivity with lower critical field ͑H c1 ͒ of around 180 Oe. By applying Ginzburg-Landau theory, the H c2 ͑0͒ value is estimated to be ϳ1840 kOe for the 90% of resistive transition. A heat capacity measurement revealed bulk superconductivity by a hump at T c near 13 K and an expected decrease in the same was observed under an applied magnetic field.

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The high field magnetization and magnetotransport measurements are carried out to determine the critical superconducting parameters of MgB 2−x C x system. The synthesized samples are pure phase and the lattice parameter evaluation is carried out using the Rietveld refinement. The R − T͑H͒ measurements are done up to a field of 140 kOe. The upper critical field values, H c2 , are obtained from these data based on the criterion of 90% of normal resistivity, i.e., H c2 = H at which =90% N , where N is the normal resistivity, i.e., resistivity of about 40K in our case. The Werthamer-Helfand-Hohenberg prediction of H c2 ͑0͒ 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 − T͑H͒ data which not only calculate the H c2 ͑0͒ value but also determine the dependence of H c2 on temperature in the low temperature high field region. The estimated H c2 ͑0͒ = 157.2 kOe for pure MgB 2 is profoundly enhanced to 297.5 kOe for the x = 0.15 sample in MgB 2−x C x series. Magnetization measurements are done up to 120 kOe at different temperatures and the other parameters such as irreversibility field H irr and critical current density J c ͑H͒ are also calculated. The nano carbon doping results in substantial enhancement of critical parameters such as H c2 , H irr , and J c ͑H͒ in comparison to the pure MgB 2 sample.

The physical property characterization of Al doped Mg 1-x Al x B 2 system with x = 0.0 to 0.50 is reported. The results related to phase formation, structural transition, resistivity ρ(T) and magnetization M(T) measurements are discussed in detail. It is shown that the addition of electrons to MgB 2 through Al results in loss of superconductivity. Also seen is a structural transition associated with the collapse of boron layers reflected by the continuous decrease in the c parameter. The main emphasis in this paper is on slow scan X-ray diffraction (XRD) results, which confirm the existence of a superstructure along the c-direction for the x = 0.50 sample. The appearance of some additional peaks, viz. [103], [004], [104] and [112], results in doubling of the lattice parameter along the c-axis. This possibly indicates the alternative ordering of Al and Mg in MgAlB 4 separated by hexagonal boron layers but still maintaining the same hexagonal AlB 2 type structure.