S. J. Singh

Enhancement in transition temperature and upper critical field of CeO0.8F0.2FeAs by yttrium doping

We report significant enhancement in superconducting properties of yttrium substituted Ce1−xYxOFFeAs superconductors. The polycrystalline samples were prepared by two step solid state reaction technique. X-ray diffraction confirmed tetragonal ZrCuSiAs structure with decrease in both a and c lattice parameters on increasing yttrium substitution (with fixed F content). With smaller ion Y in place of Ce, the transition temperature increased by 6 K. Yttrium doping also lead to higher critical fields as well as broader magnetization loops, particularly at elevated temperature.

Enhancement of the superconducting transition temperature and upper critical field of LaO0.8F0.2FeAs with antimony doping

We report the synthesis and characterization of the antimony-doped oxypnictide superconductor, LaO0.8F0.2FeAs1?xSbx (x = 0.05 and 0.10). The parent compound LaOFeAs with fluorine doping exhibits superconductivity at the maximum transition temperature ~28.5?K (at ambient pressure). Here we partially substitute As by Sb (LaO0.8F0.2FeAs1?xSbx) and observe enhancement of the transition temperature to 30.1?K. This is one rare case where Tc increases with doping in the conducting layer (FeAs) and this leads to the highest transition temperature in any La-based oxypnictide. X-ray diffraction and energy-dispersive x-ray analysis measurements confirm the phase purity of the samples and the presence of Sb. The magneto-resistance measurements show that the value of the upper critical field Hc2(0) is about 73?T, corresponding to a coherence length (?GL) of 22??. The Seebeck coefficient measurements indicate electron transport with a strong contribution from electron?electron correlation. These results provide interesting insight into the origin of superconductivity in this novel series of compounds.

Study of upper critical field in 1111-ferropnictide superconductors

In this paper we have shown the doping effects of elements Y, F and Sb in place of RE (rare earth elements), O and As respectively in 1111-ferropnictides. We study the behavior of upper critical field and superconducting transition temperatures as a function of doping. In Ce and La based compounds maximum upper critical field was found to be 147 T and 122 T respectively. Correspondingly, the maximum transition temperature was found to be 48.6 K and 34.8 K.

Study of Thermoelectric Power and Superconducting Gap in CeO0.9F0.1Fe1−yCoyAs

To understand the interplay of magnetism and superconductivity in oxypnictides, we have studied simultaneous carrier doping in different layers of CeOFeAs; F in CeO and Co in FeAs. The rf penetration depth shows exponential temperature dependence with gap values ∼1.9 and 1.6 meV for optimal F‐doped and F & Co‐doped samples respectively. The thermoelectric power data for CeOFFeCoAs can be divided into multiple regions and every region could be fitted with appropriate temperature dependence. The dominance of different scattering mechanism is discussed with respect to F and Co substitutions.

Structural and Superconducting Properties of Ce‐Based Ferropnictides

CeOFeAs is a versatile oxy‐arsenide which shows a variety of electrical and magnetic properties. Superconductivity in this system has been achieved by doping electrons through different ionic substitutions by sealed tube reactions at ∼1100 °C. The superconducting transition temperature (Tc) and upper critical field (Hc2) could be optimized by substitution of suitable ions at Ce, O and Fe site. The highest Tc obtained was ∼49 K in a Y‐doped CeO0.9F0.1FeAs.