Upper critical field, superconducting energy gaps, and Seebeck coefficient in La0.8Th0.2OFeAs

Abstract

We report the synthesis and characterization of a new electron-doped La-oxypnictide superconductor by partial substitution of lanthanum by thorium. The superconducting transition temperature at about 30.3 K was observed in La0.8Th0.2OFeAs which is the highest in La-based oxypnictide superconductors synthesized at ambient pressure. We find that the decrease in lattice parameters with Th doping in LaOFeAs is more drastic as compared to that obtained by high pressure (6 GPa) synthesis of oxygen deficient LaOFeAs. The resistivity and magnetic susceptibility measurements yield an upper critical field Hc2 (0) of 47 T. Partial substitution of Th in place of La induces electron doping as evidenced by negative Seebeck coefficient. The temperature dependent magnetic penetration depth data provides strong evidence for a nodeless low energy gap of 1.4 meV.

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• Andreev spectroscopy of iron-based superconductors: temperature dependence of the order parameters and scaling of the ${\Delta }_{L,S}$ with $T_C$
• Enhancement in transition temperature and critical current density of CeO0.8F0.2FeAs by yttrium doping
• Evidence of a multiple boson emission in Sm${}_{1-x}$Th${}_x$OFeAs
• The thermoelectric conversion efficiency problem: Insights from the electron gas thermodynamics close to a phase transition
• Tetragonal-phase SnOFeSe: A possible parent compound of FeSe-based superconductor