Abstract
We report the synthesis and characterization of a new electron-dopedLa-oxypnictide superconductor by partial substitution of lanthanum by thorium.The superconducting transition temperature at about 30.3 K was observed inLa0.8Th0.2FeAsO,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 ascompared to that obtained by high pressure (6 GPa) synthesis of oxygen-deficient LaOFeAs.The resistivity and magnetic susceptibility measurements yield an upper critical fieldHc2(0) of 47 T. Partial substitution of Th in place of La induces electron doping, as shown by anegative Seebeck coefficient. The temperature-dependent magnetic penetrationdepth data provides strong evidence for a nodeless low energy gap of 1.4 meV.
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