Yttrium doped La 1−xY xO 0.9F 0.1FeAs superconductors: Hall and thermopower studies

Abstract

The effect of yttrium substitution at the lanthanum site on the superconducting properties of La 1− x Y x O 0.9F 0.1FeAs (‘ x’ = 0, 0.10, 0.20, 0.30, 0.50 and 0.60) oxypnictides has been studied. Powder X-ray diffraction studies confirm single phases till x = 0.1 beyond which minor amount of Y 2O 3 is observed. The temperature dependence of resistivity measurements confirm the superconducting transition temperature ( T c ) of 34.8 (±0.05) K and corresponding Meissner transition at 34.3 K in the ‘ x’ = 0.3 composition which is higher than that reported for the parent phase (LaO 0.9F 0.1FeAs ( T c = 28 K)). Further increase in the concentration of yttrium leads to broadening and suppression of the superconducting transition. The value of H c 2 at zero temperature is estimated to be about 60.5 T. The Seebeck coefficient ( S) shows a negative sign indicating that the major contribution to the conductivity is by electrons. The Hall coefficient ( R H ) also remains negative throughout the temperature range supporting the thermopower results. The lattice parameters ( a and c) decreases and the charge-carrier density increases with yttrium doping.