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-xY_xO_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 Y₂O₃ 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_c2 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.