Upward Curvature of the Upper Critical Field and the V-Shaped Pressure Dependence of T c in the Noncentrosymmetric Superconductor PbTaSe2

The temperature evolution of the upper critical field H (c2)(T) in the noncentrosymmetric superconductor PbTaSe2 was determined via resistivity measurements down to 0.5 K. A pronounced positive curvature in the H (c2)-T phase diagram was observed in the whole temperature range below T (c) . The Seebeck coefficient S(T) in the temperature range 5K a parts per thousand currency sign T a parts per thousand currency sign 350 K was found to be negative in sign, modest in magnitude and non-linear in temperature. In addition, the superconducting transition temperature T (c) under hydrostatic pressure shows a marked non-monotonic variation, decreasing initially with the applied pressure up to P (c) 5-10 kbar but then rising with further pressurization. The underlying physical mechanisms of all these findings have been discussed.