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

The temperature evolution of the upper critical field Hc2(T) in the noncentrosymmetric superconductor PbTaSe2 was determined via resistivity measurements down to 0.5 K. A pronounced positive curvature in the Hc2-T phase diagram was observed in the whole temperature range below Tc. The Seebeck coefficient S(T) in the temperature range 5K ≤ T ≤ 350 K was found to be negative in sign, modest in magnitude and non-linear in temperature. In addition, the superconducting transition temperature Tc under hydrostatic pressure shows a marked non-monotonic variation, decreasing initially with the applied pressure up to Pc ∼5–10 kbar but then rising with further pressurization. The underlying physical mechanisms of all these findings have been discussed.