Critical fields and magnetoresistance in the molecular superconductors (TMTSF)₂X

The resistive transition to the superconducting state in the quasi-one dimensional molecular organic metal (TMTSF)₂PF₆ has been measured under 6 kbar pressure for magnetic fields applied along the intrachain (a), interchain (b′) and interplane (c*) directions. Pronounced positive curvature in H_c2(T) is observed, with no sign of saturation, in the basal plane directions a and b′, in contrast to previous publications on this material, but similar to recent results by Lee, et al. on a related compound, (TMTSF)₂ClO₄. A new crossover or reversal of anisotropy occurs near T_c/2, below which the critical field along b′ exceeds that along a. Moreover, H_c2 in these directions exceeds the Chandrasekhar-Clogston limit, reaching as much as 3 times H_CC. The normal state magnetoresistance displays peculiar behavior in both materials at low temperature, with a local maximum in the vicinity of the superconducting transition. In addition, we report on a new series of angular magnetoresistance oscillations observed in both the ClO₄ and PF₆ salts.