CRITICAL FIELDS AND MAGNETORESISTANCE IN THE MOLECULAR SUPERCONDUCTORS (TMTSF)(2)X

The resistive transition to the superconducting state in the quasi-one dimensional molecular organic metal (TMTSF)(2)PF6 has been measured u nder 6 kbar pressure for magnetic fields applied along the intrachain (a), interchain (b') and interplane (c) directions. Pronounced positi ve curvature in H-c2(T) is observed, with no sign of saturation, in th e basal plane directions a and b', in contrast to previous publication s on this material, but similar to recent results by Lee, et al. on a related compound, (TMTSF)(2)ClO4. A new crossover or reversal of aniso tropy occurs near Tc/2, below which the critical field along b' exceed s that along a. Moreover, H-c2 in these directions exceeds the Chandra sekhar-Clogston limit, reaching as much as 3 times H-CC. The normal st ate magnetoresistance displays peculiar behavior in both materials at low temperature, with a local maximum in the vicinity of the supercond ucting transition. In addition, we report on a new series of angular m agnetoresistance oscillations observed in both the ClO4 and PF6 salts.