ANISOTROPY OF THE UPPER CRITICAL-FIELD IN (TMTSF)(2)PF6

The temperature dependence of the upper critical magnetic field H-c2(T ) in the quasi-one-dimensional molecular superconductor (TMTSF)(2)PF6 was determined via resistivity, for the intrachain (a), interchain (b) , and interplane (c) directions. For H along a and b, H-c2(T) exhibit s pronounced positive curvature, with no sign of saturation to 0.1 K. A novel anisotropy inversion is observed, wherein H-c2(b) grows larger than H-c2(a) and exceeds the paramagnetic limit by more than 200%. Th e anomalous shape of H-c2(T) is consistent with recent predictions of unconventional behavior in anisotropic superconductors, including the possibility of triplet pairing and reentrant superconductivity in very high held.