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) exhibits 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%. The 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.