Anisotropy of the Upper Critical Field in (TMTSF)2PF6

The temperature dependence of the upper critical magnetic field $H_{c2\mathrm{}}\left(T\right)$ in the quasi-one-dimensional molecular superconductor (TMTSF${)}_2$P${\mathrm{F}}_6$ was determined via resistivity, for the intrachain (a), interchain (b), and interplane $\left({\mathbf{c}}^{*}\right)$ directions. For H along a and b, $H_{c2\mathrm{}}\left(T\right)$ exhibits pronounced positive curvature, with no sign of saturation to 0.1 K. A novel anisotropy inversion is observed, wherein $H_{c2\mathrm{}}^b$ grows larger than $H_{c2\mathrm{}}^a$ and exceeds the paramagnetic limit by more than 200%. The anomalous shape of $H_{c2\mathrm{}}\left(T\right)$ is consistent with recent predictions of unconventional behavior in anisotropic superconductors, including the possibility of triplet pairing and reentrant superconductivity in very high field.