Triplet electron pairing and anisotropic spin susceptibility in organic superconductors

We argue that the (Formula presented) compound under pressure is likely a triplet superconductor with a vector order parameter (Formula presented) It corresponds to an anisotropic spin susceptibility at (Formula presented) where (Formula presented) is its value in a metallic phase. (The spin-quantization axis, z, is parallel to a so-called (Formula presented) axis.) We show that the suggested order parameter explains why the upper critical field along the (Formula presented) axis exceeds all paramagnetic limiting fields, including that for a nonuniform superconducting state, whereas the upper critical field along the a axis (Formula presented) is limited by the Pauli paramagnetic effects [I. J. Lee, M. J. Naughton, G. M. Danner, and P. M. Chaikin, Phys. Rev. Lett. 78, 3555 (1997)]. The triplet order parameter is in agreement with the recent Knight shift measurements by I. J. Lee et al. as well as with the early results on a destruction of superconductivity by nonmagnetic impurities and on the absence of the Hebel-Slichter peak in the NMR relaxation rate.