Possible triplet order parameter in a Q1D organic superconductor (TMTSF)(2)PF6

We point out that (TMTSF)(2)PF6 compound at P similar or equal to 6 kbar is possibly a triplet superconductor with the vector order parameter d = (d(x ) not equal 0, d(y), d(z) = 0). It corresponds to an anisotropic spin susce ptibility in a superconducting state: chi (b'(z)) = chi (0), chi (a(x)) not equal chi (0), where chi (0) is its value in a metallic phase. (Here, the spin quantization axis, z, is parallel to the easy axis, b', of an antiferr omagnetic SDW phase existing at P = 0). It is shown that the suggested orde r parameter can explain why the upper critical field along b'-axis (I.J. Le e, M.J. Naughton, G. Danner, and P.M. Chaikin, Phys. Rev. Lett. 78, 3555 (1 997)) exceeds all paramagnetic limiting fields (including that for a non-un iform superconductivity) whereas the upper critical field along the a-axis is restricted by the Pauli paramagnetic effects. The triplet order paramete r is consistent with the earlier results on a destruction of superconductiv ity by nonmagnetic impurities and the absence of the Hebel-Slichter peak in the NMR data as well as with the recent Knight shift measurements by S. Br own et al and by I. J. Lee et al.