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

We point out that (TMTSF)₂PF₆ compound at P ≃ 6 kbar is possibly a triplet superconductor with the vector order parameter d = (d_x ≠ 0, d_y ,d_z = 0). It corresponds to an anisotropic spin susceptibility in a superconducting state: χ_b′(z) = χ₀, χ_a(x) ≠ χ₀, where χ₀ is its value in a metallic phase. (Here, the spin quantization axis, z, is parallel to the easy axis, b′, of an antiferromagnetic SDW phase existing at P = 0). It is shown that the suggested order parameter can explain why the upper critical field along b′-axis (I.J. Lee, M.J. Naughton, G. Danner, and P.M. Chaikin, Phys. Rev. Lett. 78, 3555 (1997)) exceeds all paramagnetic limiting fields (including that for a non-uniform superconductivity) whereas the upper critical field along the a-axis is restricted by the Pauli paramagnetic effects. The triplet order parameter is consistent with the earlier results on a destruction of superconductivity 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. Brown et al and by I. J. Lee et al.