Triplet superconductivity and stripes? In (TMTSF)₂PF₆

We report on ⁷⁷Se NMR studies of (TMTSF)₂PF₆ with field aligned along the a and b axes. We observe no change in the Knight shift on cooling well into the superconducting state, consistent with an equal spin paired triplet state. Nuclear spin lattice relaxation measurements show an increase just below T_c which we associate with a Hebel-Slichter (HS) peak and suggest P wave pairing that is nodeless. An additional study of the critical field as a function of pressure shows a strong enhancement as pressure is reduced toward the critical pressure, P_c, for spin density wave formation. We find that the superconducting T_c is not reduced as the SDW phase is entered. However, H_c2 is increased and there is a strong upward curvature of H_c2 along all axes as pressure is decreased toward P_c. We suggest that near P_c, there is a coexistence region consisting of stripes (slabs) of commingled SDW and superconducting regions and that the application of a magnetic field further finely divides the superconductor.