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 Tc 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, Pc, for spin density wave formation. We find that the superconducting Tc is not reduced as the SDW phase is entered. However, Hc2 is increased and there is a strong upward curvature of Hc2 along all axes as pressure is decreased toward Pc. We suggest that near Pc, 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.