The superconducting state in (TMTSF)₂PF₆ in high magnetic field

The resistive upper critical field along the b-axis (H_c2//b) in (TMTSF)₂PF₆ is investigated in detail in fields to 17.5 T and temperatures to T/T_c ∼ 1/60, at various pressures near the border between spin density wave and metallic states. Previously, we had obtained a novel crossover where H_c2//b exceeded both H_c2//a, in contrast to band structure, and the Pauli paramagnetic limit μ_oH_p≈ 2 T. Remarkably, in an optimum pressure setting reported here, it is found that the onset of superconductivity persists up to 9 T (> 4 H_p), which strongly suggests triplet Cooper pairing. Strong upward curvature with nearly diverging behaviour of the critical field also seem to suggest a field-dependent dimensional crossover, although the H_c2 enhancement is considerably more than that predicted by recent theories.