Unconventional superconductivity with a radial-node gap in quasi-one-dimensional metals

It is shown that a new type of superconductivity is possible in one-dimensional (1D) and quasi-one-dimensional (Q1D) metals where the pairing interaction is mediated by both spin and charge fluctuations. The gap function inevitably changes sign in the radial direction near the Fermi surface; it vanishes at the Fermi points for 1D, and for Q1D it has line nodes on the Fermi surface as in usual anisotropic pairings. The transition temperature T-c is generally higher for the singlet pairing than for the triplet, but only by a few times. This implies that T-c for the triplet pairing can exceed that of the singlet under a moderately large Zeeman magnetic field, i.e., field-induced triplet pairing. This feature seems to explain the anomalous behaviours of H-c2 and the Knight shift observed in (TMTSF)(2)PF6.