Spin-orbit interaction effects in quasi-one-dimensional conductors

Abstract

The author studies the effects of the spin-orbit interaction on the competing pairing states of a model quasi-one-dimensional conductor with hopping-type interchain coupling. Within a ladder approximation scheme use of the renormalisation-group method allows him to account explicitly for anisotropy effects involving the intrachain and interchain transfer integrals. Parametrising the spin-orbit interaction by a backward, g1n<0, and forward, g2n<0, scattering coupling constant, he concludes that its main effect is to stabilize the triplet-pairing states of spin density wave and triplet superconductivity over the corresponding singlet-pairing states of charge density wave and singlet superconductivity. In particular the boundary line between the density-wave-type states and the superconducting states is determined as: g1+g1n=2g2, where g1 and g2 are the usual spin-independent backward- and forward-scattering coupling constants. The boundary line between the triplet- and singlet-pairing states is determined as: 2g1=g1n+g2n- mod g1n-g2n mod . The direction of the triplet-pairing polarization axis m with respect to the spin-orbit-anisotropy axis n is shown to be dependent only upon the sign of the difference: g1n-g2n. The implications of these results of the (TMTSF)2X and (TMTTF)2X families of organic conductors is discussed. Finally, it is shown that the spin-dependent exchange interaction introduced recently by Abrikosov (1983) can be reduced to a simple definition of the spin-independent couplings g1 and g2. He concludes that a ferromagnetic type of exchange also favours the triplet-pairing states over the singlet-pairing states.