New Mechanisms for Phase Transitions in Quasi-One-Dimensional Conductors

We show that interchain single-particle tunnelling in quasi-one-dimensional solids invariably leads to effective interchain pair tunnelling interactions even when there is no gap in the intrachain excitation spectrum. These can lead to antiferromagnetism well before the dimensionality crossover or to superconductivity below this crossover whenever the nesting conditions of the Fermi surface are poor. We thus suggest a new mechanism for super-conductivity which would involve Cooper pairs on different chains and an isotropic gap compatible with recent nuclear-relaxation rate experiments.