On the origin of the anomalous upper critical field in quasi–one-dimensional superconductors
The upper critical field, Hc2, in quasi-1D superconductors is investigated by the weak coupling renormalization group technique. It is shown that Hc2 greatly exceeds not only the Pauli limit, but also the conventional paramagnetic limit of the Flude-Ferrell-Larkin-Ovchinnikov (FFLO) state. This increase is mainly due to the quasi-1D fluctuations effect as triggered by interference between unconventional superconductivity and density-wave instabilities. Our results give a novel viewpoint on the large Hc2 observed in TMTSF-salts in terms of a d-wave FFLO state that is predicted to be verified by Hc2 measurements under pressure.