Effects of Pauli paramagnetism on the superconducting vortex phase diagram in strong fields

Abstract

The Ginzburg-Landau functional and the resultant phase diagram of quasi-two-dimensional superconductors in strong fields perpendicular to the layers, where the Pauli paramagnetic depairing is not negligible, are examined in detail by assuming the weak coupling BCS model with a ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-like pairing. It is found that the temperature at which the mean field (MF) transition at ${H}_{c2}$ changes into a discontinuous one lies much above another temperature at which a line ${H}_{\mathrm{FFLO}}(T)$ of transition to a helical vortex solid similar to a Fulde-Ferrell-Larkin-Ovchinnikov state may begin to appear. In addition to MF results, details of a real phase diagram near the ${H}_{c2}(T)$ line are examined based on a theoretical argument and Monte Carlo simulations, and it is found that the MF discontinuous transition is changed due to the fluctuation into a crossover which is nearly discontinuous in systems with weak enough fluctuation. These results are consistent both with the MF discontinuous behavior and a suggestion of ${H}_{\mathrm{FFLO}}(T)$ in the heavy fermion superconductor ${\mathrm{CeCoIn}}_{5}$ with weak fluctuation and with their absence in cuprate and organic superconductors with strong fluctuation.