First-order phase transition between superconducting and charge/spin density wave states causes their coexistence in organic metals

The interplay between superconductivity (SC) and spin/charge density wave (DW) in organic metals shows many similarities to high-${T}_c$ superconductors. It also contains many puzzles, for example, the anisotropic SC onset observed and the severalfold increase of the upper critical field ${H}_c2$ in the coexistence region, as well as the microscopic origin of SC/DW phase separation there. In this paper, by the direct calculation of the Landau expansion for DW free energy, we argue that the phase transition between DW and metallic/SC phase in organic superconductors goes by first order at low-enough temperature, which explains the spatial segregation of DW and SC at large length scale, consistent with experimental observations. This first-order phase transition is not directly related to SC and happens even above the SC transition temperature.