Properties of superconductivity on the density wave background with small ungapped Fermi surface pockets

Abstract

We investigate the properties and the microscopic structure of superconductivity (SC), coexisting and sharing the common conducting band with density wave (DW). Such coexistence may take place when the nesting of the Fermi surface (FS) is not perfect, and in the DW state some quasi-particle states remain on the Fermi level and lead to the Cooper instability. The dispersion of such quasi-particle states is, in general, very different from that without DW. Therefore, the properties of SC on the DW background may strongly differ from those without DW. The upper critical field $H_{c2}$ in such a SC state increases as the system approaches the critical pressure, where the ungapped quasi-particles and superconductivity just appear, and it may considerably exceed the usual $H_{c2}$ value without DW. The SDW background strongly suppresses the singlet SC pairing, while it does not affect so much the triplet SC transition temperature. The results obtained explain the experimental observations in layered organic metals (TMTSF)$_{2}$PF$_{6}$ and $\alpha $-(BEDT-TTF)$_{2}$KHg(SCN)$_{4}$, where SC appears in the DW states under pressure and shows many unusual properties.