Role of a fermion condensate in the structure of high-temperature pairing in cuprates

The anomalous properties of a pairing gap in cuprate superconductors have been explained under the assumption that their electron systems in the normal phase exhibit a fermion condensate, i.e., a set of dispersionless states close to the nominal Fermi surface. It has been shown that exactly the fermion condensate is responsible for $D$-state pairing in cuprates. More specifically, the effective Coulomb repulsion in the Cooper channel, which prevents the existence of superconductivity in normal metals in the $S$ channel, makes it high-temperature in the $D$ channel.