Influence of exchange fields in hybrid pairing superconductors,I: Ground states

Hybrid pairing superconductors with two bands of localized and delocalized states in the presence of uniform molecular fields and a finite displacement of the bands at the Fermi level are analyzed in mean field theory.At zero temperature, in addition to the usual BCS like ground state, agapless superconducting state withfinite spin polarization can be realized. The appearance of this state is somewhat analogous to the occurrence of the Fulde Ferrell state in a single band superconductor with an exchange field.We comment on the relevance of this model for heavy-fermion superconductors.Work performed within the research program of Sonderforschungsbereich 125 Aachen-Jülich-Köln