Coexistence of ferromagnetism and superconductivity: The role of kinetic interactions, kinetic correlations, and external pressure

We use the Hubbard type model to describe the coexistence between superconductivity (SC) and ferromagnetism (F). Our Hamiltonian contains single-site and two-site interactions. All inter-site interactions will have included the inter-site kinetic correlation: 〈ciσ+cjσ〉 within the Hartree–Fock approximation. To obtain the SC transition temperature TSC and Curie temperature TC we use the Green’s functions method. The numerical results show that the singlet SC is eliminated by F, but the triplet SC is either enhanced or depleted by F, depending on the carrier concentration and direction of a superconducting spin pair with respect to magnetization. The kinetic correlation is capable of creating superconductivity. We find that the ferromagnetism created by change of the bandwidth can coexist with singlet superconductivity. In the case of triplet superconductivity the ferromagnetism creates different critical SC temperatures for the A1 and A2 phase (the pair’s spin parallel and antiparallel to magnetization, respectively).