Theoretical study of interplay between superconductivity and itinerant ferromagnetism

Following Green's function technique and equation of motion method, the coexistence of superconductivity (SC) and itinerant ferromagnetism (FM) is investigated in a single band homogenous system. Self consistent equations for SC and FM order parameters, Δ and m or I respectively are derived. It is shown that there generally exists a coexistent (Δ ≠ 0, and m or I ≠ 0) solutions to the coupled equations of the order parameter in the, temperature range 0 < T < min(T_C, T_FM), where T_C and T_FM are respectively the superconducting and ferromagnetic transition temperatures. Expressions for specific heat, density of states, free energy and critical field are derived. The specific heat has linear temperature dependence as opposed to the exponential decrease in the BCS theory. The density of states for a finite m increases as opposed to that of a ferromagnetic metal. Free energy study reveals that FM-SC state has lowest energy than the normal FM state and therefore realized at low enough temperature. Effect of small external field is also studied. The theory is applied to explain the observations in uranium based intermetallics systems UCoGe and UIr. The agreement between theory and experiments is quite encouraging. (author)