Coexistence of ferromagnetism and superconductivity

Abstract

A comprehensive theory is developed that describes the coexistence of p-wave, spin-triplet superconductivity, and itinerant ferromagnetism. It is shown how to use field-theoretic techniques to derive both conventional strong-coupling theory, and analogous gap equations for superconductivity induced by magnetic fluctuations. It is then shown and discussed in detail that the magnetic fluctuations are generically stronger on the ferromagnetic side of the magnetic phase boundary, which substantially enhances the superconducting critical temperature in the ferromagnetic phase over that in the paramagnetic one. The resulting phase diagram is compared with the experimental observations in ${\mathrm{UGe}}_2$ and ${\mathrm{ZrZn}}_2.$