Coexistent spin-triplet superconducting and ferromagnetic phases induced by Hund's rule coupling and electronic correlations: Effect of the applied magnetic field

The recently proposed local-correlation-driven pairing mechanism, describing ferromagnetic phases (FM1 and FM2) coexisting with spin-triplet superconductivity (SC) within a single orbitally degenerate Anderson lattice model, is extended to the situation with an applied Zeeman field. The model provides and rationalizes in a semiquantitative manner the principal features of the phase diagram observed for UGe2 in the field absence [cf., Phys. Rev. B 97, 224519 (2018)]. As spin-dependent effects play a crucial role for both the ferromagnetic and SC states, the role of the Zeeman field is to single out different stable spin-triplet SC phases. This analysis should thus be helpful in testing the proposed real-space pairing mechanism, which may be regarded as complementary to spin-fluctuation theory suitable for He-3. Specifically, we demonstrate that the presence of the two distinct phases, FM1 and FM2, and the associated field-driven metamagnetic transition between them, induces a respective metasuperconducting phase transformation. At the end, we discuss briefly how the spin fluctuations might be incorporated as a next step in the renormalized quasiparticle picture considered herein.