Spontaneous magnetization in unitary superconductors with time reversal symmetry breaking

We report the study of spontaneous magnetization (i.e., spin polarization) for time-reversal-symmetry-breaking (TRS-breaking) superconductors with unitary pairing potentials, in the absence of external magnetic fields or Zeeman fields. Spin-singlet (Δs) and spin-triplet (Δt) pairings can coexist in superconductors whose crystal structures lack inversion symmetry. The TRS can be spontaneously broken once a relative phase of ±π/2 is developed, forming a TRS-breaking unitary pairing state (Δs±iΔt). We demonstrate that such unitary pairing could give rise to spontaneous spin polarization with the help of spin-orbit coupling. Our result provides an alternative explanation for the TRS breaking, beyond the current understanding of such phenomena in the noncentrosymmetric superconductors. The experimental results of Zr3Ir and CaPtAs are also discussed in view of our theory.