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Generalized Spin Fluctuation Feedback In Correlated Fermion Superconductors

Experiments reveal that the superconductors $\text{UPt}_3$, $\text{U}_{1-x}\text{Th}_x\text{Be}_{13}$ and $\text{PrOs}_4\text{Sb}_{12}$ undergo two superconducting transitions in the absence of an applied magnetic field. The prevalence of these multiple transitions suggests a common underlying mechanism. A natural candidate theory which accounts for these two transitions is the existence of a small symmetry breaking field, however such a field has not been observed in $\text{PrOs}_4\text{Sb}_{12}$ or $\text{U}_{1-x}\text{Th}_x\text{Be}_{13}$ and has been called into question for $\text{UPt}_3$. Motivated by arguments originally developed for superfluid $^3\text{He}$ we propose that a generalized spin fluctuation feedback effect is responsible for these two transitions. We first develop a phenomenological theory for $^3\text{He}$ that couples spin fluctuations to superfluidity, which correctly predicts that a high temperature broken time-reversal superfluid $^3\text{He}$ phase can emerge as a consequence. The transition at lower temperatures into a time-reversal invariant superfluid phase must then be first order by symmetry arguments. We then apply this phenomenological approach to the three superconductors $\text{UPt}_3$, $\text{U}_{1-x}\text{Th}_x\text{Be}_{13}$ and $\text{PrOs}_4\text{Sb}_{12}$ revealing that this naturally leads to a high-temperature time-reversal invariant nematic superconducting phase, which can be followed by a second order phase transition into a broken time-reversal symmetry phase, as observed.