Generalized Spin Fluctuation Feedback In Correlated Fermion Superconductors

TL;DR

This paper proposes a generalized spin fluctuation feedback mechanism as the underlying cause of multiple superconducting transitions in certain correlated fermion superconductors, explaining experimental observations without requiring symmetry-breaking fields.

Contribution

It introduces a phenomenological theory linking spin fluctuations to superconductivity, explaining multiple transitions and time-reversal symmetry breaking in specific heavy-fermion superconductors.

Findings

Predicts high-temperature time-reversal invariant nematic phase

Explains second transition into broken time-reversal symmetry phase

Aligns with experimental observations in UPt3, U1-xThxBe13, PrOs4Sb12

Abstract

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.