Generalized spin fluctuation feedback in heavy fermion superconductors

Abstract

Experiments reveal that the superconductors UPt3, PrOs4Sb12, and U1-xThxBe13 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 PrOs4Sb12 or U1-xThxBe13 and has been called into question for UPt3. Motivated by arguments originally developed for superfluid He3, we propose that a generalized spin fluctuation feedback effect is responsible for these two transitions. We first develop a phenomenological theory for He3 that couples spin fluctuations to superfluidity, which correctly predicts that a high-temperature broken time-reversal superfluid He3 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 UPt3, PrOs4Sb12, and U1-xThxBe13, 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.