Theory of (s plus id) pairing in mixed-valent correlated metals

Motivated by the recent discovery of superconductivity in square-planar nickelates as well as by longstanding puzzling experiments in heavy-fermion superconductors, we study Cooper pairing between correlated d electrons coupled to a band of weakly correlated electrons. We perform self-consistent large N calculations on an effective t - J model for the d electrons with additional hybridization. Unlike previous studies of mixed-valent systems, we focus on parameter regimes where both hybridized bands are relevant to determine the pairing symmetry. For sufficiently strong hybridization, we find a robust (s + id) pairing which breaks time-reversal and point-group symmetries in the mixed-valent regime. Our results illustrate how intrinsically multiband systems such as heavy fermions can support a number of highly nontrivial pairing states. They also provide a putative microscopic realization of previous phenomenological proposals of (s + id) pairing and suggest a potential resolution to puzzling experiments in heavy-fermion superconductors such as U1-xThxBe13 which exhibit two superconducting phase transitions and a full gap at lower temperatures.