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

TL;DR

This paper investigates the emergence of an $s+id$ pairing symmetry in mixed-valent correlated metals, revealing how multi-band interactions can lead to complex superconducting states that may explain experimental puzzles in heavy-fermion systems.

Contribution

It provides a microscopic model demonstrating the stability of $s+id$ pairing in mixed-valent metals with hybridization, extending understanding of unconventional superconductivity.

Findings

Robust $s+id$ pairing found at strong hybridization

$s+id$ state breaks time-reversal symmetry

Potential explanation for multiple superconducting phases in heavy-fermion materials

Abstract

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 determining 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 multi-band systems such as heavy-fermions can support a number of highly non-trivial 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 U$_{1-x}$Th$_x$Be$_{13}$ which exhibit two superconducting phase transitions and a full gap at lower temperatures.