Extended $s$-wave pairing from an emergent Feshbach resonance in bilayer nickelate superconductors

TL;DR

This paper investigates the pairing mechanism in bilayer nickelate superconductors, revealing an extended s-wave pairing symmetry driven by a Feshbach resonance, and provides a mean-field framework supported by DMRG benchmarks.

Contribution

It introduces a mean-field analysis of a minimal model showing BCS-to-BEC crossover and extended s-wave pairing, linked to Feshbach resonance phenomena in bilayer nickelates.

Findings

Predicted extended s-wave pairing symmetry in LNO.

Demonstrated BCS-to-BEC crossover with Feshbach resonance.

Validated mean-field results with DMRG simulations.

Abstract

Since the discovery of unconventional superconductivity in cuprates, unraveling the pairing mechanism of charge carriers in doped antiferromagnets has been a long-standing challenge. Motivated by the discovery of high-T$_c$ superconductivity in nickelate bilayer La$_3$Ni$_2$O$_7$ (LNO), we study a minimal mixed dimensional (MixD) $t-J$ model supplemented with a repulsive Coulomb interaction $V$. When hole-doped, previous numerical simulations revealed that the system exhibits strong binding energies, with a phenomenology resembling a BCS-to-BEC crossover accompanied by a Feshbach resonance between two distinct types of charge carriers. Here, we perform a mean-field analysis that enables a direct observation of the BCS-to-BEC crossover as well as microscopic insights into the crossover region and the pairing symmetry for two-dimensional bilayers. We benchmark our mean-field description by comparing it to density-matrix renormalization group (DMRG) simulations in quasi-one dimensional settings and find remarkably good agreement. For the two-dimensional system relevant to LNO our mean-field calculations predict a BCS pairing gap with an extended $s$-wave symmetry, directly resulting from the pairing mechanism's Feshbach-origin. Our analysis hence gives insights into pairing in unconventional superconductors and, further, can be tested in currently available ultracold atom experiments.