Triplon-mediated pairing and the superconducting gap structure in bilayer nickelates

TL;DR

This paper proposes a triplon-mediated pairing mechanism in bilayer nickelates, explaining their superconducting gap structure and anisotropy, and aligning with key experimental observations.

Contribution

It introduces a novel microscopic pairing mechanism involving triplons, providing a comprehensive explanation for the superconducting properties of bilayer nickelates.

Findings

Interband s+- pairing with opposite signs on two bands

Larger superconducting gap on the band with smaller density of states

Pronounced gap anisotropy from nonlocal Kondo coupling

Abstract

We investigate the superconducting gap structure in bilayer nickelates within a model where conduction bands of dx2-y2 symmetry coexist with localized d3z2-r2 spins. Strong interlayer coupling drives the local moments into a singlet ground state, whose virtual singlet-triplet excitations ("triplons") mediate the pairing interaction between conduction electrons. This yields interband s+- pairing, with opposite signs of the order parameter on the two (alpha and beta) bands. Our theory naturally explains the key experimental features: a larger gap on the alpha band despite its smaller density of states, and pronounced gap anisotropy arising from nonlocal Kondo coupling. The results support triplon-mediated pairing as the microscopic origin of superconductivity in bilayer nickelates.