Low-lying excitations in superconducting bilayer systems

TL;DR

This paper investigates low-lying excitations in superconducting bilayer systems, revealing a collective mode linked to Josephson interaction and proposing a connection to neutron scattering resonance phenomena.

Contribution

It introduces a new superexchange pairing force and analyzes its effects alongside Josephson coupling, expanding understanding of phase transitions in bilayer superconductors.

Findings

Moderate-to-strong Josephson interaction creates a low-lying collective mode.

The collective mode is an out-of-phase oscillation of gauge angles.

Transition between phases depends on bilayer pairing strength and doping level.

Abstract

The ground and first excited state of two superconducting layers in interaction are studied considering two different coupling terms, one represented by the standard Josephson interaction, and one new, which is a superexchange pairing force between bilayer pairs. It is shown that a moderate-to strong Josephson interaction produces a low-lying collective state, pictured as an out-of-phase oscillation of the BCS gauge angles of the two layers. This antisymmetric angular oscillation might explain the 41 meV resonance observed in the neutron scattering experiments. The bilayer pairs are formed by electrons from different layers with an antiparallel orientation of the spins, being related to the antiferromagnetic arrangement. The pair operators within the layers together with the bilayer pairs generate by commutation an so(5) algebra. It is shown that the transition between the superconducting and antiferromagnetic phases can be explained assuming the dependence on concentration of the bilayer pairing strength, with maximum at half-filling.