Bound collective modes in nonuniform superconductors

TL;DR

This paper investigates bound collective excitations in nonuniform superconductors with domain walls, revealing new modes linked to broken symmetries and topological features, with implications for detection in confined systems.

Contribution

It identifies and characterizes new bound collective modes associated with domain walls in superconductors, including a Goldstone mode, expanding understanding of nonuniform superconducting states.

Findings

Two amplitude/Higgs modes lie below the bulk pairbreaking edge 2Δ.

A Goldstone mode with zero excitation energy exists.

Bound modes are related to the topological structure and stability of the domain wall.

Abstract

We study dynamics of a superconducting condensate in the presence of a domain wall defect in the order parameter. We find that broken translation and reflection symmetries result in new collective excitations, bound to the domain wall region. Two additional amplitude/Higgs modes lie below the bulk pairbreaking edge $2\Delta$; one of them is a Goldstone mode with vanishing excitation energy. Spectrum of bound collective modes is related to the topological structure and stability of the domain wall. The `unbound' bulk collective modes and transverse gauge field mostly propagate across the domain wall, but the longitudinal component of the gauge field is completely reflected. Softening of the amplitude mode suggests reduced damping and possible route to its detection in geometrically confined superfluids or in superconductor-ferromagnetic heterostructures.