Bardasis-Schrieffer-like phase mode in a superconducting bilayer

TL;DR

This paper theoretically investigates collective modes in a superconducting bilayer with strong spin-orbit coupling, revealing a Bardasis-Schrieffer-like phase mode that is affected by Coulomb interactions.

Contribution

It develops a gauge-invariant theory for phase fluctuations in a bilayer superconductor with competing pairing states, identifying a novel Bardasis-Schrieffer-like mode.

Findings

Identifies a Bardasis-Schrieffer-like phase mode within the excitation gap.

Shows Coulomb interactions convert this mode into an antisymmetric plasmon.

Suggests the mode is likely pushed into the quasiparticle continuum.

Abstract

We theoretically study the low-lying collective modes of an even-parity spin-singlet superconducting bilayer, where strong spin-orbit coupling leads to a closely competing odd-parity pairing state. We develop a gauge-invariant theory for the coupling of phase fluctuations to an external electromagnetic field and show that the competing odd-parity pairing instability gives rise to a Bardasis-Schrieffer-like phase mode within the excitation gap. Accounting for the long-range Coulomb interaction, however, we find that this mode is converted into an antisymmetric plasmon and is likely pushed into the quasiparticle continuum.