Leggett mode in a strong-coupling model of iron arsenide superconductors

TL;DR

This paper investigates the Leggett mode in a strong-coupling two-orbital model of iron arsenide superconductors, identifying conditions for its observability and potential as a probe of pairing symmetry and correlations.

Contribution

It provides a detailed analysis of the Leggett mode spectrum across different symmetries and parameter regimes in a strong-coupling model, highlighting its experimental signatures and significance.

Findings

Leggett mode exists below the two-particle continuum in certain parameter regions.

Sharp Leggett modes can distinguish different gap momentum dependencies.

Orbital character influences the mode's experimental observability.

Abstract

Using a two-orbital model of the superconducting phase of the pnictides, we compute the spectrum of the Leggett mode -- a collective excitation of the phase of the superconducting gap known to exist in multi-gap superconductors -- for different possible symmetries of the superconducting order parameter. Specifically, we identify the small regions of parameter space where the Leggett mode lies below the two-particle continuum, and hence should be visible as a sharp resonance peak. We discuss the possible utility of the Leggett mode in distinguishing different momentum dependencies of the superconducting gap. We argue that the observation of a sharp Leggett mode would be consistent with the presence of strong electron-electron correlations in iron-based superconductors. We also emphasize the importance of the orbital character of the Leggett mode, which can result in an experimental observation of the mode in channels other than $A_{1g}$.