Leggett modes in iron-based superconductors as a probe of Time Reversal Symmetry Breaking

TL;DR

This paper investigates Leggett modes in iron-based superconductors, revealing that their presence and behavior can indicate transitions to a time-reversal symmetry-breaking state, especially in multiband models with interband pairing.

Contribution

It demonstrates that Leggett modes vanish at the transition to a time-reversal symmetry-breaking s+is phase in a three-band superconductor model, providing a new probe for such states.

Findings

Leggett mode is absent in simple two-band models of pnictides.

In three-band models, a Leggett mode appears and vanishes at the TRS-breaking transition.

The transition to the s+is phase is marked by the massless Leggett-like fluctuations.

Abstract

Since their discovery, it has been suggested that pairing in pnictides can be mediated by spin fluctuations between hole and electron bands. In this view, multiband superconductivity would substantially differ from other systems like MgB2, where pairing is predominantly intraband. Indeed, interband-dominated pairing leads to the coexistence of bonding and antibonding superconducting channels. Here we show that this has profound consequences on the nature of the low-energy superconducting collective modes. In particular, the so-called Leggett mode for phase fluctuations is absent in the usual two-band description of pnictides. On the other hand, when also the repulsion between the hole bands is taken into account, a more general three-band description should be used, and a Leggett mode is then allowed. Such a model, that has been proposed for strongly hole-doped 122 compounds, can also admit a low-temperature s+is phase which breaks the time reversal symmetry. We show that the (quantum and thermal) transition from the ordinary superconductor to the s+is state is accompanied by the vanishing of the mass of Leggett-like phase fluctuations, regardless the specific values of the interaction parameters. This general result can be obtained by means of a generalized construction of the effective action for the collective degrees of freedom that allows us also to deal with the non-trivial case of dominant interband pairing.