Leggett modes in iron-based superconductors as a probe of time-reversal symmetry breaking

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 MgB 2 , 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, which has been proposed for strongly hole-doped 122 compounds, can also admit a low-temperature s + is phase that 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 nontrivial case of dominant interband pairing.

Theory of light-induced resonances with collective Higgs and Leggett modes in multiband superconductors

“…(27). As already shown, forced oscillation of the phase difference δ [θ α (t) − θ β (t)] resonates with the Leggett mode at 2Ω ≈ ω L , so that Eq.…”

“…(27). Thus a resonant excitation of the Leggett mode is also available for short-lived cases, while sharpness of the resonance will be degraded.…”

“…Study of collective modes in such multi-band superconductors should shed a new light on their order parameter and pairing interactions. For instance, it has been predicted that competing s-and d-wave interactions can result in different collective modes for different ground states 27,44 . Collective modes are also recently studied for systems where superconductivity coexists with diagonal orders such as spin-density wave 45,46 or charge-density wave 47 .…”

“…For d-wave superconductors such as high-T c cuprates, it has been group-theoretically shown 21 that they can accommodate additional amplitude (Higgs) modes coming from multiple irreducible representations of the k-dependent gap function with the D 4 point-group symmetry. As for the phase modes, multi-gap superconductors are predicted to have an out-of-phase mode between the two components of the gap function [22][23][24][25][26][27][28][29][30] , called "Leggett mode".…”

Collective modes in a two-band superfluid of ultracold alkaline-earth-metal atoms close to an orbital Feshbach resonance

“…Comparing Fig.4 (c) with Fig.4 (d), the spectral weight of Leggett mode diminishes after merging into the continuum. Investigations of the Leggett modes in multi-band superconductor have already attracted intensive interests [24][25][26][27][28][29]. It is only until very recently years that some evidences of its existence have been observed experimentally in multi-band superconductors MgB 2 , by tunneling spectroscopy techniques [30], Raman spectroscopy [31], and angle-resolved photoemission spectroscopy [32].…”