Effective action approach to the Leggett's mode in two-band superconductors

TL;DR

This paper analyzes the Leggett's mode in two-band superconductors using an effective phase-only action, finding that in MgB2 the mode occurs above the two-particle threshold, affecting its observability.

Contribution

It introduces an effective action approach to study Leggett's mode and assesses its potential observation in MgB2 superconductors.

Findings

Leggett's mode appears above the two-particle threshold in MgB2.

The effective phase-only action framework is effective for analyzing collective modes.

Observation of Leggett's mode in MgB2 may be challenging due to its energy position.

Abstract

We investigate a collective excitation (Leggett's mode) corresponding to small fluctuations of the relative phase of two condensates in two-band superconductor using the effective ``phase only'' action. We consider the possibility of observing Leggett's mode in MgB$_2$ superconductor and conclude that for the known at present values of the two-band model parameters for MgB$_2$ Leggett's mode arises above the two-particle threshold.