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Massless collective excitations in frustrated multi-band superconductors

We study collective excitations in three- and four-band superconductors with inter-band frustration, which causes neither 0 nor $π$ inter-band phases in the superconducting state. Using a low-energy spin-Hamiltonian originating from a multi-band tight-binding model, we find that a massless Leggett mode occurs in a wide parameter region of this four-band system. This massless mode is related to the fact that the mean-field energy does not depend on a continuous superconducting phase. In other words, we find a link of the massless mode with a degeneracy between a time-reversal-symmetry-breaking state (neither 0 nor $π$ phases) and a time-reversal-symmetric state (either 0 or $π$ phases). Therefore, the mass of this collective mode characterizes well the time-reversal symmetry in frustrated multi-band superconductors.