Fluctuation-assisted gap evolution in frustrated multiband superconductors

Maintaining pair entropy assists in the evolution of a gap in frustrated multiband superconductors. A finite and persistent gap is always open at finite temperatures due to the fluctuation of the interband phase difference when the interband interaction is much smaller than the intraband interaction. We demonstrate this gap evolution by combining the Bardeen–Cooper–Schrieffer (BCS) free energy with an entropy contribution originating from the internal freedom of the pair.