BCS-BEC crossover of collective excitations in two-band superfluids

We use the functional integral approach to study low energy collective excitations in a continuum model of neutral two-band superfluids at $T=0$ for all couplings with a separable pairing interaction. In the long wavelength and low frequency limit, we recover Leggett's analytical results in weak coupling (BCS) for s-wave pairing, and further obtain analytical results in strong coupling Bose-Einstein condensation for both two and three dimensional systems. We also analyze numerically the behavior of the out-of-phase exciton (finite frequency) mode and the in-phase phonon (Goldstone) mode from weak to strong coupling limits, including the crossover region. In principle, the evolution of Goldstone and finite frequency modes from weak to strong coupling may be accessible experimentally in the superfluid phase of neutral Fermi atomic gases, and could serve as a test of the validity of the theoretical analysis and approximations proposed here.