Pairing transitions in a binary Bose gas

The stable Bardeen–Schrieffer–Cooper (BCS) pairing state of a bosonic system has long been sought theoretically and experimentally. Here we propose that a stable BCS state of bosons can be realized in a binary Bose gas with s-wave intra-species repulsion and an inter-species attraction in the mean-field-stable region. We find that above the Bose–Einstein condensation (BEC) transition temperature, there is a phase transition from the normal state to a BCS state driven by inter-species pairing. When the temperature decreases, another phase transition from the BCS state to a mixed state featuring both atomic BEC and inter-species pairing occurs. As the temperature is further lowered, the mixed state is eventually taken over by the pure BEC state. We present the phase diagram of this system and discuss its experimental implications.