Berezinskii-Kosterlitz-Thouless transition in a Rabi-coupled binary Bose mixture

We discuss the Berezinskii-Kosterlitz-Thouless (BKT) transition in a 2D binary Bose mixture with a Rabi coupling under balanced densities. BKT transition is a transition originating from unbindings of vortex-antivortex pairs in 2D systems such as a 2D Bose gas. In contrast to single-component Bose superfluids, a Monte Carlo simulation of a binary Bose mixture suggests distinct vortex excitations responsible for the BKT transition in the presence of a Rabi coupling. We include the effects of the distinct topological excitations to perform the renormalization group analysis of the binary Bose mixture with a Rabi coupling. Adopting the Nelson-Kosterlitz renormalization group approach, we clarify the dependence of the BKT transition temperature on the Rabi coupling and the inter-component interaction strength. In particular, we find an amplification of the BKT transition temperature with a strong Rabi coupling due to a large gap in one branch of the elementary excitations. We also find that the Rabi coupling suppresses a quasicrossing behavior of the first sound and second sound modes in the low-temperature regime. It indicates that, in contrast to a single-component Bose superfluid, the second sound is detectable by a density probe in the low-temperature regime, which can be tested experimentally.