Dissipation-induced collective excitations and nonequilibrium phase transition in fermionic superfluids

Collective excitations in fermionic superfluids have been widely studied in condensed matter physics. In particular, recent experimental progress has enabled the studies of out-of-equilibrium dynamics of superfluid order parameters. For example, a periodic modulation of the amplitude of the order parameter excites the Higgs amplitude mode, which has been realized with ultracold atomic gases [1]. However, they inevitably suffer from atomic loss due to inelastic scattering, which has received little attention in literature.

In this talk, we theoretically investigate collective excitations and a nonequilibrium phase transition of fermionic superfluids driven by sudden switch-on of two-particle loss, for which we extend the BCS theory to fully incorporate quantum jumps [2]. We find that dissipation induces an amplitude oscillation of the superfluid order parameter accompanied by chirped phase rotation in a superfluid as a consequence of particle loss. We demonstrate that when dissipation is introduced to one of the two superfluids coupled via a Josephson junction, it gives rise to a nonequilibrium dissipative phase transition characterized by the vanishing dc Josephson current.

[1] A. Behrle et al., Nat. Phys. 14, 781 (2018).
[2] K. Yamamoto et al., arXiv: 2006.06169.