Collective excitations of superfluids have been widely studied in condensed matter physics. Recent experimental progress in ultracold atoms has enabled studies of out-of-equilibrium dynamics of superfluids. For example, a periodic modulation of the amplitude of the order parameter excites the Higgs amplitude mode, which has been observed with ultracold fermions. As for collective phase modes, the Nambu-Goldstone mode exists in neutral superfluids, and the relative-phase Leggett mode has been predicted for multiband superfluids. In particular, ultracold atoms allow for a dynamical control of various system parameters including dissipation, offering an ideal playground to investigate collective modes. However, they suffer from atom loss due to inelastic scattering in open quantum systems, which has received little attention in literature.

In this talk, we predict a new mechanism to induce collective excitations and a nonequilibrium phase transition of fermionic superfluids via a sudden switch-on of two-body loss, for which we extend the BCS theory to fully incorporate a change in particle number [1]. We find that a sudden switch-on of dissipation induces an amplitude oscillation of the superfluid order parameter accompanied by a chirped phase rotation 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 dynamical phase transition characterized by the vanishing dc Josephson current. The dissipation-induced collective modes and nonequilibrium phase transition can be realized with ultracold fermionic atoms subject to inelastic collisions. If time allows, I will also talk about the non-Hermitian generalization of BCS superfluidity [2], which is closely related to the current work.

[1] K. Yamamoto, M. Nakagawa, N. Tsuji, M. Ueda, and N. Kawakami, Phys. Rev. Lett. 127, 055301 (2021).
[2] K. Yamamoto, M. Nakagawa, K. Adachi, K. Takasan, M. Ueda, and N. Kawakami, Phys. Rev. Lett. 123, 123601 (2019).