Abstract
The spontaneous breaking of parity-time ({{{{{{{mathcal{PT}}}}}}}}) symmetry yields rich critical behavior in non-Hermitian systems, and has stimulated much interest, albeit most previous studies were performed within the single-particle or mean-field framework. Here, by studying the collective excitations of a Fermi superfluid with {{{{{{{mathcal{PT}}}}}}}}-symmetric spin-orbit coupling, we uncover an emergent {{{{{{{mathcal{PT}}}}}}}}-symmetry breaking in the Anderson-Bogoliubov (AB) collective modes, even as the superfluid ground state retains an unbroken {{{{{{{mathcal{PT}}}}}}}} symmetry. The critical point of the transition is marked by a non-analytic kink in the speed of sound, which derives from the coalescence and annihilation of the AB mode and its hole partner, reminiscent of the particle-antiparticle annihilation. The system consequently becomes immune to low-frequency external perturbations at the critical point, a phenomenon associated with the spectral topology of the complex quasiparticle dispersion. This critical phenomenon offers a fascinating route toward perturbation-free quantum states.
Highlights
The spontaneous breaking of parity-time (PT ) symmetry yields rich critical behavior in nonHermitian systems, and has stimulated much interest, albeit most previous studies were performed within the single-particle or mean-field framework
We theoretically demonstrate an emergent PT -symmetry breaking in the collective modes of a Fermi superfluid, and investigate in detail the rich many-body critical phenomena therein
At this emergent PT transition, the AB mode and its hole partner coalesce and annihilate each other, leading to the complete disappearance of low-frequency excitations, as the speed of sound vanishes in a kink at the transition. This is in sharp contrast to the case with a single-particle PT -symmetric system, where eigenmodes merely merge at the critical point. Such a manybody critical behavior is associated with the point-gap topology of the quasiparticle dispersion, which suggests a topologically robust critical state that is immune to low-frequency perturbations
Summary
The spontaneous breaking of parity-time (PT ) symmetry yields rich critical behavior in nonHermitian systems, and has stimulated much interest, albeit most previous studies were performed within the single-particle or mean-field framework. A critical SOC strength exists, separating PT -symmetry unbroken and broken phases of the AndersonBogoliubov (AB) modes that have purely real or imaginary spectra, respectively At this emergent PT transition, the AB mode and its hole partner coalesce and annihilate each other, leading to the complete disappearance of low-frequency excitations, as the speed of sound vanishes in a kink at the transition. This is in sharp contrast to the case with a single-particle PT -symmetric system, where eigenmodes merely merge at the critical point. Such a manybody critical behavior is associated with the point-gap topology of the quasiparticle dispersion, which suggests a topologically robust critical state that is immune to low-frequency perturbations
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