Abstract
A crowd of nonequilibrium entities can show phase transition behaviors that are prohibited in conventional equilibrium setups. An interesting question is whether similar activity-driven phase transitions also occur in pure quantum systems. Here we investigate a classical anisotropic lattice gas model that undergoes motility-induced phase separation and extend the model to the quantum regime. The resulting model is a quantum many-body model that undergoes quantum phase transitions induced by non-Hermiticity. The quantum phase diagram includes active phase transitions involving phase separation, microphase separation, and flocking. The quantum phase transitions are identified as the transitions of dynamical paths in the classical kinetics upon the application of biasing fields. Our approach sheds light on the useful connection between classical nonequilibrium kinetics and non-Hermitian quantum physics.
Highlights
The collective dynamics of active or self-driven components can lead to phase transitions and pattern formations that are prohibited in equilibrium systems [1]
Past works have pointed out how similar equations of time evolution may appear between classical active matter and models from other regions of physics; the Toner-Tu type equation [9] describing the dynamics of microwave-driven 2D electron liquid [10], high effective temperature realized at the interface between two 3D systems with a difference in chemical potential [11,12], and a self-propelled particle expressed by a spinor with spin-orbit coupling [13], to name a few
By investigating the quantum phase diagram via Monte Carlo (MC) simulations, we find that the model exhibits flocking and microphase-separated phases that do not appear in the embedded classical model, and further show the relation of these phases to the dynamical phases
Summary
The collective dynamics of active or self-driven components can lead to phase transitions and pattern formations that are prohibited in equilibrium systems [1]. Advances in atomic-molecular-optical experiments have allowed precise control over open quantum systems [14–17], encouraging the exploration of nonequilibrium physics in various courses including topological phases [18–25] and quantum critical phenomena [26–29] in nonHermitian setups. In light of these developments, it is sensible to consider how the quantum versions of classical nonequilibrium processes can be realized, and ask whether there exist new phases of matter induced by activity in quantum many-body systems. This work demonstrates how activity-induced phase transitions, so far exclusively studied in classical models, can occur as quantum phase transitions
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