On the nature of the non-equilibrium phase transition in the non-Markovian driven Dicke model

TL;DR

This paper analytically investigates the non-equilibrium phase transition in a non-Markovian driven Dicke model, revealing new critical exponents, the classical nature of the transition, and finite-size effects through a fractional Langevin framework.

Contribution

It provides an analytical approach to understanding the critical behavior of a non-Markovian Dicke model, including derivation of critical exponents and finite-size scaling, extending previous numerical studies.

Findings

Critical exponents match zero-temperature results at finite temperature with non-zero chemical potential.

The superradiant phase transition is classical yet inherently non-equilibrium.

Finite-size scaling reveals complex static and dynamic behaviors.

Abstract

The Dicke model famously exhibits a phase transition to a superradiant phase with a macroscopic population of photons and is realized in multiple settings in open quantum systems. In this work, we study a variant of the Dicke model where the cavity mode is lossy due to the coupling to a Markovian environment while the atomic mode is coupled to a colored bath. We analytically investigate this model by inspecting its low-frequency behavior via the Schwinger-Keldysh field theory and carefully examine the nature of the corresponding superradiant phase transition. Integrating out the fast modes, we can identify a simple effective theory allowing us to derive analytical expressions for various critical exponents, including those, such as the dynamical critical exponent, that have not been previously considered. We find excellent agreement with previous numerical results when the non-Markovian bath is at zero temperature; however, contrary to these studies, our low-frequency approach reveals that the same exponents govern the critical behavior when the colored bath is at finite temperature unless the chemical potential is zero. Furthermore, we show that the superradiant phase transition is classical in nature, while it is genuinely non-equilibrium. We derive a fractional Langevin equation and conjecture the associated fractional Fokker-Planck equation that capture the system's long-time memory as well as its non-equilibrium behavior. Finally, we consider finite-size effects at the phase transition and identify the finite-size scaling exponents, unlocking a rich behavior in both statics and dynamics of the photonic and atomic observables.