Absence of Criticality in the Phase Transitions of Open Floquet Systems

TL;DR

This paper investigates phase transitions in open Floquet systems, revealing that finite-frequency drives turn what would be second order transitions into first order due to fluctuation effects, contrasting with the infinite-frequency limit.

Contribution

It uncovers a universal mechanism causing the change from second to first order phase transitions in driven systems at finite frequencies, extending understanding of non-equilibrium critical phenomena.

Findings

Infinite drive limit exhibits second order phase transition.

Finite frequency drives induce first order transitions due to fluctuations.

Critical exponents can be experimentally probed by tuning drive frequency.

Abstract

We address the nature of phase transitions in periodically driven systems coupled to a bath. The latter enables a synchronized non-equilibrium Floquet steady state at finite entropy, which we analyse for rapid drives within a non-equilibrium RG approach. While the infinitely rapidly driven limit exhibits a second order phase transition, here we reveal that fluctuations turn the transition first order when the driving frequency is finite. This can be traced back to a universal mechanism, which crucially hinges on the competition of degenerate, near critical modes associated to higher Floquet Brillouin zones. The critical exponents of the infinitely rapidly driven system -- including a new, independent one -- can yet be probed experimentally upon smoothly tuning towards that limit.