We experimentally demonstrate vibration phase reversal transition in a discrete time-translational symmetry broken cold atomic system by the application of a pulsed bias-field opposite to the existing phase of the symmetry broken vibrational state. The reversal transition depends on the strength hp and the duration Δt of the applied pulse. Consequently, we obtain a hp−Δt phase boundary with a divergent relaxation time τ due to the critical slowdown behavior. Interestingly, the dependence of the dynamic phase boundary and relaxation time on the noise-induced switching rate implies that the system is out of equilibrium, though not so in the Ising model of a spin system.
We experimentally demonstrate vibration phase reversal transition in a discrete time-translational symmetry broken cold atomic system by the application of a pulsed bias-field opposite to the existing phase of the symmetry broken vibrational state. The reversal transition depends on the strength hp and the duration Δt of the applied pulse. Consequently, we obtain a hp−Δt phase boundary with a divergent relaxation time τ due to the critical slowdown behavior. Interestingly, the dependence of the dynamic phase boundary and relaxation time on the noise-induced switching rate implies that the system is out of equilibrium, though not so in the Ising model of a spin system.
We report the results of mean field and the Monte Carlo study of the dynamic magnetization-reversal transition in the Ising model, brought about by the application of an external field pulse applied in opposition to the existing order before the application of the pulse. The parameters controlling the transition are the strength hp and the duration $\ensuremath{\Delta}t$ of the pulse. In the mean field case, an approximate analytical expression is obtained for the phase boundary which agrees well with that obtained numerically in the small $\ensuremath{\Delta}t$ and large T limit. The order parameter of the transition has been identified, and is observed to vary continuously near the transition. The order parameter exponent $\ensuremath{\beta}$ was estimated both for the mean field $(\ensuremath{\beta}=1)$ and the Monte Carlo $(\ensuremath{\beta}=0.90\ifmmode\pm\else\textpm\fi{}0.02$ in two dimensions) cases. The transition shows a ``critical slowing down'' type behavior near the phase boundary with diverging relaxation time. The divergence was found to be logarithmic in the mean field case and exponential in the Monte Carlo case. The finite size scaling technique was employed to estimate the correlation length exponent $\ensuremath{\nu}$ $(=1.5\ifmmode\pm\else\textpm\fi{}0.3$ in two dimensions) in the Monte Carlo case.
This chapter describes the experimental and theoretical studies of a spontaneous time-translation symmetry breaking transition in a non-equilibrium cold atom system. Such studies prepare two vibrating atomic clouds by periodically modulating the laser intensity in a magneto-optical trap (MOT). For a comparatively small total number of trapped atoms the clouds have equal populations, a consequence of fluctuation-induced inter-cloud transitions. As the total number of atoms is increased, spontaneous breaking of the discrete time-translation symmetry is observed with respect to the modulation period. The measured critical exponents and the frequency dispersion of the response show an ideal mean-field transition behaviour. The symmetry breaking is due to the modification of fluctuational dynamics by the interatomic interaction. The chapter provides a theoretical model based on the activation energy and master equation which explains the experimental results well. The enhancement of the fluctuation which cannot be described by the mean-field model, is also discussed.
We discuss dynamic magnetization-reversal phenomena in the Ising model under a finite-duration external magnetic field, which competes with the existing order for . The nature of the phase boundary is estimated from the mean-field (MF) equation of motion. The susceptibility and relaxation time diverge at the MF phase boundary. A Monte Carlo study also shows divergence of the relaxation time and order-parameter fluctuations at the phase boundary. The fourth-order cumulant shows two distinct behaviors. For low temperatures and pulse durations, the value of the cumulant at the crossing point for different system sizes is far less than that for the static transition in the same dimension. This suggests a new universality class for the dynamic transition. For higher temperatures and pulse durations, the transition falls in a MF-like weak-singularity universality class.
Fluctuation-dissipation relations (FDRs) and time-reversal symmetry (TRS), two pillars of statistical mechanics, are both broken in generic driven-dissipative systems. These systems rather lead to non-equilibrium steady states far from thermal equilibrium. Driven-dissipative Ising-type models, however, are widely believed to exhibit effective thermal critical behavior near their phase transitions. Contrary to this picture, we show that both the FDR and TRS are broken even macroscopically at, or near, criticality. This is shown by inspecting different observables, both even and odd operators under time-reversal transformation, that overlap with the order parameter. Remarkably, however, a modified form of the FDR as well as TRS still holds, but with drastic consequences for the correlation and response functions as well as the Onsager reciprocity relations. Finally, we find that, at criticality, TRS remains broken even in the weakly-dissipative limit.
Fluctuation-dissipation relations (FDRs) and time-reversal symmetry (TRS), two pillars of statistical mechanics, are both broken in generic driven-dissipative systems. These systems rather lead to non-equilibrium steady states far from thermal equilibrium. Driven-dissipative Ising-type models, however, are widely believed to exhibit effective thermal critical behavior near their phase transitions. Contrary to this picture, we show that both the FDR and TRS are broken even macroscopically at, or near, criticality. This is shown by inspecting different observables, both even and odd operators under time-reversal transformation, that overlap with the order parameter. Remarkably, however, a modified form of the FDR as well as TRS still holds, but with drastic consequences for the correlation and response functions as well as the Onsager reciprocity relations. Finally, we find that, at criticality, TRS remains broken even in the weakly-dissipative limit.
In equilibrium physics, spontaneous symmetry breaking and elementary excitation are two concepts closely related with each other: the symmetry and its spontaneous breaking not only control the dynamics and spectrum of elementary excitations, but also determine their underlying structures. In this Letter, based on an exactly solvable model, we propose a phase ramping protocol to study an excitationlike behavior of a nonequilibrium quantum matter: a discrete time crystal phase with spontaneous temporal translational symmetry breaking. It is shown that slow ramping could induce a dynamical transition between two Z_{2} symmetry breaking time crystal phases in time domain, which can be considered as a temporal analog of the soliton excitation spatially sandwiched by two degenerate charge density wave states in polyacetylene. By tuning the ramping rate, we observe a critical value at which point the transition duration diverges, resembling the critical slowing down phenomenon in nonequilibrium statistic physics. We also discuss the effect of stochastic sequences of such phase ramping processes and its implication to the stability of the discrete time crystal phase against noisy perturbations.