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International Workshop on Critical Point and Onset of Deconfinement (CPOD)

Universal scaling of non-equilibrium critical fluctuations from Langevin dynamics of model A

Huichao Song, Shanjin Wu

Published Jan 16, 2018• 0 citations
Publisher

Abstract

Within the framework of the Kibble-Zurek Mechanism, we investigate the universal behavior of the non-equilibrium critical fluctuations, using the Langevin dynamics of model A. With properly located typical time, length and angle scales, $\tau_{\mbox{\tiny KZ}}$, $l_{\mbox{\tiny KZ}}$, and $\theta_{\mbox{\tiny KZ}}$, the constructed function $\bar{f}_n((\tau-\tau_c)/\tau_{\mbox{\tiny KZ}},\theta_{\mbox{\tiny KZ}})$ for the cumulants of the sigma field show universal behavior near the critical point, which are independent from some non-universal factors, such as the relaxation time or the evolution trajectory.

DOI
10.22323/1.311.0026

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Within the framework of the Kibble-Zurek Mechanism, we investigate the universal behavior of the non-equilibrium critical fluctuations, using the Langevin dynamics of model A. With properly located typical time, length and angle scales, τ\mboxKZ\tau_{\mbox{\tiny KZ}}τ\mboxKZ​, l\mboxKZl_{\mbox{\tiny KZ}}l\mboxKZ​, and θ\mboxKZ\theta_{\mbox{\tiny KZ}}θ\mboxKZ​, the constructed function fˉn((τ−τc)/τ\mboxKZ,θ\mboxKZ)\bar{f}_n((\tau-\tau_c)/\tau_{\mbox{\tiny KZ}},\theta_{\mbox{\tiny KZ}})fˉ​n​((τ−τc​)/τ\mboxKZ​,θ\mboxKZ​) for the cumulants of the sigma field show universal behavior near the critical point, which are independent from some non-universal factors, such as the relaxation time or the evolution trajectory.
Within the framework of the Kibble-Zurek Mechanism, we investigate the universal behavior of the non-equilibrium critical fluctuations, using the Langevin dynamics of model A. With properly located typical time, length and angle scales, $τ_{\mbox{KZ}}$, $l_{\mbox{KZ}}$, and $θ_{\mbox{KZ}}$, the constructed functions $\bar{f}_n((τ-τ_c)/τ_{\mbox{KZ}},θ_{\mbox{KZ}})$ (n=1...4) for the cumulants of the sigma field show universal behavior near the critical point, which are independent from some non-universal factors, such as the relaxation time or the evolution trajectory.
Near a critical point, the equilibrium relaxation time of a system diverges and any change of control/thermodynamic parameters leads to nonequilibrium behavior. The Kibble-Zurek problem is to determine the dynamical evolution of the system parametrically close to its critical point when the change is parametrically slow. The nonequilibrium behavior in this limit is controlled entirely by the critical point and the details of the trajectory of the system in parameter space (the protocol) close to the critical point. Together, they define a universality class consisting of critical exponents, discussed in the seminal work by Kibble and Zurek, and scaling functions for physical quantities, which have not been discussed hitherto. In this article, we give an extended and pedagogical discussion of the universal content in the Kibble-Zurek problem. We formally define a scaling limit for physical quantities near classical and quantum transitions for different sets of protocols. We report computations of a few scaling functions in model Gaussian and large-NNN problems and prove their universality with respect to protocol choice. We also introduce a protocol in which the critical point is approached asymptotically at late times with the system marginally out of equilibrium, wherein logarithmic violations to scaling and anomalous dimensions occur even in the simple Gaussian problem.
In context of the search for the QCD critical endpoint in heavy-ion collisions, a deep understanding of the out-of-equilibrium dynamics of the system is necessary to make well-grounded predictions for signatures in final states. To this end, we investigate the dynamic critical behavior of a classical scalar field theory with Z2\mathbb{Z}_2Z2​ symmetry in the dynamic universality class of Model A in two spatial dimensions. The critical dynamics of the system is studied under a linear quench protocol, in which the external symmetry breaking field is changed at a constant rate through the critical point. We discuss the connection to the Kibble-Zurek mechanism and determine universal scaling functions, which fully describe the non-equilibrium evolution of the system near the critical point for all quench rates under consideration. We find that, while the scaling functions are non-trivial, the corresponding scaling exponents are fully determined by static critical exponents and the dynamic critical exponent.
We study the critical dynamics of a scalar field theory with Z 2 symmetry in the dynamic universality class of Model A in two and three spatial dimensions with classical-statistical lattice simulations. In particular, we measure the non-equilibrium behavior of the system under a quench protocol in which the symmetry-breaking external field is changed at a constant rate through the critical point. Using the well-established Kibble-Zurek scaling theory we compute non-equilibrium scaling functions of cumulants of the order parameter up to fourth order. Together with the static critical exponents and the dynamic critical exponent, these fully describe the universal non-equilibrium evolution of the system near the critical point. We further extend the analysis to include finite-size effects and observe good collapse of our data onto two-dimensional universal non-equilibrium and finite-size scaling functions.
The Kibble-Zurek mechanism demands an initial adiabatic stage before an impulse stage to have a frozen correlation length that generates topological defects in a cooling phase transition. Here we study such a driven critical dynamics but with an initial condition that is near the critical point and that is far away from equilibrium. In this case, there is no initial adiabatic stage at all and thus adiabaticity is broken. However, we show that there again exists a finite length scale arising from the driving that divides the evolution into three stages. A relaxation--finite-time-scaling--adiabatic scenario is then proposed in place of the adiabatic-impulse-adiabatic scenario of the original Kibble-Zurek mechanism. A unified scaling theory, which combines finite-time scaling with critical initial slip, is developed to describe the universal behavior and is confirmed with numerical simulations of a two-dimensional classical Ising model.
We study the effects induced by the initial conditions in the driving critical dynamics. A protocol is proposed in which a system starts with a nonequilibrium initial state near the critical point. This driving critical dynamics is remarkably different from the dynamics described by the usual adiabatic--impulse--adiabatic scenario of the Kibble-Zurek mechanism. Instead, it is described by a universal relaxation--finite-time scaling--adiabatic scenario. The distinctive feature is the appearance of the relaxation stage in which the increment of the correlation time dominates and the external driving acts only as a perturbation. A unified scaling theory, which includes the initial scaling variables, is proposed to describe the scaling behaviors. The results are confirmed by numerical simulations of the two-dimensional Ising model.