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arXiv:hep-ph/0105045 · Physical review. D. Particles, fields, gravitation, and cosmology/Physical review. D. Particles and fields

Multiple-scale analysis and renormalization of quenched second order phase transitions

Sang Pyo Kim, Supratim Sengupta, F. C. Khanna

Published Oct 29, 2001• 4 citations
arXivHTMLPDF

Abstract

A quenched second order phase transition is modeled by an effective ${\ensuremath{\Phi}}^{4}$ theory with a time-dependent Hamiltonian $\ifmmode \hat{H}\else \^{H}\fi{}(t),$ whose symmetry is broken spontaneously in time. The quantum field evolves out of equilibrium (nonequilibrium) during the phase transition as the density operator significantly deviates from $\stackrel{^}{\ensuremath{\rho}}{(t)=e}^{\ensuremath{-}\ensuremath{\beta}\ifmmode \hat{H}\else \^{H}\fi{}(t)}{/Z}_{H}.$ The recently developed Liouville--von Neumann method provides various quantum states for the phase transition in terms of a complex solution to the mean-field equation, which is equivalent to the Gaussian effective potential in the static case and to the time-dependent Hartree-Fock equation in the nonequilibrium case. Using multiple-scale perturbation theory we solve the mean-field equation analytically to the first order of the coupling constant and find the quantum states during the quenched second order phase transition. We propose a renormalization scheme during the process of phase transition to regularize the divergences, which originate from the mode coupling between hard and hard modes or between soft and hard modes. The effect of mode coupling is discussed.

DOI
10.1103/physrevd.64.105026

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A quenched second order phase transition is modeled by an effective \ensuremathΦ4{\ensuremath{\Phi}}^{4}\ensuremathΦ4 theory with a time-dependent Hamiltonian \ifmmodeH^\elseHˆ\fi(t),\ifmmode \hat{H}\else \^{H}\fi{}(t),\ifmmodeH^\elseHˆ\fi(t), whose symmetry is broken spontaneously in time. The quantum field evolves out of equilibrium (nonequilibrium) during the phase transition as the density operator significantly deviates from \stackrel{^}{\ensuremath{\rho}}{(t)=e}^{\ensuremath{-}\ensuremath{\beta}\ifmmode \hat{H}\else \^{H}\fi{}(t)}{/Z}_{H}. The recently developed Liouville--von Neumann method provides various quantum states for the phase transition in terms of a complex solution to the mean-field equation, which is equivalent to the Gaussian effective potential in the static case and to the time-dependent Hartree-Fock equation in the nonequilibrium case. Using multiple-scale perturbation theory we solve the mean-field equation analytically to the first order of the coupling constant and find the quantum states during the quenched second order phase transition. We propose a renormalization scheme during the process of phase transition to regularize the divergences, which originate from the mode coupling between hard and hard modes or between soft and hard modes. The effect of mode coupling is discussed.
We use the so-called Liouville--von Neumann (LvN) approach to study the nonequilibrium quantum dynamics of time-dependent second order phase transitions. The LvN approach is a canonical method that unifies the functional Schr\"odinger equation for the quantum evolution of pure states and the LvN equation for the quantum description of mixed states of either equilibrium or nonequilibrium. As nonequilibrium quantum mechanical systems we study a time-dependent harmonic and an anharmonic oscillator and find the exact Fock space and density operator for the harmonic oscillator and the nonperturbative Gaussian Fock space and density operator for the anharmonic oscillator. The density matrix and the coherent, thermal, and coherent-thermal states are found in terms of their classical solutions, for which the effective Hamiltonians and equations of motion are derived. The LvN approach is further extended to quantum fields undergoing time-dependent second order phase transitions. We study an exactly solvable model with a finite smooth quench and find the two-point correlation functions. Because of the spinodal instability of long wavelength modes, the two-point correlation functions lead to the t1/4{t}^{1/4}t1/4-scaling relation during the quench and the Cahn-Allen scaling relation t1/2{t}^{1/2}t1/2 after completion of the quench. Further, after the finite quench the domain formation shows a time-lag behavior at the cubic power of the quench period. Finally we study the time-dependent phase transition of a self-interacting scalar field.
Several mean-field computations have revealed the existence of an out-of-equilibrium dynamical transition induced by quantum-quenching an isolated system starting from its symmetry-broken phase. In this work we focus on the quantum ${\ensuremath{\phi}}^{4}$ NNN-component field theory. By taking into account dynamical fluctuations at the Hartree-Fock level, corresponding to the leading order of the 1/N1/N1/N expansion, we derive the critical properties of the dynamical transition beyond mean-field theory (including at finite temperature). We find diverging time and length scales, dynamic scaling, and aging. Finally, we unveil a relationship with critical coarsening, an off-equilibrium regime that can be induced by quenching from the symmetric toward the symmetry-broken phase.
The $\ensuremath{\lambda}{\ensuremath{\varphi}}^{4}$ model in a finite volume is studied within a non-Gaussian, time-dependent Hartree-Fock (TDHF) approximation both at equilibrium and out of equilibrium, with particular attention to the structure of the ground state and of certain dynamical features in the broken symmetry phase. The mean-field coupled time-dependent Schr\"odinger equations for the modes of the scalar field are derived and the suitable procedure to renormalize them is outlined. A further controlled Gaussian approximation of our TDHF approach is used in order to study the dynamical evolution of the system from nonequilibrium initial conditions characterized by a uniform condensate. We find that, during the slow rolling down, the long-wavelength quantum fluctuations do not grow to a macroscopic size but do scale with the linear size of the system, in accordance with similar results valid for the large-N approximation of the O(N)O(N)O(N) model. This behavior undermines in a precise way the Gaussian approximation within our TDHF approach, which therefore appears as a viable means to correct an unlikely feature of the standard HF factorization scheme, such as the so-called ``stopping at the spinodal line'' of the quantum fluctuations. We also study the dynamics of the system in infinite volume with particular attention to the asymptotic evolution in the broken symmetry phase. We are able to show that the fixed points of the evolution cover at most the classically metastable part of the static effective potential.
Dynamics of phase transition in (3+1) dimensional scalar ϕ4ϕ^4ϕ4 theory
We use the variational approximation with double Gaussian type trial wave-functional approximation, in which we use the square root of the dispersion of the zero-mode wave-function as an order parameter, to study the out of equilibrium quantum dynamics of time-dependent second order phase transitions in (3+1) dimensions. We study the time evolution of symmetric states of scalar λϕ4λϕ^4λϕ4 theory in several situations by properly treating the effect of the λϕ4λϕ^4λϕ4 interaction. We also calculate the effective action and the effective potential of the theory with the precarious renormalization. We show that the presence of a quenching of the mass-squared leads to second order phase transition nontrivially since the vacuum structure changes by absorbing the energy required for quenching, even though there is no symmetry breaking in the effective potential of the theory without quenching process. We also calculate the equal time correlation function, and then evaluate the correlation length as a function of the mass-squared. The time dependence of the correlation length varies depending on how the mass-squared changes in time. For constant mass-squared it gives the classical Cahn-Allen relation, and it leads to different relations for other time-dependence of the mass-squared. We also show that there exists a propagating spatial correlation after termination of the phase transition process in addition to the correlation corresponding to the formation and growth of domains.
This work is devoted to the study of relaxation--dissipation processes in systems described by Quantum Field Theory. In the first part, I focus on the phi^4 scalar quantum field theory in finite volume in the large N limit. I find that the zero-mode quantum fluctuations cannot grow macroscopically large starting from microscopic initial conditions, while the long-wavelength fluctuations do scale with the linear size of the system, signalling dynamical infrared properties quite different from the equilibrium ones. I also introduce a non-gaussian Hartree-Fock approximation (tdHF) and I derive the mean-field coupled time-dependent Schroedinger equations for the modes of the scalar field. The dynamical evolution of a uniform condensate is studied in a further controlled gaussian approximation, for N=1. Results similar to the large N case show an internal inconsistency of this approximation. I also study the dynamics of the system in infinite volume in the broken symmetry phase. The data show that the fixed points of the evolution cover at most the classically metastable part of the static effective potential. I finally study the dynamical evolution for the O(N+1) nonlinear sigma model in 1+1 dimensions in the large N limit. I first of all verify that the large coupling limit of the O(N+1) phi^4 model, which renders the model non linear, commutes with the large N limit, so that the O(infty) nonlinear sigma model is uniquely defined. I then study the evolution of several observables, like the spectrum of produced particles, finding no evidence for parametric resonance. I conclude with some remarks on the ``state of art'' in gauge theories and some comments about the open issues in the subject.
The second-order quantum phase transition and quench dynamics in the two-mode Rabi model are investigated. We propose a diagonalization approach of jointly using a beam-splitter operator and a squeezed operator for each effective low-energy Hamiltonian when the ratio of the qubit transition frequency to the oscillator frequency approximates infinity. Eigenenergy and eigenstate of the normal and superradiant phases in the system are analytically derived, demonstrating the second-order quantum phase transition at a critical point. This critical point means that the requirement of coupling strength between the qubit and two oscillators is largely loosened by joint effect of two modes when compared with the standard Rabi model. The universal scaling between residual energy and quench time from ${\ensuremath{\tau}}^{\ensuremath{-}\frac{1}{3}}$ to ${\ensuremath{\tau}}^{\ensuremath{-}2}$ is confirmed.