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arXiv:0707.3949

Nonequilibrium Renormalization Theory I

D. V. Prokhorenko

Published Jul 26, 2007• 0 citations
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Abstract

In the present article we consider some general class of divergent diagrams in Keldysh diagram technique. These divergences arise for nonequilibrium matter and closely related to the divergences in the kinetic theory of gases. We suggest a scheme of renormalization of such divergences and illustrate it on some examples. In the other papers of these series we develop the general theory of renormalization of nonequilibrium diagram technique. The fact that thermal divergences in non-equilibrium diagram technique can be renormalized leads to the following consequence: to prove that the system tends to the thermal equilibrium one should to take into account the behaviour of the system on its boundary. In this paper we illustrate this fact on Bogoliubov derivation of kinetic equations.

DOI
10.48550/arxiv.0707.3949

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In the present article we consider some general class of divergent diagrams in Keldysh diagram technique. These divergences arise for nonequilibrium matter and closely related to the divergences in the kinetic theory of gases. We suggest a scheme of renormalization of such divergences and illustrate it on some examples. In the other papers of these series we develop the general theory of renormalization of nonequilibrium diagram technique. The fact that thermal divergences in non-equilibrium diagram technique can be renormalized leads to the following consequence: to prove that the system tends to the thermal equilibrium one should to take into account the behaviour of the system on its boundary. In this paper we illustrate this fact on Bogoliubov derivation of kinetic equations.
In this paper we prove that Bose gas with weak pair interaction is non ergodic system. In order to prove this fact we consider the divergences in some nonequilibrium diagram technique. These divergences are analogous to the divergences in the kinetic equations discovered by Cohen and Dorfman. We develop the general theory of renormalization of such divergences and illustrate it with some simple examples. The fact that the system is non ergodic leads to the following consequence: to prove that the system tends to the thermal equilibrium we should take into account its behavior on its boundary. In this paper we illustrate this thesis with the Bogoliubov derivation of the kinetic equations.
In this paper we prove that Bose gas with weak pair interaction is nonergodic\nsystem. In order to prove this fact we consider the divergences in some\nnonequilibrium diagram technique. These divergences are analogous to the\ndivergences in the kinetic equations discovered by Cohen and Dorfman. We develop\nthe general theory of renormalization of such divergences and illustrate it with\nsome simple examples. The fact that the system is nonergodic leads to the\nfollowing consequence: to prove that the system tends to the thermal equilibrium\nwe should take into account its behavior on its boundary. In this paper we\nillustrate this thesis with the Bogoliubov derivation of the kinetic\nequations.
In this article, we present a concise and self-contained introduction to nonequilibrium statistical mechanics with quantum field theory by considering an ensemble of interacting identical bosons or fermions as an example. Readers are assumed to be familiar with the Matsubara formalism of equilibrium statistical mechanics such as Feynman diagrams, the proper self-energy, and Dyson's equation. The aims are threefold: (i) to explain the fundamentals of nonequilibrium quantum field theory as simple as possible on the basis of the knowledge of the equilibrium counterpart; (ii) to elucidate the hierarchy in describing nonequilibrium systems from Dyson's equation on the Keldysh contour to the Navier-Stokes equation in fluid mechanics via quantum transport equations and the Boltzmann equation; (iii) to derive an expression of nonequilibrium entropy that evolves with time. In stage (i), we introduce nonequilibrium Green's function and the self-energy uniquely on the round-trip Keldysh contour, thereby avoiding possible confusions that may arise from defining multiple Green's functions at the very beginning. We try to present the Feynman rules for the perturbation expansion as simple as possible. In particular, we focus on the self-consistent perturbation expansion with the Luttinger-Ward thermodynamic functional, i.e., Baym's Phi-derivable approximation, which has a crucial property for nonequilibrium systems of obeying various conservation laws automatically. We also show how the two-particle correlations can be calculated within the Phi-derivable approximation, i.e., an issue of how to handle the "Bogoliubov-Born-Green-Kirkwood-Yvons (BBGKY) hierarchy".
We discuss the renormalization of the initial value problem in nonequilibrium quantum field theory within a simple, yet instructive, example and show how to obtain a renormalized time evolution for the two-point functions of a scalar field and its conjugate momentum at all times. The scheme we propose is applicable to systems that are initially far from equilibrium and compatible with nonsecular approximation schemes which capture thermalization. It is based on Kadanoff-Baym equations for non-Gaussian initial states, complemented by usual vacuum counterterms. We explicitly demonstrate how various cutoff-dependent effects peculiar to nonequilibrium systems, including time-dependent divergences or initial-time singularities, are avoided by taking an initial non-Gaussian three-point vacuum correlation into account.
The method of the nonequilibrium statistical operator developed by D. N. Zubarev is employed to analyze and derive generalized transport and kinetic equations. The degrees of freedom in solids can often be represented as a few interacting subsystems (electrons, spins, phonons, nuclear spins, etc.). Perturbation of one subsystem may produce a nonequilibrium state which is then relaxed to an equilibrium state due to the interaction between particles or with a thermal bath. The generalized kinetic equations were derived for a system weakly coupled to a thermal bath to elucidate the nature of transport and relaxation processes. It was shown that the "collision term" had the same functional form as for the generalized kinetic equations for the system with small interactions among particles. The applicability of the general formalism to physically relevant situations is investigated. It is shown that some known generalized kinetic equations (e.g. kinetic equation for magnons, Peierls equation for phonons) naturally emerges within the NSO formalism. The relaxation of a small dynamic subsystem in contact with a thermal bath is considered on the basis of the derived equations. The Schrödinger-type equation for the average amplitude describing the energy shift and damping of a particle in a thermal bath and the coupled kinetic equation describing the dynamic and statistical aspects of the motion are derived and analyzed. The equations derived can help in the understanding of the origin of irreversible behavior in quantum phenomena.