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arXiv:hep-ph/0204009 · Chinese Physics C

A Deduced Feynman Rule for Calculating Retarded and Advanced Green function in Closed Time Path Formalism

Xiao, Jun, E Wang

Published Apr 1, 2002• 0 citations
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Abstract

Based on the closed time path formalism, a new Feynman rule for directly calculating the retarded and advanced Green functions is deduced. This Feynman rule is used to calculate the two-point self-energy and three-point vertex correction in $ϕ^3$ theory. The generalized fluctuation-dissipation theorem for three-point nonlinear response function is verified.

DOI
10.48550/arxiv.hep-ph/0204009

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A nonlinear generalization of the fluctuation-dissipation theorem (FDT) for the n-point Green functions and the amputated one particle irreducible vertex functions at finite temperature is derived in the framework of the closed time path formalism. We verify that this generalized FDT coincides with known results for n=2n=2n=2 and 3. New explicit relations among the 4-point nonlinear response and correlation (fluctuation) functions are presented.
I present a numerical treatment for a certain class of Feynman diagrams in perturbation theory with respect to the Coulomb correlation U. This method is formally exact for the frequency integrals occurring in the expression for the retarded Green's function. I specify this method to the self-consistent second-order perturbation theory (SOPT) for the single-impurity Anderson model and to the self-consistent fluctuation exchange approximation (FLEX) for the Hubbard model in infinite dimensions d\ensuremath{\rightarrow}\ensuremath{\infty}. The presented treatment is not only restricted to the limit d\ensuremath{\rightarrow}\ensuremath{\infty}. I present the resulting equations for the retarded self-energy, numerical results for the self-energy, density of states and the interaction spectrum in the FLEX for different scalings of the real frequency axis. The numerical solution of the SOPT and the FLEX could be obtained with very little numerical effort. In this form the calculation is nearly independent of the temperature and over a wide range nearly independent of the value of U. The computational time and the memory requirement for the solution of the self-energy equation are reduced to a minimum in this numerical treatment. \textcopyright{} 1996 The American Physical Society.