My confusion relates to Appendix C of this this paper although the same derivation is presented in many others. When deriving the gradient expansion of this term arrives at a term quadratic in the scalar potential, which takes the form TrVαΥαβVβ where α,β∈{cl,q} are indices in the Keldysh matrix space, the trace is over space and time indices, and Υαβ(ω)=−12∑pTrKG(p,ϵ+ω/2)γαG(p,ϵ−ω/2)γβ
So far so good. My confusion is the decision in Appendix C to only include the products GRGR or GAGA in the integral. Using the above parametrization of the Keldysh Green's function one can see that Υcl,cl=0 but each of the other components are not identically zero and contain (GR)2, (GA)2, and GRGA terms. I don't follow the argument for why only the R-R terms should be included and in fact in the other parts of the gradient expansion it is instead the R-A terms which are kept and the R-R and A-A terms vanish due to the analytic structure of the Green's functions. Furthermore, it is not clear to me why the q-q component is zero. Am I missing something about the analytical structure of the integrands or is there a physical argument for this?
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