Wednesday, 30 August 2017

condensed matter - Derivation of the gradient expansion of the Keldysh nonlinear sigma model for disorder metals


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)=12pTrKG(p,ϵ+ω/2)γαG(p,ϵω/2)γβ

with TrK the trace over Fermionic Keldysh indices, γcl=σ0, γq=σ1 are matrices in the Fermionic Keldysh space, and G is the disorder averaged Fermionic Green's function. In particular, we write the Fermionic Green's function as ˆG=12GR(1+ˆΛ)+12GA(1ˆΛ)
where GR(A)=[ϵξ±i2τ]1 is the retarded (advanced) disorder averaged Green's function and Λ is the stationary saddle point solution of the non-linear sigma model ˆΛ=(1R2F01A)
c.f. Eq. 165. This expression is then evaluated to be iνˆσαβ1 where σ1 is the first Pauli matrix in the bosonic Keldysh space and ν is the density of states at the Fermi surface.


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?




No comments:

Post a Comment

Understanding Stagnation point in pitot fluid

What is stagnation point in fluid mechanics. At the open end of the pitot tube the velocity of the fluid becomes zero.But that should result...