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A Framework for Studying a Quantum Critical Metal in the Limit $N_f\rightarrow0$
by Petter Säterskog
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Submission summary
As Contributors:  Petter Säterskog 
Arxiv Link:  https://arxiv.org/abs/1711.04338v1 (pdf) 
Date submitted:  20171121 01:00 
Submitted by:  Säterskog, Petter 
Submitted to:  SciPost Physics 
Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
We study a model in 1+2 dimensions composed of a spherical Fermi surface of $N_f$ flavors of fermions coupled to a massless scalar. We present a framework to nonperturbatively calculate general fermion $n$point functions of this theory in the limit $N_f\rightarrow0$ followed by $k_F\rightarrow\infty$ where $k_F$ sets both the size and curvature of the Fermi surface. Using this framework we calculate the zerotemperature fermion densitydensity correlation function in real space and find an exponential decay of Friedel oscillations.
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Reports on this Submission
Anonymous Report 1 on 20171220 Invited Report
 Cite as: Anonymous, Report on arXiv:1711.04338v1, delivered 20171220, doi: 10.21468/SciPost.Report.303
Strengths
1. Treatment of the problem is technically sound.
2. Demonstration of a nonperturbative effect of the fermionboson interaction in the densitydensity response.
3. The introduction contains a good exposition of the problem.
Weaknesses
1. The conclusion foregoes a discussion of the principal result with respect to the physical properties of the system at hand.
Report
The author studies the correlation functions of a twodimensional metal which is strongly coupled to gapless bosons. Building on earlier related work, the present publication elaborates on the densitydensity response in a strictly controllable, albeit somehow arbitrary limit which is nonperturbative in the coupling strength. As the main result, the densitydensity correlation features an exponentially fast extinction in real space, which appears only beyond perturbation theory.
I can recommend the publication, but I suggest to elaborate in better detail on the significance of the result for critical metals in two dimensions.
Requested changes
The author might consider to widen the scope of the discussion to include some implications of their fast decay. What does this mean for the stability of the critical theory at Q=0, and for the stability of approximations where the Landau damping is incorporated from the outset? Can the improved densitydensity correlator be used to supplement previous N_f>0 methods?
Dear referee, Thank you for your comments.
1: "What does this mean for the stability of the critical theory at Q=0, and for the stability of approximations where the Landau damping is incorporated from the outset?" In order to thoroughly study instabilities of the $Q=0$ quantum critical metals I have found it necessary to do several extensions to this model and some rather complicated calculations so I have decided to do that in a follow up work and limit this paper to introducing the framework for calculating $N_f\rightarrow0$ correlation functions. I have now added a sentence at the end of the conclusion to underline the importance of searching for instabilities.
2: "Can the improved densitydensity correlator be used to supplement previous N_f>0 methods?" The densitydensity correlator indeed gives part of the $N_f$ corrections to the boson. However, the relevant boson selfenergy contribution comes from small momenta and as we argue at the top of p. 13 (see ref. 25), all corrections beyond one loop cancel out so we can not use this result to improve on the one loop boson selfenergy. The new nonperturbative effects found in the densitydensity correlator all appear at large momentum $2k_F$ and are thus not useful for improving on the boson selfenergy.
Best regards, Petter
In this context, I would like to bring to your attention my papers: https://arxiv.org/abs/1608.01320 (Superconducting instability in nonFermi liquids) and https://arxiv.org/abs/1608.06642.
Please ignore this message if you already knew about these works, but did not find them relevant to include in the references.