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Physical and unphysical regimes of selfconsistent manybody perturbation theory
by Kris Van Houcke, Evgeny Kozik, Riccardo Rossi, Youjin Deng, Félix Werner
Submission summary
Authors (as registered SciPost users):  Félix Werner 
Submission information  

Preprint Link:  scipost_202102_00011v3 (pdf) 
Date accepted:  20240506 
Date submitted:  20240416 08:29 
Submitted by:  Werner, Félix 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approaches:  Theoretical, Computational 
Abstract
In the standard framework of selfconsistent manybody perturbation theory, the skeleton series for the selfenergy is truncated at a finite order $N$ and plugged into the Dyson equation, which is then solved for the propagator $G_N$. We consider two examples of fermionic models, the Hubbard atom at half filling and its zero spacetime dimensional simplified version. First, we show that $G_N$ converges when $N\to\infty$ to a limit $G_\infty\,$, which coincides with the exact physical propagator $G_{\rm exact}$ at small enough coupling, while $G_\infty \neq G_{\rm exact}$ at strong coupling. This follows from the findings of [Kozik, Ferrero and Georges, PRL 114, 156402 (2015)] and an additional subtle mathematical mechanism elucidated here. Second, we demonstrate that it is possible to discriminate between the $G_\infty=G_{\rm exact}$ and $G_\infty\neq G_{\rm exact}$ regimes thanks to a criterion which does not require the knowledge of $G_{\rm exact}\,$, as proposed in [Rossi et al., PRB 93, 161102(R) (2016)].
Author comments upon resubmission
``I feel like one could improve the presentation by adding a few sentences at the end to connect the proof more explicitly with the written statements Eq.9 and Eq.10."
We implemented this suggestion by making two minor additions, listed below, and appearing in blue in the updated manuscript.
List of changes
1) We added the following sentence at the end of the appendix, to reiterate the connection with the criterion Eq.9:
``This concludes the derivation of the equality $G_\infty = G_{\rm exact}$ under the assumption (9)."
2) To connect with the simplified criterion Eq.10:
In the appendix, we prefer to stick to the derivation of the criterion Eq.9 and not to discuss Eq.10 (in accordance with the title of the Appendix and its description in the main text). Instead, we added, in the main text, the following sentence (right after Eq.10):
``Indeed, (9) implies (10), and a situation where (10) would hold while (9) would not hold seems unlikely to occur."
Current status:
Editorial decision:
For Journal SciPost Physics: Publish
(status: Editorial decision fixed and (if required) accepted by authors)