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Quantum Monte Carlo simulations in the trimer basis: firstorder transitions and thermal critical points in frustrated trilayer magnets
by L. Weber, A. Honecker, B. Normand, P. Corboz, F. Mila, S. Wessel
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Submission summary
Authors (as registered SciPost users):  Philippe Corboz · Andreas Honecker · Lukas Weber 
Submission information  

Preprint Link:  https://arxiv.org/abs/2105.05271v2 (pdf) 
Date submitted:  20210518 10:20 
Submitted by:  Weber, Lukas 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
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Abstract
The phase diagrams of highly frustrated quantum spin systems can exhibit firstorder quantum phase transitions and thermal critical points even in the absence of any longranged magnetic order. However, all unbiased numerical techniques for investigating frustrated quantum magnets face significant challenges, and for generic quantum Monte Carlo methods the challenge is the sign problem. Here we report on a quantum Monte Carlo approach that nevertheless allows us to study a frustrated model of coupled spin1/2 trimers: simulations of the trilayer Heisenberg antiferromagnet in the spintrimer basis are signproblemfree when the intertrimer couplings are fully frustrated. This model features a firstorder quantum phase transition, from which a line of firstorder transitions emerges at finite temperatures and terminates in a thermal critical point. The trimer unit cell hosts an internal degree of freedom that can be controlled to induce an extensive entropy jump at the quantum transition, which alters the shape of the firstorder line. We explore the consequences for the thermal properties in the vicinity of the critical point, which include profound changes in the lines of maxima defined by the specific heat. Our findings reveal trimer quantum magnets as fundamental systems capturing in full the complex thermal physics of the strongly frustrated regime.
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Reports on this Submission
Anonymous Report 1 on 2021720 (Invited Report)
 Cite as: Anonymous, Report on arXiv:2105.05271v2, delivered 20210720, doi: 10.21468/SciPost.Report.3264
Strengths
1. Numerical simulations are very clean and wellmade
2. Useful results to illustrate why some compounds/models could show two lines of maxima of specific heat, while some others (e.g. water) have only. Connection to recent experiments on frustrated magnets (Ref. 32).
Weaknesses
1. Unclear if the rotated Ising model discussion actually explains the scaling of the specific heat. This should be clarified.
Report
This paper describes quantum Monte Carlo simulations of a frustrated spin system, a trilateral magnet, which have for specificities to have no sign problem in a 3site (trimer) basis.
At the technical level, this is an extension of previous work (including by the authors) in Refs 2327 which were mostly concerned with bilayer systems, using a 2sites basis. There is no major breakthrough here.
Concerning physics, there is a firstorder quantum phase transition (driven by the competition between the couplings within or between trimers) similar to the one in the bilayer system, further continued at finite temperature by a line of firstorder transitions ending a critical point (of Ising type), similar to the water phase diagram. The main point of the paper is that this frustrated model has an extra tuning parameter (here one of the three couplings inside the trimer) which allows to change the slope of the firstorder line, as well as to create/enhance a second maximum in the specific heat (beyond the one existing that continues the firstorder line). This second line of maxima is not present in the water phase diagram. This is perhaps not a major physical result, but it is an interesting point that is made, specially in light of recent experiments in SrCu2(BO3)2 (Ref. 32) where the similarity between specific heat data in this frustrated magnet and water was made. This manuscript allows to rationalise (and tune) the different behaviours between models which admit one line of maxima of specific heat, and those which admit two.
The paper is very clear and otherwise well written. I think it satisfies all the criteria for publication in SciPost Physics. Below, I have only one main concern and the rest is mostly simple comments / suggestions.
Requested changes
 My main concern is the discussion on 4.3 using a “rotation” of the critical Ising model to explain the scaling behaviours of specific heat. From my understanding of this discussion, it seems that the specific heat should *always* diverge as L^{gamma / nu} for all models which admit a 2d Ising critical point in an extended phase diagram, except when the Z_2 symmetry is not explicitly broken (phi=0). This sounds a bit surprising, and I wonder whether finitesize scaling of the rotated Ising model (of the kind shown in Figure 7) actually confirm this. My concern is mostly that this perhaps true, but only very close to the critical point. I am not sure this actually explains the scaling reported in Fig 8c, where none of the data scale as expected L^\gamma/nu (neither the quartet susceptibility nor the specific heat). Can the authors report the values of the (quartet) correlation length corresponding to the data in Fig. 8 ? (I assume these must have been obtained). This would allow to see if the data presented there are actually inside the critical region. If this is not the case, I am not sure the full rotated Ising model argument is worth adding to the paper then.
 I would suggest to move the full section 3 into an Appendix, as this is mostly a technical/formal discussion. I don’t think this is a major point of the paper and I also think that quantum Monte Carlo experts (to which perhaps this discussion is aimed at) would not be too much surprised by the abstract directed loop.
 As a minor point, the value of c in Appendix A should be quoted (even if not universal).
Author: Lukas Weber on 20210817 [id 1678]
(in reply to Report 1 on 20210720)We thank the referee very much for their insightful comments. We have attached our detailed response as a pdf file.
Attachment:
response.pdf