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The Folded Spin1/2 XXZ Model: II. Thermodynamics and Hydrodynamics with a Minimal Set of Charges
by Lenart Zadnik, Kemal Bidzhiev, Maurizio Fagotti
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Submission summary
As Contributors:  Maurizio Fagotti · Lenart Zadnik 
Arxiv Link:  https://arxiv.org/abs/2011.01159v2 (pdf) 
Date submitted:  20201125 12:12 
Submitted by:  Zadnik, Lenart 
Submitted to:  SciPost Physics 
Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
We study the (dual) folded spin1/2 XXZ model in the thermodynamic limit. We focus, in particular, on a class of local macrostates that includes Gibbs ensembles. We develop a thermodynamic Bethe Ansatz description and work out generalised hydrodynamics at the leading order. Remarkably, in the ballistic scaling limit the junction of two local macrostates results in a discontinuity in the profile of essentially any local observable.
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Submission & Refereeing History
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Reports on this Submission
Anonymous Report 2 on 202121 Invited Report
 Cite as: Anonymous, Report on arXiv:2011.01159v2, delivered 20210201, doi: 10.21468/SciPost.Report.2491
Strengths
1 the paper is well written and present enough details about this new model;
2 the folded XXZ provides a nice example of a spin chain which is interacting with a simple but rich BetheAnsatz structure;
3 Comparison between numerics and analytics is shown with very good agreement;
4 Nontrivial discontinuous behavior identified in the stationary state emerging from the partitioning protocol.
Weaknesses
1 it is not reported how well the prediction of the folded XXZ would compare with the full XXZ at large anisotropy.
Report
In this work, the authors continue their study of the socalled "folded XXZ model" which emerges by considering an appropriate limit of the large anisotropy Heisenberg spin chain. The computation of the thermodynamic bethe ansatz and the generalised hydrodynamics (GHD) equation provide interesting results for both the equilibrium and outofequilibrium properties of the model.
As an application the authors consider the standard partitioning protocol, obtained joining two thermal states. In this case, a solution of the GHD equation is given in the ballistic limit. Interestingly enough, quite generically the profile of local observables shows a discontinuity. Such a property even appears in the YangYang entropy, which gives the stationary limit of finite interval entanglement entropy.
I believe that this is a nice and interesting work. Outofequilibrium properties of integrable systems have attracted tremendous interest in recent years and the authors have identified a nice model for which most of the BetheAnsatz technicalities have a rather simple form while still remain highly nontrivial.
Requested changes
1 In the introduction, could the author explain the difference between what they dub "folded picture" and the standard "interaction picture"?
2 Could it be sketched at least how Eq. 5 follows from Eq. 4?
3 I had some troubles understanding the notation introduced after Eq. 9 for the positions of the down spins, i.e. $2 \ell_j' + b_j$. It should be stated more clearly that $b_j \in \{0,1\}$, i.e. they are the parity $\mod(2)$ of the position index. Please state it more explicitly.
4 Could it be clarified why Eq. 109 is only a conjecture? What is the technical difficulty with the staggered magnetization?
5 There seems to be a problem with the caption in Fig. 8, where the labels (b) and (c) are exchanged.
Anonymous Report 1 on 2021110 Invited Report
 Cite as: Anonymous, Report on arXiv:2011.01159v2, delivered 20210110, doi: 10.21468/SciPost.Report.2387
Strengths
1  The overall motivation for the work, as well as main results of the manuscript, are well explained and sound.
2  Proper treatment of strong coupling regimes is often difficult and subject to uncontrolled approximations
Report
The manuscript by Zadnik et al studies properties of the folded spin1/2 XXZ model. It is a continuation of the initial work by two authors of the present manuscript to construct an effective Hamiltonian in the strong coupling limit of the model. In this work, the authors focus on the thermodynamic Bethe ansatz description of the model and on the generalized hydrodynamics description. Finally, they consider some specific examples in nonequilibrium dynamics where two semiinfinite chains are put into contact.
The overall motivation for the work, as well as main results of the manuscript, are well explained and sound. I understand that a proper treatment of strong coupling regimes is often difficult and subject to uncontrolled approximations. I am, however, not an expert in neither the Bethe ansatz technique nor in the generalized hydrodynamics. Still, I find the contribution of the authors as an interesting new development in the field of the wellstudied spin1/2 XXZ model and therefore recommend the manuscript for publication in SciPost Physics.
Dear Referee (2021110),
We appreciate your positive report on our manuscript.
Yours sincerely,
The Authors
(in reply to Report 2 on 20210201)
Dear Referee (202121),
Thank you for your positive report and helpful comments on the manuscript. We have incorporated the requested changes in the new version of the paper. Below we address them point by point:
(1) The folded formulation's main distinction from the standard interaction picture lies in the fact that it is the state rather than the operator that evolves with a timeindependent Hamiltonian; furthermore, both state and operators undergo an additional (stationary) unitary transformation. A comment has been added at the end of the paragraph below Eq. (3).
(2) The idea behind the derivation of the effective folded Hamiltonian is very simple: the time evolution with the full Heisenberg Hamiltonian is split as $e^{i H t}=e^{i H_I t}U(t;\kappa)$. Then, $U(t;\kappa)$ is expanded in the inverse coupling constant $\kappa$; the leading order reads $e^{i H_F t}$. This clarification has been incorporated in the form of a footnote.
(3) Appropriate changes have been made to the corresponding paragraph, below Eq. (9).
(4) The charges that pertain to the configuration degrees of freedom, namely $(b_1,\ldots,b_N)$, are not known, except for the staggered magnetisation. Their expectation values are not fixed by the momenta, which makes the derivation of the corresponding hydrodynamic equations out of scope. On the other hand, since in the TD limit the only relevant parameter referring to the configuration is $\mu$, we expect and conjecture all configurationrelated degrees of freedom to behave in a similar fashion.
(5) The caption has been corrected, thank you for noticing.
We express our appreciation for your comments again.
Yours sincerely,
The Authors