# Finite temperature and quench dynamics in the Transverse Field Ising Model from form factor expansions

### Submission summary

 Authors (as Contributors): Fabian Essler · Etienne Granet
Submission information
Date submitted: 2020-07-10 02:00
Submitted by: Granet, Etienne
Submitted to: SciPost Physics
Ontological classification
Specialties:
• Condensed Matter Physics - Theory
• Mathematical Physics
• Quantum Physics

### Abstract

We consider the problems of calculating the dynamical order parameter two-point function at finite temperatures and the one-point function after a quantum quench in the transverse field Ising chain. Both of these can be expressed in terms of form factor sums in the basis of physical excitations of the model. We develop a general framework for carrying out these sums based on a decomposition of form factors into partial fractions, which leads to a factorization of the multiple sums and permits them to be evaluated asymptotically. This naturally leads to systematic low density expansions. At late times these expansions can be summed to all orders by means of a determinant representation. Our method has a natural generalization to semi-local operators in interacting integrable models.

###### Current status:
Has been resubmitted

We are grateful to the three referees for their careful reading of the manuscript and helpful comments. All three referees note the significant progress made in our work in calculating (dynamical) correlation functions of semi-local operators at finite temperature and after quantum quenches and recommend publication.

In the following we respond to the various points raised.

We thank the referee for their report and constructive comments. The referee's main concern is that the presentation of our manuscript is rather technical. On the other hand, as they go on to point out, given the nature of the subject matter this is difficult to avoid. In light of the referee's comments we have made additional efforts to improve the readability of our manuscript. In particular we have added a summary of our main results at the end of the introduction, which hopefully will make it easier for readers to navigate their way through the technical parts that follow. We have also improved the text in the Appendix.

We thank the referee for their report and constructive comments. The referee did not request specific changes, but raised several questions to which we now reply in turn.

-The referee notes that ''Interestingly but unfortunately, the formalism does not seem to be extendable for calculating local operators even for integrable cases in the presence of interactions.'' and considers this a weakness of our work. Here we beg to differ: this is not a weakness, but a simply consequence of fundamental differences in the properties of Lehmann representations of local and semi-local operators. Which states dominate the spectral sums of response functions is ultimately a physical property, and these are simply different between local and semi-local operators. As is already mentioned in our manuscript, local operators will be the subject of a separate publication.

-The referee expresses ''a small confusion regarding the interpretations of the finite-T result.'' We discuss two completely separate physical problems: (i) Finite-temperature dynamical (time-dependent) spin-spin correlation functions and (ii) Time evolution of the order parameter after a quantum quench of the transverse field (from $h_0$ to $h$), where the initial state is taken to be the ground state at field $h_0$. These two unrelated problems can both be formulated in terms of spectral representations, and the calculations required to extract the dynamics turn out to be technically similar. The spectral representation in the quench case, and the resulting averaging in the steady state are based on the quench-action approach co-developed by one of us. There is no problem with applying it to problems where the initial density matrix is thermal, but we do not do this here.

• The referee wonders if the formalism can be extended to calculate (in a perturbative sense) semi-local operators for non-integrable interacting systems. We agree that this is an interesting question that should be addressed in future work. The current state of the art is the methodology developed in our manuscript, and in our view the most immediate application/generalization is to interacting integrable theories such as the Lieb-Liniger model.

We thank the referee for their report, pointing out a number of typos, and constructive comments. We have corrected the typos mentioned by the referee (as well as some others). We have followed the referee's suggestion and added an extended outline of the paper and summary of our key results at the end of the introduction. We hope this will help in making our work more accessible.

The referee comments that ''The description is so technical that it is difficult for non-experts on this subject to read.'' We are well aware of this issue. However, as the content of our paper is inherently technical we are afraid there is a limit to how accessible we can make our work to a wider audience.

### Submission & Refereeing History

Resubmission scipost_202003_00052v3 on 10 August 2020

Resubmission scipost_202003_00052v2 on 10 July 2020
Submission scipost_202003_00052v1 on 30 March 2020

## Reports on this Submission

### Report

This paper is certainly publishable in the journal.

### Requested changes

Authors have endeavoured to improve the quality of the presentation of the work. I recommend its publication in
the present from...

• validity: high
• significance: high
• originality: high
• clarity: good
• formatting: good
• grammar: perfect

### Anonymous Report 2 on 2020-8-5 (Invited Report)

• Cite as: Anonymous, Report on arXiv:scipost_202003_00052v2, delivered 2020-08-05, doi: 10.21468/SciPost.Report.1890

### Report

In this revised version, the introduction is modified considerably so that it includes the outline of the paper and the summary of the main results. Due to this modification, the manuscript is improving in presentation. I now recommend this manuscript for publication.

### Requested changes

1. With regard to eq. (14), give the definition of h and \varepsilon(x) or mention that they are defined in Section 2.
2. Correct next typos:
- 2nd line in the 2nd paragraph of Section 5.1: ‘Section 5.2’ -> ‘Section 3.2’
- Caption of Figure 16: h = 0.5 -> h = 1.5

• validity: -
• significance: -
• originality: -
• clarity: -
• formatting: -
• grammar: -

### Report

The authors have tried and partially succeeded in answering some of the questions I had. I stick to my previous recommendation of publishing the article in Sci Post.

• validity: -
• significance: -
• originality: -
• clarity: -
• formatting: -
• grammar: -