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Exact correlations in the LiebLiniger model and detailed balance outofequilibrium
by Jacopo De Nardis, Miłosz Panfil
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Submission summary
Authors (as registered SciPost users):  Milosz Panfil 
Submission information  

Preprint Link:  https://arxiv.org/abs/1611.00194v2 (pdf) 
Date submitted:  20161108 01:00 
Submitted by:  Panfil, Milosz 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
We study the densitydensity correlation function of the 1D LiebLiniger model and obtain an exact expression for the small momentum limit of the static correlator in the thermodynamic limit. We achieve this by summing exactly over the relevant form factors of the density operator in the small momentum limit. The result is valid for any eigenstate, including thermal and nonthermal states. We also show that the small momentum limit of the dynamic structure factors obeys a generalized detailed balance relation valid for any equilibrium state.
Current status:
Reports on this Submission
Anonymous Report 1 on 2016123 (Invited Report)
 Cite as: Anonymous, Report on arXiv:1611.00194v2, delivered 20161203, doi: 10.21468/SciPost.Report.46
Strengths
1. Very few results on GGE correlators exist. The paper is charting a new territory, with no landmarks;
2. Techniques used are difficult and demanding;
3. Given all the advances in quantum nonequilibrium dynamics, The subject is currently empirically relevant.
Weaknesses
1. No interpretation of the principal result is attempted. A hint on why the static structure factor of a quenched BEC does not depend on the interaction strength after quench, or, at least a discussion on other examples of results of such simplicity;
Report
This is an important paper, and it deserves publishing. The subject is correlation functions in representative eigenstates of the GGE ensemble, the correlators that produce values of observables in an integrable system after a relaxation form a nonequilibrium initial state. Very few results exist, all of them covering the onepoint correlators. The manuscript in question is, to my knowledge, the first attempt to approach the twopoint correlation functions. The object computed is the the infrared limit of the static structure factor, expressed through the rapidity distribution.
In an important particular case, a quench from a BEC, the commutation is performed all the way to an explicit result.
Results are important. In particular, they allow to predict the speed of sound in highly nonequilibrium integrable gases, something that can be measured. Furthermore, the techniques developed do apply to spin systems. In general, the paper is a valuable contribution to the field of quantum nonequilibrium dynamics.
Requested changes
Besides a few trivial suggestions listed below, I have one that is difficult to implement.
1.1. The $n/2$ authors obtain for the zeromomentum static structure factor of a BEC, independently of the interaction strength, suggests a deed fundamental interpretation. A minimal discussion is in order. The very least, the sign of the effect must be discussed: one would naively expect that thanks to the interatomic repulsion, the structure factor would go down from the its zero value at the noninteracting BEC. Yet, it goes up.
Minor suggestions:
2.1. Using the same letter for both $S(x,\, t)$ and $S(k)$ may be misleading;
2.2. $\Big_{\{\mu_i\}_i}$ is extremely misleading. Should be something like $\Big_{\mbox{other Lagrange multipliers}}$;
2.3. Presence of $T$ in the definition (7), but absence of any Lagrange multipliers in (9) looks like an inconsistency;
2.4. Around Eq. 11, some discussion of the relationship between the notion of the "detailed balance" used and the common
notion notion of the "detailed balance", that implies absence of loops in the probability/matter transfer in a steady state, is needed;
2.5. Fig. 1 is never referenced;
2.6. The order in which energy and momentum appear In Eq.15 is different from the one in the preceding sentence;
2.7. In Eq. 25, energy is defined as a momentum and momentum as energy;
2.8. Around Eq. 27, it should be mentioned, that $m$ stands for the number of the particlehole excitations, it has never been defined;
2.9. Some interpretation of the function $F(\lambdap,\,h)$ is needed. Is it the deformation of the rapidity distribution due to a particlehole excitation?