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Many Phases of Generalized 3D Instanton Crystals
by Matti Jarvinen, Vadim Kaplunovsky, Jacob Sonnenschein
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Authors (as registered SciPost users):  Matti Jarvinen 
Submission information  

Preprint Link:  https://arxiv.org/abs/2011.05338v2 (pdf) 
Date submitted:  20210216 18:30 
Submitted by:  Jarvinen, Matti 
Submitted to:  SciPost Physics 
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Academic field:  Physics 
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Approaches:  Theoretical, Computational 
Abstract
Nuclear matter at large number of colors is necessarily in a solid phase. In particular holographic nuclear matter takes the form of a crystal of instantons of the flavor group. In this article we initiate the analysis of the threedimensional crystal structures and the orientation patterns for the twobody potential that follows from holographic duality. The outcome of the analysis includes several unexpected results. We perform simulations of ensembles of O(10000) instantons whereby we identify the lattice structure and orientations for the different values of the weight factors of the nonAbelian orientation terms in the twobody potential. The resulting phase diagram is surprisingly complex, including a variety of ferromagnetic and antiferromagnetic crystals with various global orientation patterns, and various "nonAbelian" crystals where orientations have no preferred direction. The latter include variants of facecenteredcubic, hexagonal, and simple cubic crystals which may have remarkably large or small aspect ratios. The simulation results are augmented by analytic analysis of the longdistance divergences, and numerical computation of the (divergence free) energy differences between the nonAbelian crystals, which allows us to precisely determine the structure of the phase diagram.
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Reports on this Submission
Report #2 by Anonymous (Referee 2) on 2021427 (Invited Report)
 Cite as: Anonymous, Report on arXiv:2011.05338v2, delivered 20210427, doi: 10.21468/SciPost.Report.2848
Report
The authors consider the holographic description of nuclear matter as an ensemble of pointlike instantons interacting with a pair interaction Eq. 1, that depends on the instanton relative distances and "flavor" orientations, but is essentially repulsive. The authors suggest to parametrize the flavor dependent and repulsive part using two positive parameters alpha and beta. Their numerical studies show rich lattice structures for varying alpha and beta. For alpha=beta=0 the canonical structure is fcc.
1/ Originally, Eq. 1 describes instantons interactions in 3spatial dimensions plus 1holographic dimension, leading to 4dimensional crystal arrangements. However, the authors consider only 3spatial dimensions. This important simplification should be explained.
2/ The long range sensitivity of the pair interaction in Eq. 1 noted by the authors, is naturally cutoff in nuclear matter by the the pion mass. Eq. 1 falls short of the long range pion attraction as the authors say in their introduction, and also is in the chiral limit. So clearly, without any external force the system will fly apart. To fix this, the authors implement a meanfieldlike "attractive" force. The energetics of their crystalline structures depend on this meanfield, making certain structures energetically unfavorable in comparison to others. The authors should give a better understanding of these points.
Overall, the paper ideas and objectives are well stated. The results are interesting on their own for an ensemble of gauge quasiparticles pair interacting via Eq. 1 and worth publishing.
Report #1 by Anonymous (Referee 1) on 2021315 (Invited Report)
 Cite as: Anonymous, Report on arXiv:2011.05338v2, delivered 20210315, doi: 10.21468/SciPost.Report.2699
Report
This paper discusses 3d instanton crystals for the purpose of studying the phase structures of nuclear matter. The paper performs a lattice simulation to handle a huge number of instantons to find out the lowest energy configurations. Although the reviewer doesn't make a full understanding of the details of the numerical computation, he finds that all the results are stated in a very wellorganized and transparent manner. He thus recommends this paper to be published in SciPost. Before this, however, he asks the authors to add more comments or explanations about the following points, which he believes will help to improve the clarity of the paper.
(1) The paper takes into account only the twobody potential when analyzing the instanton crystals. This approximation seems valid when nucleons are wellseparated. On the other hand, the twobody potential was derived originally for the short distance limit of two nucleons. Is there any common parameter regime where these approximations are consistent with each other?
(2) It would be preferable to discuss the physical meaning of the two parameters that appear in the generalized twobody potential.
(3) The paper discovers a phase such as the antiferromaganetic one, which exhibits a global spherical structure. It would be better to clarify if this is due to the boundary effect in the lattice simulation or not. The paper claims to observe a very strong tendency to converge to it. What are the implications the authors extract from these observations?
On top of these points, he reminds the authors that the layout of the pdf file is broken in his own PC environment(Mac+Preview), when he looks on the Figures from one to four.
Author: Matti Jarvinen on 20210427 [id 1386]
(in reply to Report 1 on 20210315)
We thank the referee for careful reading of the manuscript and for the useful comments, and apologize for the slow response.
We will clarify the discussion of the numerical details in the second version of the manuscript.
Our response to the specific issues:
(1) The two body potential is motivated by the analysis in the WittenSakaiSugimoto model. In this context there is indeed a parametrically large range where our expression for the potential is valid. This requires that the separation $d$ between the instantons satisfies $ 1/(M\sqrt{\lambda}) \ll d \ll 1/M$ where $\lambda$ is the coupling constant and $M$ is the scale of strong interactions. The lower limit arises from the instanton size and the upper limit from neglecting curvature corrections in the AdS geometry. In the strong coupling limit $\lambda \to \infty$, where the classical dual description is valid, these two scales are therefore well separated. This is discussed in detail in ref. [15] of the manuscript.
We notice that this point is not explained in the current version of the manuscript. We suggest to add an explanation in section 2.2 where the twobody potential is introduced.
(2) We agree that there should be more discussion of the interpretation of the parameters. Basic properties of the potential can be read from the twobody interaction term, eq. (8). The $\alpha$ coefficient multiplies a term where the dependence on orientation is independent of the spatial interactions. For positive (negative) $\alpha$ perpendicular (parallel) spins of nearby neighbors are preferred, and the effect increases with increasing $\alpha$. The interaction involving $\beta$ is a bit more complicated since there is nontrivial coupling between the orientations and directions in coordinate space. Picking an instanton with unit orientation $y=1$, positive (negative) $\beta$ means that orientation $y$ of a neighboring instanton which is perpendicular (parallel) to the spatial link between the two instantons is preferred (with the understanding that e.g. the orientation $y=i$ is parallel to the $x$ axis in coordinate space).
There are some scattered comments in sections 3 and 4 about the interactions but we plan to add a more detailed discussion in section 2.2. We will also comment how the final results for the phase diagram reflect the structure of the interaction term in section 5.
(3) The presence of the spherical phases is indeed a nontrivial question. We have carried out various checks (see below) some of which are mentioned in the manuscript. Based on them, we are confident that the spherical configurations are dominant over "regular" crystals in the regions indicated in figure 8. In particular, our analytic findings (which are independent of the boundary effects) agree to a great degree with the simulation results.
However it might happen that the true ground state in the continuum limit is some other, even more complicated structure (which we do not see in our simulations due to finite size/boundary effects)  this we do not know. But even in with the largest volumes we could with relative ease simulate on a personal computer (about 40000 instantons), we did not see signs of any other structures. In addition we have checked that the spherical antiferromagnetic structure is insensitive to the shape of the cavity where we carry out the simulations. We have tried simulating i.e. in a cubic cavity and still found configurations which are clearly spherical near the center of the simulation result.
The strong tendency to form spherical structures for $\beta>\alpha>0$ seen in the simulations (the simulations converge to the final state more than 10 times faster than for $\alpha>\beta>0$) may mean that such configurations have a large energy gap with respect to regular crystals. This is in part confirmed by the IR analysis of the configurations.
We suggest to add more discussion of these points in section 3.4.
We also thank the referee for pointing out the issue with Preview. However, we did not observe issues in our own tests with this viewer, so we cannot fix the problem. (The 3D content in the figures is not available on Preview, and to our knowledge it only works with Adobe Reader.)
For the authors,
Matti Jarvinen
Anonymous on 20210430 [id 1395]
(in reply to Matti Jarvinen on 20210427 [id 1386])The reviewer agrees the revised version to be published with no further revision.
Author: Matti Jarvinen on 20210621 [id 1513]
(in reply to Report 2 on 20210427)We thank the referee for report and for the useful comments. Our response:
1/ This was not explained well in the first version of the manuscript and we thank the referee for pointing this out. Indeed also four dimensional crystals are possible. However at low density, one expects that the solitons are confined to a plane in the holographic direction so that the only relevant dimensions are the spatial ones. As the density increases, we expect a "popcorn" transition from three dimensional to four dimensional crystals, similar to the one to two dimensional transition considered, for example, in Ref. [15]. Studying the four dimensional crystals and the transition to them would be an interesting (but extremely challenging) topic for future research. There is a parametrically large region of densities where the three dimensional crystals can be embedded in the WittenSakaiSugimoto setup. We have clarified the setup by adding comments on the 3D to 4D transition in the introduction on page 3 before Eq. (1) and in the beginning of Sec. 2.
2/ Indeed due to the long range sensitivity of the twobody interaction, we add an additional external force in our simulations. In the probably most interesting cases (i.e. the nonAbelian crystals) the external force is however welldefined: there is only a single choice which makes sense. This choice is the external force due to an unoriented continuum of instantons outside the cavity of the simulation. We stress that this is not an adhoc force but added because of the setup that we want to consider: in principle we want to consider an infinite sample, but due to limited resources, we can only simulate a relatively small bubble. The external force is then the mean field force due to all those instantons which were left out of the simulation due to this limitation. For the nonAbelian (and also for the global ferromagnetic) crystals this external force also leads to crystals having constant instanton density (approximated over a large enough volume), and we see no other reasonable choice for the force in this case.
For the oriented (spherical antiferromagnetic or ferromagnetic) phases, the situation is a bit more complicated. This is because the force which leads to constant instanton density is not the same as that obtained by averaging over an (anti)ferromagnetic continuum of instantons outside the cavity of simulations. Here we have chosen the external force such that the averaged instanton density is constant. This makes it possible to carry out the IR analysis of section 4. However we have also tried simulating with other choices of the eternal forces (these attempts are not documented in detail in the article for brevity), and did not see any significant changes in the results for the crystal structures locally. Globally, there were some changes, as the density of the instanton varies are one easily develops even empty regions in the middle of the cavity.
Notice that one can also do simulation without external force, even though this is presumably less well physically motivated. In this case one some of the solitons first cluster at the surface of the simulation volume, and then one obtains crystal formation in the middle. Even though we have not studied such configurations in detail, we expect that the crystal structures would be the same even in this case as with the simulations with the additional external force.
In summary, we do not expect that any of our main results are sensitive to the choices for the external force.
We have improved the discussion of the external force in the introduction (page 4, the first item (i) of the list and discussion before it), in the first paragraph of Sec. 3, and at the end of Sec 3.4. We now point out more clearly that the external force for the nonAbelian case appears to be essentially unique, and have added references to Appendix A.2, where a detailed discussion of the forces is given.
For the authors, Matti Jarvinen