## SciPost Submission Page

# Quantum spin systems versus Schroedinger operators: A case study in spontaneous symmetry breaking

### by C. J. F. van de Ven, G. C. Groenenboom, R. Reuvers, N. P. Landsman

### Submission summary

As Contributors: | Klaas Landsman |

Arxiv Link: | https://arxiv.org/abs/1811.12109v3 |

Date submitted: | 2019-07-31 |

Submitted by: | Landsman, Klaas |

Submitted to: | SciPost Physics |

Domain(s): | Theor. & Comp. |

Subject area: | Quantum Physics |

### Abstract

Spontaneous symmetry breaking (SSB) is mathematically tied to some limit, but must physically occur, approximately, before the limit. Approximate SSB has been independently understood for Schroedinger operators with double well potential in the classical limit (Jona-Lasinio et al, 1981; Simon, 1985) and for quantum spin systems in the thermodynamic limit (Anderson, 1952; Tasaki, 2019). We relate these to each other in the context of the Curie-Weiss model, establishing a remarkable relationship between this model (for finite N) and a discretized Schroedinger operator with double well potential.

###### Current status:

### Ontology / Topics

See full Ontology or Topics database.### Author comments upon resubmission

The minor points made by the referee are uncontroversial and have been incorporated. On the whole, our dialogue with the referee and the journal has greatly improved and clarified our paper so we repeat our gratitude, simultaneously hoping that the end has now been reached! In particular, we hope that the referee and the editor accept that there is not a single valid perspective on SSB; there is a genuine difference between the standard theoretical physics approach (which the referee follows) and the standard mathematical physics approach (which we follow), each with its own advantages, and this paper is partly meant as a bridge between the two. See also the list of changes below.

### List of changes

page 17, last line added (answers minor point by referee)

page 20, sentence after (3.36) added, following referee almost verbatim

page 28, discussion after (5.26) rephrased to clarify that there are two different points of view on SSB, where we explain why we favour ours (which is the standard one in mathematical physics), upon which the entirely new paragraph in lines 9-19 on page 29 relates the two approaches again, in a way which seems to clarify the situation, leading to peaceful coexistence, or so we hope.

Reference 17 added (Note that a huge addition to the bibliography has already been made in version 2; this one was added to support our new footnote 22, see above)

### Submission & Refereeing History

## Reports on this Submission

### Report 1 by Jasper van Wezel on 2019-8-16 Invited Report

### Report

I thank the authors for the added discussions.

The new paragraphs do clarify the difference between the CM and MathPhys literature on this topic, and thus alert the reader to look into this further. However, they do not add any arguments to the discussion between the author and myself, and I am not convinced that any of my previous objections were invalid.

I also do not have any new arguments myself, beyond what I already wrote in my previous report. I therefore recommend that the editor decides whether or not they are happy to publish, taking into account the differences of opinion and arguments in the already available reports.

----

To the authors: I would like to clarify one thing - that my earlier points 1b and 2 do not contradict each other.

1b points out that the approach of the "flea" advocated by the authors is the same as that of a "symmery-breaking field" which is standard in the CM literature.

2 points out that one does have to choose an appropriate symmetry-breaking field, and that the authors have not at all proven their perturbation to remain appropriate all the way to the thermodynamic limit.

(in reply to Report 1 by Jasper van Wezel on 2019-08-16)

Hi, We agree to now leave this to the editor and emphasize that we have followed the previous editorial recommendation to (re)submit a minor revision. On the one hand, the dialogue with the referee has improved the paper almost beyond recognition, but on the other hand we regret that no complete agreement has been reached. Apparently, the subject lends itself to various interpretations, like works of modern art. We do believe that we answered all points raised by the referee and our previous replies explain in considerable detail (or at least intended to explain in detail) how we did so.

In reply to the last two points made by the referee we would like to point out that: 1) Apparently no perturbation is regarded as novel (by the referee) as long as it falls within the scope of general symmetry-breaking terms. Of course our flea perturbation falls within that scope - the novelty (or so we think) lies in its detailed mathematical form. We are not P.W. Anderson and we emphasized that this novelty is limited (but in our view still very interesting) from the start. 2) It is true that our evidence for symmetry breaking triggered by our perturbation is numerical and of course these were carried out for finite N, and hence not "all the way to the thermodynamic limit". However, the totally analogous flea perturbations for the double well potential have been rigorously proven, in the papers we cite throughout ours, viz. refs. [20], [15], and [40], to do this job for hbar -> 0, and since our perturbation is obtained from that one through the mapping explained in chapter 3, under the substitution hbar -> 1/N, we in fact collect two kinds of evidence for its validity. Hence we take issue with the claim that we "have not

at allproven [our] perturbation to remain appropriate all the way to the thermodynamic limit". Since our mapping is itself only defined for finite N, we would agree that we have not strictlyprovedthis (i.e. analytically), but inserting "at all" wrongly suggests that we have no evidence at all (sic). See especially page 26.