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LiebSchultzMattis, Luttinger, and 't Hooft  anomaly matching in lattice systems
by Meng Cheng, Nathan Seiberg
Submission summary
Authors (as Contributors):  Meng Cheng · Nathan Seiberg 
Submission information  

Arxiv Link:  https://arxiv.org/abs/2211.12543v2 (pdf) 
Date submitted:  20230110 15:51 
Submitted by:  Cheng, Meng 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
We analyze lattice Hamiltonian systems whose global symmetries have 't Hooft anomalies. As is common in the study of anomalies, they are probed by coupling the system to classical background gauge fields. For flat fields (vanishing field strength), the nonzero spatial components of the gauge fields can be thought of as twisted boundary conditions, or equivalently, as topological defects. The symmetries of the twisted Hilbert space and their representations capture the anomalies. We demonstrate this approach with a number of examples. In some of them, the anomalous symmetries are internal symmetries of the lattice system, but they do not act onsite. (We clarify the notion of "onsite action.") In other cases, the anomalous symmetries involve lattice translations. Using this approach we frame many known and new results in a unified fashion. In this work, we limit ourselves to 1+1d systems with a spatial lattice. In particular, we present a lattice system that flows to the $c=1$ compact boson system with any radius (no BKT transition) with the full internal symmetry of the continuum theory, with its anomalies and its Tduality. As another application, we analyze various spin chain models and phrase their LiebShultzMattis theorem as an 't Hooft anomaly matching condition. We also show in what sense filling constraints like Luttinger theorem can and cannot be viewed as reflecting an anomaly. As a byproduct, our understanding allows us to use information from the continuum theory to derive some exact results in lattice model of interest, such as the lattice momenta of the lowenergy states.
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Reports on this Submission
Anonymous Report 1 on 2023211 (Invited Report)
Strengths
1. The authors develop a very detailed theory of anomalies in lattice systems with translation symmetry. I certainly think that there is much value and conceptual insight to be gained in formulating these ideas precisely, and they discuss many examples in detail.
2. The results on finitesize systems, for example the relationship between the momentum of the ground state and the anomalies, are particularly welcome and I think totally new.
Weaknesses
1. Regarding section 5: From various perspectives on LSM,it is clear that for a general internal symmetry group G, there should be an "anomaly" whenever the translation unit cell transforms projectively under G. However, this seems to be far from manifest from the perspectives described here. It's clear enough if G is Abelian  then you can just read off the projective representation of a unit cell from eq. (5.4). [Indeed it's a wellknown result about the cohomology of finite Abelian groups with U(1) coefficients that if phi is a 2cocycle, then phi(g,h)  phi(h,g) contains all the information about the cohomology class of phi]. But the situation for nonAbelian G seems much less clear, especially due to the restriction that h \in C_g.
2. Throughout the paper, the authors are careful to consider systems with a finite (but large) system size L. This allows them to consider interesting properties of the finitesize systems, e.g. the momenta of the lowlying states. However, in condensed matter physics we are often interested only in the thermodynamic limit. I wonder whether there is a clean way to take the L>infty limit in order to simplify the formalism described, or if worrying about the details of finiteL is in fact unavoidable?
Report
For the reasons described above, I would recommend acceptance, subject to the authors responding to the points in the "Weaknesses" and "Requested changes" sections of this report.
Requested changes
1. In Section I, regarding the sentence "The modern view of these anomalies involves coupling the system to classical background gauge fields for these symmetries, placing the system on a closed Euclidean spacetime, e.g., a torus, and studying the partition function"
Although this is *one* way to think about anomalies, it is certainly not the *only* way as the authors seem to be implying here. For example for local anomalies (which includes at least some of the anomalies discussed in the present work) one can talk about nonconservation of charge in response to background gauge fields, or noncommutation of the local density operators. Even for anomalies of discrete symmetries there are ways to formulate them in a Hilbert space / Hamiltonian language without having to talk about partition functions.
2. In Section 1: "This picture of anomalies assumes a continuous and Lorentz invariant spacetime".
I do not believe that Lorentz invariance plays any essential role in discussing anomalies in continuum theories.
3. In Section 5.5, I can see why for sufficiently large L, there should be an exact agreement with the continuum theory regarding the momentum of the lowlying states. However for very small values of L, isn't the agreement that the authors found in this particular model merely a coincidence, or curiosity? For a generic Hamiltonian, with extremely small system sizes like L=2, there is no reason to think that the eigenstates of the Hamiltonian will have anything to do with the continuum theory.
4. The terminology and notation in section 2 was a bit confusing:
(a) In the general setup in section 2, are the authors already assuming that the internal symmetries are onsite? Otherwise it seems there may be some ambiguities in how to introduce the twist defects.
(b) In Section 2.3.2, the sentence: "Consider first the symmetry operators associated with the internal symmetry G and denote them by h \in G". Since the authors are talking about the symmetry operators in the twisted theory here, shouldn't they already be calling them "h(g)"?
(c) On the lattice, for internal onsite symmetries, is it not the case that h(g) = h(1) anyway, if h is in the centralizer of g?'
(d) In the paragraph below (2.10), the sentence "We will denote these operators as h(g)", it is unclear what exactly "these operators" is referring to.