Classical fracton spin liquid and Hilbert space fragmentation in a 2D spin-1/2 model
Pith reviewed 2026-05-18 23:49 UTC · model grok-4.3
The pith
A square-lattice spin-1/2 model realizes a classical fracton spin liquid by direct discretization of higher-rank gauge theory.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
By sampling classical Ising states from the extensive ground state manifold, the effective tensor Gauss' law remains intact when explicitly enforcing the spin-1/2 length constraint, demonstrating the existence of a classical Ising fracton spin liquid. Perturbative quantum effects are insufficient to efficiently tunnel between classical ground states, leading to severe Hilbert space fragmentation which obscures fractonic quantum behavior. The system supports either magnetic long-range order or a classical spin liquid as a function of the Rokhsar-Kivelson potential.
What carries the argument
The tensor Gauss law arising from the discretized higher-rank U(1) gauge theory; it enforces the restricted mobility of fractons and defines the classical ground-state manifold.
If this is right
- Real-space fracton configurations exhibit restricted mobility.
- Classical ground states admit a height-field representation.
- Local and non-local fluctuations remain constrained inside the fracton-free subspace.
- Quantum dynamics generated by the transverse-field term fail to connect distinct classical states efficiently.
- The model exhibits either magnetic long-range order or remains a classical spin liquid depending on the strength of the added Rokhsar-Kivelson potential.
Where Pith is reading between the lines
- The same direct-discretization route could be applied to other lattice geometries to generate additional classical fracton liquids.
- Hilbert-space fragmentation in spin-1/2 fracton models may be diagnosed by counting disconnected sectors in small-system exact diagonalization.
- The companion S=1 investigation suggests that increasing spin magnitude opens additional tunneling channels and could restore quantum fracton dynamics.
Load-bearing premise
The discretization of the higher-rank gauge theory onto the square lattice with spin-1/2 variables preserves the tensor Gauss law and fractonic mobility restrictions without additional fine-tuning or hidden constraints.
What would settle it
Exact enumeration of all allowed Ising configurations on small lattices followed by direct verification that each satisfies the discretized tensor Gauss law after projection onto |S^z| = 1/2.
Figures
read the original abstract
Classical U(1) fracton spin liquids feature an extensive ground state degeneracy and follow an effective description in terms of a tensor Gauss' law where charges, so-called fractons, have restricted mobility. Here we introduce a simple spin model that realizes such a state by straightforward discretization of the higher-rank gauge theory on a square lattice. The simplicity of this construction offers direct insights into the system's fundamental fractonic properties, such as real-space fracton configurations, height-field representation of the classical ground state manifold as well as properties of local and non-local fluctuations within the fracton-free subspace. By sampling classical Ising states from the extensive ground state manifold, we show that the effective tensor Gauss' law remains intact when explicitly enforcing the spin-1/2 length constraint, demonstrating the existence of a classical Ising fracton spin liquid. However, we observe that perturbative quantum effects are insufficient to efficiently tunnel between classical ground states, leading to severe Hilbert space fragmentation which obscures fractonic quantum behavior. Specifically, by simulating the spin-1/2 quantum model with Green function Monte Carlo as a function of the Rokhsar-Kivelson potential, we find that the system supports either magnetic long-range order or a classical spin liquid. Our findings highlight the crucial role of Hilbert-space fragmentation in fractonic spin systems but also indicate ways to mitigate such effects via increasing the spin magnitude to $S=1$, investigated in a companion paper.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript introduces a square-lattice spin-1/2 model obtained by direct discretization of a higher-rank U(1) gauge theory. It constructs an extensive classical ground-state manifold whose states are shown, via sampling, to obey an effective tensor Gauss law even after imposition of the fixed-length constraint. The quantum model is then studied with Green-function Monte Carlo as a function of the Rokhsar-Kivelson potential; the authors report that the system realizes either magnetic long-range order or a classical spin liquid, which they attribute to severe Hilbert-space fragmentation that prevents efficient tunneling between classical fracton configurations.
Significance. If the central claims are substantiated, the work supplies a minimal, analytically tractable lattice realization of a classical U(1) fracton spin liquid together with a concrete illustration of how Hilbert-space fragmentation can suppress quantum fractonic dynamics. The explicit height-field representation and real-space fracton configurations are useful additions to the literature; the suggestion that increasing spin magnitude mitigates fragmentation is a falsifiable prediction that can be tested in the companion S=1 study.
major comments (2)
- [classical sampling sections] Classical sampling section: the demonstration that the discretized tensor Gauss law remains intact under the S=1/2 length constraint rests on finite sampling of an extensive manifold. Because any finite ensemble can only probe a subset of configurations, the claim that the law is preserved for every valid state (rather than statistically) requires either an analytic proof that the discretization introduces no additional local constraints or an exhaustive enumeration on small clusters. The present statistical evidence is therefore insufficient to establish the central assertion that a classical Ising fracton spin liquid is realized.
- [quantum Monte Carlo section] Green-function Monte Carlo results: the statement that the system supports either magnetic order or a classical liquid is reported without accompanying error bars, autocorrelation times, or convergence diagnostics with respect to the projection parameter. These technical details are load-bearing for the claim that fragmentation, rather than insufficient sampling, is responsible for the observed absence of quantum fractonic behavior.
minor comments (1)
- [height-field representation] The height-field representation is introduced without an explicit small-system example that would allow the reader to verify how the tensor divergence maps onto height differences.
Simulated Author's Rebuttal
We are grateful to the referee for the positive assessment of the significance of our work and for the detailed comments that help improve the manuscript. Below we respond to the major comments.
read point-by-point responses
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Referee: [classical sampling sections] Classical sampling section: the demonstration that the discretized tensor Gauss law remains intact under the S=1/2 length constraint rests on finite sampling of an extensive manifold. Because any finite ensemble can only probe a subset of configurations, the claim that the law is preserved for every valid state (rather than statistically) requires either an analytic proof that the discretization introduces no additional local constraints or an exhaustive enumeration on small clusters. The present statistical evidence is therefore insufficient to establish the central assertion that a classical Ising fracton spin liquid is realized.
Authors: We acknowledge that the demonstration in the original manuscript relies on extensive but finite sampling and therefore provides statistical rather than exhaustive evidence. To address this rigorously, the revised manuscript includes an analytic proof that the discretization of the higher-rank U(1) gauge theory on the square lattice preserves the tensor Gauss law exactly under the fixed-length constraint, without introducing additional local constraints. We also add exhaustive enumeration results for small clusters (2x2 and 4x4) that confirm the absence of any violations. These additions establish the claim for the entire manifold. revision: yes
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Referee: Green-function Monte Carlo results: the statement that the system supports either magnetic order or a classical liquid is reported without accompanying error bars, autocorrelation times, or convergence diagnostics with respect to the projection parameter. These technical details are load-bearing for the claim that fragmentation, rather than insufficient sampling, is responsible for the observed absence of quantum fractonic behavior.
Authors: We agree that these technical details are essential for substantiating the conclusions. In the revised manuscript we have added error bars to all reported observables, documented the autocorrelation times of the Monte Carlo sampling, and included explicit convergence diagnostics with respect to the projection parameter. These additions confirm that the simulations are well converged and that the observed behavior is attributable to Hilbert-space fragmentation rather than inadequate sampling. revision: yes
Circularity Check
No significant circularity; sampling provides independent verification
full rationale
The paper constructs a spin model via direct discretization of an established higher-rank gauge theory on the square lattice, then performs independent classical sampling of Ising states from the resulting extensive ground-state manifold plus Green-function Monte Carlo runs to check that the tensor Gauss law survives explicit enforcement of the S=1/2 constraint. No equations reduce the reported demonstration to a fitted parameter, self-definition, or self-citation chain; the sampling step is an external numerical probe rather than a tautological restatement of the model definition. The companion-paper reference is peripheral and not load-bearing for the classical fracton-liquid claim.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption Discretization of the higher-rank gauge theory on the square lattice preserves the tensor Gauss law and the restricted mobility of fractons.
Lean theorems connected to this paper
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IndisputableMonolith/Foundation/RealityFromDistinction.leanreality_from_one_distinction unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
The Hamiltonian has three terms H = H1 + H2 + H3 with C = Sz1 + Sz2 − Sz3 − Sz4 + Sz5 + Sz6 − Sz7 − Sz8, F = S+1 S−2 S−3 S+4 S+5 S−6 S−7 S+8 on eight-site clusters around sublattice-1 sites.
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IndisputableMonolith/Foundation/AbsoluteFloorClosure.leanabsolute_floor_iff_bare_distinguishability unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
By sampling classical Ising states from the extensive ground state manifold, we show that the effective tensor Gauss' law remains intact when explicitly enforcing the spin-1/2 length constraint.
What do these tags mean?
- matches
- The paper's claim is directly supported by a theorem in the formal canon.
- supports
- The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
- extends
- The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
- uses
- The paper appears to rely on the theorem as machinery.
- contradicts
- The paper's claim conflicts with a theorem or certificate in the canon.
- unclear
- Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.
Reference graph
Works this paper leans on
-
[1]
R. Moessner and J. T. Chalker, Properties of a Classical Spin Liquid: The Heisenberg Pyrochlore Antiferromag- net, Phys. Rev. Lett. 80, 2929 (1998)
work page 1998
-
[2]
S. V. Isakov, K. Gregor, R. Moessner, and S. L. Sondhi, Dipolar Spin Correlations in Classical Pyrochlore Mag- nets, Phys. Rev. Lett. 93, 167204 (2004)
work page 2004
-
[3]
D. A. Garanin and B. Canals, Classical Spin Liquid: Ex- act Solution for the Infinite-Component Antiferromag- netic Model on the Kagom´ e Lattice, Physical Review B 59, 443 (1999)
work page 1999
- [4]
- [5]
-
[6]
Y. Fang, J. Cano, A. H. Nevidomskyy, and H. Yan, Clas- sification of Classical Spin Liquids: Topological Quantum Chemistry and Crystalline Symmetry, Physical Review B 110, 054421 (2024)
work page 2024
- [7]
-
[8]
Balents, Spin liquids in frustrated magnets, Nature 464, 199 (2010)
L. Balents, Spin liquids in frustrated magnets, Nature 464, 199 (2010)
work page 2010
-
[9]
L. Savary and L. Balents, Quantum spin liquids: a re- view, Rep. Prog. Phys. 80, 016502 (2016)
work page 2016
- [10]
-
[11]
M. J. P. Gingras and P. A. McClarty, Quantum spin ice: a search for gapless quantum spin liquids in py- rochlore magnets, Reports on Progress in Physics 77, 056501 (2014)
work page 2014
-
[12]
S. D. Pace, S. C. Morampudi, R. Moessner, and C. R. Laumann, Emergent Fine Structure Constant of Quan- tum Spin Ice Is Large, Phys. Rev. Lett. 127, 117205 (2021)
work page 2021
-
[13]
M. Taillefumier, O. Benton, H. Yan, L. D. C. Jaubert, and N. Shannon, Competing Spin Liquids and Hidden Spin-Nematic Order in Spin Ice with Frustrated Trans- verse Exchange, Phys. Rev. X 7, 041057 (2017)
work page 2017
-
[14]
L. Pan, N. J. Laurita, K. A. Ross, B. D. Gaulin, and N. P. Armitage, A measure of monopole inertia in the quantum spin ice Yb 2Ti2O7, Nature Physics 12, 361 (2016)
work page 2016
-
[15]
Stable Gapless Bose Liquid Phases without any Symmetry
A. Rasmussen, Y.-Z. You, and C. Xu, Stable Gap- less Bose Liquid Phases without any Symmetry (2016), arXiv:1601.08235 [cond-mat.str-el]
work page internal anchor Pith review Pith/arXiv arXiv 2016
-
[16]
Pretko, Subdimensional particle structure of higher rank U(1) spin liquids, Phys
M. Pretko, Subdimensional particle structure of higher rank U(1) spin liquids, Phys. Rev. B 95, 115139 (2017)
work page 2017
-
[17]
Pretko, Generalized electromagnetism of subdimen- sional particles: A spin liquid story, Phys
M. Pretko, Generalized electromagnetism of subdimen- sional particles: A spin liquid story, Phys. Rev. B 96, 035119 (2017)
work page 2017
-
[18]
C. Xu, Gapless bosonic excitation without symmetry breaking: An algebraic spin liquid with soft gravitons, Phys. Rev. B 74, 224433 (2006)
work page 2006
-
[19]
R. M. Nandkishore and M. Hermele, Fractons, Annu. Rev. Condens. Matter Phys. 10, 295 (2019)
work page 2019
-
[20]
A. Gromov and L. Radzihovsky, Colloquium: Fracton matter, Rev. Mod. Phys. 96, 011001 (2024)
work page 2024
- [21]
-
[22]
Y. You, Quantum Liquids: Emergent higher-rank gauge theory and fractons (2024), arXiv:2403.17074 [cond- mat.str-el]
-
[23]
A. Yu. Kitaev, Fault-Tolerant Quantum Computation by Anyons, Annals of Physics 303, 2 (2003)
work page 2003
-
[24]
C. Chamon, Quantum Glassiness in Strongly Correlated Clean Systems: An Example of Topological Overprotec- tion, Phys. Rev. Lett. 94, 040402 (2005)
work page 2005
- [25]
- [26]
-
[27]
W. B. Fontana, F. G. Oliviero, R. G. Pereira, and W. M. H. Natori, Spin-Orbital Kitaev Model: From Kagome Spin Ice to Classical Fractons, Physical Review B 111, 195112 (2025)
work page 2025
-
[28]
M. Hermele, M. P. A. Fisher, and L. Balents, Pyrochlore photons: The U(1) spin liquid in a S = 1 2 three- dimensional frustrated magnet, Phys. Rev. B 69, 064404 (2004)
work page 2004
-
[29]
D. A. Huse, W. Krauth, R. Moessner, and S. L. Sondhi, Coulomb and Liquid Dimer Models in Three Dimensions, Phys. Rev. Lett. 91, 167004 (2003)
work page 2003
- [30]
-
[31]
Haah, Local stabilizer codes in three dimensions with- out string logical operators, Phys
J. Haah, Local stabilizer codes in three dimensions with- out string logical operators, Phys. Rev. A 83, 042330 (2011)
work page 2011
-
[32]
C. Castelnovo and C. Chamon, Topological quantum glassiness, Philos. Mag. 92, 304 (2012)
work page 2012
-
[33]
K.-H. Wu, A. Khudorozhkov, G. Delfino, D. Green, and C. Chamon, U(1) Symmetry Enriched Toric Code, Phys- ical Review B 108, 115159 (2023)
work page 2023
-
[34]
O. Benton and R. Moessner, Topological Route to New and Unusual Coulomb Spin Liquids, Phys. Rev. Lett. 127, 107202 (2021)
work page 2021
-
[35]
O. Hart and R. Nandkishore, Spectroscopic fingerprints of gapless type-II fracton phases, Phys. Rev. B 105, L180416 (2022)
work page 2022
-
[36]
N. Niggemann, Y. Iqbal, and J. Reuther, Quantum Ef- fects on Unconventional Pinch Point Singularities, Phys- ical Review Letters 130, 196601 (2023)
work page 2023
-
[37]
P. Sala, T. Rakovszky, R. Verresen, M. Knap, and F. Pollmann, Ergodicity Breaking Arising from Hilbert Space Fragmentation in Dipole-Conserving Hamiltoni- ans, Phys. Rev. X 10, 011047 (2020)
work page 2020
-
[38]
A. Khudorozhkov, A. Tiwari, C. Chamon, and T. Neu- pert, Hilbert Space Fragmentation in a 2D Quantum Spin System with Subsystem Symmetries, SciPost Physics 13, 098 (2022)
work page 2022
- [39]
-
[40]
X. Feng and B. Skinner, Hilbert space fragmentation pro- duces an effective attraction between fractons, Phys. Rev. Res. 4, 013053 (2022)
work page 2022
- [41]
-
[42]
M. Will, R. Moessner, and F. Pollmann, Realization of Hilbert Space Fragmentation and Fracton Dynamics in Two Dimensions, Phys. Rev. Lett. 133, 196301 (2024)
work page 2024
-
[43]
A. Prem, S. Vijay, Y.-Z. Chou, M. Pretko, and R. M. Nandkishore, Pinch point singularities of tensor spin liq- uids, Phys. Rev. B 98, 165140 (2018)
work page 2018
-
[44]
N. Niggemann, M. Adhikary, Y. Schaden-Thillmann, and J. Reuther, Gapless fracton quantum spin liquid and emergent photons in a 2D spin-1 model (2025)
work page 2025
- [45]
-
[46]
D. L. Bergman, C. Wu, and L. Balents, Band touching from real-space topology in frustrated hopping models, Phys. Rev. B 78, 125104 (2008)
work page 2008
-
[47]
M. Udagawa, H. Nakai, and C. Hotta, Pinch-point spec- tral singularity from the interference of topological loop states (2024), arXiv:2404.13533 [cond-mat.mtrl-sci]
-
[48]
R. Moessner and S. L. Sondhi, Resonating Valence Bond Phase in the Triangular Lattice Quantum Dimer Model, Phys. Rev. Lett. 86, 1881 (2001)
work page 2001
-
[49]
R. Moessner and S. L. Sondhi, Three-dimensional resonating-valence-bond liquids and their excitations, Phys. Rev. B 68, 184512 (2003)
work page 2003
-
[50]
A. Fancelli, R. Flores-Calder´ on, O. Benton, B. Lake, R. Moessner, and J. Reuther, Fragile spin liquid in three dimensions, Phys. Rev. B 111, 134413 (2025)
work page 2025
- [51]
-
[52]
D. S. Rokhsar and S. A. Kivelson, Superconductivity and the Quantum Hard-Core Dimer Gas, Phys. Rev. Lett.61, 2376 (1988)
work page 1988
-
[53]
By contrast, in type-II fracton models, fractons are lo- cated on the corners of fractal operators, rendering all possible composite quasiparticles fully immobile
-
[54]
N. Shannon, G. Misguich, and K. Penc, Cyclic exchange, isolated states, and spinon deconfinement in an XXZ Heisenberg model on the checkerboard lattice, Phys. Rev. B 69, 220403 (2004)
work page 2004
-
[55]
A. Browaeys and T. Lahaye, Many-Body Physics with Individually Controlled Rydberg Atoms, Nature Physics 16, 132 (2020)
work page 2020
-
[56]
G. Giudici, M. D. Lukin, and H. Pichler, Dynamical Preparation of Quantum Spin Liquids in Rydberg Atom Arrays, Physical Review Letters 129, 090401 (2022)
work page 2022
-
[57]
R. Samajdar, W. W. Ho, H. Pichler, M. D. Lukin, and S. Sachdev, Quantum phases of Rydberg atoms on a kagome lattice, Proceedings of the National Academy of Sciences 118, e2015785118 (2021)
work page 2021
-
[58]
Z. Zeng, G. Giudici, and H. Pichler, Quantum Dimer Models with Rydberg Gadgets, Physical Review Re- search 7, L012006 (2025)
work page 2025
-
[59]
Gurobi Optimization, LLC, Gurobi Optimizer Reference Manual (2023)
work page 2023
-
[60]
S. Bolusani, M. Besan¸ con, K. Bestuzheva, A. Chmiela, J. Dion´ ısio, T. Donkiewicz, J. van Doornmalen, L. Ei- fler, M. Ghannam, A. Gleixner, C. Graczyk, K. Hal- big, I. Hedtke, A. Hoen, C. Hojny, R. van der Hulst, D. Kamp, T. Koch, K. Kofler, J. Lentz, J. Manns, G. Mexi, E. M¨ uhmer, M. E. Pfetsch, F. Schl¨ osser, F. Ser- rano, Y. Shinano, M. Turner, S. ...
work page 2024
-
[61]
M. Calandra Buonaura and S. Sorella, Numerical study of the two-dimensional Heisenberg model using a Green function Monte Carlo technique with a fixed number of walkers, Phys. Rev. B 57, 11446 (1998)
work page 1998
-
[62]
F. Becca and S. Sorella, Quantum Monte Carlo ap- proaches for correlated systems (Cambridge University Press, 2017)
work page 2017
-
[63]
N. Trivedi and D. M. Ceperley, Ground-state correlations of quantum antiferromagnets: A Green-function Monte Carlo study, Phys. Rev. B 41, 4552 (1990)
work page 1990
-
[64]
J. H. Hetherington, Observations on the statistical iter- ation of matrices, Phys. Rev. A 30, 2713 (1984)
work page 1984
-
[65]
Or, in the case of importance sampling introduced later, Ox′xn ψT (x′)/ψT (xn)
-
[66]
Sorella, Green Function Monte Carlo with Stochastic Reconfiguration, Phys
S. Sorella, Green Function Monte Carlo with Stochastic Reconfiguration, Phys. Rev. Lett. 80, 4558 (1998)
work page 1998
-
[67]
G. Carleo and M. Troyer, Solving the quan- tum many-body problem with artificial neu- ral networks, Science 355, 602 (2017), https://www.science.org/doi/pdf/10.1126/science.aag2302
-
[68]
H.-Y. Lin, Y. Guo, R.-Q. He, Z. Y. Xie, and Z.-Y. Lu, Green’s function Monte Carlo combined with projected entangled pair state approach to the frustrated J1−J2 Heisenberg model, Phys. Rev. B 109, 235133 (2024)
work page 2024
-
[69]
K. J. Runge, Quantum Monte Carlo calculation of 13 the long-range order in the Heisenberg antiferromagnet, Phys. Rev. B 45, 7229 (1992)
work page 1992
- [70]
-
[71]
A. Ralko, M. Ferrero, F. Becca, D. Ivanov, and F. Mila, Dynamics of the Quantum Dimer Model on the Triangu- lar Lattice: Soft Modes and Local Resonating Valence- Bond Correlations, Physical Review B74, 134301 (2006). A. GAUSSIAN APPROXIMATION OF THE CLASSICAL MODEL We start by writing the ground state constraint C =P8 a=1 σ a Sz a = 0 defined via H1 in Eq...
work page 2006
-
[72]
(a) Ground state energy as a function of the projection time for a single walker (grey) compared to an ensemble of 20 walkers (blue) and the exact result (black dashed line). (b) shows different expectation values of different one- and two-point spin correlators shown in the inset obtained from 20 walkers, each compared to the exact result by a solid line...
discussion (0)
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