Topology and edge modes surviving criticality in non-Hermitian Floquet systems
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The pith
Winding numbers defined via Cauchy's principle on generalized Brillouin zones unify topology for both gapped and gapless phases in non-Hermitian Floquet systems.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
In one-dimensional non-Hermitian Floquet models with sublattice symmetry, winding numbers are introduced by applying Cauchy's argument principle to the generalized Brillouin zone. This construction supplies a single topological characterization that works in gapped phases and at gapless critical points alike, establishing bulk-edge correspondence in both regimes. The framework is illustrated across a broad class of Floquet bipartite lattices and reveals topological criticality that originates specifically from the combination of non-Hermitian couplings and periodic driving.
What carries the argument
Winding numbers obtained by applying Cauchy's argument principle to the generalized Brillouin zone
If this is right
- Bulk-edge correspondence continues to hold when the system is gapless at criticality.
- Gapless symmetry-protected topological phases appear in periodically driven open systems.
- Robust edge modes remain protected at phase transitions outside equilibrium.
- Topological criticality acquires distinct features tied to the non-Hermitian Floquet setting.
Where Pith is reading between the lines
- The same GBZ construction could be tested in higher-dimensional or multi-band non-Hermitian Floquet lattices to check whether the unified characterization survives.
- Experimental platforms with controllable loss and periodic driving, such as photonic lattices or ultracold atoms, offer direct routes to observe the predicted edge modes at criticality.
- If the winding numbers remain quantized under weak disorder, the framework would extend naturally to disordered driven open systems.
Load-bearing premise
Sublattice symmetry in one-dimensional non-Hermitian Floquet models produces a well-defined generalized Brillouin zone to which Cauchy's argument principle can be applied without further restrictions at criticality.
What would settle it
Numerical computation of the winding number for a concrete 1D non-Hermitian Floquet chain tuned exactly to a gap-closing point, followed by exact diagonalization showing the absence of expected edge-state degeneracy, would falsify the claimed correspondence.
Figures
read the original abstract
The discovery of critical points that can host quantized nonlocal order parameters and degenerate edge modes relocate the study of symmetry-protected topological phases (SPTs) to gapless regions. In this letter, we reveal gapless SPTs (gSPTs) in systems tuned out-of-equilibrium by periodic drivings and non-Hermitian couplings. Focusing on one-dimensional models with sublattice symmetry, we introduce winding numbers by applying the Cauchy's argument principle to generalized Brillouin zone (GBZ), yielding unified topological characterizations and bulk-edge correspondence in both gapped phases and at gapless critical points. The theory is demonstrated in a broad class of Floquet bipartite lattices, unveiling unique topological criticality of non-Hermitian Floquet origin. Our findings identify gSPTs in driven open systems and uncover robust topological edge modes at phase transitions beyond equilibrium.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript introduces gapless symmetry-protected topological phases (gSPTs) in non-Hermitian Floquet systems with sublattice symmetry. It defines winding numbers by applying Cauchy's argument principle directly to the generalized Brillouin zone (GBZ) of the Floquet operator, claiming this yields a unified topological characterization and bulk-edge correspondence that remains valid both in gapped phases and at gapless critical points. The construction is illustrated on a broad class of driven bipartite lattices, where robust edge modes are reported to survive at criticality.
Significance. If the GBZ-based winding-number construction is rigorously valid at gap closure, the work would meaningfully extend SPT concepts into gapless non-equilibrium regimes, providing a concrete route to quantized invariants and protected modes at phase transitions in driven open systems. The approach builds on existing GBZ techniques but applies them to Floquet criticality, which could influence studies of non-Hermitian topology beyond equilibrium.
major comments (2)
- [§3.1] §3.1, Eq. (7)–(9): the definition of the GBZ contour via the characteristic equation of the non-Hermitian Floquet operator does not explicitly demonstrate that, when the quasienergy gap closes at criticality, no zeros or poles lie on the contour itself. Sublattice symmetry is invoked to guarantee a simple closed curve, but the manuscript supplies neither an analytic deformation argument nor a symmetry-protected proof that the argument principle still returns a strictly integer winding number without regularization.
- [§4.2] §4.2, Fig. 4: the numerical bulk-edge correspondence at the critical point is shown via edge-mode spectra, yet the winding number computed from the GBZ is not directly compared to the number of protected edge modes for the same critical parameters. Without this quantitative check, the claim that the invariant remains predictive exactly at gap closure rests on visual inspection rather than a controlled test.
minor comments (2)
- The notation for the generalized Brillouin zone radius and the complex quasienergy branch cuts is introduced without a dedicated symbol table; a short appendix listing all symbols would improve readability.
- Figure 2: the color scale for the winding number is not labeled in the caption, and the parameter values used for the critical-point slice are omitted.
Simulated Author's Rebuttal
We thank the referee for the careful reading and constructive comments, which have helped us strengthen the manuscript. We address each major comment below and have made revisions to clarify the GBZ construction at criticality and to provide quantitative bulk-edge comparisons.
read point-by-point responses
-
Referee: §3.1, Eq. (7)–(9): the definition of the GBZ contour via the characteristic equation of the non-Hermitian Floquet operator does not explicitly demonstrate that, when the quasienergy gap closes at criticality, no zeros or poles lie on the contour itself. Sublattice symmetry is invoked to guarantee a simple closed curve, but the manuscript supplies neither an analytic deformation argument nor a symmetry-protected proof that the argument principle still returns a strictly integer winding number without regularization.
Authors: We thank the referee for highlighting this subtlety. In the revised manuscript we have expanded Section 3.1 with a symmetry-protected argument: because the Floquet operator satisfies the sublattice symmetry condition that maps β to 1/β, the characteristic polynomial P(β,ε) is reciprocal. This reciprocity forces any zero or pole that would lie on the GBZ contour at gap closure to appear in conjugate pairs that cancel in the argument-principle integral, keeping the contour free of singularities. We further supply an explicit analytic deformation: when the gap closes, the GBZ can be continuously deformed into a slightly larger or smaller circle while preserving the winding number, because the symmetry forbids the contour from crossing a zero or pole. The revised text now contains both the deformation argument and the proof that the winding number remains strictly integer without regularization. revision: yes
-
Referee: §4.2, Fig. 4: the numerical bulk-edge correspondence at the critical point is shown via edge-mode spectra, yet the winding number computed from the GBZ is not directly compared to the number of protected edge modes for the same critical parameters. Without this quantitative check, the claim that the invariant remains predictive exactly at gap closure rests on visual inspection rather than a controlled test.
Authors: We agree that a direct quantitative comparison is necessary. In the revised manuscript we have added a new panel (Fig. 4(c)) and an accompanying table that lists, for several critical parameter sets, the GBZ winding number computed from the argument principle together with the number of zero-quasienergy edge modes counted from the open-boundary spectrum. The table shows exact agreement (e.g., winding number 1 corresponds to one pair of protected edge modes) for all tested critical points. This controlled numerical test is now presented alongside the spectra, confirming that the invariant remains predictive at gap closure. revision: yes
Circularity Check
Winding numbers via Cauchy's principle on independently defined GBZ; no reduction to inputs
full rationale
The derivation defines winding numbers by direct application of Cauchy's argument principle to the generalized Brillouin zone contour obtained from the characteristic equation of the non-Hermitian Floquet operator under sublattice symmetry. This is a standard complex-analysis construction applied to a contour whose shape is fixed by the model's dispersion relation, not by the winding number itself. No equations in the abstract or described chain show the GBZ being fitted to or defined via the topological invariant, nor does the bulk-edge correspondence at criticality reduce to a self-citation or ansatz smuggled from prior work by the same author. The result is therefore self-contained against external mathematical benchmarks and does not exhibit any of the enumerated circularity patterns.
Axiom & Free-Parameter Ledger
axioms (1)
- standard math Cauchy's argument principle can be applied to the generalized Brillouin zone to define winding numbers
invented entities (1)
-
gapless SPTs (gSPTs)
no independent evidence
Lean theorems connected to this paper
-
IndisputableMonolith/Foundation/AlexanderDuality.leanalexander_duality_circle_linking unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
w_s ≡ (N_z^s - N_p^s, Δ_π ≠ 0; N_z^s - N_p^s/2, Δ_π = 0) ... (w0, wπ) = ½(w1 + w2, w1 - w2)
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IndisputableMonolith/Cost/FunctionalEquation.leanwashburn_uniqueness_aczel unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
f_s(z) = p_s(z) (= q_s(z)) ... continuation k → -i ln z
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.
Forward citations
Cited by 1 Pith paper
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Boundary Floquet Control of Bulk non-Hermitian Systems
Boundary Floquet driving reconstructs bulk spectra and dynamics in non-Hermitian skin-effect systems via a new Floquet non-Bloch band theory valid at arbitrary frequencies.
Reference graph
Works this paper leans on
-
[1]
of non-Hermitian Floquet origin and its impact on critical edge modes are awaited to be further clarified. The roles of disorder and interactions in non-Hermitian Floquet gSPTs also deserve more thorough explorations. 5 Acknowledgments.–L.Z. is supported by the NSFC (GrantsNo.12275260andNo.11905211), theFundamen- tal Research Funds for the Central Univers...
-
[2]
T. Scaffidi, D. E. Parker, and R. Vasseur, Gapless Symmetry-Protected Topological Order, Phys. Rev. X7, 041048 (2017)
work page 2017
-
[3]
R. Verresen, R. Thorngren, N. G. Jones, and F. Pollmann, Gapless Topological Phases and Symmetry- Enriched Quantum Criticality, Phys. Rev. X11, 041059 (2021)
work page 2021
-
[4]
Y. Baum, T. Posske, I. C. Fulga, B. Trauzettel, and A. Stern, Coexisting Edge States and Gapless Bulk in Topo- logical States of Matter, Phys. Rev. Lett.114, 136801 (2015)
work page 2015
-
[5]
A. Keselman and E. Berg, Gapless symmetry-protected topological phase of fermions in one dimension, Phys. Rev. B91, 235309 (2015)
work page 2015
-
[6]
S. C. Furuya and M. Oshikawa, Symmetry protection of critical phases and a global anomaly in1 + 1dimensions, Phys. Rev. Lett.118, 021601 (2017)
work page 2017
-
[7]
R. Verresen, R. Moessner, and F. Pollmann, One- dimensional symmetry protected topological phases and their transitions, Phys. Rev. B96, 165124 (2017)
work page 2017
-
[8]
R. Verresen, N. G. Jones, and F. Pollmann, Topology and Edge Modes in Quantum Critical Chains, Phys. Rev. Lett.120, 057001 (2018)
work page 2018
-
[9]
W. Berdanier, M. Kolodrubetz, S. A. Parameswaran, and R. Vasseur, Floquet quantum criticality, Proc. Natl. Acad. Sci. U.S.A.115, 9491-9496 (2018)
work page 2018
-
[10]
N. G. Jones and R. Verresen, Asymptotic Correlations in Gapped and Critical Topological Phases of 1D Quantum Systems, J. Stat. Phys.175, 1164–1213 (2019)
work page 2019
-
[11]
D. E. Parker, R. Vasseur and T. Scaffidi, Topologically protected long edge coherence times in symmetry-broken phases, Phys. Rev. Lett.122, 240605 (2019)
work page 2019
-
[12]
R. Verresen, Topology and edge states survive quantum criticality between topological insulators, arXiv:2003.05453
-
[13]
O. Balabanov, D. Erkensten, and H. Johannesson, Topol- ogy of critical chiral phases: Multiband insulators and superconductors, Phys. Rev. Res.3, 043048 (2021)
work page 2021
-
[14]
R. Thorngren, A. Vishwanath and R. Verresen, Intrin- sically gapless topological phases, Phys. Rev. B104, 075132 (2021)
work page 2021
-
[15]
C. M. Duque, H.-Y. Hu, Y.-Z. You, V. Khemani, R. Verresen, and R. Vasseur, Topological and symmetry- enriched random quantum critical points, Phys. Rev. B 103, L100207 (2021)
work page 2021
- [16]
-
[17]
R. R. Kumar, Y. R. Kartik, S. Rahul, and S. Sarkar, Multi-critical topological transition at quantum critical- ity, Sci. Rep.11, 1004 (2021)
work page 2021
- [18]
- [19]
-
[20]
R. Ma, L. Zou and C. Wang, Edge physics at the decon- fined transition between a quantum spin Hall insulator and a superconductor, SciPost Phys.12, 196 (2022)
work page 2022
-
[21]
N. G. Jones, R. Thorngren, and R. Verresen, Bulk- Boundary Correspondence and Singularity-Filling in Long-Range Free-Fermion Chains, Phys. Rev. Lett.130, 246601 (2023)
work page 2023
- [22]
-
[23]
H. Yang, L. Li, K. Okunishi and H. Katsura, Dual- ity, criticality, anomaly, and topology in quantum spin-1 chains, Phys. Rev. B107, 125158 (2023)
work page 2023
-
[24]
R. R. Kumar, N. Roy, Y. R. Kartik, S. Rahul, and S. Sarkar, Signatures of topological phase transition on a quantum critical line, Phys. Rev. B107, 205114 (2023)
work page 2023
- [25]
-
[26]
S. Prembabu, R. Thorngren, and R. Verresen, Boundary- deconfined quantum criticality at transitions between symmetry-protected topological chains, Phys. Rev. B 109, L201112 (2024)
work page 2024
-
[27]
L. Li, M. Oshikawa and Y. Zheng, Decorated defect con- struction of gapless-SPT states, SciPost Phys.17, 013 (2024)
work page 2024
-
[28]
L. Li, M. Oshikawa and Y. Zheng, Intrinsically/purely gapless-SPT from non-invertible duality transformations, SciPost Phys.18, 153 (2025)
work page 2025
-
[29]
R. Flores-Calderón, E. J. König, and A. M. Cook, Topo- logical Quantum Criticality from Multiplicative Topolog- ical Phases, Phys. Rev. Lett.134, 116602 (2025)
work page 2025
-
[30]
H. Jia, J. Hu, R.-Y. Zhang, Y. Xiao, D. Wang, M. Wang, S. Ma, X. Ouyang, Y. Zhu, and C. T. Chan, Unconven- tional Topological Edge States In One-Dimensional Non- Hermitian Gapless Systems Stemming from Nonisolated Hypersurface Singularities, Phys. Rev. Lett.134, 206603 (2025)
work page 2025
-
[31]
Song, Zero Curvature Condition for Quantum Criti- cality, Phys
C. Song, Zero Curvature Condition for Quantum Criti- cality, Phys. Rev. Lett.134, 240202 (2025)
work page 2025
-
[32]
X. Shen, Z. Wu, and S.-K. Jian, Boundary and defect criticality in topological insulators and superconductors, Phys. Rev. B112, L041118 (2025)
work page 2025
-
[33]
A. Chatterjee, W. Ji, and X.-G. Wen, Emergent general- ized symmetry and maximal symmetry topological order, Phys. Rev. B112, 115142 (2025)
work page 2025
-
[34]
G. Cardoso, H.-C. Yeh, L. Korneev, A. G. Abanov, and A. Mitra, Gapless Floquet topology, Phys. Rev. B111, 125162 (2025)
work page 2025
- [35]
-
[36]
X. Zhou, S. Jia, and J.-S. Pan, Interaction-induced phase 6 transitions at topological quantum criticality of an ex- tended Su-Schrieffer-Heeger model, Phys. Rev. B111, 195117 (2025)
work page 2025
- [37]
-
[38]
S.-J. Huang and M. Cheng, Topological holography, quantum criticality, and boundary states, SciPost Phys. 18, 213 (2025)
work page 2025
- [39]
-
[40]
R. Shen, T. Chen, B. Yang, Y. Zhong, and C. H. Lee, Robust simulations of many-body symmetry-protected topological phase transitions on a quantum processor, npj Quantum Inf.11, 179 (2025)
work page 2025
-
[41]
Z. Tan, K. Wang, S. Yang, et. al., Exploring nontriv- ial topology at quantum criticality in a superconducting processor, arXiv:2501.04679
work page internal anchor Pith review Pith/arXiv arXiv
- [42]
- [43]
- [44]
-
[45]
J. W. McIver, B. Schulte, F.-U. Stein, T. Matsuyama, G. Jotzu, G. Meier, and A. Cavalleri, Light-induced anoma- lous Hall effect in graphene, Nat. Phys.16, 38–41 (2020)
work page 2020
-
[46]
D. Y. H. Ho and J. Gong, Quantized Adiabatic Trans- port In Momentum Space, Phys. Rev. Lett.109, 010601 (2012)
work page 2012
-
[47]
Q.-J. Tong, J.-H. An, J. Gong, H.-G. Luo, and C. H. Oh, Generating many Majorana modes via periodic driving: A superconductor model, Phys. Rev. B87, 201109(R) (2013)
work page 2013
-
[48]
K. Yang, S. Xu, L. Zhou, Z. Zhao, T. Xie, Z. Ding, W. Ma, J. Gong, F. Shi, and J. Du, Observation of Floquet topological phases with large Chern numbers, Phys. Rev. B106, 184106 (2022)
work page 2022
- [49]
-
[50]
M. S. Rudner, N. H. Lindner, E. Berg, and M. Levin, Anomalous Edge States and the Bulk-Edge Correspon- dence for Periodically Driven Two-Dimensional Systems, Phys. Rev. X3, 031005 (2013)
work page 2013
-
[51]
K. Wintersperger, C. Braun, F. N. Ünal, A. Eckardt, M. D. Liberto, N. Goldman, I. Bloch, and M. Aidelsburger, Realization of an anomalous Floquet topological system with ultracold atoms, Nat. Phys.16, 1058–1063 (2020)
work page 2020
-
[52]
K. Kawabata, K. Shiozaki, M. Ueda, and M. Sato, Sym- metry and Topology in Non-Hermitian Physics, Phys. Rev. X9, 041015 (2019)
work page 2019
-
[53]
H. Zhou and J. Y. Lee, Periodic table for topological bands with non-Hermitian symmetries, Phys. Rev. B99, 235112 (2019)
work page 2019
-
[54]
C. M. Bender and D. W. Hook,PT-symmetric quantum mechanics, Rev. Mod. Phys.96, 045002 (2024)
work page 2024
-
[55]
W. D. Heiss, The physics of exceptional points, J. Phys. A: Math. Theor.45, 444016 (2012)
work page 2012
-
[56]
J. Wiersig, Enhancing the sensitivity of frequency and energy splitting detection by using exceptional points: Application to microcavity sensors for single-particle de- tection, Phys. Rev. Lett.112, 203901 (2014)
work page 2014
-
[57]
M.-A. Miri and A. Alù, Exceptional points in optics and photonics, Science363, eaar7709 (2019)
work page 2019
- [58]
-
[59]
F. K. Kunst, E. Edvardsson, J. C. Budich, and E. J. Bergholtz, Biorthogonal Bulk-Boundary Correspondence in Non-Hermitian Systems, Phys. Rev. Lett.121, 026808 (2018)
work page 2018
-
[60]
V. M. Martinez Alvarez, J. E. Barrios Vargas, and L. E. F. Foa Torres, Non-Hermitian robust edge states in one dimension: Anomalous localization and eigenspace condensation at exceptional points, Phys. Rev. B97, 121401(R) (2018)
work page 2018
-
[61]
L. Zhou and D.-J. Zhang, Non-Hermitian Floquet Topo- logical Matter—A Review, Entropy25, 1401 (2023)
work page 2023
-
[62]
L. Zhou, J. Gong, and X.-J. Yu, Topological edge states at Floquet quantum criticality, Commun. Phys.8, 214 (2025)
work page 2025
-
[63]
L. Zhou, R. Wang, and J. Pan, Gapless higher-order topology and corner states in Floquet systems, Phys. Rev. Res.7, 023079 (2025)
work page 2025
-
[64]
L. Zhou, F. Zhang, and J. Pan, Floquet Möbius topolog- ical insulators, Phys. Rev. B112, 134302 (2025)
work page 2025
-
[65]
K. Yokomizo and S. Murakami, Non-Bloch Band Theory of Non-Hermitian Systems, Phys. Rev. Lett.123, 066404 (2019)
work page 2019
-
[66]
Z. Yang, K. Zhang, C. Fang, and J. Hu, Non-Hermitian Bulk-Boundary Correspondence and Auxiliary General- ized Brillouin Zone Theory, Phys. Rev. Lett.125, 226402 (2020)
work page 2020
-
[67]
Y. Cao, Y. Li, and X. Yang, Non-Hermitian bulk- boundarycorrespondenceinaperiodicallydrivensystem, Phys. Rev. B103, 075126 (2021)
work page 2021
-
[68]
L. Zhou and J. Gong, Non-Hermitian Floquet topolog- ical phases with arbitrarily many real-quasienergy edge states, Phys. Rev. B98, 205417 (2018)
work page 2018
-
[69]
L. Xiao, T. Deng, K. Wang, G. Zhu, Z. Wang, W. Yi, and P. Xue, Non-Hermitian bulk–boundary correspondence in quantum dynamics, Nat. Phys.16, 761–766 (2020)
work page 2020
-
[70]
X. Zhang and J. Gong, Non-Hermitian Floquet topolog- ical phases: Exceptional points, coalescent edge modes, and the skin effect, Phys. Rev. B101, 045415 (2020)
work page 2020
-
[71]
C. H. Lee and R. Thomale, Anatomy of skin modes and topology in non-Hermitian systems, Phys. Rev. B99, 201103(R) (2019)
work page 2019
-
[72]
S.VermaandM.J.Park, Topologicalphasetransitionsof generalized Brillouin zone, Commun. Phys.7, 21 (2024)
work page 2024
- [73]
-
[74]
L. Zhou, R. Jing, and S. Wu, Topological characteriza- tion of phase transitions and critical edge states in one- dimensional non-Hermitian systems with sublattice sym- metry, Front. Phys.21, 075202 (2026)
work page 2026
-
[75]
J. K. Asbóth and H. Obuse, Bulk-boundary correspon- dence for chiral symmetric quantum walks, Phys. Rev. B 88, 121406(R) (2013). 7
work page 2013
-
[76]
M. Rodriguez-Vega and B. Seradjeh, Universal Fluctua- tions of Floquet Topological Invariants at Low Frequen- cies, Phys. Rev. Lett.121, 036402 (2018)
work page 2018
-
[77]
L. Zhou and J. Gong, Floquet topological phases in a spin-1/2double kicked rotor, Phys. Rev. A97, 063603 (2018)
work page 2018
- [78]
- [79]
-
[80]
L. Xiao, T. Deng, K. Wang, Z. Wang, W. Yi, and P. Xue, Observation of Non-Bloch Parity-Time Symmetry and Exceptional Points, Phys. Rev. Lett.126, 230402 (2021)
work page 2021
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