{"id":"f019749e-7927-4684-9744-97ad9a16b5f7","arxiv_id":"2608.06786","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":5,"one_line_summary":"Light amplitude can flip the sign of current-induced spin polarization in topological surface states.","lead":"This paper predicts that shining high-frequency circularly polarized light on a topological insulator surface can reverse the direction of the spin polarization generated by an electric current. If correct, it offers an all-optical control knob for spin-based devices.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The predicted sign reversal of χ^s_xy rests on the unpublished FEWT occupation ansatz (Eqs. 10-11); without a test against an alternative steady-state occupation scheme, the central claim remains conditional.","rationale":"The reader's weakest assumption is also the point I would stress. The electric spin susceptibility is obtained from G^<, and G^< is fixed by the FEWT ansatz rather than derived from a bath model or from the dynamics. A sign reversal is the most sensitive kind of prediction: both the position of the zero crossing and the direction of the crossing can depend on which Floquet sectors are occupied, so the unpublished status of [36] is a genuine correctness risk. I do not find an internal inconsistency in the high-frequency effective Hamiltonian; Appendix A is explicit and the algebra appears internally coherent. The spin-texture reversal is a plausible mechanism if the susceptibility calculation is robust. I also note that the topological phase transition claim is under-supported: Fig. 4 shows Berry curvature sign patterns but no computed Chern invariant, so the word 'topological' is suggestive rather than demonstrated. That is a secondary concern about the explanation, not the primary load-bearing point. The proposed reservoir-occupation test directly isolates the FEWT assumption while keeping the rest of the calculation fixed. If the test confirms robustness, the conditional verdict can be upgraded; if not, the central claim is not yet established.","tokens_in":11250,"tokens_out":15543,"duration_ms":170690,"concrete_test":"Recompute χ^s_xy(A) and χ^s_yx(A) from Eqs. (8)-(16) with the same impurity SCBA, but replace the FEWT non-interacting lesser Green's function by the steady-state solution of a driven system coupled to a wide-band fermionic reservoir: take Σ^r = -iΓ/2 and Σ^< = iΓ f(ω-μ0), solve Eqs. (12)-(13) self-consistently, and scan Γ/v_imp from 0.1 to 10. If the zero crossing remains at A≈1.8 ± 0.1 for all Γ, the reversal is not controlled by the FEWT ansatz; if it shifts, disappears, or changes slope, the central claim is FEWT-dependent and the paper should be revised to justify the occupation scheme.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The sign reversal that constitutes the central claim is computed from the lesser Green's function G^<, whose only non-equilibrium input is the FEWT ansatz in Eqs. (10)-(11). That ansatz assigns each Floquet sector a chemical potential shifted by ⟨N⟩_{μnk}Ω and is imported from the authors' unpublished preprint [36]; no derivation or independent validation is provided. Because Eq. (16) assembles χ^s from spectral weights of G^<, the zero crossing at A≈1.8 is precisely the kind of observable that can move or disappear when the occupation rule changes. The high-frequency choice Ω=8 eV makes FEWT predominantly populate the n=0 sector, so the decisive question is whether the true driven steady state really reduces to that sector. If photon-assisted reservoir processes populate sidebands, G^< changes and the computed transverse susceptibility could change sign at a different A or not at all. The spin-texture and Berry-curvature discussion in Figs. 3-4 does not resolve this, because it addresses the band geometry, not the occupation of the response kernel.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper studies the current-induced spin polarization (Edelstein effect) in the surface states of a three-dimensional topological insulator (Bi2Se3) under illumination by high-frequency circularly polarized light. Using Floquet theory, the authors compute the electric spin susceptibility χ^s_{αβ} from a nonequilibrium Green's function formalism with impurity scattering in the self-consistent Born approximation, assuming the Floquet eigenstate-wise thermalization (FEWT) hypothesis for the occupation of Floquet sidebands. They find that the transverse susceptibility χ^s_{xy} and χ^s_{yx} change sign at normalized field strength A≈1.8, while the longitudinal components remain near zero. They attribute this sign reversal to a reversal of the in-plane spin texture of the high-frequency conduction band, driven by topological phase transitions at A=1.88 and A=1.98 where the quasienergy gap closes. The paper also provides an analytic high-frequency effective Hamiltonian via the Floquet-Magnus expansion, with the derivation given in Appendix A.","tokens_in":11483,"tokens_out":8783,"duration_ms":81298,"significance":"If the predicted sign reversal survives scrutiny of the occupation model, the result would be of genuine interest to the Floquet engineering and spintronics communities, as it offers an optical route to controlling the Edelstein effect in topological surface states. The paper's strengths include an explicit derivation of the high-frequency effective Hamiltonian (Appendix A) and a fixed-parameter calculation in which the sign change is not fitted to the target observable. However, the central calculation rests on the FEWT hypothesis from an unpublished preprint and on a Kubo formula also referenced to that preprint, so the significance is conditional on the validation of those ingredients. The spin-texture and Berry-curvature analysis provides a plausible microscopic mechanism, but the quantitative link to the susceptibility sign change is not fully demonstrated.","major_comments":[{"comment":"The FEWT hypothesis, which assigns each Floquet eigenstate a chemical potential shifted by ⟨N⟩_{μnk}Ω, is the sole non-equilibrium occupation rule in the calculation and is imported from the authors' unpublished arXiv:2512.01346 [36]. The sign reversal in Fig. 2(a) is computed from the lesser Green's function G^<, which depends entirely on this ansatz. No derivation or independent validation is provided. Because Eq. (16) assembles χ^s from spectral weights of G^<, the zero crossing at A≈1.8 is precisely the kind of observable that can move or disappear when the occupation rule changes. Please either derive FEWT from a microscopic coupling-to-reservoir model or benchmark it against an alternative steady-state scheme (e.g., Floquet-Born-Markov or a photon-assisted reservoir) and show that the sign reversal persists. Without such a test, the central claim remains conditional.","section":"Nonequilibrium Green's functions, Eqs. (10)-(11)"},{"comment":"The Floquet generalization of the Kubo formula is justified only by a reference to the same unpublished preprint [36] ('can be derived following the derivation of the similar nonequilibrium Kubo formula [36]'). Since Eq. (16) is the central response formula, the paper should include the derivation in an appendix or at least outline the key steps. As it stands, the calculation is not independently verifiable, and a reader cannot check whether the energy-derivative structure and the Floquet representation of the current operator are correct.","section":"Kubo formula, Eq. (16)"},{"comment":"The statement that Ω=8 eV is '1,000 times the minimum energy gap of the quasienergy bands' appears inconsistent with the data shown. At A=0 the effective surface Hamiltonian is gapless at the Γ point (h_x=h_y=h_z=0 at k=0), so the minimum gap is zero; the ratio is then infinite, not 1,000. If a different definition of the gap is intended (e.g., the gap at the Fermi level or the gap for the range of A used), please specify it and give the numerical value. This matters because the high-frequency condition and the FEWT simplification (⟨N⟩≈n) are invoked to justify the occupation scheme.","section":"High-frequency condition, Fig. 3(a) and text near Eq. (11)"},{"comment":"The causal connection between the spin-texture reversal and the susceptibility sign change is qualitative. The spin texture begins rotating counterclockwise between A=1.65 and A=1.88, while χ^s crosses zero at A≈1.8, i.e., before the topological transitions at A=1.88 and A=1.98. To substantiate the claim that the reversal of the spin texture causes the sign change, please provide a quantitative relation, for example by evaluating χ^s from the high-frequency spin texture alone and comparing with the full Kubo result, or by decomposing the k-resolved contributions to the Kubo integral and showing that they track the spin-texture orientation.","section":"Spin texture reversal and topological phase transitions"}],"minor_comments":[{"comment":"The symbol labeled \"David's star\" should be rendered as \"Star of David\" for clarity.","section":"Fig. 3(b)"},{"comment":"The notation for the self-energy is slightly ambiguous: the left-hand side has indices (an,bm) while the right-hand side has δ_{ab} and a Green's function with indices (an,am). Consider writing the expression as [Σ^{r,<}(ω)]_{an,am} after applying δ_{ab} to avoid confusion.","section":"Eq. (14)"},{"comment":"The chemical potential is stated to be 0.28 eV above the Dirac point at A=0, and it is adjusted to maintain the same electron density under illumination. It would be helpful to give the corresponding density or Fermi wavevector, as this affects the physical interpretation of the filling.","section":"Text near Eq. (9)"},{"comment":"The author name \"A. Echardt\" appears to be a typo for \"A. Eckardt\" (A. Eckardt and E. Anisimovas, New J. Phys. 17, 093039 (2015)).","section":"Reference [44]"},{"comment":"The sentence about controlling and reversing the electric spin susceptibility appears nearly verbatim in the abstract and again in the introduction and conclusion. Consider reducing the duplication in the introduction.","section":"Abstract and Introduction"}],"recommendation":"major_revision","confidential_remarks":"The manuscript leans very heavily on the authors' own unpublished preprint arXiv:2512.01346 [36] for two key elements: the FEWT occupation hypothesis and the Floquet Kubo formula. This is a significant verifiability and novelty-disclosure concern. If the preprint is not publicly available or not yet accepted, other readers will not be able to reproduce the central calculation. I recommend asking the authors to either include the necessary derivations in the paper or provide a publicly accessible version of [36] with the relevant material. The topic is within the journal's scope, and the fixed-parameter prediction is a strength, but the load-bearing assumptions must be independently checkable."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The new result here is concrete: for the Bi2Se3 surface Hamiltonian under high-frequency circularly polarized light, the transverse electric spin susceptibility chi^s_xy changes sign near A ≈ 1.8, with the mechanism traced to a reversal of the in-plane spin texture between two Dirac-like band touchings. The high-frequency effective Hamiltonian in Appendix A is worked out explicitly and is internally coherent; the Floquet–Magnus expansion is standard, and the resulting spin-texture plots give a plausible qualitative mechanism for the sign change. That is a legitimate piece of Floquet engineering, not a re-coordinatization of prior results, and the paper is honest that the susceptibility is computed for a fixed model with no parameter tuned to hit the reversal.\n\nThe soft spot is exactly where the stress-test note puts it. The central response calculation rests on the FEWT ansatz in Eqs. (10)–(11), taken from the authors' unpublished preprint [36], and no independent derivation or validation is given. Since G^< encodes all the non-equilibrium occupation information, and Eq. (16) builds chi from G^<, the sign reversal at A≈1.8 inherits the FEWT occupation rule. If a different steady-state scheme populates the n≠0 sidebands appreciably, the crossing could shift or disappear. That genuinely makes the prediction conditional, not wrong. Two additional minor points: the paper claims topological phase transitions but never computes a Chern number, relying instead on Berry-curvature sign flips; and the numerical details of the Floquet truncation are not stated, which makes the susceptibility curves hard to reproduce from the text alone.\n\nProportionately, this is a solid paper with one load-bearing assumption. The analytic framework is transparent, the spin-texture mechanism is well argued, and the claimed effect is sharp and falsifiable in principle. The FEWT issue is real but is also the kind of thing that can be addressed by testing an alternative occupation scheme or by deriving FEWT from a microscopic coupling to a reservoir.\n\nWho gets value from this: people working on Floquet spintronics or light-controlled responses in topological insulators. It deserves a serious referee—the calculation is explicit enough to check, and the central claim is interesting enough to spend referee time on, provided the referee pushes for the FEWT sensitivity analysis before publication.","headline":"A clean, coherent Floquet calculation of an amplitude-driven sign reversal in the Edelstein susceptibility of topological surface states, whose only real weakness is the load-bearing occupation ansatz imported from the authors' own unpublished work.","tokens_in":12023,"tokens_out":613,"would_cite":false,"duration_ms":8036,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"This paper predicts that high-frequency circularly polarized light can reverse the current-induced spin polarization of topological surface states through Floquet topological phase transitions.","keywords":["Edelstein effect","Floquet spintronics","topological surface states","circularly polarized light","electric spin susceptibility","spin texture reversal","Floquet topological phase transition","current-induced spin polarization"],"falsifier":"Measure the current-induced spin polarization of an illuminated Bi2Se3 surface, for example by Kerr rotation or spin-sensitive optical detection, while sweeping the normalized field strength through $A\\simeq 1.8$ and look for the predicted sign change; equivalently, replace the FEWT occupation rule by a kinetic or master-equation solution of the Floquet steady state and check whether $\\chi^s_{xy}$ still crosses zero near the same field strength.","tokens_in":11004,"feed_emoji":"🔄","tokens_out":11491,"duration_ms":98346,"temperature":0.7,"pith_summary":"This paper predicts that high-frequency circularly polarized light provides an optical control knob for the Edelstein effect in topological surface states, the current-induced spin polarization that arises from spin-momentum locking. Concretely, the transverse electric spin susceptibilities $\\chi^s_{xy}$ and $\\chi^s_{yx}$ change sign when the normalized field strength $A=eE_0 a/\\hbar\\Omega$ reaches about 1.8, so raising the light intensity alone can reverse the direction of the induced spin polarization. The reversal is traced to a sign flip of the in-plane spin texture in momentum space, which occurs through two Floquet topological phase transitions at $A=1.88$ and $A=1.98$ where the quasienergy gap closes and the Berry curvature changes sign. If the prediction holds, Floquet engineering would extend from modifying band structures to actively switching spintronic signals without magnetic fields.","feed_headline":"Light reverses spin polarization on topological surfaces","feed_subtitle":"At field strength A=1.8, the Edelstein spin response of Bi2Se3 flips sign, offering all-optical control.","key_machinery":"The central object is the electric spin susceptibility tensor $\\chi^s_{\\alpha\\beta}$, defined by $\\langle S_\\alpha\\rangle=\\sum_\\beta \\chi^s_{\\alpha\\beta} E_\\beta$ and computed in the Floquet representation through a Kubo formula built from retarded, advanced, and lesser Green's functions. The calculation is carried by the high-frequency effective Hamiltonian $H_{\\mathrm{high}}(k)$ from the Floquet-Magnus expansion, whose spin Bloch vector $\\tilde{\\mathbf{h}}(k)$ fixes the conduction-band spin texture as $\\mathbf{s}(k)=\\tilde{\\mathbf{h}}(k)/|\\tilde{\\mathbf{h}}(k)|$. The occupation of Floquet sidebands is set by the Floquet eigenstate-wise thermalization (FEWT) hypothesis, which shifts each sideband's chemical potential by the expectation value of the Floquet index operator; this rule converts the band-structure change into the specific sign-reversal prediction for the susceptibility.","core_discovery":"Starting from the effective surface Hamiltonian of Bi2Se3 and coupling it to circularly polarized light through Peierls substitution, the paper derives a high-frequency Floquet effective Hamiltonian and computes the linear-response electric spin susceptibility from nonequilibrium Green's functions with impurity scattering treated in the self-consistent Born approximation. The central result is that $\\chi^s_{xy}$ and $\\chi^s_{yx}$ are not merely renormalized by the light but change sign at $A\\simeq 1.8$, while the longitudinal components $\\chi^s_{xx}$ and $\\chi^s_{yy}$ remain essentially zero. The sign change is explained by the momentum-space spin texture of the conduction band: the in-plane spin winds clockwise for $A\\lesssim 1.65$ and counterclockwise for $A\\gtrsim 2.05$, with the transition passing through two gap-closing events at $A=1.88$ and $A=1.98$ where the Berry curvature switches from predominantly positive to predominantly negative. The paper therefore identifies the reversal as a Floquet topological phase transition with a direct electric-transport signature.","pith_inferences":["A natural extension, not tested in the paper, is to compute the same $\\chi^s_{xy}(A)$ curve with a microscopic kinetic equation for Floquet occupations; if the sign change persists, the effect would be a strong candidate for all-optical spintronic switching on ultrafast timescales.","Because the mechanism is governed by the high-frequency effective Hamiltonian's spin Bloch vector, similar light-induced spin-texture reversals should appear in other spin-momentum-locked surfaces and possibly in Rashba two-dimensional electron gases.","The prediction that the Berry curvature changes from predominantly positive to negative could be probed independently through the circular photogalvanic effect or the anomalous Hall response of the illuminated surface, giving a transport signature separate from the Edelstein spin polarization."],"forward_implications":["The sign of the current-induced spin polarization on a topological surface becomes switchable by light intensity at fixed current direction and fixed material.","The two gap-closing events at $A=1.88$ and $A=1.98$ constitute Floquet topological phase transitions that should be observable as sharp changes in Berry-curvature-related response functions.","The longitudinal spin susceptibility stays essentially zero across the whole range, so the predicted control is a purely transverse Hall-like effect.","The sign change is obtained with impurity scattering included at $v_{\\mathrm{imp}}=1$ in the self-consistent Born approximation, so the prediction is made for a disordered surface rather than only a clean one."],"supporting_citations":[{"why":"Supplies the four-parameter surface Hamiltonian for Bi2Se3 and the phenomenological parameter values used throughout the calculation.","marker":"[34]"},{"why":"Fixes the chemical potential at 0.28 eV above the Dirac point from experimental surface-state observations, defining the Fermi surface for the susceptibility calculation.","marker":"[35]"},{"why":"Defines the Edelstein effect, the current-induced spin polarization that the paper sets out to control.","marker":"[16]"},{"why":"Motivates and supports the expectation that high-frequency circularly polarized light can drive topological phase transitions in illuminated Dirac-like systems.","marker":"[21–25]"},{"why":"Supplies the Floquet eigenstate-wise thermalization (FEWT) hypothesis, used without derivation in this paper, that fixes the Floquet sideband occupations entering the lesser Green's function.","marker":"[36]"},{"why":"Provides the equilibrium Kubo formula for current-induced spin polarization that the paper generalizes to the Floquet nonequilibrium case.","marker":"[42]"},{"why":"Provides the Floquet-Magnus high-frequency expansion used to derive the effective Hamiltonian that determines the spin texture.","marker":"[43]"},{"why":"Supplies the Floquet-space high-frequency approximation used alongside the Floquet-Magnus expansion in the derivation.","marker":"[44]"},{"why":"Provides the self-consistent Born approximation for delta-function impurity scattering used to include disorder in the Green's functions.","marker":"[40, 41]"}],"fun_headline_variants":["Light flips spin polarization on topological surfaces","All-optical control of spin polarization in Bi2Se3","Light tunes spin susceptibility of topological surface states","Floquet light reverses Edelstein spin response in Bi2Se3","Circularly polarized light flips spin polarization on topological surfaces"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing assumption is that electrons distribute among the light-dressed Floquet states according to the FEWT rule, which shifts each sideband's chemical potential by the Floquet index operator; if the true steady-state occupation is different, the computed magnitude and even the sign of the transverse susceptibility could change.","fun_headline_variants_meta":{"raw":{"variants":["Light flips spin polarization on topological surfaces","All-optical control of spin polarization in Bi2Se3","Light tunes spin susceptibility of topological surface states","Floquet light reverses Edelstein spin response in Bi2Se3","Circularly polarized light flips spin polarization on topological surfaces"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000757,"raw_usage":{"total_tokens":3329,"prompt_tokens":877,"completion_tokens":2452,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":493,"completion_tokens_details":{"reasoning_tokens":2371}},"tokens_in":493,"tokens_out":2452,"duration_ms":17562,"temperature":1.0,"reasoning_tokens":2371,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-10T20:49:07.899433+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the current-induced spin polarization of an illuminated Bi2Se3 surface, for example by Kerr rotation or spin-sensitive optical detection, while sweeping the normalized field strength through $A\\simeq 1.8$ and look for the predicted sign change; equivalently, replace the FEWT occupation rule by a kinetic or master-equation solution of the Floquet steady state and check whether $\\chi^s_{xy}$ still crosses zero near the same field strength.","supporting_citations":[{"cited_title":"Baykusheva, A","cited_arxiv_id":null,"evidence_quote":"Supplies the four-parameter surface Hamiltonian for Bi2Se3 and the phenomenological parameter values used throughout the calculation."},{"cited_title":"Bianchi, D","cited_arxiv_id":null,"evidence_quote":"Fixes the chemical potential at 0.28 eV above the Dirac point from experimental surface-state observations, defining the Fermi surface for the susceptibility calculation."},{"cited_title":"Kim and K","cited_arxiv_id":null,"evidence_quote":"Supplies the Floquet eigenstate-wise thermalization (FEWT) hypothesis, used without derivation in this paper, that fixes the Floquet sideband occupations entering the lesser Green's function."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the equilibrium Kubo formula for current-induced spin polarization that the paper generalizes to the Floquet nonequilibrium case."},{"cited_title":"Echardt and E","cited_arxiv_id":null,"evidence_quote":"Supplies the Floquet-space high-frequency approximation used alongside the Floquet-Magnus expansion in the derivation."}],"review_version":1}