{"id":"cf7762e3-4954-4630-8ca3-e1a6cdc129a5","arxiv_id":"2608.05887","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"FeSb2 is shown to host topological-polarization surface states whose correlation-controlled behavior is tracked by nonreciprocal transport and which can be gated into a ferromagnetic-like surface state.","lead":"Experiments and calculations on the narrow-gap insulator FeSb2 indicate that its metallic surface states come from topological polarization, and that strong electron correlations in the bulk control them. The authors also report that electric-field gating can switch the surface into a magnetic state, suggesting a route to combine topology and correlations in 3d compounds.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The R2ω fingerprint is not uniquely tied to the topological-polarization surface state; a trivial EDLT/interface channel could produce the same signal, and the WCC invariant is computed only in noninteracting DFT, leaving the correlated topological assignment unproven.","rationale":"The paper makes a rich, multi-technique case. The DFT WCC analysis and slab surface states (Fig. 1g–i) provide genuine computational support for the existence of topological-polarization surface states, and the CDFS/CHOD orbital reconstruction combined with DFT+DMFT (Fig. 2) is a plausible observation of correlation-driven change. The reproducibility on film B, apart from the sign reversal, is also a positive control. My concern is not that the data are fabricated or internally inconsistent; it is that the central inference—that the R2ω signal is caused by the topological-polarization surface state and that its temperature onset proves bulk-edge correspondence—is not uniquely determined. The unexplained sign reversal in film B is a self-reported anomaly that directly tests the fixed-polarization assumption, and the transport formula is generic to any polar channel. In addition, the topological invariant itself is computed noninteracting; since the paper's central concept is a correlated topological polarization, the invariant's interaction robustness should be checked. Neither issue forces rejection: a quantitative calculation of γ from the computed surface bands, or a direct surface-spectroscopy experiment, could close the gap. Until then, CONDITIONAL is the right verdict and no adjustment is needed.","tokens_in":13054,"tokens_out":10137,"duration_ms":108661,"concrete_test":"Compute the nonreciprocal (second-order) conductivity tensor γ for the (101) slab surface bands of Fig. 1g using the standard quantum kinetic formula, with the DFT+DMFT self-energy (U=5 eV, J=0.8 eV) at 50 K and 200 K, and compare the sign, temperature dependence, and Fermi-level (gate) dependence with Fig. 3e and with film B's gating sign reversal. If the computed γ vanishes or disagrees in sign or trend, the R2ω data do not uniquely reflect the topological-polarization surface state, and the bulk-edge correspondence claim would require direct surface-spectroscopy support; if it matches, the reader's main objection is answered.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The load-bearing step is the assignment of the measured nonreciprocal resistance to the topological-polarization surface states computed in Fig. 1g–i. Two gaps make this assignment insecure. First, the WCC/Zak-phase and slab calculations are PBE-DFT results on Kohn–Sham bands; the paper never recalculates the WCC or the surface states with the U=5 eV, J=0.8 eV interactions used in its own DFT+DMFT (Fig. 2e). Since the central concept is a correlated topological polarization, its survival under strong correlations is exactly what must be shown. Second, the transport test is not specific to the computed surface state: R=R0[1+γ0(P×H)·I] holds for any inversion-broken conductive channel, and the [DEME][TFSI]/FeSb2 interface is itself a strongly polar, chemically modified surface. The paper reports that in a second film the nonreciprocal signal reverses sign on gating, with the origin 'remains to be clarified' (Methods)—an unexplained sign change for a fixed bulk polarization is the kind of behavior expected from a trivial interface channel, not from a bulk-derived topological polarization. Consequently, the claimed 'direct evidence of bulk-edge correspondence' reduces to a temperature coincidence between γ appearing below ~50 K and the bulk CDFS orbital reconstruction at 100–200 K. The DFT slab/WCC calculation is genuine supporting evidence, but it does not establish that the current-carrying channel is that state.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper reports a combined computational and experimental study of FeSb2, a narrow-gap 3d insulator, and argues that its (101) surface hosts metallic polar surface states of topological-polarization origin, generated by the bulk Zak phase / obstructed-atomic-insulator character rather than by spin-orbit coupling. PBE slab calculations show surface bands crossing the bulk gap with charge floating off the outermost atoms; a WCC Wilson-loop analysis places valence Wannier centers in interatomic positions. Temperature-resolved synchrotron CDFS maps show a redistribution of Fe 3d valence electron density between 100 and 200 K, quantified by CHOD as a decrease of the d_xy/d_yz occupation and an increase of the d_z2 occupation, which the authors attribute to correlations and reproduce qualitatively with DFT+DMFT (U=5 eV, J=0.8 eV). In ionic-liquid-gated 23.6-nm FeSb2(101) films, the authors observe a gate-tunable surface conduction channel, a second-harmonic nonreciprocal resistance appearing below about 50 K and peaking near 25 K, and, above 4 V gate bias, a hysteretic anomalous Hall effect indicating an emergent surface magnetic order with Tc up to 50 K. They interpret these observations as demonstrations that the topological-polarization surface states inherit the bulk correlation-driven orbital reconstruction (bulk-edge correspondence) and that gating drives the surface into a ferromagnetic or possibly altermagnetic state.","tokens_in":13342,"tokens_out":6767,"duration_ms":66522,"significance":"If the interpretation is correct, this is a significant advance: it would be one of the first explicit demonstrations that a topological polarization, rather than spin-orbit band inversion, can stabilize metallic surface states in a correlated 3d compound, and that the surface states can be tuned electrostatically through a magnetic quantum phase transition. The paper's computational backbone is partly parameter-free: the PBE WCC and slab calculations use no adjustable parameters, the CDFS maps are raw diffraction-derived density maps, and the DFT+DMFT calculations use standard values of U and J. The experiments are internally consistent in that the gate-induced resistance drop, carrier-density increase, nonreciprocal transport, and anomalous Hall effect are qualitatively reproduced in a second film. However, the central claim of 'direct evidence' currently relies on a correlation between temperature scales and on a transport fingerprint that is not unique to topological-polarization surface states; the strongest claim is therefore conditional. If the identified gaps are addressed in revision, the work would merit publication in a high-impact journal.","major_comments":[{"comment":"The topological-polarization assignment is based entirely on PBE-DFT WCC and slab calculations, while the correlation-driven reconstruction that the paper claims governs this polarization is established by DFT+DMFT with U = 5.0 eV, J = 0.8 eV (Methods). The WCC/Zak-phase invariant and the floating surface-state wavefunctions are never recomputed with these interactions. Since the paper's central concept is a correlated topological polarization whose magnitude is asserted to be modulated by the bulk orbital reconstruction, the survival of the invariant under strong correlations is precisely what must be demonstrated; without such a calculation or a clear adiabatic-continuity argument, the step from 'noninteracting WCC' to 'correlated bulk-edge correspondence' is not established.","section":"First-principles calculations on bulk and surface electronic states in FeSb2; Fig. 1g–i"},{"comment":"The measured second-harmonic resistance is interpreted through R = R0[1 + γ0(P×H)·I], but this form applies to any inversion-broken conducting channel. The EDLT interface, involving a strongly polar [DEME][TFSI] ionic liquid on a chemically reactive FeSb2 surface, could itself produce such a signal. The Methods report that in film B the nonreciprocal signal reverses sign upon gating, with the origin left unexplained; a fixed bulk polarization would not be expected to reverse sign under gating, whereas a trivial interface channel could. The transport data therefore do not uniquely identify the topological-polarization surface state as the current-carrying channel, and the phrase 'direct evidence' overstates what the experiment demonstrates without additional corroboration, such as a direct probe of the surface-state dispersion or a systematic control on the interface condition.","section":"Nonreciprocal transport from the polar surface state; Methods, Reproducibility"},{"comment":"The claimed bulk-edge correspondence is supported mainly by a temperature coincidence—nonreciprocal transport emerges below about 50 K, whereas the CDFS/CHOD orbital reconstruction occurs between 100 and 200 K. The paper does not provide a quantitative relation between γ(T) and the reconstructed occupancies, and the two temperature scales differ by roughly a factor of two. A direct link, such as a calculation of the WCC or surface-state weight as a function of the correlated orbital occupations, or a control measurement on a surface whose bulk reconstruction is suppressed, is needed to elevate the observed concurrence from correlation to evidence.","section":"Nonreciprocal transport from the polar surface state; Fig. 3d,e compared with Fig. 2a–d"}],"minor_comments":[{"comment":"The minimization expression contains garbled notation ('minimize,,.' and misplaced sub-/superscripts); please restore the intended mathematical form.","section":"Methods, CHOD analysis"},{"comment":"'e.g., G-Z' and 'X=-S?' should be typeset as Γ–Z and X–S (or an explicit k-path label) to be readable.","section":"Fig. 1 and main text, bulk band structure discussion"},{"comment":"'G.H. and K.K.' names a K.K. who is not in the author list; this is presumably a typo for T.K. or K.W. and should be corrected.","section":"Author contributions"},{"comment":"'The ionic liquids used in this study was' should read 'The ionic liquid used in this study was' or 'The ionic liquids used in this study were'.","section":"Methods, Device fabrication"},{"comment":"Supplementary Fig. 3 is referenced both for thermal expansion and for θ–2θ XRD; please renumber to avoid ambiguity.","section":"Supplementary figure references"},{"comment":"The repository URL appears as a placeholder 'https://...'; the full link should be provided.","section":"Data availability"}],"recommendation":"major_revision","confidential_remarks":"Editor: This is a strong paper that is currently overclaimed at the 'direct evidence' level. The main issues are the PBE-only WCC calculation and the non-uniqueness of the nonreciprocal transport fingerprint, both of which can be repaired in revision. I would also ask the authors to disclose clearly that the CHOD method is introduced in an author-overlapping preprint (ref. 49) and to provide a fuller description or benchmark of the method, since the orbital-reconstruction quantification depends on it. The scope fits the journal and the experimental/computational backbone is generally careful; with the requested revisions, the paper would be suitable for publication."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Dear colleague,\n\nThe thing to know: this paper makes a serious, multi-technique case that FeSb2 hosts topological-polarization surface states whose behavior tracks bulk correlations, and that gating can make the surface magnetic. It is not a slam dunk, but it deserves a real referee.\n\nWhat's actually new: previous ARPES saw metallic surface states but left their origin unsettled. This paper adds three things: parameter-free PBE WCC/slab calculations showing displaced Wannier centers and floating surface bands; synchrotron CDFS maps showing a real-space orbital reconstruction between 100 K and 200 K with DFT+DMFT support; and nonreciprocal surface transport plus a gate-induced hysteretic AHE. If the interpretation holds, that is a correlated topological surface in a 3d compound without SOC, which would be a nice advance.\n\nThe DFT is the strongest part. The WCC calculation is clean and the slab bands decay as expected. The CDFS maps are convincing because the orbital change is visible in the raw maps, not just in the fitted occupancies.\n\nThe soft spots are real but not fatal. The nonreciprocal transport is the load-bearing experimental evidence for the topological-polarization assignment, but the formula R = R0[1+γ0(P×H)·I] holds for any inversion-broken conductor, and the EDLT interface is itself polar and chemically modified. The paper does not measure the surface-state dispersion or spin texture, so a trivial interface channel could in principle produce the same signal. The unexplained sign reversal in film B on gating (admitted in Methods) does not help.\n\nSecond, the paper never recomputes the WCC or surface states with the U = 5 eV, J = 0.8 eV interactions used in its own DFT+DMFT. Since the central concept is a correlated topological polarization, its survival under strong correlations should be checked. That's an omission the authors can fix.\n\nThird, the \"direct evidence of bulk-edge correspondence\" is weaker than claimed. The nonreciprocal signal appears below about 50 K, while the CDFS orbital reconstruction is characterized at 100–200 K. That is a coarse correlation with a factor-of-two offset, not a tight coincidence. It suggests a connection, but it does not nail it.\n\nThe gate-induced AHE with hysteresis is suggestive of surface magnetism, but it is inferred from transport alone; the altermagnetic speculation is clearly labeled as such.\n\nWho this is for: anyone working on topological polarization, correlated topological materials, or FeSb2. The paper deserves a serious referee: the theory is solid, the experiments are careful, and the flaws are addressable rather than fatal. I'd send it out.\n\nBest,","headline":"Not a slam dunk, but a serious multi-technique case for correlated topological surface states in FeSb2; the transport evidence is correlative rather than unique, yet the paper deserves a real referee.","tokens_in":13931,"tokens_out":3484,"would_cite":true,"duration_ms":33132,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"The metallic surface states of the narrow-gap insulator FeSb2 are polar surface states of topological-polarization origin, controlled by bulk electron correlations, and electrostatic gating drives them into a ferromagnetic or possibly…","keywords":["topological polarization","FeSb2","correlated topological surface","nonreciprocal transport","orbital reconstruction","obstructed atomic insulator","electrostatic gating","altermagnetism"],"falsifier":"An angle-resolved photoemission measurement on a clean FeSb2(101) surface that failed to find floating surface bands inside the gap, or a control experiment on a non-topological polar film that showed the same nonreciprocal transport, would falsify the assignment. A more targeted test: measure the second-harmonic resistance as a function of temperature through the orbital reconstruction onset and check whether its onset tracks the reconstructed orbital occupation shift in a single sample.","tokens_in":12842,"feed_emoji":"🧲","tokens_out":8980,"duration_ms":82913,"temperature":0.7,"pith_summary":"This paper argues that the metallic surface states of the narrow-gap insulator FeSb2 are polar surface states of topological-polarization origin, and that strong electron correlations in the bulk control when they exist and what they do. If true, FeSb2 becomes a working example of a correlated topological surface, a place where two usually separate worlds, band topology and strong interactions, meet on one surface. The authors support the claim by showing that the bulk's valence electron density carries part of its charge in interatomic regions, by imaging a temperature-driven reconstruction of the iron 3d orbital occupation that strengthens the polarization at low temperature, and by detecting nonreciprocal surface transport that turns on only in that low-temperature regime. Electrostatic gating then drives the same surface into a ferromagnetic or possibly altermagnetic state, while the polarization survives.","feed_headline":"Topological-polarization surface states found in correlated FeSb2","feed_subtitle":"Surface transport tracks a bulk orbital reconstruction; gating yields ferromagnetic or altermagnetic order.","key_machinery":"The central object is topological polarization, the quantized displacement of valence Wannier charge centers into the interatomic region, equivalent to a nonzero Zak phase in the bulk bands. This displaced charge is what appears at a surface termination as floating polar surface states. The paper's measurement machinery is nonreciprocal transport: in a polar system the resistance acquires a term $R = R_0[1 + \\gamma_0 (\\mathbf{P}\\times\\mathbf{H})\\cdot\\mathbf{I}]$, detected as a second-harmonic resistance that is forbidden in the centrosymmetric bulk. The correlation control comes from the temperature dependence of the iron 3d orbital occupation, imaged in real space and reproduced by dynamical mean-field calculations, which modulates the amount of bonding charge available for the polarization.","core_discovery":"In FeSb2, the metallic surface states seen in earlier experiments are assigned to polar surface states of topological-polarization origin. The bulk valence Wannier charge centers lie in interatomic positions rather than on the Fe and Sb atoms, so cutting the crystal exposes displaced charge as floating surface states within the gap. The paper further claims a bulk-edge correspondence in a correlated system: as temperature rises, a correlation-driven reconstruction of the Fe 3d orbital occupation mixes antibonding weight into the occupied states and depletes the bonding charge that generates the polarization, and the nonreciprocal transport signal of the polar surface disappears in step with that reconstruction. Finally, gate voltages above a few volts drive the correlated surface across a quantum phase transition into a ferromagnetic or possibly altermagnetic state, with the nonreciprocal signal persisting into the ordered phase.","pith_inferences":["A direct test of the topological-polarization assignment would be spin-resolved photoemission on a clean (101) surface to look for the predicted floating surface bands and their polarization; the paper infers the surface state from transport and calculations rather than imaging its dispersion.","If the surface is indeed altermagnetic, the anomalous Hall effect should show a characteristic dependence on the in-plane field direction relative to the crystal axes, which the current data do not yet isolate.","The persistence of the nonreciprocal signal across the magnetic transition suggests that the polarization and the order parameter are coupled; measuring the nonreciprocal coefficient as a function of gate voltage through the transition could reveal whether the magnetic order enhances or competes with the polarization.","The same nonreciprocal transport protocol could be applied to other correlated narrow-gap insulators predicted to be obstructed atomic insulators, offering a bulk-transport substitute for surface-sensitive spectroscopy in materials where clean surfaces are hard to prepare."],"forward_implications":["The previously unexplained metallic surface states of FeSb2 are identified as topological-polarization states, resolving their origin and connecting them to the bulk electronic structure.","Bulk-edge correspondence holds in a correlated insulator: the surface's polar transport tracks the bulk orbital reconstruction, so the surface state is not a purely surface-localized phenomenon.","Electrostatic gating is a viable control knob for a correlated topological surface, driving it into a ferromagnetic or possibly altermagnetic state with transition temperatures up to 50 K.","Because topological polarization needs no heavy elements or spin-orbit coupling, the same physics should appear in other 3d transition-metal compounds with obstructed atomic insulator character, including FeSi.","The correlated surface, with its narrow bands and tunable carrier density, sits between correlated oxides and moiré systems as a platform for two-dimensional strongly correlated phases such as superconductivity or density waves."],"supporting_citations":[{"why":"Defines the Zak phase, the bulk quantity behind topological polarization.","marker":"18"},{"why":"Classifies obstructed atomic insulators and shows Wannier centers displaced from atoms, the basis for assigning FeSb2's polarization.","marker":"23"},{"why":"Shows polar surface states emerge from Zak-phase charge in nonmagnetic materials.","marker":"26"},{"why":"Reports a topological-polarization surface state in the correlated insulator FeSi, the closest precedent for FeSb2.","marker":"28"},{"why":"Establishes FeSb2 as a correlated narrow-gap semiconductor and supplies the gap scale.","marker":"32"},{"why":"Reports the metallic surface states whose origin this paper assigns to topological polarization.","marker":"37"},{"why":"Derives the nonreciprocal transport formula that the paper uses to detect the polar surface.","marker":"52"},{"why":"Frames nonreciprocal responses as a probe of non-centrosymmetric quantum materials.","marker":"53"},{"why":"Predicts altermagnetism in doped FeSb2, supporting the possible altermagnetic surface state.","marker":"58"}],"fun_headline_variants":["FeSb2's surface states traced to topological polarization","Gate voltage flips correlated FeSb2 surface to magnetic state","Bulk-edge correspondence in correlated FeSb2 topological polarization","Correlated FeSb2 surface turns magnetic under gate voltage"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The claim rests on interpreting the measured second-harmonic nonreciprocal resistance as the signature of the topological-polarization surface state; if that signal actually came from the electric-double-layer interface or from a trivial inversion-breaking surface state, the bulk-edge correspondence conclusion would not follow from the data.","fun_headline_variants_meta":{"raw":{"variants":["FeSb2's surface states traced to topological polarization","Gate voltage flips correlated FeSb2 surface to magnetic state","Bulk-edge correspondence in correlated FeSb2 topological polarization","Correlated FeSb2 surface turns magnetic under gate voltage"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000895,"raw_usage":{"total_tokens":3815,"prompt_tokens":861,"completion_tokens":2954,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":477,"completion_tokens_details":{"reasoning_tokens":2887}},"tokens_in":477,"tokens_out":2954,"duration_ms":22486,"temperature":1.0,"reasoning_tokens":2887,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-07T21:43:02.242109+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"An angle-resolved photoemission measurement on a clean FeSb2(101) surface that failed to find floating surface bands inside the gap, or a control experiment on a non-topological polar film that showed the same nonreciprocal transport, would falsify the assignment. A more targeted test: measure the second-harmonic resistance as a function of temperature through the orbital reconstruction onset and check whether its onset tracks the reconstructed orbital occupation shift in a single sample.","supporting_citations":[],"review_version":1}