{"id":"07d9f696-e937-4975-a4ca-b910b98b39c3","arxiv_id":"2608.09309","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":7.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":6,"one_line_summary":"A magnetic exchange field can induce a circular photogalvanic effect in a centrosymmetric monolayer without changing its lattice structure.","lead":"This paper predicts that a magnetic substrate can make a structurally symmetric two-dimensional crystal produce a photocurrent that depends on the handedness of circularly polarized light. The mechanism is demonstrated numerically for antimony hydride and is tunable by the magnetization direction.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Per-valley CPGE tensors are opposite (β^K = −β^{−K}), so the total β_ij in Eq. (4) sums to zero; the valley-selection argument double-counts the helicity sign and yields helicity-independent currents.","rationale":"The reader's weakest-assumption analysis focused on the necessity of staggered, in-plane exchange fields. That is a valid physical concern, but it is not the most load-bearing issue. The manuscript itself provides equations sufficient to expose a deeper inconsistency: with identical isoenergy contours at ±K and opposite Berry curvatures, the per-valley CPGE contributions sum to zero in Eq. (4). The paper's counter-argument, that valley-selective excitation selects one valley per helicity, is mathematically incompatible with Eq. (3), because the sign of [E×E*] already encodes the helicity. Combining the opposite per-valley tensors with the opposite pseudovector for σ− gives j_+ = j_−, not j_+ = −j_−. This is not a question of missing parameters or an unverified substrate configuration; it is an internal inconsistency in the central derivation. If the full-BZ sum is indeed zero, the primary numerical evidence in Fig. 2 does not support the headline claim of a magnetically induced circular photogalvanic effect in a symmetric material. The out-of-plane case may still yield a valley-split photocurrent, but that is not the mechanism emphasized in the abstract and introduction. For these reasons, the verdict should move from CONDITIONAL to REJECT, contingent on the concrete test confirming the cancellation.","tokens_in":10703,"tokens_out":29093,"duration_ms":316263,"concrete_test":"Evaluate the full Brillouin-zone sum of Eq. (4) for the pure in-plane field configuration (θ = 0, ϕ = π/2) using the same parameters as Fig. 2, without restricting the sum to individual valleys. If the total β_ij(ω) is zero, the MCPGE for the in-plane case is an artifact of valley-resolved bookkeeping. Independently, compute the injection currents for σ+ and σ− light directly from the interband velocity matrix elements and compare j_+ with j_−; if they are equal in magnitude and direction, the response is helicity-independent and the central claim fails.","verdict_should_be":"REJECT","load_bearing_attack":"Equations (3) and (4) define the total CPGE tensor as β_ij(ω) = Σ_k iπe^3/(ħ²A) ∂_{k_i}E_{k,12} Ω^v_j δ(ħω−E_{k,21}). The paper computes β^τ_ij per valley and states around Fig. 2(h) that β^K_ij = −β^{−K}_ij, with identical isoenergy contours at ±K. Hence the full-Brillouin-zone sum in Eq. (4) is β^K + β^{−K} = 0, so the total CPGE vanishes in the pure in-plane configuration that is the paper's main demonstration. The paper tries to recover the effect by invoking valley-selective circular dichroism, but the helicity is already contained in [E×E*]_j in Eq. (3). For σ+ the current is j_+ = β^K s, with s = [E×E*]; for σ−, only −K is excited, so j_− = β^{−K}(−s) = (−β^K)(−s) = β^K s = j_+. Thus opposite helicities produce equal, not opposite, currents. The paper's opposite conclusion follows from dropping the sign of E×E* when selecting the valley, double-counting the helicity. A correct treatment of the optical matrix elements (which the Berry-curvature formula already encodes) gives cancellation between symmetric valleys. Only an out-of-plane exchange component that splits the valley gaps (Fig. 3) lets one valley dominate at fixed photon energy, which is a different, valley-split configuration rather than the 'symmetric material with in-plane field' scenario claimed.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript proposes a magneto-circular photogalvanic effect (MCPGE) in a centrosymmetric monolayer SbH placed on a magnetic substrate. The authors use a four-band k·p Hamiltonian with staggered magnetic exchange, a staggered potential, and Rashba spin-orbit coupling; compute per-valley CPGE tensors from the standard injection-current formula in Eq. (4); and find that the exchange field shifts the valence-band isoenergy contours, producing β^K_ij = −β^{−K}_ij together with valley-contrast Berry curvature. They argue that valley-selective circular dichroism converts these opposite per-valley tensors into a helicity-dependent photocurrent. The paper also computes single-particle and excitonic absorption via a Bethe-Salpeter equation and discusses tunability with magnetization direction.","tokens_in":11115,"tokens_out":12336,"duration_ms":143200,"significance":"If correct, the proposal would broaden CPGE to magnetically proximitized centrosymmetric monolayers and offer a tunable, valley-selective photocurrent. The paper has clear strengths: it applies the standard CPGE tensor formula rather than fitting to a target, it presents explicit numerical maps of band shifts and Berry curvature, and it includes a BSE treatment of excitonic absorption. However, the central in-plane demonstration is internally inconsistent: the full-Brillouin-zone sum in Eq. (4) cancels exactly for the configuration shown in Fig. 2. The out-of-plane valley-split configuration in Fig. 3 could in principle support a nonzero effect, but the manuscript would need to be substantially reframed around that case. As written, the main claim is not established.","major_comments":[{"comment":"The in-plane configuration shown in Fig. 2 yields a vanishing total CPGE, and the manuscript's valley-selection argument double-counts the helicity sign. In Eq. (4) the CPGE tensor is a sum over the full Brillouin zone. The manuscript states that, for in-plane exchange, the isoenergy contours at ±K are identical, ∂_{k_y}E_{12} is identical at the two valleys, and β^K_ij = −β^{−K}_ij. Summing the two valley contributions in Eq. (4) therefore gives β^K_ij + β^{−K}_ij = 0 identically. The subsequent argument that valley-selective circular dichroism produces a helicity-dependent current is not a valid rescue: writing s_j = [E×E*]_j, σ+ excitation gives j_i = β^K_ij s_j, while σ− excitation gives j_i = β^{−K}_ij(−s_j) = (−β^K_ij)(−s_j) = β^K_ij s_j. The two helicities thus produce the same current, not opposite currents. Since the full-BZ sum already vanishes, the in-plane configuration of Figs. 2(a)–2(h), which is the stated proof of principle, yields zero magneto-CPGE.","section":"Eq. (4), Fig. 2(h), and the text after Fig. 2(h)"},{"comment":"The model is not a centrosymmetric electronic Hamiltonian, so the claim that magnetism overcomes the inversion-symmetry limitation is not demonstrated. Eq. (2) assumes staggered exchange fields with M_A ≠ M_B, and the Hamiltonian also contains HU = U σ_z. Both terms are odd under A↔B sublattice exchange, which is precisely the operation that realizes inversion for this buckled honeycomb structure. The text even states that the exchange 'breaks the symmetry between the two sublattices.' The abstract's claim that the limitation of inversion-symmetry breaking is overcome is therefore an input assumption, not a consequence derived from a centrosymmetric starting point. To support the central claim, the authors would need to start from an inversion-invariant Hamiltonian (for example, M_A = M_B and U = 0) or else explicitly reframe the work as CPGE controlled by an inversion-breaking magnetic proximity field.","section":"Eq. (2) and the definition of HU"},{"comment":"The statement that 'linearly polarized light is sufficient to induce a nonzero injection current' contradicts Eq. (3). The injection rate is proportional to [E×E*], which vanishes identically for linearly polarized light. The valley asymmetry shown in Fig. 3 makes β^K and β^{−K} unequal in magnitude, but a CPGE current still requires circular polarization or another chiral excitation. If this is a typo, it must be corrected; as written, it indicates a confusion between valley polarization and helicity selection. This issue is load-bearing because the paper's tunability discussion relies on it.","section":"Text near Fig. 3(b)"},{"comment":"The central numerical results are not reproducible from the information given. The values of ħv_F, λ_SO, λ_R, U, M_A, M_B, the assumed constant ratio M_A/M_B, the Lorentzian broadening Γ, and the dielectric parameters entering the BSE kernel are all omitted from the main text; only a reference to Supplemental Material [38] is given. The authors should provide the complete parameter set, the integration domain used for the per-valley β^τ tensors, and the dielectric screening model, either in the paper or in an accessible supplement. Without these inputs, Figs. 2–4 cannot be checked or reproduced.","section":"Eqs. (4), (5), and Figs. 2–4"}],"minor_comments":[{"comment":"There is a typographical error in the word 'sufficient' in the paragraph after Fig. 3(b); it should be corrected.","section":"Throughout"},{"comment":"The sentence 'Since ±K valleys have opposite Berry curvatures, the net currents at the two valleys are in opposite directions' is only true for the per-valley tensors before the full-Brillouin-zone sum is taken; in the full sum of Eq. (4) these contributions cancel. The wording should be revised to avoid implying a nonzero total for the in-plane case.","section":"Fig. 2 discussion"},{"comment":"The CPGE calculation uses a two-band model while the absorption calculation uses the four-band Hamiltonian. The authors should clarify why the additional bands included in the absorption do not affect the low-frequency CPGE response, or estimate their contribution.","section":"Eqs. (4) and (6)"},{"comment":"The paper states that 'a constant ratio M_A/M_B has been kept' when increasing M_A, but neither the ratio nor the absolute values of M_A and M_B are given. A quantitative estimate from a specific magnetic substrate would substantially strengthen the proposal.","section":"Fig. 3 and text"},{"comment":"Reference [38] is to Supplemental Material; if the supplement is not included with the submitted manuscript, the numerical details are effectively unavailable to the reader. The supplement should accompany the submission.","section":"References"}],"recommendation":"major_revision","confidential_remarks":"The in-plane cancellation in Eq. (4)/Fig. 2 is decisive and rules out the paper's main demonstration as stated. The manuscript could nevertheless be salvaged by restricting the claim to the tilted/out-of-plane configuration of Fig. 3, where valley splitting makes β^K and β^{−K} unequal in magnitude, and by explicitly acknowledging that staggered magnetic exchange and U are inversion-breaking inputs rather than a replacement for inversion breaking. If the authors are unwilling to make that reframing, the paper should not be accepted. The missing parameter values must also be supplied before any positive recommendation."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"This one is worth a look, but the central claim as presented doesn't survive a careful pass. The paper's idea—use a magnetic exchange field to break effective inversion symmetry in a centrosymmetric monolayer and thereby get a circular photogalvanic effect—is genuinely new, as far as I can tell, and the model calculations are standard and internally consistent. However, the main demonstration, for a pure in-plane exchange field, contains a sign error. In Fig. 2(h) they show β^K = −β^{−K}, and they explicitly say the isoenergy contours at ±K are identical. That means the full-Brillouin-zone sum in Eq. (4) is zero. The paper tries to rescue the effect by invoking valley-selective circular dichroism, but that double-counts the helicity: for σ+ light you excite K, current j = β^K s; for σ− light you excite −K, but now s → −s and β^{−K} = −β^K, so j = β^K s again. The two helicities give equal, not opposite, currents. So there is no helicity-dependent current in the in-plane configuration they emphasize.\n\nThe out-of-plane component in Fig. 3 is different: it splits the valley gaps, so at a fixed photon energy only one valley is resonant. That does give a helicity-dependent current through the valley polarization. But that is a valley-split mechanism, not the \"symmetric material with in-plane field\" scenario advertised in the abstract and Fig. 2. The paper conflates the two.\n\nOther soft spots are the missing Hamiltonian parameters and broadening in the main text (all in the Supplemental Material), no code or data shipped, and the staggered exchange MA ≫ MB is assumed, not derived. These are minor compared with the sign issue.\n\nWhat's good: the symmetry analysis of the band shifts is clear, the BSE absorption is a nice addition, and the authors honestly distinguish their proposal from magneto-gyrotropic effects and the CrI3 magnetic photovoltaic effect. The tuning of the response with magnetization direction is a useful idea.\n\nWho should read it: people working on magnetic proximity and nonlinear optoelectronics. The flaw is subtle enough that a careful referee might catch it; it deserves peer review because the core idea is worth testing, and the out-of-plane scenario may be publishable after major revision.\n\nMy recommendation: send it to review, but the referee should be asked to verify the sign of [E×E*] in the valley-selection argument, and the paper should be reframed around the out-of-plane valley-split case if that survives.","headline":"A creative mechanism for helicity-dependent currents in a centrosymmetric monolayer, but the in-plane configuration cancels between valleys; the paper's sign error undermines its central claim.","tokens_in":11636,"tokens_out":4962,"would_cite":false,"duration_ms":51638,"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":"A magnetic exchange field from a substrate can break inversion symmetry in the band dispersion of a centrosymmetric monolayer, generating a helicity-dependent circular photogalvanic effect without any lattice asymmetry.","keywords":["circular photogalvanic effect","magneto-circular photogalvanic effect","magnetic proximity effect","valley-contrast photocurrent","centrosymmetric 2D material","SbH monolayer","Bethe-Salpeter equation","helicity-dependent photocurrent"],"falsifier":"Measure the helicity-dependent photocurrent in a centrosymmetric SbH monolayer on a magnetic substrate while rotating the in-plane magnetization: the theory predicts the current should follow the in-plane component of the exchange field, vanishing when the field is perpendicular to the current direction ($\\theta = \\pi/2$) and reversing sign when the field direction is reversed ($\\theta = \\pi$). Observing no such angular dependence, or no current when $M_A = M_B$, would falsify the proposed mechanism.","tokens_in":10530,"feed_emoji":"🧲","tokens_out":10639,"duration_ms":89395,"temperature":0.7,"pith_summary":"Circular photogalvanic effect normally requires a material whose lattice lacks inversion symmetry; this paper proposes that a magnetic effect can do the same job in a structurally symmetric material. As a proof of principle, it studies a monolayer of SbH on a magnetic substrate, where staggered magnetic exchange fields from the substrate shift the energy bands at the two valleys so that circularly polarized light excites unbalanced currents. The paper shows the resulting photocurrent is tunable by the magnitude and direction of the substrate magnetization, and it computes the accompanying single-particle and excitonic optical absorption with a Bethe–Salpeter equation. If the mechanism holds, it removes the structural inversion-breaking requirement for helicity-dependent photocurrents and makes magnetic proximity a switch for valley-selective optoelectronic responses.","feed_headline":"Magnetic exchange creates helicity photocurrents in symmetric crystals","feed_subtitle":"Magnetism replaces lattice asymmetry to let circularly polarized light drive currents in a centrosymmetric monolayer.","key_machinery":"The argument is carried by a four-band effective Hamiltonian $H_{\\rm tot} = H_0 + H_{\\rm ex} + H_U + H_R$ for the low-energy states at the $\\pm K$ valleys of monolayer SbH. $H_0$ is the bare Dirac-type band structure with spin-orbit coupling $\\lambda_{\\rm SO}$; $H_{\\rm ex}$ is the staggered magnetic exchange field $M_A$ ($M_B$) acting on the A (B) sublattice along the substrate magnetization direction; $H_U$ is a staggered sublattice potential; and $H_R$ is Rashba spin-orbit coupling. The load-bearing mechanism is the competition between the Zeeman splitting of the exchange field and the Rashba coupling, which shifts the bands perpendicular to the in-plane field and makes the integrand of the CPGE tensor $\\beta_{ij}(\\omega) = \\sum_k (i\\pi e^3 / \\hbar^2 A)\\,\\partial_{k_i}E_{k,12}\\,\\Omega^v_j(k)\\,\\delta(\\hbar\\omega - E_{k,21})$ asymmetric in $k$-space. For the absorption side, the Bethe–Salpeter equation yields the exciton envelope functions and, through them, the circularly resolved absorption spectra.","core_discovery":"The paper's central claim is that a magnetic effect can effectively break the symmetry of the energy dispersion in a centrosymmetric material and thereby generate helicity-dependent photocurrents, an effect it names the magneto-circular photogalvanic effect (MCPGE). For monolayer SbH on a magnetic substrate, the substrate induces staggered exchange fields $M_A$ and $M_B$ on the two sublattices; with $M_A \\gg M_B$ and a finite in-plane component, the bands at the $\\pm K$ valleys shift in opposite directions perpendicular to the field. This breaks $k$-space inversion symmetry, so the resonant optical transition contour becomes asymmetric and the contributions from opposite momenta no longer cancel. Because the two valleys couple exclusively to opposite circular polarizations and carry opposite Berry curvature, a given helicity excites one valley and produces a net current. An out-of-plane field component further splits the valleys, yielding near-100% valley polarization, and the same physics appears in the excitonic absorption obtained from the Bethe–Salpeter equation.","pith_inferences":["The staggered-exchange requirement suggests the mechanism should generalize to any buckled centrosymmetric monolayer in which a magnetic substrate couples differently to the two sublattices; functionalized bismuth and antimony monolayers, and possibly two-dimensional altermagnets, are natural candidates.","Because the current direction and magnitude are set by the exchange field direction and strength, MCPGE could serve as a non-invasive optical readout of magnetic ordering or magnetization reversal in van der Waals heterostructures.","If the exchange field can be switched electrically or by spin-orbit torque, the same structure becomes a helicity-controlled optoelectronic switch, and the valley-selective excitation could be extended to helicity-controlled terahertz emission in centrosymmetric Dirac semimetal films.","The calculation assumes ballistic injection-current generation; in real devices, disorder, finite temperature, and the dielectric environment will modify the magnitude, but the angular dependence and valley-contrast signatures should remain."],"forward_implications":["Centrosymmetric 2D materials with magnetic proximity can host helicity-dependent photocurrents without any lattice inversion breaking, widening the material pool for circular photogalvanic devices.","The MCPGE current is tunable by the strength and direction of the substrate magnetization, including a current reversal when the in-plane field is reversed.","With an out-of-plane exchange-field component, valley polarization approaches ±100%, and linearly polarized light alone suffices to generate a nonzero injection current.","Excitonic absorption peaks follow the single-particle absorption and reverse their circular polarization after each band crossing, so the optical response can reveal band crossings induced by magnetic proximity.","The effect also adds an externally tunable contribution to photocurrents in non-centrosymmetric materials and can serve as an optical probe of magnetic proximity in 2D materials."],"supporting_citations":[{"why":"Identifies monolayer SbH as a large-gap topological insulator with the buckled hexagonal lattice used for the proof-of-principle model.","marker":"[29]"},{"why":"Supplies the effective Hamiltonian with staggered magnetic exchange and staggered potential on the A/B sublattices, the core symmetry-breaking ingredient.","marker":"[31]"},{"why":"Gives the CPGE tensor formula used to compute the helicity-dependent injection current.","marker":"[10]"},{"why":"Provides a valley-polarized CPGE system in BiAsI2 whose tensor magnitudes are used as a comparison scale.","marker":"[35]"},{"why":"Shows how an in-plane magnetic field reshapes the band structure of a Rashba system, the Zeeman–Rashba competition behind the band shifts.","marker":"[36]"},{"why":"Supplies the Berry-curvature formalism used for the valence-band Berry curvature in the CPGE tensor.","marker":"[44]"},{"why":"Sets up magnetic proximity effects on excitons in monolayers, the basis for the Bethe–Salpeter excitonic absorption calculation.","marker":"[28]"},{"why":"Provides the Rashba spin-orbit coupling term for hexagonal quantum spin Hall candidates used in the Hamiltonian.","marker":"[40]"}],"fun_headline_variants":["Magnetism swaps in for asymmetry in helicity photocurrents","Symmetric materials get photocurrents when magnets twist bands","Magnetic exchange breaks inversion for circular photogalvanic currents","MCPGE: Magnets enable helicity currents in centrosymmetric 2D","Lose the asymmetry, keep the photocurrent: magnetism's trick"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that the magnetic substrate produces staggered exchange fields with $M_A \\neq M_B$ and with a finite in-plane component; if the exchange were uniform across the two sublattices or purely out-of-plane, the band shifts and the magneto-circular photogalvanic effect would vanish.","fun_headline_variants_meta":{"raw":{"variants":["Magnetism swaps in for asymmetry in helicity photocurrents","Symmetric materials get photocurrents when magnets twist bands","Magnetic exchange breaks inversion for circular photogalvanic currents","MCPGE: Magnets enable helicity currents in centrosymmetric 2D","Lose the asymmetry, keep the photocurrent: magnetism's trick"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000681,"raw_usage":{"total_tokens":3078,"prompt_tokens":917,"completion_tokens":2161,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":533,"completion_tokens_details":{"reasoning_tokens":2067}},"tokens_in":533,"tokens_out":2161,"duration_ms":18080,"temperature":1.0,"reasoning_tokens":2067,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-11T19:38:36.181930+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the helicity-dependent photocurrent in a centrosymmetric SbH monolayer on a magnetic substrate while rotating the in-plane magnetization: the theory predicts the current should follow the in-plane component of the exchange field, vanishing when the field is perpendicular to the current direction ($\\theta = \\pi/2$) and reversing sign when the field direction is reversed ($\\theta = \\pi$). Observing no such angular dependence, or no current when $M_A = M_B$, would falsify the proposed mechanism.","supporting_citations":[{"cited_title":"Song, C.-C","cited_arxiv_id":null,"evidence_quote":"Identifies monolayer SbH as a large-gap topological insulator with the buckled hexagonal lattice used for the proof-of-principle model."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the effective Hamiltonian with staggered magnetic exchange and staggered potential on the A/B sublattices, the core symmetry-breaking ingredient."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides a valley-polarized CPGE system in BiAsI2 whose tensor magnitudes are used as a comparison scale."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Shows how an in-plane magnetic field reshapes the band structure of a Rashba system, the Zeeman–Rashba competition behind the band shifts."},{"cited_title":"Xiao, M.-C","cited_arxiv_id":null,"evidence_quote":"Supplies the Berry-curvature formalism used for the valence-band Berry curvature in the CPGE tensor."},{"cited_title":"Scharf, G","cited_arxiv_id":null,"evidence_quote":"Sets up magnetic proximity effects on excitons in monolayers, the basis for the Bethe–Salpeter excitonic absorption calculation."},{"cited_title":"Dominguez, B","cited_arxiv_id":null,"evidence_quote":"Provides the Rashba spin-orbit coupling term for hexagonal quantum spin Hall candidates used in the Hamiltonian."}],"review_version":1}