{"id":"2d0f68a4-f6e8-4869-8ebf-3c69dc0fea56","arxiv_id":"2608.07305","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"Interlayer sliding in magnetic sliding ferroelectrics is predicted to generate a pure spin current with a quantized spin electronic polarization change.","lead":"This paper predicts that sliding one layer of a magnetic two-dimensional crystal past the other can produce a pure spin current, a flow of electron spin without any net electric current. The effect, called the sliding spin current, could offer a new all-electrical way to generate spin currents for spintronics.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Spin U(1) is the load-bearing assumption: SOC in realistic candidates like CrI3 and VTe2 may make the spin-resolved polarization and the pure spin current ill-defined.","rationale":"The reader's weakest-assumption analysis identifies spin U(1) symmetry, and my reading agrees that this is the most load-bearing condition for the central claim. Without spin conservation, the separation of the electric polarization into spin-up and spin-down contributions in Eqs. (1)-(2) is not gauge-invariant, the fractional quantization of P_s is not protected, and the 'pure spin current' is not a conserved quantity because spin-orbit coupling introduces torque source terms. The paper explicitly assumes spin U(1) and reports only collinear calculations, so the quantitative predictions for realistic materials (CrI3, VX2) rest on an unvalidated assumption. This concern is concrete and testable via SOC-inclusive DFT. The paper is otherwise internally consistent: the symmetry analysis for the spin-U(1) limit is clear, the model in Eq. (4) supports the claimed fractional spin-polarization change, and the DFT results match the symmetry predictions. The remaining issues—such as transient charge currents during switching or the assumed 1 ns switching time—are secondary because the symmetry argument makes the endpoint charge cancellation exact in the spin-U(1) limit, and the switching time only scales the magnitude of the spin-current density, not its existence. Since the reader's conditional verdict already requires additional validation of the spin-U(1) assumption, my analysis does not change the verdict. I therefore recommend keeping the verdict unchanged and adding a concrete SOC test as a condition for full acceptance.","tokens_in":16198,"tokens_out":17147,"duration_ms":165015,"concrete_test":"Perform noncollinear DFT+U calculations including SOC for H-stacked bilayer CrI3 (and, as a second check, bilayer VTe2). Compute the in-plane spin electronic polarization along the sliding path using a spin-projected Berry phase, for example by projecting the Kohn-Sham spinors onto the S_z eigenstates and applying the modern theory of polarization separately to each spin channel. Compare δP_s with the collinear result (-2Q_a,0 for CrI3). If δP_s changes by more than about 10% or loses its quantized multiplicity of Q_a, the spin U(1) assumption is not justified and the predicted pure spin current is correspondingly weakened. If the SOC result is quantitatively unchanged, the concern is resolved.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central mechanism and all quantitative predictions (for example, δP_s = -2Q_a in CrI3 and δP_s = (1/3,-1/3)Q in VS2) are derived under the stated assumption that the magnetic SFEs have spin U(1) symmetry. This assumption is essential for Eqs. (1)-(2): only when spin is conserved can occupied bands be assigned a definite spin, making P_up and P_down separately gauge-invariant Berry phases whose difference P_s is well defined. In the proposed materials the assumption is not validated. CrI3 contains heavy iodine and exhibits strong magnetic anisotropy, indicating sizable spin-orbit coupling; VTe2 (and to a lesser extent VSe2) also contain heavy chalcogens. With SOC, spin is not conserved, the decomposition P_s = P_up - P_down loses gauge invariance, and the spin current is not a conserved current—spin-orbit torque terms enter the spin continuity equation. The paper provides no noncollinear or SOC calculations to show that the fractional quantization survives, so the claim of a symmetry-protected pure spin current in these materials is not yet established.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper introduces the concept of fractional spin ferroelectricity (FSFE) in magnetic sliding ferroelectrics (SFEs) and proposes that ferroelectric switching in such systems generates a pure spin current, the \"sliding spin current.\" Under the assumption of spin U(1) symmetry, the electric polarization is decomposed into ionic and spin-resolved electronic parts, and the difference between up- and down-spin electronic polarizations defines the spin electronic polarization P_s. The authors argue that during interlayer sliding the ionic and valence-electron contributions to the in-plane charge polarization cancel, so the electric polarization does not change, while the in-plane spin polarization changes by a (fractional) polarization quantum, giving a pure spin current J_s = δP_s/Δt. The symmetry argument is illustrated with a toy model and supported by DFT+U calculations for H-stacked bilayer CrI3 and R-stacked bilayer 2H-VX2 (X = S, Se, Te), as well as several other magnetic SFEs listed in Table I. The reported spin-current densities are on the order of 10^9 (ℏ/2e) A/m² for CrI3 and 10^8 (ℏ/2e) A/m² for VX2 at a 1 ns switching time.","tokens_in":16435,"tokens_out":7340,"duration_ms":74752,"significance":"If the central mechanism survives scrutiny, this is a conceptually new and appealing route to all-electrical pure spin-current generation, distinct from the spin Hall effect and spin pumping. The paper's strengths include a clean symmetry-based derivation, a simple solvable toy model, path-resolved first-principles calculations of P_ion, P↑, P↓, P, and P_s for multiple candidate materials, and concrete quantitative predictions that are in principle testable. The main risk is that the entire framework relies on spin U(1) symmetry, while the proposed realistic materials contain heavy elements with non-negligible spin-orbit coupling; the manuscript does not test whether the quantization and the pure-spin-current property survive SOC. A second, more local concern is whether the purity of the current is guaranteed along the entire switching path or only in the endpoint-averaged sense of Eq. (3). These issues are load-bearing for the material-specific predictions but appear addressable within the scope of a revision.","major_comments":[{"comment":"The derivation of P_s = P↑ - P↓ and the subsequent quantization statements assume spin U(1) symmetry, i.e., conserved spin with well-defined up and down channels. This assumption is stated at the start of the symmetry analysis but is not validated for the proposed materials. CrI3 contains iodine and has strong magnetic anisotropy, and VTe2 (and to a lesser extent VSe2) contain heavy chalcogens, so spin-orbit coupling is not negligible. Without SOC, the decomposition into spin-resolved Berry phases is gauge-invariant and the spin current is conserved; with SOC, P_s as defined here loses its gauge-invariant meaning and spin-orbit torque terms enter the spin continuity equation. The manuscript provides no noncollinear or SOC calculations to show that the reported δP_s values (e.g., -2Q_a for CrI3 and (1/3,-1/3)Q for VS2) survive. I request a concrete test: perform noncollinear DFT with SOC along the sliding path, using a global spin projection to evaluate the spin polarization, and check whether the quantization and the cancellation of charge current remain; alternatively, restrict the quantitative material claims to strictly negligible-SOC cases and clearly label the CrI3 and VTe2 predictions as an idealized U(1) limit.","section":"Symmetry analysis, Eqs. (1)-(3); Figs. 3-4"},{"comment":"Equation (3) defines J_c and J_s as mean currents over the switching time using only the endpoint difference δP = OP - P. A vanishing δP guarantees zero time-integrated charge transfer, but not zero instantaneous charge current during the switching process. In the CrI3 section the text states that along the path the total electric polarization is \"nearly unchanged and returns to zero\" while individual contributions show \"sizable variations.\" This implies dP/dt is generally nonzero along the path, so the charge current is not strictly zero at every instant; the \"pure\" character holds only in a time-averaged or endpoint-integrated sense. The central claim of the paper is a pure spin current without charge transport, so the authors should either demonstrate that J_c(t) = 0 along the entire adiabatic path (e.g., by showing the path-resolved total P is constant to numerical precision for every material) or explicitly redefine \"pure spin current\" as zero net charge transfer over the full switching event. This distinction is important because the mechanism's novelty rests on the absence of charge transport.","section":"Eq. (3) and Figs. 3(c), 4(c)"},{"comment":"The claim that \"most currently known magnetic SFEs turn out to be FSFEs\" is supported only by the eight materials listed in Table I and discussed in the text and Supplemental Material. This is an extrapolation that is not justified by an exhaustive survey of known magnetic SFEs. The statement should be softened to \"all magnetic SFEs examined here\" or supported by a broader materials screening.","section":"Table I and Discussion"}],"minor_comments":[{"comment":"The conversion from the 2D current-per-width δP_s/Δt (units A/m) to the quoted 3D current densities of 10^9 and 10^8 (ℏ/2e) A/m² is not specified. The effective thickness used for each material should be stated explicitly, because it directly affects the quantitative comparison with spin Hall experiments.","section":"Discussion, spin-current density estimates"},{"comment":"In Eq. (4), the quantity δ' = 1 - δ/2 is introduced without explanation. The text should clarify what units δ is measured in and why δ' takes this particular form.","section":"Toy model, Eq. (4)"},{"comment":"Reference [35] is cited as an arXiv preprint from 2026. If this work has not yet appeared in a peer-reviewed venue, the citation should be marked as a preprint to avoid giving the impression of a published reference.","section":"References"},{"comment":"The figure captions for the DFT results do not define the reference state with respect to which the plotted variations are computed, nor the modulus convention used for P and P_s modulo Q. Defining these conventions would improve reproducibility.","section":"Figures 3 and 4"},{"comment":"The text states that δP_s = (-2Q_a, 0) and calls this an integer multiple of Q. Since the paper's title and central concept emphasize fractional spin ferroelectricity, it would be helpful to clarify in the text that integer multiples of Q are included in the FSFE classification and that the genuinely fractional case is realized in VX2 and the other Table I entries.","section":"Section 'H-stacked bilayer CrI3'"}],"recommendation":"major_revision","confidential_remarks":"The paper presents a clean and elegant symmetry framework with nontrivial DFT results, and the idea is likely to be of interest to the readership. The main blocker is the absence of any SOC or noncollinear test for the proposed materials, given that the entire formalism is built on spin U(1). The second issue, concerning time-resolved versus endpoint purity of the current, is also worth resolving before publication. I do not see the circularity concern raised by the reader's report as a serious problem: the spin current is computed from DFT and the symmetry operator, and the ionic-electron cancellation is a computed result rather than an input."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Here's my take. The paper introduces the idea of a spin electronic polarization P_s = P_up - P_down and shows, for a class of magnetic sliding ferroelectrics, that sliding switching can leave the in-plane charge polarization unchanged while changing P_s by a quantized amount. That yields a pure spin current with no charge current. This is genuinely new as far as I know, and the symmetry analysis is clean. The toy model in Fig. 2 is convincing, and the DFT calculations for CrI3 and VS2 support the predicted behavior: δP stays near zero along the sliding path while δP_s jumps by the expected multiple or fraction of the polarization quantum. The calculations are standard and reproducible with the details given.\n\nNow the soft spots. The paper states upfront that it assumes spin U(1) symmetry, but it never tests that assumption for the materials it proposes. CrI3 in particular contains heavy iodine and is a strongly anisotropic Ising magnet, so spin-orbit coupling is clearly not negligible. With SOC, the split into P_up and P_down is not gauge-invariant and the spin current is not conserved. So the quantitative predictions and the 'pure' tag are not yet established for CrI3. This is the main gap, and it is addressable: noncollinear or SOC-including DFT would show whether the quantization of δP_s and the cancellation of δP survive. I'd push back a little on the reader's claim that the cancellation is only at endpoints; the plotted δP appears small along the whole path in Figs. 3-4, so that concern seems weaker. The current density estimates rest on an assumed switching time and an effective thickness for the 3D conversion; that's fine for an order-of-magnitude estimate but worth flagging. Also, for CrI3 the computed δP_s is an integer multiple of the polarization quantum, so 'fractional' is a bit of a misnomer there, though the VS2 case does give a true fractional quantum.\n\nBottom line: the mechanism is plausible and the paper deserves a serious referee. I'd send it to review and ask for SOC calculations before acceptance, because the central claim is a symmetry-protected pure spin current in these specific materials, and the symmetry argument as presented lives in a limit that likely doesn't hold for CrI3. If the SOC check comes back clean, this would be a solid contribution to the 2D spintronics literature.","headline":"A fresh mechanism for pure spin current in magnetic sliding ferroelectrics, with clean symmetry arguments and DFT support, but an untested spin-U(1) assumption that likely breaks down in the headline material CrI3.","tokens_in":16972,"tokens_out":7120,"would_cite":true,"duration_ms":61854,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":["72.25.-b","77.80.-e"],"model":"deepseek-v4-flash","headline":"Sliding magnetic bilayers convert an electric field into a pure spin current with zero charge current.","keywords":["fractional spin ferroelectric","sliding spin current","pure spin current","sliding ferroelectricity","spin electronic polarization","fractional polarization quantum","bilayer CrI3","bilayer 2H-VX2"],"falsifier":"A first-principles calculation of the sliding path with spin-orbit coupling included, or a spin-transport measurement on bilayer CrI$_3$ during switching, that yields a nonzero charge current or a spin electronic polarization change that is not an integer multiple of the fractional polarization quantum would contradict the central claim.","tokens_in":16023,"feed_emoji":"⚡","tokens_out":9667,"duration_ms":68940,"temperature":0.7,"pith_summary":"The paper argues that a new class of materials, magnetic sliding ferroelectrics, can convert an out-of-plane electric field into a pure spin current with no accompanying charge current. During ferroelectric switching, the top layer slides and the charge contributions of ions and valence electrons cancel, while the spin contribution, carried only by electrons, changes by a quantized fractional polarization quantum. The authors call this the sliding spin current and verify it with first-principles calculations in bilayer CrI$_3$, bilayer $2H$-V$X_2$ ($X$ = S, Se, Te), and several other magnetic bilayers. If correct, this gives an all-electrical, low-dissipation route to generating spin currents that does not require heavy elements or large charge currents, with predicted densities comparable to the spin Hall effect.","feed_headline":"Sliding magnets generate pure spin current","feed_subtitle":"Ion and electron charge motions cancel during switching, leaving a quantized spin flow driven by an electric field.","key_machinery":"The central object is the spin electronic polarization $P_s = P^\\uparrow - P^\\downarrow$, the difference of the Berry-phase electric polarizations of the two spin channels, together with the symmetry operator $O = \\{g \\mid \\tau\\}$ (a point-group operation followed by a translation) that maps one ferroelectric state onto the other. The argument requires two conditions: $O P_s - P_s$ must equal a sum of the symmetry-allowed fractional polarization quanta $F_i$, and the translation $\\tau$ must be a fractional lattice translation. When both hold, $\\delta P_s$ is quantized while the charge polarization change $\\delta P$ vanishes because the van der Waals layers are nearly charge neutral, so each layer's ionic and electronic charge centers move together and cancel; the spin current then follows from $J_s = \\delta P_s / \\Delta t$.","core_discovery":"The paper's central claim is that magnetic sliding ferroelectrics, whose two degenerate ground states differ by an interlayer sliding, are simultaneously fractional spin ferroelectrics: during ferroelectric switching the in-plane spin electronic polarization $P_s = P^\\uparrow - P^\\downarrow$ changes by a quantized amount, an integer multiple of the fractional polarization quantum, while the in-plane electric polarization $P$ returns to the same value because the ionic and valence-electron charge transfers exactly cancel. As a result, the switching produces a pure spin current $J_s = \\delta P_s / \\Delta t$ with zero charge current, which the authors term the sliding spin current. The claim is verified by symmetry analysis and first-principles calculations for H-stacked bilayer CrI$_3$, R-stacked bilayer $2H$-V$X_2$ ($X$ = S, Se, Te), and several other magnetic bilayers, where the estimated spin current densities at 1 ns switching reach $10^9$ and $10^8$ in units of $(\\hbar/2e)\\,\\mathrm{A/m^2}$.","pith_inferences":["In real iodides such as CrI$_3$, spin-orbit coupling will mix the up and down spin channels, so the exact quantization and the perfectly charge-free character of the current may soften into a spin current that also exerts a torque; a natural test is to recompute the spin-resolved polarization along the sliding path with spin-orbit coupling included.","The same cancellation logic should apply to orbital angular momentum, since the ions carry no orbital moment, so materials with nonzero orbital electronic polarization could host a sliding orbital current.","The symmetry criterion, a point-group operator plus a fractional translation that maps $P_s$ to a sum of symmetry-allowed quanta, provides a fast screening rule for discovering new fractional spin ferroelectrics without full first-principles calculations.","The formula $J_s = \\delta P_s / \\Delta t$ assumes the switching is slow enough for the adiabatic polarization-change relation to hold; at picosecond timescales, non-adiabatic and phonon-drag effects may change the instantaneous current and should be checked in time-dependent simulations."],"forward_implications":["Applying a periodic out-of-plane electric field to a magnetic sliding ferroelectric drives repeated sliding and produces an alternating pure spin current with no charge current.","The estimated spin current densities reach about $10^9\\,(\\hbar/2e)\\,\\mathrm{A/m^2}$ in H-stacked bilayer CrI$_3$ and $10^8\\,(\\hbar/2e)\\,\\mathrm{A/m^2}$ in bilayer $2H$-V$X_2$ at 1 ns switching, comparable to spin Hall sources without requiring large charge currents.","The effect operates in materials with negligible spin-orbit coupling, unlike the spin Hall effect, widening the material base for all-electrical spin current generation.","The compensation between ionic and valence-electron charge transfer is symmetry-protected, so the pure spin current does not depend on the detailed sliding path.","If picosecond light-induced switching is achieved, the spin current density could rise to about $10^{12}\\,(\\hbar/2e)\\,\\mathrm{A/m^2}$ in bilayer CrI$_3$."],"supporting_citations":[{"why":"Introduces sliding ferroelectricity in van der Waals bilayers, the material class the paper builds on.","marker":"[28]"},{"why":"Supplies the concept of fractional polarization quantum that quantizes the variation of spin electronic polarization.","marker":"[34]"},{"why":"Provides the symmetry-allowed fractional polarization quanta used in the two quantization conditions.","marker":"[51]"},{"why":"Gives the general theory of bilayer stacking ferroelectricity that defines the two degenerate states of CrI$_3$.","marker":"[46]"},{"why":"The experimental confirmation that few-layer H-stacked CrI$_3$ is a magnetic sliding ferroelectric, anchoring the main candidate.","marker":"[55]"},{"why":"Predicted the R-stacked bilayer $2H$-V$X_2$ family as sliding ferroelectrics, providing the second set of candidate materials.","marker":"[56]"},{"why":"Establishes the roughly 1 ns switching time used to estimate the spin current densities.","marker":"[44]"},{"why":"Provides sub-nanosecond switching kinetics that support the switching-time scale and fatigue resistance.","marker":"[45]"},{"why":"Supports the light-induced picosecond switching scenario that yields the enhanced spin current estimate.","marker":"[72]"}],"fun_headline_variants":["Sliding spin current from magnetic ferroelectrics","Pure spin current via sliding magnets","Electric field drives pure spin current in sliding magnets","Quantized spin flow from magnetic sliding ferroelectrics"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The derivation assumes spin U(1) symmetry, meaning collinear magnetic order with negligible spin-orbit coupling, so that up-spin and down-spin polarizations are separately well defined and the spin current is conserved.","fun_headline_variants_meta":{"raw":{"variants":["Sliding spin current from magnetic ferroelectrics","Pure spin current via sliding magnets","Electric field drives pure spin current in sliding magnets","Quantized spin flow from magnetic sliding ferroelectrics"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000525,"raw_usage":{"total_tokens":2595,"prompt_tokens":1065,"completion_tokens":1530,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":681,"completion_tokens_details":{"reasoning_tokens":1482}},"tokens_in":681,"tokens_out":1530,"duration_ms":10560,"temperature":1.0,"reasoning_tokens":1482,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-10T10:39:05.778472+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A first-principles calculation of the sliding path with spin-orbit coupling included, or a spin-transport measurement on bilayer CrI$_3$ during switching, that yields a nonzero charge current or a spin electronic polarization change that is not an integer multiple of the fractional polarization quantum would contradict the central claim.","supporting_citations":[{"cited_title":"Li and M","cited_arxiv_id":null,"evidence_quote":"Introduces sliding ferroelectricity in van der Waals bilayers, the material class the paper builds on."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the concept of fractional polarization quantum that quantizes the variation of spin electronic polarization."},{"cited_title":"Pang and L","cited_arxiv_id":null,"evidence_quote":"Provides the symmetry-allowed fractional polarization quanta used in the two quantization conditions."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Gives the general theory of bilayer stacking ferroelectricity that defines the two degenerate states of CrI$_3$."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"The experimental confirmation that few-layer H-stacked CrI$_3$ is a magnetic sliding ferroelectric, anchoring the main candidate."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Predicted the R-stacked bilayer $2H$-V$X_2$ family as sliding ferroelectrics, providing the second set of candidate materials."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides sub-nanosecond switching kinetics that support the switching-time scale and fatigue resistance."},{"cited_title":"Yang and S","cited_arxiv_id":null,"evidence_quote":"Supports the light-induced picosecond switching scenario that yields the enhanced spin current estimate."}],"review_version":1}