{"id":"23b370b7-db98-4c4a-87fb-6a65cf9666fd","arxiv_id":"2603.21969","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"Spin-selective STM with SmB6 tips detects bias-reversing anisotropy in quasiparticle interference on KV2Se2O, interpreted as d-wave altermagnetic spin splitting.","lead":"Using a topological-insulator STM tip, the authors report real-space and momentum-space signatures of spin-split bands in the candidate altermagnet KV2Se2O. The result matters because it offers a non-magnetic, atomic-scale probe for spin textures in compensated magnets.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The central claim rests on the SmB6-tip spin-filtering mechanism, which the paper itself describes as unresolved; without a bias-dependent calibration of the tip's spin polarization, the Fig. 4 sign reversal could be a tip artifact rather than altermagnetic spin splitting.","rationale":"The reader's weakest assumption is exactly the SmB6-tip spin-filtering mechanism, and my independent read reaches the same conclusion. The paper contains a direct admission that the mechanism is 'not resolved yet' [49], yet the central interpretation of Figs. 3 and 4 depends on that mechanism. The W-tip control is well designed and rules out simple total-LDOS anisotropy, but it cannot rule out bias-dependent tip matrix-element effects that are specific to the SmB6 nanowire. The 'unambiguous' claim in the abstract and summary is therefore stronger than the evidence currently supports. This does not invalidate the work; it means the conclusion should be accepted only conditionally on an independent calibration of the tip's spin selectivity and bias dependence. Since the reader already rendered a CONDITIONAL verdict, I recommend no change to that verdict.","tokens_in":11275,"tokens_out":7318,"duration_ms":80145,"concrete_test":"Calibrate the identical SmB6 nanowire tip on a well-characterized spin-polarized surface (e.g., Fe(001) or Cr(001)) over the full bias range used in Figs. 3–4 (±70 mV), extracting the tip's spin polarization direction and magnitude from the bias-dependent asymmetry in the same D(q) analysis. If the tip's spin polarization does not reverse sign across the bias range, or if the measured spin selectivity is too weak to account for the observed contrast, the assignment in Fig. 4 to altermagnetic spin splitting is not supported. Conversely, a verified bias-reversing spin polarization would remove the main obstacle to the interpretation.","verdict_should_be":"UNCHANGED","load_bearing_attack":"At the core of the 'unambiguous' evidence is the assertion that the SmB6 nanowire tip acts as a spin filter whose quantization axis flips with bias. The manuscript explicitly states that this mechanism is 'not resolved yet' (ref. [49]) and cites an earlier report [48] for the bias-flip behavior. No in-situ calibration of the tip's spin polarization on a known spin-polarized surface is provided. Therefore the bias-reversing qx/qy anisotropy in Fig. 4, while striking, is not uniquely attributable to altermagnetic spin splitting. A bias-dependent anisotropy in the SmB6 tip's tunneling matrix elements (e.g., orbital selectivity of the topological surface states, or interference effects in the nanowire geometry) could produce the same D(q) pattern without any sample spin polarization. The W-tip baseline excludes only total-LDOS effects; it does not control for spin-dependent tip artifacts. This is the load-bearing assumption because if it fails, the central claim of visualizing d-wave AM spin splitting in momentum space is not established.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper reports scanning tunneling microscopy (STM) and quasi-particle interference (QPI) measurements on the candidate altermagnet KV2Se2O. Using a conventional W tip and a SmB6 nanowire tip, the authors compare real-space dI/dV maps and their Fourier transforms around native impurities. They find that the W tip yields isotropic QPI between qx and qy, while the SmB6 tip yields a bias-dependent anisotropy that reverses sign with bias polarity. This anisotropy is interpreted as spin-selective tunneling from the SmB6 tip, revealing momentum-dependent d-wave altermagnetic spin splitting in KV2Se2O. A minimal tight-binding model including altermagnetic and SDW order is used to simulate the W-tip QPI, and the paper claims 'unambiguous real-space and momentum space evidence' for the AM spin splitting, with 'd-wave type AM spin-splitting band structures directly visualized.'","tokens_in":11563,"tokens_out":3081,"duration_ms":32941,"significance":"If the central interpretation holds, this would be a significant advance: a direct, microscopic visualization of altermagnetic spin polarization in a candidate d-wave altermagnet, using a non-magnetic topological-insulator tip as a spin filter. The main experimental strengths are the controlled comparison between a spin-insensitive W tip and a spin-sensitive SmB6 tip, the bias-reversal control, the use of multiple impurities, and the minimal-model QPI simulation that captures the W-tip data. However, the load-bearing assumption—that the SmB6 nanowire tip acts as a reliable, bias-flippable spin filter—is explicitly stated in the manuscript as 'not resolved yet.' No in-situ calibration on a known spin-polarized surface is provided, and the model simulation is only for the total-LDOS W-tip channel, not for the spin-selective SmB6 channel. Thus, the evidence is suggestive and well-controlled at the level of sample LDOS, but it does not yet uniquely establish the spin-polarization interpretation.","major_comments":[{"comment":"The central claim rests on the SmB6 tip acting as an intrinsic spin filter whose spin orientation can be flipped by bias voltage. The paper explicitly states that this mechanism is 'not resolved yet' (ref. [49]), and no in-situ calibration on a known spin-polarized surface is provided. The W-tip baseline controls for total-LDOS effects, but it does not control for bias-dependent tip matrix-element effects, orbital selectivity of SmB6 surface states, or nanowire-geometry interference. A bias-dependent anisotropy in the SmB6 tip alone could reproduce the D(q) sign reversal without any altermagnetic spin splitting. Please provide a calibration of the SmB6 tip on a known ferromagnetic or spin-polarized surface, or an independent confirmation (e.g., spin-resolved STS with a ferromagnetic tip) to verify the spin-filtering assumption.","section":"Spin-selective tunneling mechanism (around Figs. 3G,H and 4; text near 'we attributed this behavior to spin-selective tu"},{"comment":"The real-space comparison between W and SmB6 tips is made at different bias voltages: 60 mV for the W tip and 70 mV for the SmB6 tip. Since QPI patterns are strongly bias-dependent, the observed anisotropy could be partially a voltage effect rather than a spin effect. The authors should acquire both tips at the same bias voltages, or at a series of matched voltages, to rule out this trivial source of anisotropy.","section":"Fig. 3C,D"},{"comment":"The theoretical QPI simulation shown in Fig. 2D is for the total LDOS (spin-insensitive W-tip channel) and reproduces the W-tip data. It does not model the spin-selective tunneling of the SmB6 tip. Therefore, it cannot validate the interpretation of the bias-dependent D(q) maps in Fig. 4G,H. A spin-resolved QPI simulation—including a spin-dependent tunneling matrix element that flips with bias—should be provided and compared quantitatively to the SmB6-tip data. Without this, the connection between the observed D(q) sign reversal and the d-wave spin structure remains an assumption.","section":"Fig. 2D and QPI simulation"},{"comment":"The interpretation of the π phase shift as spin polarization assumes that impurity scattering is spin-conserved and that the d-wave spin structure from prior theory is valid. This is reasonable but not demonstrated. The paper should present a direct calculation of the spin-resolved QPI for the proposed d-wave altermagnet with the SmB6 tip, showing that the expected D(q) pattern has opposite signs for qx and qy and reverses with bias. This would make the assignment from data to spin polarization less circular and more quantitative.","section":"Spin-conserved scattering assumption (Fig. 3G,H and accompanying text)"}],"minor_comments":[{"comment":"Typo: 'have of K atoms' should be 'half of K atoms.'","section":"Fig. 1C caption"},{"comment":"Typo: 'an novel spin-sensitive tunneling process' should be 'a novel spin-sensitive tunneling process.'","section":"Page 5, top"},{"comment":"Typo: 'spin-polarized stipes' should be 'spin-polarized stripes.'","section":"Page 8, bottom"},{"comment":"Typo: 'high spin sensitively' should be 'high spin sensitivity.'","section":"Page 10, text near Fig. 4"},{"comment":"The relation q = 2k is used to extract dispersion. This is only valid for simple intra-band scattering without umklapp or surface reconstruction effects. The text should clarify why this relation applies to the dominant scattering vectors here, especially given the sqrt(2)-reconstruction and SDW folding.","section":"Fig. 2F caption/text"},{"comment":"The text says the SDW gap is approximately 40 meV, but the dI/dV spectra show a gap feature within -15 to +30 mV. Please clarify how 40 meV is extracted (peak-to-peak? half-width?) to avoid ambiguity.","section":"Fig. 1E and SDW gap"}],"recommendation":"major_revision","confidential_remarks":"The paper is timely and the experimental controls are generally thoughtful, but the central spin-filtering assumption is explicitly unresolved and no in-situ calibration is provided. The overclaim of 'unambiguous' evidence is not justified by the data shown. I would encourage the editor to send this back for major revision rather than reject, because the experimental approach is potentially important and the required controls—same-bias comparisons, spin-resolved QPI modeling, and tip calibration—are realistically achievable. The authors should also soften the 'unambiguous' language unless the requested controls close the loophole."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The paper reports STM QPI on KV2Se2O using both W and SmB6 nanowire tips. What's new: a claimed direct real-space and momentum-space visualization of altermagnetic spin splitting via spin-selective tunneling, with a bias-reversal anisotropy in the SmB6-tip QPI that is absent with the W tip. They also map dispersion, see an SDW gap, and simulate QPI with a minimal d-wave AM model. The W-tip baseline and bias-reversal control are sensible experimental checks, and the data seem carefully taken. The \"Note added\" honestly flags two concurrent STM papers with similar results, so the true novelty is reduced but not zero.\n\nThe soft spot is where the load-bearing claim lives. The interpretation of the SmB6-tip bias-dependent anisotropy as spin-selective tunneling rests on a mechanism the authors themselves say is \"not resolved yet\" (ref. 49). There is no in-situ calibration of the tip's spin polarization on a known spin-polarized surface, so a bias-dependent tip artifact (orbital selectivity or nanowire interference) could in principle create the same D(q) pattern. The W-tip baseline excludes total-LDOS effects but not spin-dependent tip artifacts. Also, the real-space comparison in Fig. 3 uses different bias voltages (60 mV W vs 70 mV SmB6), which is a small but real weakness. Error bars are absent from the key difference maps, and the \"unambiguous\" language in the summary is stronger than the evidence. The theoretical QPI simulation agrees, but the model parameters live in the SM and the fit is not quantified.\n\nNone of this sinks the paper. The central observation—a bias-reversing qx/qy anisotropy that only appears with the SmB6 tip—is striking and, if the tip story holds, it is a genuine advance. The missing calibration is fixable in principle, and the paper is honest about its main uncertainty. The concurrent preprints make it a race, not a rout.\n\nI'd send this to referees. The right referees will push on the tip calibration and ask for statistics, and that's exactly what this needs. It's a useful contribution for the altermagnet-STM community either way, and a fair referee can separate the solid control from the interpretive leap.","headline":"A plausible spin-selective STM visualization of a d-wave altermagnet, but the central claim rests on a tip mechanism the authors admit is unresolved—worth refereeing, not desk rejecting.","tokens_in":12121,"tokens_out":1560,"would_cite":true,"duration_ms":15601,"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":"Spin-selective tunneling with a topological insulator tip reveals d-wave altermagnetic spin splitting in KV2Se2O.","keywords":["altermagnetism","KV2Se2O","spin-selective tunneling","scanning tunneling microscopy","quasi-particle interference","d-wave spin splitting","SmB6 nanowire tip","spin-density wave"],"falsifier":"Spin-resolved ARPES on a single magnetic domain of KV2Se2O that resolves the predicted d-wave spin-split bands would directly confirm the electronic structure; alternatively, applying the same SmB6-tip protocol to a nonmagnetic isostructural compound and observing the same bias-dependent QPI anisotropy would indicate the effect is tip-related or orbital rather than altermagnetic in origin.","tokens_in":11200,"feed_emoji":"🔬","tokens_out":2430,"duration_ms":21757,"temperature":0.7,"pith_summary":"This paper aims to establish that KV2Se2O is a metallic d-wave altermagnet by directly visualizing its momentum-dependent spin splitting. Using a SmB6 nanowire tip as a spin-selective scanning tunneling microscopy probe, the authors observe a bias-reversing anisotropy in quasi-particle interference patterns that is absent with a conventional tungsten tip. They interpret this anisotropy as the signature of oppositely spin-polarized Fermi-surface segments along orthogonal directions, matching the d-wave form factor predicted by symmetry. If correct, the work provides microscopic, real-space and momentum-space evidence linking crystal symmetry, electronic structure, and spin polarization in an altermagnet, and introduces a minimally invasive probe for such materials.","feed_headline":"Spin-selective STM maps d-wave altermagnet in KV2Se2O","feed_subtitle":"Bias-reversing anisotropy reveals momentum-dependent spin splitting without a net magnetic moment.","key_machinery":"The central object is the SmB6 nanowire tip, which acts as an intrinsic directional spin filter owing to the helical spin–momentum locking of its topological surface states; the tunneling current is spin-polarized and its orientation can be flipped by bias voltage rather than by an external magnetic field. The quantitative tool is the QPI difference map D(q) = I(q) − R·I(q), where R is a π/2 rotation operator, which isolates the anisotropy between orthogonal scattering directions. This is supplemented by a minimal tight-binding model incorporating both altermagnetic and spin-density-wave order, which reproduces the measured QPI patterns and confirms the d-wave form factor of the spin splitti","core_discovery":"The central claim is that KV2Se2O exhibits d-wave altermagnetic spin splitting that can be visualized with an SmB6 nanowire STM tip. The authors report that QPI maps acquired with the spin-sensitive SmB6 tip show a pronounced anisotropy between the qx and qy directions, with a π phase shift between standing waves along x and y, and that this anisotropy reverses sign when the bias voltage is reversed. No such bias-dependent anisotropy appears with the spin-degenerate W-tip. Because the SmB6 tip acts as an intrinsic spin filter whose spin orientation flips with bias, the observed reversal is attributed to spin-selective tunneling into oppositely spin-polarized d-wave bands. Combined with band-","pith_inferences":["If the bias-flipping spin-filter mechanism of the SmB6 tip is independently confirmed, this approach could become a standard tool for nanoscale imaging of altermagnetic order without the stray-field perturbations inherent to ferromagnetic tips.","The paper notes two concurrent STM studies reporting similar results; if these are mutually consistent, the d-wave altermagnetic interpretation of KV2Se2O would be substantially strengthened.","Because the reported AM spin splitting (~1.8 eV) is much larger than the SDW gap (~40 meV), the material likely remains a robust d-wave altermagnet even when the spin-density-wave order reconstructs the surface, which could be tested by applying strain or doping to tune the SDW without destroying the spin splitting."],"forward_implications":["KV2Se2O is established as a tunable platform for studying spin–valley locking, Fermi-surface instability, and unconventional magnetism without net magnetization.","SmB6-tip-based STM becomes a general method for imaging compensated spin textures and could detect the hidden altermagnetic even–odd layer effect in few-layer samples.","The observation opens pathways to anomalous Hall and Nernst effects, nonrelativistic spin-polarized currents, magneto-optical Kerr effect, and chiral magnon excitations in this material class.","The van der Waals layered structure and robust altermagnetic order make KV2Se2O a candidate for efficient spin-current generation, tunable magnetic tunnel junctions, and low-dissipation magnonic devices."],"fun_headline_variants":["Spin-selective STM reveals d-wave altermagnet KV2Se2O","Imaging d-wave spin splitting in altermagnet KV2Se2O","KV2Se2O: seeing d-wave altermagnetism via spin-polarized tip","Bias-reversing anisotropy maps d-wave altermagnet KV2Se2O","Visualizing altermagnet KV2Se2O with spin-selective tunneling"],"cache_read_input_tokens":2304,"weakest_assumption_plain":"The entire spin-selective interpretation rests on the assumption that the SmB6 nanowire tip is a dependable intrinsic spin filter whose tunneling spin orientation reverses with bias voltage, a mechanism the paper itself states is not yet resolved.","fun_headline_variants_meta":{"raw":{"variants":["Spin-selective STM reveals d-wave altermagnet KV2Se2O","Imaging d-wave spin splitting in altermagnet KV2Se2O","KV2Se2O: seeing d-wave altermagnetism via spin-polarized tip","Bias-reversing anisotropy maps d-wave altermagnet KV2Se2O","Visualizing altermagnet KV2Se2O with spin-selective tunneling"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.001026,"raw_usage":{"total_tokens":4126,"prompt_tokens":674,"completion_tokens":3452,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":418,"completion_tokens_details":{"reasoning_tokens":3346}},"tokens_in":418,"tokens_out":3452,"duration_ms":21775,"temperature":1.0,"reasoning_tokens":3346,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-02T17:38:01.888101+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Spin-resolved ARPES on a single magnetic domain of KV2Se2O that resolves the predicted d-wave spin-split bands would directly confirm the electronic structure; alternatively, applying the same SmB6-tip protocol to a nonmagnetic isostructural compound and observing the same bias-dependent QPI anisotropy would indicate the effect is tip-related or orbital rather than altermagnetic in origin.","supporting_citations":[],"review_version":1}