{"id":"301fc4c2-a4a7-4de1-b622-e27b7ce8a464","arxiv_id":"2411.19237","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":1,"one_line_summary":"First-principles calculations predict four monolayer altermagnets in which uniaxial strain controls valley polarization, topological band crossings, and doping-induced piezomagnetism.","lead":"This paper uses computer simulations to propose four new two-dimensional magnetic materials whose electrons can be controlled through both spin and valley properties. Stretching or squeezing these materials could create polarized valleys, topological states, and strain-controlled magnetism for future spintronics and valleytronics devices.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The topological semimetal prediction depends on spinless band crossings; SOC is only asserted weak at zero strain, never checked at the strained nodal loops/Weyl points.","rationale":"The paper's strongest claim is that four monolayers are altermagnetic valleytronic semiconductors in which uniaxial strain induces valley polarization, topological states, and piezomagnetism. The altermagnetic semiconducting part is reasonably supported by phonon/AIMD stability checks and by HSE06 and U-value cross-checks in the supplementary material. The topological part, however, is the least secure: it relies on band crossings in PBE+U spin-collinear calculations without SOC, and the paper's only justification is a general statement that SOC has little effect on valleys. That statement is made for the unstrained compounds, while the topological features appear only under large compressive strain. For a heavy-element material like V2Te2O, SOC energy scales can easily be comparable to the 68 meV PBE gap, so the crossings could open or shift. The reader's weakest assumption correctly identified this. A concrete SOC-included calculation at the strained geometries would settle whether the nodal loops and Weyl points survive; without it, the topological-semimetal claim remains conditional, exactly as the reader's CONDITIONAL verdict states. I therefore recommend no change to the verdict.","tokens_in":12814,"tokens_out":7424,"duration_ms":69505,"concrete_test":"Recompute the strained electronic structure of V2Te2O at -5% and -1.7% strain and of V2STeO at -9% and -7% strain with SOC included, using the same PBE+U=4 eV setup with noncollinear spin and moments constrained along z. Evaluate the minimum direct gap along the claimed nodal loop around the Y point in V2Te2O and at the X-M/Y-M Weyl points in V2STeO. If the minimum gap is larger than roughly 10 meV (or if double-group irreps no longer force a crossing), the topological-semimetal prediction is not realized as described.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central topological claims are made without SOC. The paper states: 'Since the spin-orbit coupling (SOC) has little effect on the band structure and valleys, we consider the band structure in the absence of SOC in the main text', deferring the SOC band structures to the supplementary material. But the nodal loop in V2Te2O (Fig. 5, at -1.7% to -5% strain) and the Weyl points in V2STeO (Fig. 6, at -7% and -9%) are identified from nonrelativistic bands and are protected only by mirror eigenvalues in the spin-collinear calculation. With SOC, spin is not a good quantum number and the symmetry analysis must be redone with double-group irreps. Because V2Te2O contains heavy Te and has a PBE gap of only 0.068 eV, SOC can plausibly open gaps of the order of tens of meV at the claimed crossings. The supplementary SOC band structures are for the unstrained compounds; no strained SOC calculation is shown. If SOC gaps these crossings, the abstract's 'topological states' claim for V2Te2O and V2STeO fails, even though the altermagnetic valleytronic semiconducting picture may survive.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper uses first-principles DFT+U calculations to propose four exfoliable monolayers—V2Te2O, V2STeO, V2SSeO, and V2S2O—as two-dimensional altermagnetic semiconductors. The central claims are (i) an altermagnetic ground state with spin-split, spin-valley-locked bands and a pair of valleys at X and Y; (ii) uniaxial strain breaks the mirror symmetry M110 and produces valley polarization; (iii) at larger compressive strains, V2Te2O develops nodal loops and V2STeO develops Weyl points, both protected by mirror symmetries and (for the Weyl points) characterized by Berry phase pi; and (iv) hole doping of the strained, valley-polarized systems yields a net magnetization, interpreted as piezomagnetism. Stability is supported by phonon and AIMD checks, and the zero-strain electronic structure is cross-checked with HSE06.","tokens_in":12948,"tokens_out":6259,"duration_ms":57749,"significance":"If correct, the paper would enlarge the scarce family of 2D altermagnetic valleytronic materials and demonstrate simultaneous strain control of valley, topological, and magnetic properties. The computation is generally careful: convergence parameters are reported, U dependence is tested (in the supplementary material), HSE06 is used at zero strain, symmetry irreps are computed with irvsp, and phonon/AIMD stability checks are included. The main caveat is that the topological semimetal claims rest entirely on nonrelativistic (no-SOC) band structure, with SOC only examined at zero strain; this is a central, fixable gap.","major_comments":[{"comment":"The topological state claims for V2Te2O (nodal loops) and V2STeO (Weyl points) are established entirely from spin-collinear calculations without SOC, while the statement that 'spin-orbit coupling (SOC) has little effect on the band structure and valleys' is only supported at zero strain. The crossings occur under compressive strain, and because V2Te2O has a PBE+U gap of only 0.068 eV, SOC can plausibly open a gap of tens of meV at the nodal loops or Weyl points. Please provide SOC-included band structures at the representative strained conditions (e.g., -5% for V2Te2O and -9% for V2STeO) and recompute the mirror-eigenvalue protection using double-group irreps, or quantitatively bound the SOC-induced gap at the crossing points.","section":"Band structure and strain effects (Figs. 5 and 6)"},{"comment":"The HSE06 calculation at zero strain changes the V2Te2O gap from 0.068 eV (PBE+U) to 0.3095 eV, a factor of about 4.5. Since the topological transitions are driven by gap closure under strain, the strain thresholds reported in Figs. 5 and 6 (-1.7% and -5% for V2Te2O; -4%, -7%, and -9% for V2STeO) are not directly validated by the hybrid functional. Please test whether the qualitative topological semimetal state survives in HSE06 for at least one representative strained structure, or discuss quantitatively how the larger initial gap shifts the thresholds.","section":"HSE06 cross-check (band gap values in Section 4)"}],"minor_comments":[{"comment":"The claim that the Weyl points in V2STeO have Berry phase gamma_C = pi is asserted but not demonstrated; please include the integration details or a small figure showing the phase accumulation.","section":"Berry phase calculation (paragraph after Eq. (1))"},{"comment":"The SOC band structures for the unstrained compounds should be described in the main text at least to the extent of quoting the maximum SOC splitting near the gap; that would make the 'little effect' claim more concrete.","section":"SOC discussion (Section 4)"},{"comment":"The schematic of the nodal loops in the Brillouin zone is difficult to interpret; a higher-resolution plot with the two spin-channel loops drawn separately would improve readability.","section":"Fig. 5(h)"},{"comment":"The term 'Weyl point' in two dimensions should be qualified as a 2D linear touching point with Berry phase pi, to distinguish it from genuine 3D Weyl points.","section":"Terminology for 2D crossings"},{"comment":"The U-dependence test is reported only for V2Te2O; please state whether the other three compounds are expected to be equally insensitive to the choice of U, or provide similar tests for at least one more compound.","section":"U-dependence test (supplementary material)"},{"comment":"Minor typographical issues: 'fermi level' should be capitalized as 'Fermi level', and the sentence about 'magnetic ground state and stable of the monolayer' should read 'magnetic ground state and stability of the monolayer'.","section":"Throughout"}],"recommendation":"major_revision","confidential_remarks":"The paper is within the scope of the journal. The required revision is focused: the SOC check under strain is the load-bearing missing calculation, and the HSE06 strain sensitivity should be addressed either by an additional calculation or a careful quantitative discussion."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Short take: this is a workmanlike DFT prediction of four altermagnetic valleytronic monolayers. The altermagnetic semiconductor picture and strain-induced valley polarization are probably robust; the topological semimetal claims are the fragile part and need the supplemental SOC data under strain.\n\nWhat is new: the four compositions themselves, and the strain-driven transitions—nodal loop in V2Te2O, Weyl points in V2STeO, gap-type changes in the other two. Prior work on V2Se2O/V2SeTeO covered valley polarization and piezomagnetism but not these compounds or the topological transitions. The paper does several things right: a standard, well-parameterized DFT+U workflow with convergence checks, U=4 eV sensitivity tests, HSE06 cross-checks, phonon and AIMD stability, and symmetry analysis with IRs and Berry phase. There is no fitting of target results; the valleys and crossings are direct DFT outputs. That is honest computational work.\n\nSoft spots, in proportion. The main-text band structures are spinless; SOC is deferred to the supplementary material and asserted to be weak. But the nodal loops and Weyl points are identified at strained geometries (−1.7% to −5% for V2Te2O, −7% and −9% for V2STeO), and no strained SOC calculation is shown. V2Te2O has heavy Te and a PBE gap of only 0.068 eV, so SOC could plausibly gap those crossings on the scale of tens of meV. If that happens, the 'topological states' headline fails, even though the valley polarization and piezomagnetism would survive. That is a genuine gap in evidence, not a manufactured flaw. Also, the 'giant piezomagnetism' wording oversells a rigid-band estimate; the magnetization numbers are fine, but calling them giant is not supported by the calculation. Minor: no code or data deposit, and the arXiv version lacks the supplementary material, so several load-bearing checks are not independently verifiable from the preprint alone.\n\nWho benefits: anyone cataloging 2D altermagnetic valleytronic candidates or designing strain-based valley devices. It is a useful subfield-level addition rather than a breakthrough. The paper deserves a serious referee, with the specific request that the referee check SOC at the strained crossings and confirm the supplement contains what the main text promises.\n\nRecommendation: send to peer review, conditional on supplement access and strained SOC verification.","headline":"Workmanlike DFT prediction of four altermagnetic valleytronic monolayers; the valley picture is likely robust, but the strain-driven topological states rest on SOC-free bands and need checking.","tokens_in":13597,"tokens_out":2573,"would_cite":true,"duration_ms":23057,"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":"Four monolayer vanadium oxychalcogenides are predicted to be altermagnetic valleytronic semiconductors whose valleys, band topology, and magnetization can all be controlled by uniaxial strain and doping.","keywords":["altermagnetism","valleytronics","two-dimensional magnets","strain engineering","valley polarization","topological semimetal","piezomagnetism","first-principles calculations"],"falsifier":"A relativistic calculation with spin-orbit coupling at the critical compressive strains for V2Te2O and V2STeO would settle the topological claim: if the crossings at X and Y develop a gap, the nodal loops and Weyl points are artifacts of the nonrelativistic approximation. On the experimental side, spin-resolved ARPES on a strained monolayer (or exfoliated bulk) that fails to show opposite-spin valleys at X and Y would disprove the altermagnetic valley picture.","tokens_in":12519,"feed_emoji":"🧲","tokens_out":7122,"duration_ms":57183,"temperature":0.7,"pith_summary":"The paper predicts four two-dimensional vanadium oxychalcogenide monolayers—V2Te2O, V2STeO, V2SSeO, and V2S2O—as altermagnetic semiconductors, a class of collinear magnets whose opposite-spin sublattices are related by crystal symmetry rather than by inversion or translation. In these materials the conduction and valence band edges form a pair of valleys at the X and Y points of the Brillouin zone, connected by a mirror symmetry. Because the two valleys are not linked by time-reversal symmetry, uniaxial strain can break the connecting mirror symmetry, lift the valley degeneracy, and produce valley polarization. The paper further shows that compressive strain drives V2Te2O and V2STeO into topological semimetal states and that, once valley polarization is achieved, hole doping yields a sizable piezomagnetic response. If correct, these monolayers would provide a single material platform in which valley, spin, band topology, and magnetization are all tunable by strain.","feed_headline":"Strain tunes valleys, topology, and magnetization in four 2D magnets","feed_subtitle":"Four monolayer materials show strain-controlled valley polarization, topological states, and piezomagnetism.","key_machinery":"The load-bearing object is the altermagnetic order parameter: the two V atoms with opposite spins are connected by mirror symmetry $M_{110}$, which allows spin-split bands without net magnetization and places two degenerate, spin-opposite valleys at the time-reversal-invariant momenta X and Y. Uniaxial strain breaks $M_{110}$, so the valleys split. The topological states are then protected by residual mirror symmetries: nodal loops around X and Y in V2Te2O are protected by $M_z$, and Weyl points on the Y–M and X–M paths in V2STeO are protected by $M_y$ and $M_x$. Piezomagnetism arises from the combination of strain-induced valley polarization and carrier doping, which moves the Fermi level into the single spin-polarized valley.","core_discovery":"The central claim is that monolayer V2Te2O, V2STeO, V2SSeO, and V2S2O are altermagnets: their two V sublattices carry opposite moments and are connected by the mirror symmetry $M_{110}$, which produces spin-split nonrelativistic bands without net magnetization. The band edges form degenerate valleys at X and Y, with opposite spin character at the two valleys, so the valleys exhibit spin-valley locking. Uniaxial strain breaks $M_{110}$, lifting the X/Y degeneracy in opposite directions for tensile and compressive strain; beyond threshold strains, band crossings appear, namely nodal loops protected by $M_z$ in V2Te2O and Weyl points protected by $M_x$/$M_y$ in V2STeO, each with a Berry phase of $\\pi$. With valley polarization in place, hole doping populates only one spin channel, producing a strain-dependent net magnetization (piezomagnetism) whose sign reverses between tensile and compressive strain. The same qualitative strain response is reported for V2SSeO and V2S2O, and HSE06 calculations confirm the main band features and adjust the gap values.","pith_inferences":["If the predictions hold, these four monolayers would be rare examples where valley, spin, topology, and magnetism respond to one external knob (strain), which could simplify valleytronic devices that currently need magnetic fields or optical pumping.","The symmetry mechanism is general: any altermagnet whose valleys sit at time-reversal-invariant momenta connected by a mirror symmetry should show strain-induced valley polarization, so this family may serve as a template for searching other monolayers.","A direct experimental test would be spin-resolved ARPES on strained or exfoliated V2Te2O: it should show spin-split bands with opposite spin at X and Y, and uniaxial strain should shift one valley relative to the other."],"forward_implications":["Uniaxial tensile strain increases the global gap and makes the Y-valley gap larger than the X-valley gap; compressive strain does the reverse, so valley polarization is continuously tunable and reverses sign with strain.","For V2Te2O, compressive strain beyond about -1.1% closes the spin-down gap at Y, and beyond about -1.7% nodal loops protected by $M_z$ form around both Y and X points with opposite spin channels.","For V2STeO, compressive strain of -7% creates a spin-down Weyl point near Y, and at -9% Weyl points appear at both X and Y, each with Berry phase $\\pi$ and protected by $M_y$/$M_x$.","Hole doping of the strained monolayers produces a net magnetization that grows with doping and strain, saturates at large strain, and reverses between tensile and compressive strains.","Phonon spectra and 300 K ab initio molecular dynamics simulations show the monolayers are dynamically and thermally stable, and the main band features survive HSE06 and the tested Hubbard U values."],"supporting_citations":[{"why":"Defines altermagnetism and the spin-group symmetry that allows nonrelativistic spin-split bands; supplies the criterion for classifying these monolayers as altermagnets.","marker":"[1, 2]"},{"why":"Previous 2D altermagnet V2Se2O; provides the comparison material, the valley-polarization definition $P=E(X)-E(Y)$, and the piezomagnetism baseline.","marker":"[15]"},{"why":"Previous strain-induced valley polarization in V2SeTeO; supplies the U=4 eV convention and the strain-response comparison.","marker":"[38]"},{"why":"Reports synthesis of bulk V2Te2O, the experimental parent of one predicted monolayer, supporting the materials' feasibility.","marker":"[52]"},{"why":"HSE06 hybrid functional calculation used to cross-check band gaps and main band features.","marker":"[49]"},{"why":"DFT+U method used to treat V-3d correlations in the band-structure and total-energy calculations.","marker":"[45, 46]"},{"why":"Earlier V-based altermagnet studies that set the Hubbard U = 4 eV value used here.","marker":"[47, 48]"}],"fun_headline_variants":["Strain tunes valleys, topology, and magnetization in 2D altermagnets","Uniaxial strain drives valley polarization and topology in 2D altermagnets","Strain switches valley polarization and topology in 2D altermagnets","Strain-induced valleys, topology and magnetism in 2D altermagnets","Strain controls valley polarization, topology, and piezomagnetism in 2D altermagnets"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The calculations assume the low-energy band structure is correctly captured by density-functional theory with a Hubbard $U$ of 4 eV and with spin-orbit coupling neglected; if spin-orbit coupling opens a gap at the predicted band crossings, or if a different $U$ changes the band ordering, the topological semimetal states would not be realized as described.","fun_headline_variants_meta":{"raw":{"variants":["Strain tunes valleys, topology, and magnetization in 2D altermagnets","Uniaxial strain drives valley polarization and topology in 2D altermagnets","Strain switches valley polarization and topology in 2D altermagnets","Strain-induced valleys, topology and magnetism in 2D altermagnets","Strain controls valley polarization, topology, and piezomagnetism in 2D altermagnets"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.001163,"raw_usage":{"total_tokens":4879,"prompt_tokens":1077,"completion_tokens":3802,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":693,"completion_tokens_details":{"reasoning_tokens":3692}},"tokens_in":693,"tokens_out":3802,"duration_ms":24570,"temperature":1.0,"reasoning_tokens":3692,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-12T10:23:58.922068+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A relativistic calculation with spin-orbit coupling at the critical compressive strains for V2Te2O and V2STeO would settle the topological claim: if the crossings at X and Y develop a gap, the nodal loops and Weyl points are artifacts of the nonrelativistic approximation. On the experimental side, spin-resolved ARPES on a strained monolayer (or exfoliated bulk) that fails to show opposite-spin valleys at X and Y would disprove the altermagnetic valley picture.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Previous 2D altermagnet V2Se2O; provides the comparison material, the valley-polarization definition $P=E(X)-E(Y)$, and the piezomagnetism baseline."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Previous strain-induced valley polarization in V2SeTeO; supplies the U=4 eV convention and the strain-response comparison."},{"cited_title":"Ablimit, Y.-L","cited_arxiv_id":null,"evidence_quote":"Reports synthesis of bulk V2Te2O, the experimental parent of one predicted monolayer, supporting the materials' feasibility."}],"review_version":1}