{"id":"c006df4e-e41b-4ab2-a2e4-574a08f66cf7","arxiv_id":"2509.03854","paper_version":1,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":1,"one_line_summary":"Uniaxial strain can switch monolayer MnBi2S2Te2 from a Chern insulator with Chern number 2 into states with Chern number 1, 0, or a metal, depending on strain direction.","lead":"Stretching a single layer of the magnetic compound MnBi2S2Te2 could switch off or redirect its lossless edge current, according to density functional theory. The result suggests mechanical strain could control topological circuits without magnetic fields.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Fixed Poisson ratio (-0.3) sets the transverse lattice vector; without zero-stress relaxation, the reported C=2→C=1→C=0 switching may be an artifact of an assumed strain path.","rationale":"The reader's weakest-assumption analysis correctly identifies the fixed Poisson ratio as the most load-bearing issue. The DFT calculations are standard, and the use of gap closing/reopening, edge-mode counting, and anomalous Hall conductivity provides mutually supporting evidence for the topological transitions at the chosen lattice geometries. The device-level boundary claim is explicitly a suggestion and does not undermine the central bulk-switching result. What remains unsecured is the mapping from 'uniaxial strain' as a physical operation to the specific lattice path used in the simulation. The transverse lattice vector is set by a generic Poisson ratio, not by the material's actual elastic response. This could alter the phase boundaries in Fig. 4e and potentially remove the C=1 phase, which is the only phase unique to this work. The proposed test is straightforward and directly settles whether the central claim is robust to realistic relaxation. Since this concern supports the reader's conditional verdict rather than changing it, the verdict should remain unchanged.","tokens_in":7633,"tokens_out":5726,"duration_ms":63587,"concrete_test":"Repeat the DFT calculations with the same VASP settings under a uniaxial-stress protocol: apply εxx=6% while relaxing the transverse lattice vector and all internal coordinates with the in-plane transverse stress σ_yy=0 (and similarly for εyy=6%). Then recompute the Wannier band structure, Chern number, edge modes, and σxy. If the Chern sequence remains C=2→C=1/C=0 at similar strains, the fixed-ν path is representative; if the phase changes, the reported switching is an artifact of the assumed Poisson ratio. Also report the relaxed Poisson ratio at each strain.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central switching result is computed under a prescribed strain path: one lattice vector is strained and the orthogonal vector is scaled by a Poisson ratio of -0.3, described only as 'a typical value for 2D materials' (Results). The band structure, Wannier interpolation, edge-mode counting, and anomalous Hall plateaus all follow from that particular geometric path. However, a physical uniaxial-stress experiment lets the transverse lattice vector relax according to the material's actual Poisson response. For a Janus material with inequivalent x/y directions, that response has not been computed or measured; it could differ substantially from -0.3. Since the C=1 and C=0 phases in Fig. 4e are identified only along this assumed path, a different Poisson ratio could move the gap-closing strains by more than the interval between the reported ~3% and ~3.5% thresholds, narrow or eliminate the C=1 window, or shift the metal/C=0 boundary. The headline claim rests on an unvalidated structural assumption rather than on a zero-stress-relaxed calculation. No internal inconsistency is apparent in the DFT/Wannier pipeline, but the protocol dependence is the weakest load-bearing point.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript uses DFT with a standard VASP/Wannier pipeline to study strain-induced topological transitions in monolayer MnBi2S2Te2. Under zero strain the material is predicted to be a C=2 Chern insulator. The authors report that uniaxial tensile strain along x closes and reopens the gap, yielding a C=1 phase, while strain along y yields a C=0 trivial insulator. They also report metallic behavior at larger strains and combine these results with earlier biaxial-strain data from Ref. [13] to construct a strain-phase diagram. The paper proposes that a boundary between strained and unstrained regions can act as a new dissipationless edge, with possible device applications as a topological transistor or switch. The paper also reports unsuccessful attempts on MnBi2Te4 and V2WS4, which are placed in the Supplementary Materials.","tokens_in":7883,"tokens_out":8422,"duration_ms":91529,"significance":"If the predicted C=2 → C=1 → C=0 switching is robust, the paper would establish a concrete mechanism for mechanically controlling quantum anomalous Hall edge conduction in a 2D material, and would introduce a strain-domain topological boundary as a designable element. The strengths are the use of multiple independent diagnostics — bulk gap closing/reopening, edge-mode counting, and anomalous Hall conductivity plateaus — and the fact that the Chern numbers are computed directly from the Wannier Hamiltonian rather than fitted to a target. The predicted phase diagram is falsifiable by transport measurements under controlled uniaxial stress. The main weakness is that the entire switching sequence is computed along a prescribed strain path with a fixed, unvalidated transverse lattice response, which limits the current strength of the quantitative claim.","major_comments":[{"comment":"The reported topological switching is computed along a strain path in which the transverse lattice vector is scaled by a Poisson ratio of -0.3, described only as 'a typical value for 2D materials.' This is a load-bearing structural assumption for a Janus monolayer whose actual Poisson response may be anisotropic and strain-dependent. The gap-closing strains (~3% and ~3.5%) and the existence of the C=1 and C=0 phases in Fig. 4(e) depend on the full lattice geometry. A different transverse contraction could shift those thresholds, narrow or eliminate the C=1 window, or change the metal/C=0 boundary. I ask the authors to remove this assumption by either (i) computing the transverse lattice vector under zero in-plane stress for uniaxial loading, or (ii) scanning the Poisson ratio over a plausible range (e.g., 0.1–0.5) and showing that the reported Chern sequence and edge-mode counts persist.","section":"Results, first paragraph; Fig. 4(e)"},{"comment":"The phase diagram combines a small number of new uniaxial calculations with prior biaxial points from Ref. [13], and the critical strains are given only approximately. The boundaries in Fig. 4(e) appear to be interpolated, and the reader cannot determine whether the C=1 phase occupies a wide enough strain window to be experimentally relevant or whether the C=0 phase is robust over an extended range. Please provide a table listing every computed strain value, the corresponding Chern number (or metal/insulator status), and the method used to assign it, and add points in the immediate vicinity of the transitions. This is important because the claim of clean switching rests on the sequence and ordering of these phases.","section":"Fig. 4(e) and text near 'around 3%'"}],"minor_comments":[{"comment":"In the abstract, 'MnBi2S2T2' should be 'MnBi2S2Te2'.","section":"Abstract"},{"comment":"The use of the phrase 'Poisson ratio of -0.3' is confusing because the conventional Poisson ratio is positive for most 2D materials and is defined as -ε_transverse/ε_axial. Please define the convention explicitly (e.g., ε_transverse = -ν ε_axial) and state that the value used is ν=0.3 in that convention, or correct the sign.","section":"Results, first paragraph"},{"comment":"The caption should clarify that the phase boundaries are guides to the eye, since the filled and empty circles represent discrete calculations, and should specify which parameters are varied along the axes (applied strain along x and y).","section":"Fig. 4(e) caption"},{"comment":"The statement that the proposed strain-boundary edge mode is a consequence of the difference in Chern numbers is correct, but it would be useful to state explicitly that the bulk gap must remain finite on both sides of the boundary, as also illustrated in Fig. 1(a).","section":"Discussion"},{"comment":"The title and abstract say '2D materials,' but the positive demonstration is for a single material, MnBi2S2Te2, while two other materials are reported to show metallization rather than a topological switch. The scope of the claim could be qualified in the title or abstract to avoid overgeneralization.","section":"Title/Introduction"}],"recommendation":"major_revision","confidential_remarks":"The manuscript is technically sound in its DFT/Wannier execution and the topological invariants are computed honestly. The key risk is not circularity but protocol dependence: the entire phase-switching picture rests on a single imposed Poisson ratio. This is fixable with a sensitivity scan or a proper zero-stress relaxation, so I do not recommend rejection. The editor may wish to require such a check before acceptance, since the paper's signature result is the quantitative strain phase diagram."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague,\n\nThis is a DFT study of monolayer MnBi2S2Te2 under uniaxial strain. The genuinely new result is that strain along the zigzag direction drives a C=2 to C=1 transition, and along the armchair direction drives C=2 to C=0, with a combined phase diagram. The calculations are standard and solid: gap closing/reopening, edge-state counting, and anomalous Hall plateaus all agree. They also report failed attempts in MnBi2Te4 and V2WS4, which is honest reporting of negative results.\n\nThe main soft spot is the transverse lattice vector under uniaxial strain. They scale it by a fixed Poisson ratio of -0.3, described as typical. That choice directly sets the phase boundaries in Fig. 4e. The reported gap-closing strains are close (3% vs 3.5%), and the C=1 window is narrow, so a different Poisson ratio could shift or even eliminate the C=1 phase. This is a legitimate concern. It doesn't undermine the qualitative claim that strong uniaxial strain can drive the material trivial, but it does mean the exact phase boundaries are predictions of an assumed strain path, not results of a zero-stress relaxation. A sensitivity study or simply computing the relaxed transverse vector would settle this.\n\nThe phase diagram is also sparse—only a handful of points—so the boundaries are guessed between them, and the device idea of a strain-domain wall edge is a suggestion, not a simulation. Those are minor.\n\nOverall, a serious and reproducible computational paper. It deserves a real referee. I'd ask for the Poisson-ratio robustness check before final acceptance, but I would not desk reject. Anyone working on strain-tuning of topological insulators will find it useful.","headline":"Uniaxial strain can switch the Chern number in MnBi2S2Te2, but the fixed Poisson ratio makes the exact phase boundaries a prediction rather than a robust derivation.","tokens_in":8410,"tokens_out":3001,"would_cite":true,"duration_ms":29473,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":["73.43.-f","71.15.Mb"],"model":"deepseek-v4-flash","headline":"The central claim is that uniaxial strain can switch the Chern number of monolayer MnBi2S2Te2 from C=2 to C=1 for strain along the zigzag axis and to C=0 for strain along the armchair axis, with the bulk band gap closing and reopening at ea","keywords":["Chern insulator","quantum anomalous Hall effect","uniaxial strain","MnBi2S2Te2","Janus 2D materials","topological phase transition","density functional theory","dissipationless edge modes"],"falsifier":"Recompute the band structure of MnBi2S2Te2 under uniaxial strain with the transverse lattice vector relaxed until the transverse stress vanishes, or measure the material's actual Poisson ratio. If the C=2 to C=1 gap reopening along x or the C=2 to C=0 reopening along y disappears, the reported switching is an artifact of the assumed Poisson ratio. Experimentally, measure the Hall conductivity of a strained flake: the quantized plateau should step from 2e^2/h to e^2/h or to zero as strain crosses the critical values.","tokens_in":7535,"feed_emoji":"⚡","tokens_out":7217,"duration_ms":70383,"temperature":0.7,"pith_summary":"The paper argues that mechanical strain, not a magnetic field or gate voltage, can be a practical switch for dissipationless edge currents in a 2D Chern insulator. Using first-principles calculations, it shows that tensile uniaxial strain in monolayer MnBi2S2Te2 closes and reopens the topological band gap, changing the Chern number from C=2 to C=1 under strain along one axis and to C=0 under strain along the perpendicular axis. The paper further proposes that the boundary between a strained and an unstrained region of the same sample can host a dissipationless, topologically protected edge mode. If correct, this opens a route to strain-controlled topological transistors and current switches.","feed_headline":"Uniaxial strain drops a 2D Chern insulator from C=2 to C=1 or C=0","feed_subtitle":"DFT shows the Janus monolayer MnBi2S2Te2 switches between C=2, C=1, and C=0 edge conduction under realistic strain.","key_machinery":"The central object is the doubly inverted band gap at the Gamma point of monolayer MnBi2S2Te2, carried by Bi p-orbital and Te p-orbital states with opposite parity, which gives a Chern number of C=2. Uniaxial strain is modeled in a rectangular 1x√3 supercell, with the transverse lattice vector scaled by a fixed Poisson ratio of -0.3. The mechanism works because the Janus structure breaks inversion symmetry, allowing the two band inversions to unwind cleanly rather than merely metallize. The paper uses Wannier-interpolated tight-binding models to count edge modes in a slab geometry and to compute the anomalous Hall conductivity, which together identify the Chern number at each strain value.","core_discovery":"The paper's central discovery is that in the Janus monolayer MnBi2S2Te2, which has a doubly inverted band gap and two chiral edge modes (C=2), uniaxial tensile strain unwinds those inversions in a direction-dependent way. Around 3% strain along the zigzag axis the gap closes and reopens with a single inversion, leaving one edge mode (C=1); around 3.5% strain along the armchair axis it closes and reopens with no inversion, leaving no edge mode (C=0). The paper confirms these assignments by counting Fermi-level-crossing edge modes in slab tight-binding calculations and by computing the anomalous Hall conductivity, which plateaus at (e^2/h)C. Larger strain drives the system metallic. Combining","pith_inferences":["A testable extension: other Janus magnetic topological insulators, or heterostructures with asymmetric chalcogen layers, should show similar strain-uninversion behavior if the uninverted gap can be restored without metallizing.","The x/y asymmetry suggests the two band inversions couple to different orbital characters, so tuning the S/Te ratio or layer stacking could widen the strain window of the C=1 phase.","If a strain boundary carries a quantized chiral mode, nonlocal transport across a patterned strain profile should show conductance steps in units of e^2/h, with the sign determined by the Chern-number difference.","Because the phase boundaries depend on the transverse lattice response, substrate clamping or encapsulation could serve as an additional experimental tuning knob by changing the effective Poisson ratio."],"forward_implications":["Strain becomes a reversible, external control knob for quantized Hall conduction: a single flake can be toggled between C=2, C=1, and C=0 by stretching along different in-plane axes.","A sample under a strain gradient should contain a one-dimensional dissipationless edge mode at the line where the band gap closes, requiring no physical cut, magnetic domain wall, or thickness step.","The proposed topological transistor and topological current switch have concrete operating principles: current is blocked or rerouted when a strained region becomes trivial or hosts an internal conducting boundary.","The combined uniaxial and biaxial phase diagram shows that strain alone can navigate all accessible topological phases of MnBi2S2Te2, including C=-1 and a metallic phase, making the phase space continuously tunable.","The contrast with MnBi2Te4, which only metallizes under uniaxial strain, identifies inversion-symmetry breaking as the likely condition that lets the gap close and reopen as a trivial insulator."],"supporting_citations":[{"why":"Earlier first-principles prediction of the doubly inverted band gap and biaxial-strain topological transitions in MnBi2S2Te2, which this paper extends to uniaxial strain.","marker":"[13]"},{"why":"Textbook result that regions of different Chern number must be separated by a conductive edge mode, grounding the strain-boundary proposal.","marker":"[1]"},{"why":"A separate quantum anomalous Hall candidate material that the paper tests and finds only metallizes under strain, setting off the successful switching in MnBi2S2Te2.","marker":"[12]"},{"why":"Experimental measurements showing graphene and transition metal dichalcogenides can withstand large uniaxial strain, supporting the strain magnitudes used here.","marker":"[14]"},{"why":"Review of strain engineering in 2D materials, backing the feasibility of uniaxial strains up to 10%.","marker":"[15]"},{"why":"Supplies the method for constructing maximally localized Wannier functions and the tight-binding model used to compute edge states.","marker":"[28]"},{"why":"Supplies the method for computing edge states and anomalous Hall conductivity from the tight-binding model.","marker":"[29]"}],"fun_headline_variants":["Strain flips 2D Chern number from 2 to 1 or 0","Strain tunes 2D Chern insulator: C=2 to C=1 or C=0","Strain switches Chern number in 2D: 2, 1, or 0","Uniaxial strain toggles Chern number in 2D insulator","Uniaxial strain sets Chern number to 2, 1, or 0 in 2D"],"cache_read_input_tokens":2688,"weakest_assumption_plain":"The calculations set the transverse lattice spacing under uniaxial strain by assuming a Poisson ratio of -0.3, a typical value for 2D materials, instead of relaxing it to zero transverse stress for this specific compound.","fun_headline_variants_meta":{"raw":{"variants":["Strain flips 2D Chern number from 2 to 1 or 0","Strain tunes 2D Chern insulator: C=2 to C=1 or C=0","Strain switches Chern number in 2D: 2, 1, or 0","Uniaxial strain toggles Chern number in 2D insulator","Uniaxial strain sets Chern number to 2, 1, or 0 in 2D"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.002048,"raw_usage":{"total_tokens":7775,"prompt_tokens":669,"completion_tokens":7106,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":413,"completion_tokens_details":{"reasoning_tokens":6990}},"tokens_in":413,"tokens_out":7106,"duration_ms":46266,"temperature":1.0,"reasoning_tokens":6990,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-05T10:35:47.545928+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Recompute the band structure of MnBi2S2Te2 under uniaxial strain with the transverse lattice vector relaxed until the transverse stress vanishes, or measure the material's actual Poisson ratio. If the C=2 to C=1 gap reopening along x or the C=2 to C=0 reopening along y disappears, the reported switching is an artifact of the assumed Poisson ratio. Experimentally, measure the Hall conductivity of a strained flake: the quantized plateau should step from 2e^2/h to e^2/h or to zero as strain crosses the critical values.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Earlier first-principles prediction of the doubly inverted band gap and biaxial-strain topological transitions in MnBi2S2Te2, which this paper extends to uniaxial strain."},{"cited_title":"Jiang, H","cited_arxiv_id":null,"evidence_quote":"A separate quantum anomalous Hall candidate material that the paper tests and finds only metallizes under strain, setting off the successful switching in MnBi2S2Te2."},{"cited_title":"Rold ´an, A","cited_arxiv_id":null,"evidence_quote":"Review of strain engineering in 2D materials, backing the feasibility of uniaxial strains up to 10%."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the method for constructing maximally localized Wannier functions and the tight-binding model used to compute edge states."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the method for computing edge states and anomalous Hall conductivity from the tight-binding model."}],"review_version":1}