{"id":"21663403-2df8-458a-b523-09f0e6bec559","arxiv_id":"2501.09990","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":7.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":1,"one_line_summary":"Monolayer beta-prime In2Se3 shows temperature-invariant thermal conductivity near 0.6 W/mK from 150 to 800 K, with an electric field able to switch the heat flow by a factor of 2.5.","lead":"Monolayer beta-prime In2Se3, a two-dimensional ferroelectric, keeps its heat conductivity near 0.6 W/mK from 150 to 800 K, about as low as glass. The temperature-independent value comes from a balance between ordinary phonon propagation and wave-like tunneling, and an electric field can switch it by a factor of 2.5.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"PTI claim rests on an NEP potential not validated for β′-In2Se3; the 1.8× WTE/MD gap leaves the phonon balance unconfirmed.","rationale":"I read the paper as making a concrete prediction: β′-In2Se3 is an intrinsic crystalline PTI material. The reader’s conditional verdict is appropriate because the two supporting methods agree on the plateau but share one fitted potential. My stress-test finds no internal contradiction in the logic, and the α-phase cross-check is real evidence of NEP quality, but it does not cover the phase whose anharmonic balance is the whole story. The 1.8× κWTE/κMD gap reinforces the need for a direct DFT benchmark. A single DFT-based WTE recomputation of β′ would settle the question. Therefore I recommend no change to the reader’s CONDITIONAL verdict.","tokens_in":10423,"tokens_out":5108,"duration_ms":54162,"concrete_test":"Compute β′-In2Se3 harmonic and third-order IFCs from DFT (e.g., 4×4×1 supercell, 60×60×1 q-grid, same settings as the α-phase validation) and run the WTE calculation at 150, 300, and 600 K. If the DFT-based κp, κc, and total κ match the NEP results to within ~20% with a preserved temperature plateau, the validation gap is closed; if the coherence fraction shifts by more than ~10 percentage points or the plateau disappears, the PTI claim is not established.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim is not just that κ is low, but that its temperature invariance arises from a specific compensation between particle-like population transport and wave-like coherence transport. Both quantitative supports (WTE and HNEMD-MD) use the same NEP model, and the only direct DFT-level validation shown is for the α phase, where transport is conventional and coherence is negligible. For β′-In2Se3 there is no DFT-derived IFC check, no comparison of third-order force constants, and no independent anharmonic benchmark. The force RMSE of 0.147 eV/Å is large enough that a systematic error in the low-frequency anharmonic couplings—precisely the modes dominating κp and the flat bands contributing κc—could change the κp/κc ratio enough to remove the compensation. The fact that NEP-based WTE gives ~0.34 W/mK while NEP-based MD gives ~0.60 W/mK is not itself fatal, but it is unexplained at the quantitative level; the authors attribute it to zero-K IFCs missing negative frequency shifts, which means the WTE-derived balance is not independently confirmable without finite-temperature IFCs. Since the PTI mechanism is inferred from that WTE decomposition, the claim is only as secure as the β′-specific NEP. This is a tool-validation gap, not an inconsistency; the temperature plateau appears in both methods, which is encouraging, but the shared potential makes them correlated.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports a combined first-principles-informed study of thermal transport in monolayer In2Se3. The authors train a neuroevolution potential (NEP) on DFT data, then use both the generalized Wigner transport equation (WTE) and homogeneous nonequilibrium molecular dynamics (HNEMD) to compute the in-plane thermal conductivity. For monolayer β′-In2Se3 they find a temperature-invariant ultralow conductivity of about 0.6 W/mK between 150 and 800 K (about 0.34 W/mK in WTE), and they attribute this plateau to a propagating-tunneling-invariant (PTI) mechanism in which particle-like population transport and wave-like coherence transport compensate each other. They contrast this with monolayer α-In2Se3, which shows conventional T−1 behavior, and they further report that an in-plane electric field can tune the anharmonicity, restoring a power-law temperature dependence and producing a thermal switching ratio of about 2.5. The central claim is that intrinsic ferroelectric dipolar disorder, arising from a Mexican-hat potential energy surface, produces a glass-like temperature-independent thermal conductivity in a crystalline material without extrinsic disorder.","tokens_in":10834,"tokens_out":4250,"duration_ms":46508,"significance":"If the result holds, monolayer β′-In2Se3 would be a rare intrinsic crystalline system exhibiting PTI heat transport, extending the previously proposed disorder-based PTI concept to a simple stoichiometric ferroelectric. The paper has several notable strengths: the NEP is trained on a broad DFT database spanning multiple In2Se3 phases; the WTE and MD routes are methodologically independent and both produce a plateau; the α-phase WTE result is validated against DFT-derived IFCs; and the MD snapshots are consistent with experimental observations of nanostriped antiferroelectric ordering. The electric-field control of the temperature scaling law is a falsifiable and potentially useful prediction. The principal weakness is that the entire β′-In2Se3 transport claim rests on a single machine-learned potential with a force RMSE of 0.147 eV/Å, and the only direct DFT-level transport validation is for the α phase, where coherence is negligible. The unexplained factor of about 1.8 between WTE and MD further means that the WTE-based decomposition into population and coherence contributions, which is the evidence for the PTI mechanism, is not independently confirmed.","major_comments":[{"comment":"The central PTI claim for β′-In2Se3 rests entirely on thermal conductivities computed with the NEP model, but the paper validates the potential only for the α phase, where κc is negligible and the transport is conventional. Since the β′ claim depends on a precise balance between κp and κc at low frequencies and in flat bands, a systematic error in the low-frequency anharmonic couplings could remove the compensation. Please add a direct β′-specific validation: for example, compare NEP-derived second- and third-order IFCs for β′-In2Se3 against DFT supercell calculations at several representative q points, perform a DFT-based HNEMD or ab initio MD run at one temperature, or benchmark against an independently trained MLIP. Reporting the uncertainty in the NEP phonon linewidths and in κc would also help establish that the PTI decomposition is not an artifact of the potential.","section":"Methods, NEP training paragraph (p. 4)"},{"comment":"The two methods give a plateau but differ quantitatively: κWTE ≈ 0.34 W/mK versus κMD ≈ 0.60 W/mK. The explanation that zero-Kelvin IFCs miss negative frequency shifts is plausible, but it implies that the WTE-based decomposition, which is the only quantitative evidence for the PTI mechanism, is not independently verifiable as reported. Please quantify this effect: for example, extract finite-temperature IFCs from MD snapshots (e.g., with Dynaphopy) and recompute the WTE result, or estimate the increase in κc required to close the factor of 1.8 and check that this changes only κc and not κp. As written, the factor of about 1.8 remains an unexplained quantitative gap in a claim whose mechanism is inferred from the WTE decomposition.","section":"Fig. 1e and the WTE/MD comparison paragraph (p. 5)"},{"comment":"The claimed 150–800 K PTI plateau spans the β′→β phase transition at 400 K, so part of the reported range is not actually the ferroelectric β′ phase. If the thermal conductivity remains flat across the transition, then the paraelectric β phase also exhibits the plateau, and the abstract's phrase 'monolayer β′-In2Se3 … over a broad temperature range (150<T<800 K)' is misleading. Please either restrict the PTI claim to the β′ phase and treat the 400–800 K range as a separate result for the β phase, or explicitly argue and verify that the same compensation mechanism persists in the high-temperature nonpolar phase.","section":"Fig. 1e and Fig. 3c (phase transition at 400 K)"},{"comment":"The field-tuning interpretation assumes that the β′ phase remains the equilibrium structure under an in-plane field of up to 4 MV/cm. In a ferroelectric, such a field could instead stabilize a poled state or modify the order-disorder distribution, in which case the increase in κx would reflect a structural change rather than a pure modulation of anharmonicity. Please report the Se-displacement order parameter and phase identification under each field strength, and check whether the β′ phase is retained at 4 MV/cm. This is important because the conclusion that the field 'increases lattice harmonicity' is currently inferred only indirectly from longer phonon lifetimes.","section":"Electric field results, Fig. 4"}],"minor_comments":[{"comment":"The abstract reports κ ≈ 0.6 W/mK while the WTE value is about 0.34 W/mK. Please state both values explicitly and clarify that the MD and WTE routes differ by a factor of about 1.8.","section":"Abstract and Fig. 1e"},{"comment":"There are several typos and grammatical slips, including 'dose not' (p. 4), 'calcualtions' (p. 5), 'menefest' (p. 7), and 'via the using' (p. 8). A careful proofread is needed.","section":"Throughout"},{"comment":"The three-phonon lifetimes are plotted for the α and β′ phases, but the figure caption does not state the temperature or the method of lifetime extraction. Please specify these details in the caption or main text.","section":"Fig. 2d"},{"comment":"The fit κ(T) ∝ T−0.96 is shown as a line, but the fitting range and the statistical uncertainty of the exponent are not stated. Please provide this information, especially since the exponent is used to claim reemergence of a T−1 scaling.","section":"Fig. 4a"}],"recommendation":"major_revision","confidential_remarks":"The paper is within the scope of the journal and the central idea is interesting, but the validation gap for the β′-In2Se3 NEP is substantial enough that the PTI mechanism is not yet established. The self-citations (Refs. 38 and 48) are not used in a load-bearing way, so I do not see a novelty or fairness issue. I would encourage the editor to request the additional β′-specific validation and the finite-temperature WTE check described in the major comments; these are feasible within the manuscript's scope."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague,\n\nHere's the short version: this paper deserves a serious referee, but the headline claim is only as strong as the NEP potential, and that potential is not directly tested for the phase that matters.\n\nWhat's new: they report temperature-invariant (plateau) thermal conductivity in monolayer β'-In2Se3, an ordered ferroelectric, without extrinsic disorder. That's a meaningful extension of the PTI idea from meteorites and furnace bricks to a chemically simple crystalline system with intrinsic dipolar disorder from a Mexican-hat landscape. They also show an electric field can tune the behavior, with a switching ratio ≈2.5 and a return to T^-1 scaling at 4 MV/cm. If correct, that is a clean design principle.\n\nWhat's good: two routes, WTE and HNEMD-MD, both show the plateau, though at different absolute values. The α-phase comparison against DFT-derived IFCs validates the potential in that regime. The mechanistic analysis (λ parameter, Wigner vs Ioffe-Regel limits) is thoughtful, and the authors openly discuss the WTE/MD discrepancy.\n\nThe soft spots are real but not fatal. The biggest is that β' has no direct DFT-level validation of the anharmonic couplings that set the κp/κc balance. The force RMSE is 0.147 eV/Å, and the only direct validation is for α, where coherence is negligible. The 1.8× gap between WTE and MD is left as a plausible but untested explanation (finite-T frequency shifts). Since both methods share the same NEP, the plateau could be a property of the potential rather than the material. The field-switching results use a single method (finite-field Kubo MD) with no error bars or finite-size tests, and no code or data are released. These are addressable, not disqualifying.\n\nWho it's for: people in phonon coherence, 2D ferroelectrics, and ML potentials for transport. It is a good reading-group choice because the claims and the tool-validation gap are both discussable. I would send it to a referee with a request for β'-specific DFT checks (even a small-supercell IFC calculation) plus convergence tests, and I would want the NEP and scripts available.\n\nRecommendation: accept for peer review. My own verdict is conditional — the PTI mechanism is plausible but not yet nailed down.","headline":"Serious computational prediction of PTI thermal conductivity in a 2D ferroelectric, worth refereeing, but the mechanism rests on an NEP potential not directly validated for the β' phase.","tokens_in":11246,"tokens_out":3877,"would_cite":true,"duration_ms":35540,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":["63.20.-e","66.70.-f","77.80.-e"],"model":"deepseek-v4-flash","headline":"A single layer of β'-In2Se3 has glass-like heat conductivity that stays constant from 150 to 800 K.","keywords":["temperature-invariant thermal conductivity","two-dimensional ferroelectric","In2Se3","phonon coherence","Wigner transport equation","lattice anharmonicity","machine-learned potential","thermal switching"],"falsifier":"Measure the in-plane thermal conductivity of a suspended monolayer $\\beta'$-In$_2$Se$_3$ sample from 150 K to 800 K; a variation of more than about 20 percent over that range would rule out the claimed plateau. A complementary computation would recalculate the $\\beta'$ phase third-order force constants directly from density functional theory and check whether the coherence contribution still reaches roughly 40 percent of the total at 300 K.","tokens_in":10183,"feed_emoji":"🌡️","tokens_out":8641,"duration_ms":73677,"temperature":0.7,"pith_summary":"This paper reports that a monolayer of the ferroelectric crystal $\\beta'$-In$_2$Se$_3$ has an in-plane thermal conductivity of about 0.6 W/mK that stays nearly constant from 150 K to 800 K, a behavior normally associated with glasses. It attributes the plateau to a balance between two heat-carrying mechanisms: particle-like phonon propagation and wave-like tunneling between vibrational modes, a regime called propagating-tunneling-invariant (PTI). Unlike previous PTI materials, this one is a simple crystalline compound; the strong anharmonicity comes from the crystal's own ferroelectric dipolar fluctuations, so no structural disorder is needed. The paper also shows that an applied electric field can suppress this anharmonicity, restoring the usual $1/T$ decrease of thermal conductivity and yielding a thermal switching ratio of about 2.5.","feed_headline":"Ultrathin crystal keeps heat conductivity constant from 150 to 800 K","feed_subtitle":"In β'-In2Se3, wave-like and particle-like phonon transport balance out, holding heat flow at glass-like levels.","key_machinery":"The argument runs on the generalized Wigner transport equation, which separates thermal conductivity into a particle-like population term $\\kappa_p$ and a wave-like coherence term $\\kappa_c$, and on the ratio $\\lambda = (\\kappa_p - \\kappa_c)/(\\kappa_p + \\kappa_c)$ used to label phonon eigenstates as propagating or tunneling. In $\\beta'$-In$_2$Se$_3$, flat low-frequency phonon branches with large linewidths sit in a transitional regime between the quantum-coherence limit and the localization limit, giving $\\kappa_c$ about 40% of the total thermal conductivity at 300 K. The underlying source of anharmonicity is a Mexican-hat potential energy surface for the central-layer selenium atoms, whose thermally driven orientational disorder creates nanoscale antiferroelectric domains; a machine-learned interatomic potential fitted to density-functional-theory forces makes the large-scale molecular dynamics feasible.","core_discovery":"The central claim is that intrinsic ferroelectric dipolar disorder in monolayer $\\beta'$-In$_2$Se$_3$, arising from a Mexican-hat potential energy surface, generates strong lattice anharmonicity that activates wave-like coherence transport at temperatures as low as roughly 150 K. Because the coherence contribution rises with temperature while the particle-like contribution falls roughly as $T^{-1}$, the two nearly compensate, giving a temperature-invariant total conductivity of approximately 0.6 W/mK from 150 to 800 K. The paper contrasts this with the $\\alpha$-In$_2$Se$_3$ monolayer, which has the same stoichiometry but shows the conventional $\\kappa \\propto T^{-1}$ behavior. It further shows that an external electric field suppresses the dipolar disorder, lengthens low-frequency phonon lifetimes, weakens the coherence channel, and restores a $1/T$ scaling with a thermal switching ratio near 2.5.","pith_inferences":["Editorial inference: the same Mexican-hat mechanism may appear in other two-dimensional ferroelectrics and polar soft-mode materials, so temperature-invariant heat conduction could be a general property of crystals with shallow, multiply-degenerate polar minima rather than a quirk of In$_2$Se$_3$.","Editorial inference: the field-tuning results predict a smooth crossover in the temperature-scaling exponent from roughly 0 to -1 as the electric field increases, which could be tested as a tunable exponent in the same material.","Editorial inference: because the WTE calculation uses zero-temperature force constants and sits below the molecular-dynamics value, the true plateau may lie closer to 0.6 W/mK than to the WTE estimate; an experiment measuring the absolute value would discriminate between the two."],"forward_implications":["Thermal-management devices could use $\\beta'$-In$_2$Se$_3$ as a heat spreader whose conductivity does not drift as the device heats up.","The electric-field switching could enable on-demand heat routing, with a thermal switch ratio near 2.5 between low- and high-conductance states.","The work proposes a design principle: crystals whose soft ferroelectric modes create dipolar disorder can mimic glasses thermally without needing defects or alloying.","The $\\alpha$ versus $\\beta'$ comparison shows that stoichiometry alone does not determine the transport regime; the shape of the potential energy surface matters."],"supporting_citations":[{"why":"Supplies the generalized Wigner transport equation that separates thermal conductivity into population and coherence contributions.","marker":"[9]"},{"why":"Introduces the propagating-tunneling-invariant concept of temperature-invariant heat conductivity that the paper applies to a crystal.","marker":"[12]"},{"why":"Provides the neuroevolution potential method used to build the machine-learned force field for In2Se3.","marker":"[25]"},{"why":"Gives the homogeneous nonequilibrium molecular dynamics method used to compute the MD thermal conductivity.","marker":"[32]"},{"why":"Establishes monolayer In2Se3 as an intrinsic two-dimensional ferroelectric, the material platform.","marker":"[13]"},{"why":"Provides atomic-resolution imaging of ferroelectric order in beta-In2Se3 films that supports the structural picture.","marker":"[15]"},{"why":"Reports experimental nanostripe antiferroelectric ordering in In2Se3, matching the simulated dipolar disorder.","marker":"[46]"},{"why":"Provides prior DFT thermal-conductivity values for alpha-In2Se3 used to benchmark the alpha-phase calculations.","marker":"[37]"}],"fun_headline_variants":["2D ferroelectric crystal locks heat flow at glass level across 650 K","Glass-like heat flow held constant in ultrathin ferroelectric layer","Balanced phonon transport freezes heat conductivity in 2D crystal","Electric field tunes heat flow in ferroelectric monolayer","2D ferroelectric holds glass-like heat flow from 150 to 800 K"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"All thermal-conductivity results come from a machine-learned interatomic potential fitted to density-functional-theory energies and forces, and that potential is validated directly on the $\\alpha$ phase rather than on the $\\beta'$ phase; if it misrepresents the anharmonic balance in $\\beta'$-In$_2$Se$_3$, the temperature-invariant plateau collapses.","fun_headline_variants_meta":{"raw":{"variants":["2D ferroelectric crystal locks heat flow at glass level across 650 K","Glass-like heat flow held constant in ultrathin ferroelectric layer","Balanced phonon transport freezes heat conductivity in 2D crystal","Electric field tunes heat flow in ferroelectric monolayer","2D ferroelectric holds glass-like heat flow from 150 to 800 K"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000757,"raw_usage":{"total_tokens":3401,"prompt_tokens":1015,"completion_tokens":2386,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":631,"completion_tokens_details":{"reasoning_tokens":2289}},"tokens_in":631,"tokens_out":2386,"duration_ms":17192,"temperature":1.0,"reasoning_tokens":2289,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-10T19:26:51.422540+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the in-plane thermal conductivity of a suspended monolayer $\\beta'$-In$_2$Se$_3$ sample from 150 K to 800 K; a variation of more than about 20 percent over that range would rule out the claimed plateau. A complementary computation would recalculate the $\\beta'$ phase third-order force constants directly from density functional theory and check whether the coherence contribution still reaches roughly 40 percent of the total at 300 K.","supporting_citations":[{"cited_title":"Simoncelli, D","cited_arxiv_id":null,"evidence_quote":"Introduces the propagating-tunneling-invariant concept of temperature-invariant heat conductivity that the paper applies to a crystal."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the neuroevolution potential method used to build the machine-learned force field for In2Se3."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Gives the homogeneous nonequilibrium molecular dynamics method used to compute the MD thermal conductivity."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Establishes monolayer In2Se3 as an intrinsic two-dimensional ferroelectric, the material platform."},{"cited_title":"Zhang, J","cited_arxiv_id":null,"evidence_quote":"Provides atomic-resolution imaging of ferroelectric order in beta-In2Se3 films that supports the structural picture."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Reports experimental nanostripe antiferroelectric ordering in In2Se3, matching the simulated dipolar disorder."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides prior DFT thermal-conductivity values for alpha-In2Se3 used to benchmark the alpha-phase calculations."}],"review_version":1}