{"id":"4ad26e23-f9d2-48ce-9cf6-6f39223e8478","arxiv_id":"2605.29501","paper_version":1,"verdict":"UNVERDICTED","confidence":"LOW","novelty_score":7.0,"correctness_risk":"unknown","formal_verification":"none","parameter_count":1,"one_line_summary":"Diagrammatic Kubo calculation at van Hove singularity yields Re[η(Ω)] ∼ (|Ω|^{3/2} + T^{3/2})/Ω² and dc shear viscosity ∼ T, differing from Boltzmann transport theory.","lead":"The paper uses a diagrammatic Kubo formalism to calculate shear viscosity at a van Hove singularity in strongly correlated systems. It finds that while conductivity matches Boltzmann theory, shear viscosity shows different behavior and offers specific predictions for strange metals.","discovery_kind":"new_application","skeptic_critique":{"model":"grok-4.3","headline":"Central viscosity claim uses fermion sharp peak approx that paper states breaks down in dirty critical limit","rationale":"The reader's weakest_assumption is exactly the load-bearing point; the paper's internal caveat directly limits the reliability of the reported viscosity deviation in the regime of interest, leaving the central claim unverified without further work.","tokens_in":1701,"tokens_out":307,"duration_ms":15233,"concrete_test":"Recompute the Kubo bubble and vertex corrections for shear viscosity (the diagrams that yield the |Ω|^{3/2} and T^{3/2} terms) while retaining finite fermion broadening Γ_f comparable to boson broadening; check whether the dc η∼T scaling survives or crosses over to the Boltzmann form once the sharp-peak assumption is relaxed.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The headline result is that Kubo gives Re[η(Ω)]∼(|Ω|^{3/2}+T^{3/2})/Ω² and Re[η(0)]∼T while conductivity matches Boltzmann. This difference is obtained from diagrammatic Kubo under the fermion sharp peak approximation. The text explicitly notes that this approximation 'strictly breaks down in the dirty critical limit'—precisely the regime where the qualitative deviation for viscosity is asserted. The justification that Kubo is 'more reliable' rests on showing reduction to Boltzmann only inside the approximation; no controlled expansion or alternative calculation outside it is provided for the viscosity channel.","agreement_with_reader":"agree"},"referee_report":{"model":"grok-4.3","summary":"The manuscript computes several critical transport coefficients at a van Hove singularity using diagrammatic Kubo formalism. It reports that conductivity agrees with the Boltzmann result, while the dc shear viscosity shows qualitatively different scaling: Re[η(Ω)] ∼ (|Ω|^{3/2} + T^{3/2})/Ω² and Re[η(Ω=0)] ∼ T. These results are obtained under the fermion sharp peak approximation, which the abstract states strictly breaks down in the dirty critical limit; the same critical model is said to also account for strange metal behavior and yields experimentally testable predictions for optical and dc shear viscosities.","tokens_in":1831,"tokens_out":401,"duration_ms":18553,"significance":"If the viscosity results can be established beyond the noted approximation, the work would supply concrete, falsifiable predictions distinguishing Kubo from Boltzmann transport in strongly correlated critical systems, providing an additional experimental handle on the validity of the underlying model for strange metals.","major_comments":[{"comment":"Abstract: The headline claim of qualitatively different dc shear viscosity behavior (Re[η(Ω=0)] ∼ T and the optical form) is derived under the fermion sharp peak approximation. The manuscript itself states that this approximation 'strictly breaks down in the dirty critical limit'—the precise regime in which the deviation from Boltzmann is asserted—yet no controlled expansion or alternative calculation outside the approximation is provided for the viscosity channel.","section":"Abstract"},{"comment":"Abstract: The assertion that 'the diagrammatic Kubo results are more reliable' rests on demonstrating reduction of leading Feynman diagrams to the Boltzmann equation inside the fermion sharp peak approximation. This internal consistency does not establish reliability in the dirty critical limit where the approximation fails, leaving the central distinction for viscosity without independent support.","section":"Abstract"}],"minor_comments":[],"recommendation":"major_revision","confidential_remarks":null},"author_rebuttal":{"model":"grok-4.3","summary":"We thank the referee for their careful reading of our manuscript and for highlighting important points regarding the scope of our approximations. We address each major comment below and agree that revisions to the abstract are necessary to clarify the limitations.","responses":[{"response":"We fully acknowledge that our calculations are performed within the fermion sharp peak approximation, which the manuscript explicitly states breaks down in the dirty critical limit. The reported scalings for the shear viscosity are derived under this approximation. We do not provide a controlled expansion beyond it, as this would require a different theoretical framework. To address the concern, we will revise the abstract to state more clearly that the predictions apply within the fermion sharp peak approximation and note the breakdown in the dirty critical limit.","revision_made":"yes","referee_comment":"[Abstract] Abstract: The headline claim of qualitatively different dc shear viscosity behavior (Re[η(Ω=0)] ∼ T and the optical form) is derived under the fermion sharp peak approximation. The manuscript itself states that this approximation 'strictly breaks down in the dirty critical limit'—the precise regime in which the deviation from Boltzmann is asserted—yet no controlled expansion or alternative calculation outside the approximation is provided for the viscosity channel."},{"response":"The claim of greater reliability is based on the fact that, within the approximation, the Kubo formalism reproduces the Boltzmann result for conductivity, serving as a consistency check. We agree that this does not extend to establishing reliability in the regime where the approximation fails. We will revise the abstract to qualify or remove the statement about the diagrammatic Kubo results being more reliable in the dirty critical limit.","revision_made":"yes","referee_comment":"[Abstract] Abstract: The assertion that 'the diagrammatic Kubo results are more reliable' rests on demonstrating reduction of leading Feynman diagrams to the Boltzmann equation inside the fermion sharp peak approximation. This internal consistency does not establish reliability in the dirty critical limit where the approximation fails, leaving the central distinction for viscosity without independent support."}],"tokens_in":1390,"tokens_out":454,"duration_ms":27098,"standing_objections":["Developing a controlled calculation of the viscosity outside the fermion sharp peak approximation."]},"desk_editor":{"model":"grok-4.3","letter":"The main takeaway is that the claimed qualitative difference in shear viscosity from Boltzmann theory is obtained under an approximation the authors themselves flag as invalid in the regime of interest.\n\nThe calculation is new: a diagrammatic Kubo treatment of shear viscosity at the van Hove singularity that produces Re[η(Ω)] ∼ (|Ω|^{3/2} + T^{3/2})/Ω² and Re[η(0)] ∼ T while conductivity matches the semiclassical result. The work shows explicitly how the leading diagrams reduce to the Boltzmann equation inside the sharp-peak limit, which is a useful check.\n\nThe soft spot is central. The paper notes that the fermion sharp peak approximation strictly breaks down in the dirty critical limit, yet that is the regime where the viscosity deviation is asserted. No controlled expansion or alternative route outside the approximation is given for the viscosity channel, so the justification that Kubo is more reliable does not land. The tie to a strange-metal model also means the predictions are not fully independent of fitted quantities.\n\nThis is for theorists tracking transport calculations near van Hove points. A reader seeking robust, approximation-independent results on viscosity will not find them here.\n\nI would not recommend sending this to peer review in its current form; the load-bearing assumption needs to be resolved first.","headline":"The viscosity scaling result rests on the fermion sharp peak approximation that the paper states breaks down in the dirty critical limit.","tokens_in":2294,"tokens_out":335,"would_cite":false,"duration_ms":16134,"reading_group":"no","serious_thinker":"no","would_accept_peer_review":false},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"grok-4.3","headline":"At a van Hove singularity the dc shear viscosity scales linearly with temperature while conductivity follows the Boltzmann result.","keywords":["shear viscosity","van Hove singularity","Kubo formalism","Boltzmann transport","strange metal","critical transport","strongly correlated electrons","viscosity scaling"],"falsifier":"A measurement of dc shear viscosity at a van Hove singularity in a strange-metal candidate that does not scale linearly with temperature.","tokens_in":2592,"feed_emoji":"","tokens_out":626,"duration_ms":23176,"temperature":0.7,"pith_summary":"The paper examines transport coefficients at a van Hove singularity in strongly correlated systems where quasiparticle excitations are ill-defined. Using a diagrammatic Kubo formalism it calculates both conductivity and shear viscosity. Conductivity happens to agree with semiclassical Boltzmann theory, but the shear viscosity exhibits different scaling with temperature and frequency. The results supply concrete predictions for optical and dc shear viscosities in a model that also describes strange metals and thereby offer a test of when the Boltzmann approach breaks down.","feed_headline":"Shear viscosity scales linearly with temperature at van Hove singularity","feed_subtitle":"Diagrammatic Kubo results give Re[η(0)] ~ T while conductivity matches Boltzmann theory, yielding testable predictions for strange-metal mod","key_machinery":"Diagrammatic Kubo formalism for critical transport coefficients at the van Hove singularity, which reduces to the Boltzmann equation only under the fermion sharp peak approximation.","core_discovery":"Using the diagrammatic Kubo formalism the authors show that at the van Hove singularity the conductivity agrees with the Boltzmann result, yet the dc shear viscosity scales as T and the real part of the optical shear viscosity scales as (|Ω|^{3/2} + T^{3/2})/Ω². Under the fermion sharp peak approximation the leading Feynman diagrams reduce to the Boltzmann equation, but the diagrammatic method remains reliable in the dirty critical limit where that approximation fails.","pith_inferences":["Shear-viscosity measurements could distinguish regimes where semiclassical transport holds from those where it fails near van Hove points.","The same diagrammatic approach may reveal similar discrepancies for other transport coefficients when fermion broadening dominates.","Candidate materials with van Hove singularities near the Fermi level offer direct experimental tests of the predicted scalings."],"forward_implications":["Dc shear viscosity scales linearly with temperature rather than following Boltzmann predictions.","Optical shear viscosity follows the specific form Re[η(Ω)] ∼ (|Ω|^{3/2} + T^{3/2})/Ω².","Conductivity remains consistent with the Boltzmann result even when viscosity does not.","The same critical model yields testable predictions for both optical and dc shear viscosities."],"fun_headline_variants":["Shear viscosity scales as T at van Hove singularity","Kubo formalism yields T-linear shear viscosity at van Hove singularity","dc shear viscosity linear in T at van Hove via diagrammatic Kubo","Conductivity matches Boltzmann but shear viscosity ~T at van Hove singularity"],"cache_read_input_tokens":2112,"weakest_assumption_plain":"The fermion sharp peak approximation remains valid enough that leading diagrams reduce to the Boltzmann equation.","fun_headline_variants_meta":{"raw":{"variants":["Shear viscosity scales as T at van Hove singularity","Kubo formalism yields T-linear shear viscosity at van Hove singularity","dc shear viscosity linear in T at van Hove via diagrammatic Kubo","Conductivity matches Boltzmann but shear viscosity ~T at van Hove singularity"]},"model":"grok-4.3","cost_usd":0.007882,"raw_usage":{"total_tokens":3600,"prompt_tokens":679,"num_sources_used":0,"completion_tokens":75,"cost_in_usd_ticks":78824500,"prompt_tokens_details":{"text_tokens":679,"audio_tokens":0,"image_tokens":0,"cached_tokens":256},"completion_tokens_details":{"audio_tokens":0,"reasoning_tokens":2846,"accepted_prediction_tokens":0,"rejected_prediction_tokens":0}},"tokens_in":679,"tokens_out":75,"duration_ms":20594,"temperature":1.0,"reasoning_tokens":2846,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-06-29T05:48:58.260967+00:00","model_set":{"reader":"grok-4.3"},"falsifier":"A measurement of dc shear viscosity at a van Hove singularity in a strange-metal candidate that does not scale linearly with temperature.","supporting_citations":[],"review_version":1}