{"id":"45f7cdbf-a6fd-4cad-8ec0-f8a5a3e9bc29","arxiv_id":"2505.21064","paper_version":1,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":0,"one_line_summary":"In a non-superconducting UTe2 crystal, filamentary superconductivity seen in resistivity follows the pressure evolution of bulk SC phases while calorimetry shows no bulk transition.","lead":"A UTe2 crystal without bulk superconductivity still shows superconductivity-like resistivity drops under pressure, even though its heat capacity shows no bulk superconducting transition. The result warns that resistivity alone can overstate superconductivity and hints that one of UTe2's pressure-induced superconducting phases survives disorder better than others.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The central SC2-robustness claim rests on identifying a zero-resistance filament with bulk SC2; percolation and extrinsic-phase alternatives are not excluded by the data.","rationale":"The paper's experimental core is sound: resistivity shows a zero-resistance filament while calorimetry shows no bulk transition, and the result is a useful cautionary tale. The strongest interpretive claim, however, goes beyond the data: 'comparatively increased robustness of Tc in the pressure-induced SC2 phase.' For that claim to hold, the filamentary path above 0.3 GPa must be the same thermodynamic phase as bulk SC2, and its measured midpoint Tc must reflect the intrinsic SC2 transition rather than a disorder- or strain-limited percolation path. Neither condition is demonstrated. The overlay in Fig. 2c is suggestive, but the low-pressure filament is already strongly suppressed relative to bulk SC1, so a high-pressure match with bulk SC2 could reflect pressure-dependent filament connectivity or an extrinsic phase rather than intrinsic robustness of SC2. The reader's weakest-assumption statement identifies this same identification problem, and the requested changes—softening the robustness language and providing raw data/uncertainties—are the appropriate remedy. I therefore keep the reader's CONDITIONAL verdict; relative to the reader, no change is needed. A field-dependent Hc2 anisotropy measurement is the cleanest single experiment that would decide whether the filament is bulk-like SC2.","tokens_in":767,"tokens_out":736,"duration_ms":123658,"concrete_test":"Perform resistivity under magnetic field on the same NSC crystal at p ≈ 1.0 GPa, measuring Tc(H) for H along the a, b, and c axes. Bulk SC2 in UTe2 has a distinctive, strongly anisotropic Hc2; if the filament's Hc2(θ) and Tc(H) match the bulk SC2 response reported in Ref. 5 rather than an isotropic impurity phase, the assignment is supported. If the transition simply shifts with field as a weak-link percolation path, the SC2-robustness conclusion is not established.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The most load-bearing link is the identification, in Fig. 2c, of the pressure-induced zero-resistance filament with the bulk SC2 phase. The final paragraph's claim of 'comparatively increased robustness of Tc in the pressure-induced SC2 phase' requires that the Tc(p) points above 0.3 GPa reflect the intrinsic SC2 transition rather than the properties of a particular percolating path. This is not established. The NSC crystal was chosen for its ~5% uranium vacancies and suppressed ambient SC1; the ambient filament already has Tc ≈ 1 K, well below bulk SC1. Under pressure, the same filamentary path could shift Tc through strain redistribution, pressure-medium nonhydrostaticity, contact/geometric effects, or formation of an extrinsic impurity or stress-stabilized superconducting phase. The 'similar critical pressure' language is equally fragile, resting on a single non-observation at 1.62 GPa with no reported uncertainty. Therefore the robustness conclusion is under-determined by the data.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This manuscript reports resistivity and ac-calorimetry measurements on a single crystal of UTe2 grown by CVT under conditions that suppress bulk superconductivity, showing filamentary zero-resistance at ~1 K at ambient pressure. Under hydrostatic pressure up to ~1.6 GPa, resistivity shows a nonmonotonic Tc(p) that resembles the pressure-temperature phase diagram reported for bulk superconducting UTe2, while heat capacity shows no bulk superconducting or magnetic anomaly. The authors interpret the data as evidence that the pressure-induced SC2 phase is more robust to disorder than SC1 and long-range magnetism, and discuss implications for the superconducting order parameter.","tokens_in":5542,"tokens_out":3400,"duration_ms":40972,"significance":"If the identification of the filamentary superconducting signal with the bulk SC2 phase is accepted, the paper provides a novel constraint: SC2 appears robust to the ~5% uranium vacancies that suppress SC1 and magnetically ordered phases. This would support distinct gap functions for the two superconducting phases and would challenge a proposed connection between SC2 and the disorder-sensitive field-reinforced phase. The manuscript is honest about its limitations and uses standard techniques, but the central inference is not uniquely determined by the data, as detailed below.","major_comments":[{"comment":"The central claim of \"increased robustness of Tc in the pressure-induced SC2 phase\" presupposes that the zero-resistance filament above 0.3 GPa is the intrinsic SC2 phase of UTe2. The sample already hosts a filamentary SC state with Tc ≈ 1 K at ambient pressure, far below bulk SC1's Tc, and the pressure-induced zero-resistance path could equally be an extrinsic phase, a strain-stabilized filament, or a percolating artifact. Because heat capacity shows no anomaly at the corresponding temperatures, the data do not establish that the Tc(p) points in Fig. 2c reflect the thermodynamic SC2 transition. The authors should either provide evidence identifying the filamentary path as SC2, or temper the robustness conclusion accordingly.","section":"Results, Fig. 2c"},{"comment":"The inference of a similar critical pressure for the disappearance of SC2 is based on a single non-observation at p = 1.62 GPa, with no reported pressure uncertainty and no upper bound on the possible Tc if a transition were missed. A suppression of the percolating path could also result from nonhydrostatic pressure or pressure-induced cracking. This load-bearing point needs replicate data or an explicit sensitivity analysis before the claimed similarity to bulk samples is supported.","section":"§4, 'suggesting a similar critical pressure'"},{"comment":"The pressure measurements used Daphne 7373 oil in a piston-clamp cell, which is known to become nonhydrostatic at low temperatures; the paper gives no estimate of pressure inhomogeneity or the pressure at which solidification occurs. Since Tc of a filamentary SC path can be shifted by local strain, the quantitative comparison of Tc(p) with the bulk phase diagram in Fig. 2c is affected. Please state the hydrostaticity limits, the accuracy of the Pb manometer at low temperature, and any evidence that the pressure medium remained hydrostatic across the measured range.","section":"§3, Methods (pressure cell)"},{"comment":"Only one NSC crystal was measured. Given the acknowledged large sample-to-sample variability in CVT-grown UTe2 (Refs. 12, 14), a single sample cannot, by itself, support a general statement about SC2 robustness. Reproducibility on at least one additional NSC crystal, or an explicit argument for why this particular sample is representative of the NSC class, is needed for the paper's broader conclusions.","section":"§2, sample selection and generality"}],"minor_comments":[{"comment":"The growth temperatures \"775 0C\" and \"685 0C\" should be written as \"775 °C\" and \"685 °C\".","section":"§2, growth conditions"},{"comment":"The phrase \"This results indicates\" should be corrected to \"This result indicates\".","section":"§2, sentence grammar"},{"comment":"The yellow symbols marking Tc from the present study have no error bars and no stated pressure uncertainty; at minimum, symbol size or a note on estimated uncertainty should be included.","section":"Fig. 2c"},{"comment":"The discussion of Anderson's theorem and its generalization would benefit from a footnote or sentence clarifying that the theorem applies to specific disorder symmetries and that the multi-band extension invoked here is nontrivial; the current phrasing could mislead readers into thinking the robustness follows without assumptions.","section":"§4, last paragraph"}],"recommendation":"major_revision","confidential_remarks":"The measurements appear carefully done and the paper is a useful data point, but the central claim about SC2 robustness goes beyond what a single-sample resistivity measurement on a filamentary path can establish. I would urge the editor to require either an explicit reframing of the conclusion as hypothesis-generating, or additional evidence (second sample, hydrostaticity checks, or a percolation analysis) before publication. The authors' use of their own prior phase diagram as a benchmark is appropriate and not circular, since no parameters are fitted to it."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The genuinely new thing here is the first pressure–temperature phase diagram of a CVT-grown UTe2 crystal that does not show bulk superconductivity at ambient pressure, measured with both resistivity and ac calorimetry on the same sample up to 1.6 GPa. The central observation is clean: resistivity shows a zero-resistance filament with Tc ≈ 1 K, heat capacity shows no anomaly, and under pressure the filament's Tc follows roughly the same non-monotonic trajectory as bulk SC1/SC2. That is worth having on record, especially given the known sample-dependence problems in this material. The paper also functions as a good methodological caution about trusting resistivity alone for phase boundary determination.\n\nThe experimental work itself looks solid: standard four-probe resistivity, ac calorimetry, piston-clamp cell with a Pb manometer, and a clear presentation of the raw data. The authors are appropriately grounded in the prior sample-dependence literature (Rosa, Weiland, etc.) and do not overstate the descriptive parts of their result. The specific choice of a disorder-limited NSC crystal is sensible for asking whether pressure-induced phases survive in poor samples.\n\nThe soft spot is the interpretive leap at the end. The claim that the pressure-induced SC2 phase is 'comparatively increased robustness' relative to SC1 and the magnetic phases rests entirely on identifying the resistivity filament above 0.3 GPa with the bulk SC2 phase. That identification is not established. The ambient filament has Tc ≈ 1 K, well below bulk SC1; the same percolating path could shift under pressure through strain redistribution, nonhydrostaticity, or geometric/contact effects. The 'similar critical pressure' comment rests on a single non-observation at 1.62 GPa with no reported uncertainty. There are no error bars on Tc, only one sample, and no disorder series. So the robustness conclusion is under-determined. The authors do flag the small volume fraction and use cautious language in the abstract, but the final paragraph states the robustness finding as a result rather than a suggestion.\n\nWho benefits: specialists in UTe2 sample dependence and pressure phase diagrams, and experimentalists who need a reminder that resistivity filaments are not bulk superconductivity. The paper deserves a serious referee—it is a real dataset, not a trivial observation—but the referee should push for a tempered interpretation of SC2 robustness and for the raw data and uncertainties. My recommendation: send to peer review, accept after revision that either softens the central claim or adds supporting evidence (e.g., a second NSC sample, or explicit analysis of percolation scenarios).","headline":"A useful cautionary pressure study of a non-superconducting UTe2 crystal, but the headline claim about SC2 robustness outruns the filamentary evidence.","tokens_in":6121,"tokens_out":1729,"would_cite":true,"duration_ms":22538,"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":"A UTe2 crystal that lacks bulk superconductivity still hosts a pressure-induced SC2 phase whose transition temperature matches bulk samples above 0.3 GPa, showing SC2 is comparatively robust to disorder.","keywords":["UTe2","filamentary superconductivity","pressure phase diagram","uranium vacancies","disorder robustness","SC2 phase","chemical vapor transport","AC calorimetry"],"falsifier":"Measure the pressure dependence of the superconducting volume fraction, for example with AC susceptibility or muon spin rotation, in the same cold-end CVT-grown UTe2 crystals. If the SC transition seen in resistivity is accompanied by a growing bulk diamagnetic or muon signal, or if the filamentary Tc(p) curve does not match the bulk SC2 boundary when measured in samples with controlled uranium-vacancy concentrations, the claim that the filamentary phase is the robust SC2 phase would be falsified.","tokens_in":5202,"feed_emoji":"⚡","tokens_out":11641,"duration_ms":114786,"temperature":0.7,"pith_summary":"This paper studies a chemical-vapor-transport-grown UTe2 crystal that shows zero resistance near 1 K at ambient pressure but no heat-capacity anomaly, meaning superconductivity occupies only a tiny volume fraction. Under hydrostatic pressure, the resistivity-derived superconducting transition temperature first decreases, then rises into a pressure-induced phase, and then is suppressed near 1.6 GPa, matching the trajectory of bulk superconducting UTe2 samples. AC calorimetry finds no bulk superconducting or magnetic transitions up to 1.6 GPa, so the pressure-induced superconductivity remains filamentary. The authors conclude that the pressure-induced SC2 phase is comparatively robust to the disorder that destroys the ambient-pressure SC1 phase and long-range magnetism, and they caution that resistivity alone can mistake filamentary superconductivity for a bulk thermodynamic transition.","feed_headline":"Filamentary UTe2 superconductivity tracks bulk pressure phase diagram","feed_subtitle":"The pressure-induced SC2 phase survives in a crystal with only filamentary superconductivity, matching bulk Tc values.","key_machinery":"The load-bearing object is filamentary superconductivity: zero electrical resistance produced by a small volume fraction of the sample, with no corresponding heat-capacity anomaly. The measurements pair four-probe electrical resistivity, which detects percolating superconducting filaments, with AC calorimetry, which is sensitive to bulk thermodynamic transitions; their disagreement is the evidence that superconductivity is not bulk. The experiment uses a hydrostatic pressure cell with an oil pressure medium and a lead manometer, and the analysis maps the resistivity-derived $T_c(p)$ onto the established bulk pressure-temperature phase diagram of UTe2. The named conceptual mechanism at the end is a generalization of the classic theorem that fully gapped superconductors are insensitive to nonmagnetic disorder, extended to unconventional superconductors with multiple internal degrees of freedom such as orbitals or sublattices; the authors invoke it to explain why SC2 survives in the disordered crystal.","core_discovery":"The central claim is that the filamentary superconducting state in a non-superconducting UTe2 crystal evolves under pressure in a way that mirrors the behavior of bulk superconducting samples, and that this comparison reveals the pressure-induced SC2 phase to be more disorder-tolerant than both SC1 and the pressure-induced magnetic phases. The evidence is a set of electrical-resistivity and AC-calorimetry measurements on a CVT-grown crystal with significant uranium vacancies. While the ambient-pressure zero-resistance drop at about 1 K is not accompanied by any heat-capacity feature, its pressure dependence traces the same non-monotonic $T_c(p)$ curve as bulk UTe2, and above 0.3 GPa the filamentary SC2 transition temperature is comparable to the bulk value even though the ambient-pressure $T_c$ is much lower. At 1.62 GPa the resistive transition disappears, suggesting the same critical pressure as in bulk samples. The authors take this as evidence that SC1 and SC2 have distinct superconducting gap functions and that SC2's robustness may come from a generalized no-effect-of-nonmagnetic-disorder theorem for unconventional superconductors with multiple internal degrees of freedom.","pith_inferences":["A testable extension: measure the superconducting volume fraction of a cold-end CVT crystal under pressure using AC susceptibility or muon spin rotation; if the volume fraction grows with pressure or if the filamentary $T_c$ deviates from the bulk SC2 trajectory in samples with different vacancy concentrations, the identification of the filamentary phase with SC2 would need revision.","If SC2 is disorder-robust while SC1 is not, then samples that appear non-superconducting at ambient pressure may still host SC2 at high pressure, offering a route to study SC2's gap structure in crystals where SC1 is absent.","The broad 3-6 K heat-capacity hump at high pressure could be short-range magnetic correlations; following it to higher pressure or lower temperature might show whether disorder merely frustrates long-range order or suppresses the magnetic phase entirely.","Controlled electron irradiation of bulk UTe2 would provide a direct test: if SC2 is protected by a generalized disorder-insensitivity theorem, irradiation should suppress SC1 and magnetism while leaving pressure-induced SC2 largely intact."],"forward_implications":["Resistivity can be used to locate the SC2 phase boundary even in samples where the superconducting volume fraction is too small to be seen in heat capacity.","The distinct disorder responses of SC1 and SC2 imply that the two phases have different superconducting gap functions.","The robustness of SC2 is difficult to reconcile with identifying SC2 with the disorder-sensitive field-reinforced SC phase proposed by NMR work.","The lack of long-range magnetic order in this sample under pressure suggests that the magnetic phases of bulk UTe2 require a cleaner lattice, while SC2 does not.","If the generalized disorder-insensitivity mechanism applies, pressure-induced SC2 should survive in deliberately disordered UTe2 even as SC1 and magnetism are suppressed."],"supporting_citations":[{"why":"Discovery of odd-parity superconductivity in UTe2; defines the material system and the zero-field SC phase that this paper's NSC sample lacks.","marker":"1)"},{"why":"Provides the bulk pressure-temperature phase diagram of UTe2 showing SC1, pressure-induced SC2, and magnetic phases; the filamentary Tc(p) data are mapped onto this diagram.","marker":"5)"},{"why":"Reports the CVT growth method and how growth-temperature variations change Tc and RRR; the NSC crystal used here was grown at the cold end under these conditions.","marker":"12)"},{"why":"Documents uranium vacancies up to 5%, reduced unit-cell volume, and enlarged atomic displacement parameters in cold-end CVT crystals; used to attribute disorder effects.","marker":"14)"},{"why":"Shows the high-field reentrant SC phase appears even in CVT crystals that are not superconducting at zero field; motivates the idea that some UTe2 SC phases survive disorder.","marker":"17)"},{"why":"Reports that the field-reinforced SC phase is disorder-sensitive; used as a contrast to argue SC2 is instead robust and distinct.","marker":"16)"},{"why":"Additional bulk pressure-dependent data for UTe2 used together with Ref. 5 to construct the phase diagram onto which the filamentary Tc values are overlaid.","marker":"19)"},{"why":"NMR study under pressure reporting no Knight-shift drop at Tc; used to rule out a fully gapped isotropic gap as the explanation of SC2's robustness.","marker":"20)"},{"why":"Presents a generalized disorder-insensitivity theorem for unconventional superconductors with multiple internal degrees of freedom; invoked to explain SC2's insensitivity to disorder.","marker":"21)"},{"why":"The AC calorimetry technique; the method used to show that neither bulk superconductivity nor magnetism develops under pressure.","marker":"18)"}],"fun_headline_variants":["UTe2 filamentary SC tracks bulk pressure phase diagram","Non-bulk UTe2 shows pressure-driven SC2","SC2 in UTe2 survives without bulk superconductivity","Filamentary UTe2 superconductivity mirrors bulk Tc under pressure","Pressure reveals disorder-tolerant SC2 in UTe2"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The small superconducting volume fraction seen in resistivity under pressure is the same SC2 phase that appears in bulk UTe2 samples, not an extrinsic or strain-stabilized phase; if it were something else, the paper's conclusions about SC2's robustness would not follow.","fun_headline_variants_meta":{"raw":{"variants":["UTe2 filamentary SC tracks bulk pressure phase diagram","Non-bulk UTe2 shows pressure-driven SC2","SC2 in UTe2 survives without bulk superconductivity","Filamentary UTe2 superconductivity mirrors bulk Tc under pressure","Pressure reveals disorder-tolerant SC2 in UTe2"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.001258,"raw_usage":{"total_tokens":5124,"prompt_tokens":886,"completion_tokens":4238,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":502,"completion_tokens_details":{"reasoning_tokens":4156}},"tokens_in":502,"tokens_out":4238,"duration_ms":29576,"temperature":1.0,"reasoning_tokens":4156,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-07T13:36:03.509672+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the pressure dependence of the superconducting volume fraction, for example with AC susceptibility or muon spin rotation, in the same cold-end CVT-grown UTe2 crystals. If the SC transition seen in resistivity is accompanied by a growing bulk diamagnetic or muon signal, or if the filamentary Tc(p) curve does not match the bulk SC2 boundary when measured in samples with controlled uranium-vacancy concentrations, the claim that the filamentary phase is the robust SC2 phase would be falsified.","supporting_citations":[],"review_version":1}