{"id":"8e494b7e-e594-492e-96f1-dc53b2c47937","arxiv_id":"2507.09779","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"high","formal_verification":"none","parameter_count":3,"one_line_summary":"In the 3D Kagome compound CeRu2, uniaxial stress creates a dome in Tc and drives the superconducting gap from anisotropic to isotropic, while hydrostatic pressure preserves Tc but changes the low-temperature superfluid density to a linear form, suggesting nodal pairing.","lead":"Muon spin rotation shows that squeezing CeRu2 along one direction changes its superconducting temperature in a dome shape and makes its superconducting gap more uniform. High pressure instead changes how the material carries supercurrent at low temperature, suggesting a different form of superconductivity.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Pairing-symmetry claims lack statistical model comparison; the anisotropic-to-isotropic and nodeless-to-nodal transitions rest on fits whose superiority is not demonstrated.","rationale":"The reader's verdict is CONDITIONAL, and I do not disagree. My chosen concern is not the unknown in-plane stress direction (which affects interpretation of the stress axis but not necessarily the existence of the dome) but the unsupported model-selection step underlying both pairing-symmetry claims. The dome itself is a direct AC-susceptibility observation, so it is comparatively robust. In contrast, the anisotropic-to-isotropic crossover and the pressure-induced nodal state are conclusions drawn entirely from which gap model fits sigma_SC(T) best. The manuscript gives no quantitative goodness-of-fit comparison and no parameter-error discussion for model choice. Table I is particularly suggestive: at every stressed pressure the isotropic and anisotropic fits return identical lambda0 and Delta_max, so the 'crossover' may be a null result rather than evidence for a distinct isotropic state. Table II at 1.9 GPa is a single high-pressure point; the nodal and s-wave fits are both listed without any statistical basis for preference. This is not a claim that the data are wrong, only that the central fingerprint claims are not yet demonstrated. The proposed check, a formal model comparison on the published data, would settle it. If the fits genuinely differ, the conclusion stands; if not, the paper's headline should be weakened. Hence the reader's CONDITIONAL verdict remains appropriate.","tokens_in":12214,"tokens_out":5816,"duration_ms":66320,"concrete_test":"Obtain the sigma_SC(T) data and fit code for the 0, 0.07, 0.10, 0.13, 0.18 GPa and 1.5, 1.9 GPa runs. Re-fit each dataset with isotropic s-wave, anisotropic s-wave, and nodal models using the same London formula (Methods Eq. 5) and report chi-square_red, AIC/BIC, and F-test p-values. Specifically, check whether the anisotropic model at 0.07-0.18 GPa yields a Delta_min consistent with Delta_max within errors (isotropic) and whether the nodal model at 1.9 GPa beats the s-wave model at p < 0.05. If the model-selection evidence is not significant, the crossover and nodal claims should be withdrawn or explicitly labeled as not statistically resolvable.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The most load-bearing assumption is that the muon-spin-rotation relaxation-rate data actually distinguish the competing gap models. The paper reports no chi-square, Delta-chi-square, or likelihood metric for any of the fits. For the stress series, Table I lists identical lambda0 and Delta_max for isotropic and anisotropic s-wave fits at 0.07-0.18 GPa and gives no Delta_min; the claimed 'crossover to isotropic s-wave' could simply reflect that the data no longer constrain the anisotropy parameter. For the hydrostatic series, the nodal conclusion at 1.9 GPa rests on a single pressure point; Table II lists nodal Delta_max = 1.22(4) meV and s-wave Delta_max = 0.92(3) meV with no indication that the nodal model is statistically superior. The abstract's '2.2 GPa' versus the body's '1.9 GPa' adds a consistency question. Since the headline claims a pairing-symmetry fingerprint, this missing quantitative support is load-bearing.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports muon spin rotation (μSR) measurements on single-crystalline CeRu2 under uniaxial in-plane stress and hydrostatic pressure. The authors claim three main results: (i) uniaxial stress up to 0.22 GPa produces a dome-shaped evolution of Tc, with an initial plateau followed by enhancement and suppression; (ii) uniaxial stress drives a crossover from anisotropic to isotropic s-wave pairing, inferred from fits to the temperature-dependent superfluid density; and (iii) hydrostatic pressure up to 1.9–2.2 GPa leaves Tc largely unchanged but changes the low-temperature superfluid density from exponential to linear behavior, suggesting a nodeless-to-nodal transition. The paper combines a unique experimental setup and compares fits of isotropic s-wave, anisotropic s-wave, and nodal models to the extracted σSC(T).","tokens_in":12362,"tokens_out":4483,"duration_ms":49546,"significance":"If the central claims are substantiated, the paper would provide a valuable demonstration of mechanical tuning of the superconducting pairing in a three-dimensional kagome-type correlated metal, with distinct responses to uniaxial stress versus hydrostatic pressure. The work has clear strengths: it uses high-quality single crystals, combines AC susceptibility with μSR, checks the elastic regime through force-displacement linearity (Fig. 2c), and the ambient-pressure anisotropic s-wave gap ratio (Δmin/Δmax = 0.41) is consistent with previous μSR, NMR, and ARPES reports. The two-axis tuning approach and the proposed connection to flat-band physics are timely and interesting. However, the central pairing-symmetry claims currently rest on model fits without quantitative statistical comparison, a sparse Tc-stress dataset without error bars, and a single pressure point for the purported nodal behavior; these issues must be addressed before the conclusions can be considered established.","major_comments":[{"comment":"The manuscript reports no goodness-of-fit metric (χ², Δχ², AIC, BIC, or residuals) for any of the gap-model fits. In Table I, the anisotropic and isotropic s-wave fits return identical values of λ0 and Δmax at 0.07, 0.10, 0.13, and 0.18 GPa, with no Δmin value listed; the claimed 'crossover to isotropic s-wave' could simply reflect that the data no longer constrain the anisotropy parameter. In Table II, the nodal fit at 1.9 GPa gives Δmax = 1.22(4) meV versus 0.92(3) meV for the s-wave fit, but without a statistical comparison the assertion that the nodal model is 'best' is unsupported. Please provide quantitative model comparisons for all stress and pressure points, and explicitly state whether Δmin was a free parameter, fixed to zero, or constrained during the fits.","section":"§II.A, §II.B, Tables I and II, Eq. (5)"},{"comment":"The Fig. 1 caption states that the exact direction of the uniaxial stress within the (111) plane is not known. The interpretation that stress shifts flat bands and drives both the Tc dome and the anisotropic-to-isotropic crossover assumes that the uncontrolled in-plane direction is either irrelevant or reproducibly fixed. If the stress direction varies, or if the stress is not purely uniaxial along the intended plane, the observed Tc variation and the apparent evolution of the fitted gap structure could be artifacts of the stress cell rather than intrinsic properties of CeRu2. Please specify how the sample orientation was verified, how the stress direction was calibrated, and discuss the sensitivity of the conclusions to the unknown in-plane direction.","section":"Fig. 1 caption, §II.A"},{"comment":"The claimed dome-shaped Tc evolution is based on five stress values (0, 0.10, 0.13, 0.19, and 0.22 GPa) with no quoted uncertainties on Tc or on the transition midpoint. Given the definition of Tc as the 50% flux-exclusion midpoint and the absence of error bars, the enhancement between 0.10 and 0.13 GPa is not demonstrably significant. Please report Tc with errors and provide a statistical test (e.g., a fit to a dome function or a comparison of transition widths) to support the non-monotonic claim.","section":"§II.A, Fig. 2(a)"},{"comment":"The abstract states that hydrostatic pressure was applied 'up to 2.2 GPa', whereas the body text and Methods state the maximum pressure is 1.9 GPa, and all presented hydrostatic data are at 0, 1.5, and 1.9 GPa. This inconsistency is confusing and should be corrected. More importantly, the conclusion that hydrostatic pressure induces a nodeless-to-nodal transition rests on a single pressure point at 1.9 GPa, where only one σSC(T) curve supports the linear low-temperature behavior. Please either add at least one additional pressure point above 1.5 GPa or temper the claim by explicitly acknowledging that the evidence for nodal superconductivity is based on one pressure value.","section":"Abstract, §II.B, §IV (Hydrostatic Pressure Cell), Fig. 4"}],"minor_comments":[{"comment":"For the anisotropic s-wave rows at 0.07–0.18 GPa, the values of λ0 and Δmax are identical to the s-wave rows and Δmin is left blank; please state explicitly whether Δmin was fixed to Δmax during these fits or whether the fit returned a boundary value, and define the meaning of the blank entry.","section":"§II.A, Table I"},{"comment":"In Fig. 3, the solid and dashed lines are labelled only in panel (a); please ensure that every panel (a–e) clearly identifies which line corresponds to the isotropic and anisotropic s-wave fits, since the visual comparison is central to the crossover claim.","section":"Fig. 3"},{"comment":"There is a typographical error in 'degress of freedom' in the closing paragraph; it should read 'degrees of freedom'.","section":"§III (Conclusion)"},{"comment":"The phrase 'alters the superfluid density from exponential to linear behavior' is used for the hydrostatic-pressure result, but the paper does not show an explicit fit to an exponential form at ambient pressure in the hydrostatic series; please clarify that this refers to the temperature dependence of σSC(T) (or λ−2(T)) and specify the functional forms used.","section":"Abstract and §II.B"}],"recommendation":"major_revision","confidential_remarks":"The paper is potentially interesting but the pairing-symmetry claims are not yet supported by quantitative model comparison. The stress-direction uncertainty and the abstract/body pressure discrepancy should be resolved. I would urge the editor to require the authors to provide the statistical metrics and, if that is not possible, to soften the central claims accordingly."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Worth a look if you care about tunable superconductivity in kagome or pyrochlore systems. What is actually new: the first μSR study combining uniaxial stress and hydrostatic pressure on CeRu2. The stress-induced Tc dome and the pressure-induced change in low-temperature superfluid density are new for this compound and likely real. The experimental work is demanding, the crystal growth and multiple techniques are solid, and the ambient-pressure results reproduce earlier μSR, NMR, and ARPES reports, so the baseline is credible.\n\nThe soft spots are mostly about what the model fits can support. The crossover from anisotropic to isotropic s-wave pairing is inferred from fits to σSC(T) with no chi-square, likelihood, or other statistical comparison between models. In Table I, the anisotropic-s-wave entries above 0.07 GPa are reported with parameters identical to the isotropic ones and no Δmin, which suggests the data no longer constrain the anisotropy; the claimed crossover may be an underdetermined fit rather than a real change in gap symmetry. The hydrostatic-pressure nodal claim rests on a single pressure point at 1.9 GPa, where the nodal and s-wave fits are again not statistically compared. There is also a consistency issue: the abstract says 2.2 GPa while the body and methods say 1.9 GPa. The admitted unknown direction of the uniaxial stress within the (111) plane further weakens the flat-band interpretation, though it does not undermine the basic observation of a stress response.\n\nThe authors are appropriately cautious in places: they call the pressure result \"indicative of nodal superconductivity\" and explicitly note that nodal behavior does not require d-wave symmetry. But the headline claim of a \"fingerprint\" distinguishing stress and pressure effects needs quantitative support. As it stands, the data are a useful experimental contribution, while the symmetry-crossover and nodal-transition conclusions are plausible but not demonstrated.\n\nWho this is for: experimentalists working on CeRu2 or kagome superconductors, and theorists who want new tuning data. I would send it to peer review, but with a clear request for statistical model comparison and clarification of the pressure value and stress direction. The core measurements deserve to be published; the interpretive claims need to be scaled to the evidence.","headline":"Fresh μSR data on CeRu2 under stress and pressure, but the pairing-symmetry claims outrun the statistics.","tokens_in":13019,"tokens_out":2477,"would_cite":true,"duration_ms":29897,"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":"Uniaxial stress reshapes the superconducting gap of CeRu2 into a Tc dome, while hydrostatic pressure switches the same compound to nodal pairing.","keywords":["CeRu2","3D Kagome lattice","uniaxial stress","hydrostatic pressure","muon spin rotation","superconducting gap symmetry","nodal superconductivity","flat bands"],"falsifier":"Apply uniaxial stress with a cell that independently measures the in-plane strain tensor while tracking Tc and σSC(T); observing a different dome shape or no gap isotropization for different known stress directions would invalidate the flat-band-shift interpretation. Alternatively, hydrostatic pressure measurements at finer intervals near 1.9 GPa could check whether the linear low-temperature superfluid density emerges discontinuously or smoothly.","tokens_in":11991,"feed_emoji":"🧲","tokens_out":5681,"duration_ms":59470,"temperature":0.7,"pith_summary":"This paper reports that the superconducting state of the pyrochlore compound CeRu2 responds very differently to two mechanical knobs. Uniaxial stress applied in the (111) Kagome plane raises and then lowers the critical temperature, forming a dome, and changes the superconducting gap from anisotropic s-wave to isotropic s-wave with no structural transition. Hydrostatic pressure up to 2.2 GPa leaves Tc nearly unchanged but switches the low-temperature superfluid density from exponential to linear behavior, a signature of nodal pairing. The authors position CeRu2 as a 3D Kagome material where flat bands and correlations can be tuned by stress and pressure, offering a way to study how lattice geometry controls pairing symmetry.","feed_headline":"Stress and pressure steer CeRu2 superconductivity in opposite ways","feed_subtitle":"Muon spin rotation finds a Tc dome under strain but nodal pairing under isotropic pressure.","key_machinery":"The central observable is the superconducting muon spin relaxation rate σSC, related to the magnetic penetration depth by σSC/γμ = 0.06091 Φ0/λ². Fits of its temperature dependence to anisotropic s-wave, isotropic s-wave, and nodal gap models distinguish the pairing symmetry. The two tuning mechanisms are a piezoelectric uniaxial stress cell for in-plane strain along the Kagome plane and a piston-cylinder hydrostatic pressure cell for isotropic compression; the paper interprets the stress response through stress-induced shifts of flat bands near the Fermi level.","core_discovery":"Using muon spin rotation on a single crystal of CeRu2, the paper claims that uniaxial stress within the (111) Kagome planes produces a non-monotonic, dome-shaped Tc(σ) with a plateau, a maximum near 0.13 GPa, and a 16% suppression at 0.22 GPa, and that the gap anisotropy ratio Δmin/Δmax drops from 0.41 at ambient pressure to a fully isotropic value above 0.07 GPa. The same crystal under hydrostatic pressure up to 1.9 GPa shows essentially constant Tc, but the superfluid density, extracted from the muon relaxation rate, changes from exponential to linear low-temperature behavior, indicating the minimum gap closes and nodal quasiparticles appear. The coexistence of these two pathways—a pairing-symmetry crossover under anisotropic stress and a nodeless-to-nodal transition under isotropic pressure—is presented as a distinct fingerprint of tunable superconductivity in a 3D Kagome lattice.","pith_inferences":["If the flat-band-shift explanation is correct, a stress cell with a controlled in-plane direction should produce an anisotropic dome, with the largest Tc change when strain aligns with the Kagome bond directions.","The pressure-induced nodal state could be tested by low-temperature specific heat or penetration depth measurements searching for a T-linear term or a T² dependence expected from line nodes.","The paramagnetic upturn seen below 1.5 K under pressure, if intrinsic, may indicate field-induced magnetism or a competing order that couples to the nodal state, rather than pairing alone."],"forward_implications":["Uniaxial stress can tune both Tc and gap symmetry of CeRu2 without any structural phase transition, so the lattice's electronic structure alone responds to strain.","Hydrostatic pressure provides an independent knob that preserves Tc but changes the gap structure from nodeless to nodal, meaning pairing symmetry is decoupled from Tc in this material.","The stress scale (≤0.22 GPa) is an order of magnitude smaller than the pressure scale (up to 2.2 GPa) for comparable microscopic changes, showing that directional strain is a far more sensitive control parameter.","CeRu2 becomes a testbed for theories connecting Kagome flat bands and heavy-fermion correlations to pairing symmetry, since both a smooth-gap crossover and a nodal transition are accessible in one compound."],"supporting_citations":[{"why":"ARPES identification of nearly dispersionless flat bands in CeRu2, the electronic features the paper invokes to explain the stress-induced Tc dome.","marker":"[10]"},{"why":"Observation in CaNi2 that a low-lying flat band can be shifted to the Fermi level by small doping, used as precedent for stress shifting flat bands.","marker":"[12]"},{"why":"Previous µSR study of CeRu2 providing the ambient-pressure anisotropic s-wave gap and time-reversal-symmetry-breaking anomaly that this work extends under stress and pressure.","marker":"[14]"},{"why":"Earlier µSR characterization of CeRu2's vortex-state superfluid density and gap symmetry under ambient conditions, the baseline for pressure fits.","marker":"[4]"},{"why":"NMR determination of the gap anisotropy ratio Δmin/Δmax ≈ 0.33, corroborating the anisotropic s-wave model at zero stress.","marker":"[22]"},{"why":"Hydrostatic pressure cell design used for the µSR pressure experiments, enabling the pressure data.","marker":"[28]"},{"why":"Piezoelectric uniaxial stress cell setup adapted for µSR, providing the stress application method for the dome and crossover measurements.","marker":"[38]"}],"fun_headline_variants":["CeRu2: stress shapes Tc dome, pressure opens nodes","Uniaxial stress vs. hydrostatic pressure: divergent CeRu2 superconductivity","Stress and pressure fingerprint distinct pairing in Kagome CeRu2","CeRu2 shows stress dome but pressure-induced nodal gap","Distinct stress and pressure effects on CeRu2 superconductivity"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing assumption is that the uniaxial stress is truly uniaxial and acts along a well-defined direction within the (111) plane; the paper notes the exact in-plane direction is not known, so any unintended shear or misalignment could in principle produce the observed dome and gap changes without reflecting intrinsic CeRu2 physics.","fun_headline_variants_meta":{"raw":{"variants":["CeRu2: stress shapes Tc dome, pressure opens nodes","Uniaxial stress vs. hydrostatic pressure: divergent CeRu2 superconductivity","Stress and pressure fingerprint distinct pairing in Kagome CeRu2","CeRu2 shows stress dome but pressure-induced nodal gap","Distinct stress and pressure effects on CeRu2 superconductivity"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000288,"raw_usage":{"total_tokens":1728,"prompt_tokens":1021,"completion_tokens":707,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":637,"completion_tokens_details":{"reasoning_tokens":619}},"tokens_in":637,"tokens_out":707,"duration_ms":7482,"temperature":1.0,"reasoning_tokens":619,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-06T17:47:14.209340+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Apply uniaxial stress with a cell that independently measures the in-plane strain tensor while tracking Tc and σSC(T); observing a different dome shape or no gap isotropization for different known stress directions would invalidate the flat-band-shift interpretation. Alternatively, hydrostatic pressure measurements at finer intervals near 1.9 GPa could check whether the linear low-temperature superfluid density emerges discontinuously or smoothly.","supporting_citations":[{"cited_title":"Huang , author C","cited_arxiv_id":null,"evidence_quote":"ARPES identification of nearly dispersionless flat bands in CeRu2, the electronic features the paper invokes to explain the stress-induced Tc dome."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Observation in CaNi2 that a low-lying flat band can be shifted to the Fermi level by small doping, used as precedent for stress shifting flat bands."},{"cited_title":"Mielke III , author H","cited_arxiv_id":null,"evidence_quote":"Previous µSR study of CeRu2 providing the ambient-pressure anisotropic s-wave gap and time-reversal-symmetry-breaking anomaly that this work extends under stress and pressure."},{"cited_title":"Mielke III , author Y","cited_arxiv_id":null,"evidence_quote":"Earlier µSR characterization of CeRu2's vortex-state superfluid density and gap symmetry under ambient conditions, the baseline for pressure fits."},{"cited_title":"Kittaka , author T","cited_arxiv_id":null,"evidence_quote":"NMR determination of the gap anisotropy ratio Δmin/Δmax ≈ 0.33, corroborating the anisotropic s-wave model at zero stress."},{"cited_title":"Khasanov , author R","cited_arxiv_id":null,"evidence_quote":"Hydrostatic pressure cell design used for the µSR pressure experiments, enabling the pressure data."}],"review_version":1}