{"id":"eb9501bc-1c9e-4e38-b03a-12090eb2bae0","arxiv_id":"2411.15333","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"CsV3Sb5 shows two superconducting regimes with a sharp low-temperature upturn in the upper critical field and a 90-degree rotation of the thermal conductivity anisotropy, interpreted as nearly decoupled two-gap superconductivity.","lead":"Measurements on the kagome superconductor CsV3Sb5 reveal two distinct superconducting regimes at low temperatures, marked by a sharp upturn in the upper critical field and a 90-degree switch in thermal conductivity anisotropy. If confirmed as separate nearly decoupled superconducting gaps, the finding would point to band-selective pairing, a scenario that could reconcile conflicting reports on this material family.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The 0.6 K heat-capacity anomaly, a key thermodynamic pillar for two nearly decoupled gaps, may be an artifact of subtracting a 9 T ab-plane reference that lies below Hc2 at low temperatures; a safe reference field is needed.","rationale":"The reader's weakest assumption identifies exactly the same load-bearing issue: the 9 T ab-plane background reference may fall below Hc2 at the lowest temperatures, making the second heat-capacity anomaly unreliable. My stress-test focuses on this single point because it is the most concrete and checkable threat to the central claim. The Hc2 upturn itself is reproduced on two samples and is robust, and the thermal-conductivity anisotropy switch is an independent observation, so the paper has real experimental content. However, the thermodynamic evidence is the only direct measurement that ties the low-temperature feature to the opening of a second gap on a distinct Fermi surface, rather than to a field-induced or extrinsic effect. The background-subtraction flaw, if confirmed, would invalidate the second heat-capacity anomaly and would leave the two-gap scenario as one of several possible explanations, not a conclusion. I do not recommend rejecting the paper: the measurements are valuable and the interpretation may be correct, but the central claim should be conditioned on a clean normal-state reference measurement. The reader's verdict of CONDITIONAL remains appropriate, so no change is needed.","tokens_in":20970,"tokens_out":3085,"duration_ms":30146,"concrete_test":"Re-measure the heat-capacity background using a field safely above the zero-temperature Hc2,||, for example mu0H = 12 T applied along the ab-plane, or measure C/T at several fields (9, 12, and 16 T) and verify that the subtracted electronic C/T becomes field-independent. If the ~0.6 K anomaly persists with a 12 T reference, the second-gap interpretation is supported; if it disappears or shifts, the anomaly is an artifact of subtracting a superconducting-state trace and the thermodynamic evidence for a second nearly decoupled gap must be withdrawn. As a cross-check, verify entropy conservation: the superconducting and normal-state entropies integrated up to Tc should match; a spurious subtraction will violate this condition.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim is that CsV3Sb5 hosts two nearly decoupled superconducting gaps, and the strongest thermodynamic evidence is the second heat-capacity anomaly near 0.6 K. That evidence depends on the background subtraction described in Methods Section X: the normal-state reference is C/T measured at mu0H = 9 T applied along the ab-plane (Extended Fig. 8a). However, the paper's own phase diagram (Fig. 2c) and Fig. 1d show Hc2,|| at the lowest temperatures is nearly 10 T, i.e., above 9 T. Therefore the reference field lies inside the superconducting state precisely in the temperature range where the second anomaly appears. Subtracting a superconducting-state C/T trace from the zero-field trace cannot cleanly cancel the lattice contribution and can produce a spurious low-temperature feature or distort the true electronic term. The abstract's statement that 'no evidence for a phase transition' separates the two regimes is also in tension with the observed peak at ~0.6 K; if this peak is a genuine thermodynamic anomaly, a phase transition is the default interpretation unless the two-gap crossover scenario can quantitatively explain its absence. The Hc2 upturn and the thermal-conductivity slope change are real experimental observations, but the two-gap interpretation requires all three signatures to arise from the same gap-opening process. If the heat-capacity anomaly is an artifact, the thermodynamic pillar of the band-selective pairing claim is removed, leaving transport-only evidence that admits other explanations such as a field-induced or inhomogeneity-driven effect.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports combined electrical transport, thermal transport, and specific heat measurements on the kagome superconductor CsV3Sb5, claiming the discovery of two distinct superconducting regimes with a boundary near 1 K. In the proposed scenario, a primary superconducting gap opens near Tc ≈ 3.5 K on one Fermi surface, while a second, nearly decoupled gap opens near 0.8–0.6 K on a different band, producing a sharp upturn in Hc2 at low temperatures, a second anomaly in C/T, a slope change in κxx/T, and a 90° rotation of the in-plane thermal conductivity anisotropy. The authors argue against a phase transition separating the two regimes and propose band-selective pairing with possible nodal structure in the second gap.","tokens_in":21339,"tokens_out":2447,"duration_ms":24702,"significance":"If the central claim holds, this would be an important contribution to the physics of multiband superconductors, suggesting that orbital-selective or band-selective pairing with nearly decoupled gaps can occur in a kagome lattice and that the conventional proximity-induced multigap scenario fails in CsV3Sb5. The experimental work is substantial: the Hc2 phase diagrams are cross-validated on two separate exfoliated samples and with three different resistive criteria (50%, 10%, and 90% of the normal-state resistivity), and the thermal conductivity angular data are carefully checked against accidental sample canting. The paper is also appropriately cautious in some places, noting that more complete thermal conductivity and modeling work is needed to establish the pairing symmetry. However, the central interpretation rests on thermodynamic evidence whose reliability is undermined by the normal-state reference field choice, and the paper's own assertion that no phase transition separates the two regimes is in tension with the observed heat capacity peak. These issues affect the load-bearing claim and require additional measurements or modeling.","major_comments":[{"comment":"The 9 T ab-plane field used as the normal-state reference for the heat capacity subtraction is not above Hc2 in the temperature range where the second anomaly appears. The manuscript states that 9 T is 'above and very close to μ0Hc2,||', but Fig. 2c shows μ0Hc2,|| exceeding 9 T below roughly 1 K and extrapolating to nearly 10 T at zero temperature. Therefore, at T ≲ 1 K the 9 T trace is itself in the superconducting state, and subtracting it from the zero-field trace does not cleanly remove the lattice/phonon background. The second specific-heat anomaly near 0.6 K could be an artifact of this incomplete subtraction. A safe reference field (clearly above Hc2 at all measured temperatures, e.g., 14 T or 16 T) is needed to confirm the thermodynamic signature. Because this second anomaly is the strongest thermodynamic support for the nearly decoupled two-gap scenario, this issue is load-bearing for the central claim.","section":"Methods Section X, Extended Fig. 8, Fig. 2c"},{"comment":"The abstract states that the two superconducting regimes are separated 'while finding no evidence for a phase transition', yet Fig. 3c shows a peak in the electronic contribution to C/T near 0.6 K. A genuine thermodynamic peak in C/T at a characteristic temperature is the standard signature of a phase transition, so the default interpretation of a real peak is a phase transition unless the two-gap crossover model quantitatively explains the peak shape and why it does not constitute a thermodynamic transition. The current manuscript does not provide such a quantitative analysis; it only shows a model calculation in Fig. 3d with a qualitative caption. The authors should either demonstrate that a nearly decoupled two-gap model (without an intervening phase transition) quantitatively reproduces the observed C/T peak of the correct shape and height, or soften the claim that there is no phase-transition-like feature.","section":"Abstract, Fig. 3c"},{"comment":"The two-gap model is presented as a consistency check but the connection between the model and the experimentally observed Hc2 upturn is not established. The model calculations in Fig. 3b and 3d are described only qualitatively (see SI), and the Hc2(T) curves in Fig. 2c and 2f are not reproduced from the same model. To support the claim that band-selective pairing with nearly decoupled gaps explains the full set of observations, the authors need to show that a single set of model parameters produces (i) the approximate temperature scale of the second gap opening, (ii) the sharp Hc2 upturn with its near-isotropy across field orientations, and (iii) the thermal conductivity slope change. Without this quantitative link, the scenario remains one of several possible interpretations of the transport data.","section":"Sec. 'Two-gap model', Fig. 3b and 3d"}],"minor_comments":[{"comment":"The caption states that the 9 T field is 'close to μ0Hc2^{ab}', but does not mention that Hc2,|| exceeds 9 T at low temperatures; this should be clarified to warn the reader about the subtraction issue.","section":"Fig. 3c caption"},{"comment":"The phrase 'with both values being above and very close to μ0Hc2,⊥ and μ0Hc2,|| respectively' is imprecise for the ab-plane case; consider specifying the temperature range over which 9 T is above Hc2.","section":"Methods Section X"},{"comment":"The sentence 'The extrapolation of the heat capacity to T = 0 K suggests the near absence of residual electronic contribution' is presented without an error estimate or the extrapolation procedure; adding these would strengthen the claim.","section":"Main text, paragraph on heat capacity"},{"comment":"The three listed sources of thermal conductivity anisotropy are stated to be exhaustive, but this claim should be justified or softened, as other mechanisms (e.g., multiband quasiparticle scattering, vortex-channel contributions) could also affect κxx/T anisotropy.","section":"Methods Section XI"},{"comment":"The values '450' and '1350' in the figure caption appear to be typos for 45° and 135°; please correct them.","section":"Fig. 4c"},{"comment":"The fit function cos 2φ + cos 2(φ + φ0) is declared but the amplitudes of the two components are not stated; the fit would be more meaningful if the relative amplitude were reported.","section":"Methods Section XIII"}],"recommendation":"major_revision","confidential_remarks":"The paper reports a striking and potentially important set of observations, and the transport data appear to be of high quality and well cross-validated. The central concern is the heat capacity subtraction: the 9 T ab-plane reference field is likely below Hc2 at low temperatures, which could generate the second thermodynamic anomaly and invalidate the strongest support for nearly decoupled two-gap superconductivity. This is a correctable issue but it requires new measurements or a re-analysis with a safe reference field. In addition, the 'no phase transition' phrasing is at odds with the observed C/T peak, and the two-gap model is not currently tied quantitatively to the Hc2 data. I therefore recommend major revision rather than rejection, because the transport-derived evidence for two regimes is real and the proposed scenario is falsifiable with the requested control measurements."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Two things to know. First, the Hc2 upturn around 1 K and the 90-degree rotation of the in-plane thermal conductivity anisotropy are new, real-looking observations in CsV3Sb5. Second, the band-selective two-gap interpretation is plausible but not proven, and the key thermodynamic support—the heat-capacity peak at ~0.6 K—has a possible background-subtraction problem.\n\nThe paper does a lot well. The transport data are cross-validated on two samples and with three different Hc2 criteria; the angular controls for accidental c-axis canting are careful. The thermal conductivity field dependence shows a steep, roughly linear increase that is consistent with nodal quasiparticles, and the authors explicitly list alternative explanations and admit the second anomaly is modest. That is honest.\n\nThe soft spot is the heat capacity. The second anomaly at ~0.6 K is the main thermodynamic evidence for a second nearly decoupled gap, but the normal-state reference is C/T at 9 T along the ab-plane. The paper's own phase diagram puts Hc2,|| close to 10 T at low temperature, so 9 T may be inside the superconducting state in exactly the temperature range where the anomaly appears. Subtracting a superconducting-state trace from the zero-field trace can create a spurious low-temperature feature. The abstract's 'no evidence for a phase transition' also sits uneasily with the observed peak; a crossover is possible, but the authors should show that quantitatively. I also could not find the two-gap model parameters in the main text—they may be in the SI, but the 'remarkable decoupling' claim needs those numbers in front of me.\n\nThe 90-degree rotation is intriguing but the Volovik/nodes interpretation is one of several; the authors themselves say a heavily anisotropic s-wave cannot be fully discarded. That is a minor concern, not fatal.\n\nBottom line: this is a specialist paper for the kagome community and anyone working on multiband superconductivity. The experimental work is careful and the two-regime phenomenology deserves a serious referee. The interpretation needs more work—especially a safe heat-capacity reference field and a quantitative two-gap fit—but that is what peer review is for. I would accept it for review.","headline":"A careful experimental study with a novel two-regime phenomenology in CsV3Sb5, but the band-selective pairing interpretation rests on a heat-capacity anomaly whose background subtraction may be compromised.","tokens_in":21951,"tokens_out":3188,"would_cite":true,"duration_ms":28540,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"A kagome superconductor hosts two nearly independent superconducting gaps.","keywords":["kagome superconductor","CsV3Sb5","two-gap superconductivity","band-selective pairing","upper critical field","thermal conductivity","heat capacity","nodal gap"],"falsifier":"Measure the heat capacity of a high-quality single crystal down to about 50 mK at several fixed fields: the two-gap scenario predicts that the 0.6 K anomaly tracks the upturn in $H_{c2}$ and disappears once the field exceeds $H_{c2}$, whereas a separate phase transition or an inhomogeneity artifact would leave the anomaly position or shape essentially unchanged with field.","tokens_in":20767,"feed_emoji":"🧲","tokens_out":8959,"duration_ms":71043,"temperature":0.7,"pith_summary":"The paper studies the kagome-lattice superconductor CsV$_3$Sb$_5$ and argues that its superconducting state consists of two nearly independent superconducting regimes, each tied to a different band of the Fermi surface. The evidence includes a pronounced upturn of the upper critical field below about 1 K, a second small heat-capacity anomaly near 0.6 K, a slope change in the thermal conductivity near 0.8 K, and a 90-degree rotation of the thermal-conductivity anisotropy between the two regimes. The authors interpret these observations as the sequential opening of two nearly decoupled gaps, with the lower-temperature gap possibly nodal. If correct, this band-selective pairing would replace the usual multiband picture in which one dominant gap induces pairing in other bands, and it would reconcile conflicting reports of nodal and nodeless superconductivity in this family.","feed_headline":"A kagome superconductor hosts two nearly independent gaps","feed_subtitle":"Heat capacity and transport place the second gap below 1 K, with hints of a nodal structure.","key_machinery":"The argument is carried by the split Fermi surface of CsV$_3$Sb$_5$, specifically the circular pocket around the $\\Gamma$ point derived mostly from Sb $p$ orbitals and the hexagonal pocket derived mostly from V $d$ orbitals. The paper assigns a primary gap to the Sb-derived band and a secondary, nearly decoupled gap to the V-derived band, reproducing the measured heat capacity and thermal conductivity with a minimal two-gap model. The experimental load is carried by four observations: the low-temperature upturn of the upper critical field $\\mu_0H_{c2}$ for both in-plane and out-of-plane fields, the second heat-capacity anomaly near 0.6 K, the change in slope of $\\kappa_{xx}/T$ near 0.8 K, and the 90-degree rotation of the in-plane thermal-conductivity anisotropy while the $\\mu_0H_{c2}$ anisotropy direction remains fixed, which rules out ordinary field-anisotropy explanations and points to a strongly anisotropic or nodal gap on one band.","core_discovery":"CsV$_3$Sb$_5$ displays two distinct superconducting regimes separated near 1 K, with no evidence of a phase transition between them. In the higher-temperature regime, a first gap opens on the Sb-derived Fermi surface pocket while substantial quasiparticle weight remains visible in thermodynamics and thermal transport. Below roughly 1 K, a second, nearly decoupled gap opens on a V-derived band with a much higher upper critical field, removing that residual weight. The band that acquires the second gap continues to host low-energy quasiparticles, possibly because that gap has nodes. The authors conclude that superconductivity here is band-selective: pairing develops essentially independently on separate bands rather than being induced from a dominant primary gap.","pith_inferences":["If the second gap is nodal, thermal-conductivity measurements below roughly 0.1 K should reveal a residual linear-in-$T$ term whose magnitude reflects the nodal quasiparticle density; this cleanly distinguishes nodal from fully gapped scenarios.","Because the gaps are nearly decoupled, perturbations that act on one band only — disorder, strain, or doping — should alter one gap's $T_c$ and $H_{c2}$ while leaving the other nearly unchanged, a direct test of band selectivity.","The 90-degree rotation of the thermal-conductivity pattern may track the orientation of the nematic charge order; applying uniaxial strain to reorient the nematic domains should rotate the low-temperature thermal-conductivity anisotropy by the same amount."],"forward_implications":["The standard multiband scenario, in which one dominant gap induces pairing in the other bands, fails for CsV$_3$Sb$_5$; the two gaps behave as nearly decoupled.","The lower-temperature gap is likely strongly anisotropic or nodal, which would explain the residual density of states reported by tunneling experiments.","The second gap carries a much higher upper critical field, possibly because that band is in the dirty limit or has a shorter coherence length, analogous to MgB$_2$.","The 90-degree switch of the thermal-conductivity anisotropy, without a corresponding rotation of the $H_{c2}$ anisotropy, offers a new way to detect gap anisotropy in multi-gap superconductors.","The two-fold symmetric, nematic character of the normal state is inherited by the superconducting state, visible in both $H_{c2}$ and thermal transport."],"supporting_citations":[{"why":"Introduces the AV$_3$Sb$_5$ family and its crystal structure, setting the material context for the study.","marker":"[6]"},{"why":"Provides the band structure and Fermi surface pockets (Sb $p$ and V $d$) used in the two-gap model.","marker":"[7]"},{"why":"Reports two-fold symmetric c-axis resistivity and nematic behavior in the normal state, which the paper builds on for the two-fold $H_{c2}$.","marker":"[15]"},{"why":"Proposes orbital-selective electron-phonon coupling as a mechanism for band-selective pairing.","marker":"[23]"},{"why":"Documents orbital-selective Cooper pairing in FeSe, the comparative experimental case.","marker":"[24]"},{"why":"Supplies the Tinkham formula and Ginzburg-Landau framework used to fit $H_{c2}$ angular and temperature dependence.","marker":"[25]"},{"why":"Provides the phenomenological two-gap model for the specific heat of MgB$_2$ that the authors adapt for CsV$_3$Sb$_5$.","marker":"[38]"},{"why":"Explains very high $H_{c2}$ in MgB$_2$ via selective impurity scattering, the analog they invoke for the second gap's high critical field.","marker":"[39]"},{"why":"Reviews angle-resolved thermal transport as a probe of nodal gap structure, underpinning the $\\kappa_{xx}$ anisotropy interpretation.","marker":"[40]"}],"fun_headline_variants":["Kagome superconductor splits its gap into two independent regimes","Two separate superconducting gaps in a kagome lattice","Kagome superconductor reveals band-selective pairing with two gaps","Two distinct superconducting gaps appear without a phase transition","Band-selective pairing in kagome superconductor yields two gaps"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The conclusion rests on the assumption that the second heat-capacity bump near 0.6 K and the sharp rise in the magnetic field needed to destroy superconductivity below about 1 K are two signatures of the same event — a second superconducting gap opening on a separate band — and not a different phase transition, sample variation, or an artifact of the background subtraction.","fun_headline_variants_meta":{"raw":{"variants":["Kagome superconductor splits its gap into two independent regimes","Two separate superconducting gaps in a kagome lattice","Kagome superconductor reveals band-selective pairing with two gaps","Two distinct superconducting gaps appear without a phase transition","Band-selective pairing in kagome superconductor yields two gaps"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.0014,"raw_usage":{"total_tokens":5677,"prompt_tokens":977,"completion_tokens":4700,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":593,"completion_tokens_details":{"reasoning_tokens":4616}},"tokens_in":593,"tokens_out":4700,"duration_ms":29072,"temperature":1.0,"reasoning_tokens":4616,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-12T14:24:56.400219+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the heat capacity of a high-quality single crystal down to about 50 mK at several fixed fields: the two-gap scenario predicts that the 0.6 K anomaly tracks the upturn in $H_{c2}$ and disappears once the field exceeds $H_{c2}$, whereas a separate phase transition or an inhomogeneity artifact would leave the anomaly position or shape essentially unchanged with field.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Introduces the AV$_3$Sb$_5$ family and its crystal structure, setting the material context for the study."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the band structure and Fermi surface pockets (Sb $p$ and V $d$) used in the two-gap model."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Reports two-fold symmetric c-axis resistivity and nematic behavior in the normal state, which the paper builds on for the two-fold $H_{c2}$."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Proposes orbital-selective electron-phonon coupling as a mechanism for band-selective pairing."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Documents orbital-selective Cooper pairing in FeSe, the comparative experimental case."},{"cited_title":"Introduction to Superconductivity 2nd edn (McGraw-Hill, New York, 1996)","cited_arxiv_id":null,"evidence_quote":"Supplies the Tinkham formula and Ginzburg-Landau framework used to fit $H_{c2}$ angular and temperature dependence."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the phenomenological two-gap model for the specific heat of MgB$_2$ that the authors adapt for CsV$_3$Sb$_5$."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Explains very high $H_{c2}$ in MgB$_2$ via selective impurity scattering, the analog they invoke for the second gap's high critical field."},{"cited_title":"& Vekhter, I","cited_arxiv_id":null,"evidence_quote":"Reviews angle-resolved thermal transport as a probe of nodal gap structure, underpinning the $\\kappa_{xx}$ anisotropy interpretation."}],"review_version":1}