{"id":"4c0d0303-a4bc-4548-bfb4-64dbffce1cf4","arxiv_id":"2411.18744","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":5,"one_line_summary":"Nb doping separates time-reversal symmetry breaking from charge order in the bulk of CsV3Sb5, and pressure induces a time-reversal-breaking superconducting state with nodeless pairing.","lead":"Muon experiments on niobium-doped cesium-vanadium-antimony show that in the bulk of the crystal, time-reversal symmetry breaks at a lower temperature than the charge order does, while near the surface the two happen together. Applying pressure turns this material into a superconductor that itself breaks time-reversal symmetry, a rare and sought-after state.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The claim of TRS-breaking superconductivity above 0.85 GPa rests on an unverified assumption that charge order is fully suppressed in Nb0.07-CVS under pressure.","rationale":"The reader's weakest assumption is exactly the load-bearing concern I identify: the interpretation of the high-pressure ZF-μSR signal below Tc as intrinsic TRS-breaking superconductivity relies on the unverified claim that charge order is fully suppressed above 0.85 GPa. This is the single most important point because the central novelty of the paper is the pressure-induced transition from a chiral charge-ordered normal state to a TRS-breaking superconducting state; if charge order persists at high pressure, the observed internal fields below Tc could simply be a continuation of the normal-state TRS breaking that the authors themselves document at ambient pressure. The paper acknowledges the assumption explicitly but provides no direct high-pressure evidence for CDW suppression in this specific compound, instead citing other AV3Sb5 systems. The proposed μSR test is feasible and directly settles the ambiguity. Secondary issues, such as the inconsistency between text and Table I for ambient Tc and penetration depth, are real but do not undermine the central symmetry argument. Because the concern is addressable and the existing evidence is otherwise strong, the conditional verdict remains appropriate.","tokens_in":16486,"tokens_out":4298,"duration_ms":41296,"concrete_test":"Perform ZF-μSR on Nb0.07-CVS at 1.2 GPa at temperatures from just above Tc (≈7 K) up to 100 K. If the relaxation rate Γ shows an upturn at ~40 K similar to ambient pressure, charge-order-related TRS breaking persists at this pressure and the below-Tc signal cannot be uniquely attributed to the superconducting state. A null result—flat Γ above Tc—would support the authors' interpretation. A complementary check is to fit the SC-state Γ(T) with and without a constant residual relaxation term representing any un-suppressed normal-state contribution.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim that the superconducting state breaks time-reversal symmetry depends on the assertion in Discussion §IV that hydrostatic pressure above pcr ≈ 0.85 GPa fully suppresses charge order, so the weak internal fields observed below Tc are intrinsic to the SC state. However, no direct measurement of charge order under pressure is reported for Nb0.07-CVS. The pressure phase diagram (Fig. 6a) shows a jump and broadening of Tc, but a first-order-like Tc jump is not evidence of CDW suppression. At ambient pressure, the same compound shows a normal-state TRS-breaking signal below T* = 40 K; if a similar signal persists at 1.2 GPa above Tc, the below-Tc ZF-μSR increase could be a remnant of normal-state TRS breaking rather than a new SC order. The authors cite other AV3Sb5 compounds, but Nb doping may alter the critical pressure. This assumption is load-bearing because the reported 'transition from chiral charge order to TRS-breaking superconducting state' collapses if charge order coexists with superconductivity at high pressure.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper presents a muon spin rotation/relaxation (μSR), AC susceptibility, and scanning tunnelling microscopy study of the Nb-doped kagome superconductor Cs(V0.93Nb0.07)3Sb5 (Nb0.07-CVS) under doping, hydrostatic pressure, magnetic field, and muon implantation depth. The authors report three main findings: (1) in the normal state at ambient pressure, time-reversal symmetry (TRS) breaking occurs below T* = 40 K in the bulk, decoupled from the charge-order onset at TCO = 58 K, while near the surface (within ~20 nm) the TRS-breaking signal is doubled and onsets at TCO; (2) hydrostatic pressure raises Tc from 4.5 K to about 7 K, doubles the superfluid density, and changes the superconducting gap structure from a single nodeless s-wave gap to a two-gap structure at 0.8 GPa and back to a single nodeless gap at 1.2 GPa; and (3) zero-field μSR at 1.2 and 1.5 GPa shows an increase in the relaxation rate below Tc, which the authors interpret as TRS breaking in the superconducting state, possibly indicating chiral superconductivity.","tokens_in":16669,"tokens_out":4177,"duration_ms":39502,"significance":"If the central claims hold, this paper would provide a comprehensive and depth-resolved picture of how TRS breaking evolves from the normal-state charge-ordered phase to the superconducting phase in a kagome superconductor, supported by an unusually complete combination of μSR techniques (ZF, TF, high-field, and low-energy depth profiling) and pressure tuning. The authors explicitly acknowledge the importance of ruling out electric-field-gradient effects by using high-field μSR controls, which is a methodological strength. The reported pressure phase diagram and the gap-structure analysis are useful additions to the kagome superconductivity literature. However, the most novel claim — that the superconducting state above pcr breaks TRS — rests on an assumption that charge order is fully suppressed under pressure in Nb0.07-CVS, which is not directly measured in this compound. Because the normal state of the same material already exhibits TRS breaking at ambient pressure, this missing control is load-bearing. The paper is therefore significant if the assumption can be substantiated, but the current evidence is conditional rather than conclusive.","major_comments":[{"comment":"The central claim of a TRS-breaking superconducting state above pcr relies on the assumption that charge order is fully suppressed for p > 0.85 GPa. The manuscript states in Section IV: 'we assume that the charge order is suppressed with pressure, as has been measured in other AV3Sb5 compounds', and in Section III: 'At 0.8 GPa, the pressure brings the system into the optimal Tc region of the phase diagram, where charge order is fully suppressed'. No direct measurement of charge order under pressure (e.g., XRD, neutron, or STM at pressure) is reported for Nb0.07-CVS. Since the same compound exhibits normal-state TRS breaking below T* = 40 K at ambient pressure, a remnant of this signal above Tc at 1.2 GPa could be misidentified as a superconducting-order-induced increase. To support the claim, the authors should measure the ZF-μSR relaxation rate in the normal state just above Tc at p > pcr, or provide a direct probe of charge-order suppression under pressure for this specific stoichiometry without relying on extrapolation from other AV3Sb5 compounds.","section":"§IV, Discussion and §III"},{"comment":"The high-field μSR data are presented only in normalized form, so the reader cannot judge the absolute magnitude of the 8-T relaxation enhancement or its relation to the nuclear baseline. The authors argue that the 8-T increase confirms the magnetic origin of the TRS-breaking signal, but without absolute relaxation rates and a quantitative estimate of the nuclear contribution remaining at 8 T, it is difficult to exclude field-dependent artifacts or a partial suppression of the nuclear signal. Please provide the unnormalized σ(T) for each applied field, including the 0.01 T and 8 T data, with the nuclear contribution explicitly modeled.","section":"§II, Fig. 2c"},{"comment":"The zero-field μSR measurements under pressure are performed in a double-wall MP35N/CuBe pressure cell, yet the manuscript does not describe how the pressure-cell background was separated from the sample signal or whether the cell contributes a temperature-dependent relaxation. The reported increase in Γ below Tc is small (roughly 0.02–0.03 μs−1 in Fig. 6b), comparable in magnitude to the ambient-pressure normal-state signal. Given this, a pressure-cell background with any temperature dependence around Tc could mimic or mask the effect. The authors should show the raw ZF asymmetry spectra at 1.2 GPa, state the sample signal fraction for the pressure runs, and discuss the temperature stability of the cell background.","section":"§III, Fig. 6b and Methods"},{"comment":"The statement that 'the sharp and well-defined transition from the low-TC to high-TC state strongly indicates a first-order phase transition' is not fully supported by the presented AC susceptibility data. The transition at 0.8 GPa is broad and the phase diagram is constructed from discrete pressure points, so a continuous evolution or a two-phase coexistence region cannot be excluded. The presence of a first-order transition is not essential to the main conclusions, but the claim should be moderated unless thermodynamic evidence (e.g., hysteresis in pressure) is provided.","section":"§IV, Discussion point (2)"}],"minor_comments":[{"comment":"In Section III, the word 'superconuctor' should be 'superconductor'.","section":"§III"},{"comment":"The caption contains the fragment 'LH2nmBias=1V' and 'LH2nmBias=20mV', which appear to be formatting artifacts; these should be readable axis labels (e.g., 'Bias (mV)').","section":"Fig. 1 caption"},{"comment":"In Table I, the header 'λ(T >0) (nm)' is ambiguous and should be clarified; presumably the quantity is the zero-temperature London penetration depth λ(0), not a value at arbitrary positive temperature.","section":"Methods and Table I"},{"comment":"The author name 'J. X. YIn' in reference 13 should be 'J.-X. Yin'.","section":"Reference 13"},{"comment":"The data availability statement includes the phrase 'named incorrectly in file'; this is informal and should be corrected or removed.","section":"Data availability"}],"recommendation":"major_revision","confidential_remarks":"The paper is a strong experimental tour de force, but the key claim of TRS-breaking superconductivity above pcr depends on an unverified assumption about charge-order suppression in Nb0.07-CVS. I would recommend the editor solicit at least one referee with specific expertise in high-pressure μSR on pressure cells, since the small signals in Fig. 6b and the lack of a pressure-cell background analysis are the most critical technical points. If the authors can provide normal-state ZF-μSR data at 1.2 GPa above Tc (even a narrow temperature window) or an explicit pressure-cell subtraction, the central claim would be considerably more robust."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Two things you should know about this paper. First, the depth-resolved muon data are the real deal: the bulk TRS-breaking onset at T* = 40 K, decoupled from TCO = 58 K, while the surface tracks TCO, is a genuinely new observation and the most solid part of the work. Second, the claim that the superconducting state above 0.85 GPa breaks time-reversal symmetry rests on an assumption the authors state openly: that charge order is fully suppressed under pressure in Nb0.07-CVS. They do not directly test that in this compound, and the stress-test note is right that a first-order-looking jump in Tc is not the same as CDW suppression.\n\nWhat the paper does well: the experimental coverage is thorough. ZF, LF, TF, high-field up to 8 T, low-energy muons at multiple depths, pressure up to 2.2 GPa, plus comparisons with Ta-doped and undoped CsV3Sb5. The 8 T control is a sensible way to separate magnetic relaxation from electric-field-gradient effects, and it supports the normal-state TRS interpretation. The surface-versus-bulk decoupling is presented clearly and backed by implantation-depth simulations. Data availability is good, with direct links to the muon data.\n\nSoft spots, in proportion. The clearest problem is internal inconsistency: the text quotes TC = 4.70(3) K and lambda = 316(5) nm for ambient pressure, while Table I lists TC = 3.000(6) K and lambda = 381 nm. That will trip up any reader and suggests the analysis needs a careful pass. More substantively, the headline claim about TRS-breaking superconductivity depends on charge order being absent at 1.2 GPa. The authors assume this based on other AV3Sb5 compounds, but they do not report any pressure-dependent measurement of the CDW in Nb0.07-CVS. If the normal-state TRS signal persists above Tc at pressure, the below-Tc increase in Gamma could be a continuation of normal-state TRS breaking rather than a new superconducting order parameter. The right fix is to show ZF data above Tc at 1.2 GPa, or a direct CDW probe under pressure. The nodeless-gap conclusion is plausible given the low-temperature plateau, but an explicit fit to a d-wave or nodal model would make it much stronger; the reduced chi-squared of 1.74 at 1.2 GPa is not convincing on its own.\n\nWho this is for: people working on kagome superconductors, muon spectroscopy, and TRS breaking. The depth-resolved result is worth the read even if the superconducting claim turns out to need qualification. The paper deserves a serious referee, not a desk reject. The authors should be asked to fix the numbers and to directly address the CDW-suppression assumption. I would send it to review.","headline":"A serious experimental paper with a genuinely new depth-resolved finding, but the headline TRS-breaking superconducting claim rests on an unverified assumption about charge order suppression, and the text has a numeric inconsistency that must be fixed.","tokens_in":17329,"tokens_out":3753,"would_cite":true,"duration_ms":35985,"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":"Muon-spin and pressure data argue that in Nb-doped CsV3Sb5, suppressing the chiral charge order with about 0.85 GPa reveals a superconducting state that breaks time-reversal symmetry.","keywords":["kagome superconductor","CsV3Sb5","Nb doping","charge order","time-reversal symmetry breaking","hydrostatic pressure","muon spin rotation","superconducting gap"],"falsifier":"Look for the 2×2 charge-order superstructure or its ~20 meV gap under pressure: if x-ray scattering, STM, or transport measurements at 1.2 GPa show that charge order persists or is only partially suppressed, then the TRS-breaking signal below Tc cannot be cleanly assigned to superconductivity. Alternatively, measure zero-field µSR in a Nb-doped sample with charge order fully suppressed chemically; the absence of any internal-field increase below Tc would directly contradict the claim.","tokens_in":16260,"feed_emoji":"🧲","tokens_out":6276,"duration_ms":68287,"temperature":0.7,"pith_summary":"The paper sets out to show that in the kagome superconductor Cs(V0.93Nb0.07)3Sb5, hydrostatic pressure converts the normal-state chiral charge order into a superconducting state that spontaneously breaks time-reversal symmetry. In the bulk of the crystal at ambient pressure, muon-spin rotation detects internal magnetic fields below T* ≈ 40 K, well below the charge-order onset at TCO ≈ 58 K, whereas near the surface the magnetic signal appears at TCO and is about twice as strong. Applying pressure raises both Tc (from 4.5 K to 7 K) and the superfluid density, and once the pressure exceeds about 0.85 GPa the superconducting state exhibits a nodeless gap together with weak internal fields below Tc, which the authors read as evidence for chiral pairing. If correct, the result would show that a single tuning parameter, pressure, can dial a kagome metal from a charge-ordered normal state into a time-reversal-breaking superconductor, and it would strengthen the case that such pairing is a generic feature of the AV3Sb5 family.","feed_headline":"0.85 GPa flips kagome metal into a TRS-breaking superconductor","feed_subtitle":"Muon-spin data show nodeless superconductivity with weak internal fields below Tc once charge order is suppressed.","key_machinery":"The experimental engine is muon spin rotation (µSR), in which spin-polarized muons implanted in the crystal precess in the local magnetic field; in zero field an exponential relaxation rate Γ measures the distribution of static internal fields, and an increase in Γ below a transition temperature is the standard signature of spontaneously broken time-reversal symmetry. The paper combines three variants: zero-field and high-field µSR to separate electronic from nuclear contributions, low-energy µSR to tune the muon implantation depth between about 1 nm and 120 nm (revealing the surface-bulk difference), and transverse-field µSR under hydrostatic pressure to extract the superfluid density from the muon depolarization rate. The data-analysis identity connecting the superconducting relaxation rate to the London penetration depth, σsc(T)/γµ = 0.06091 Φ0/λ²(T), then converts measured relaxation rates into gap structure and superfluid density values.","core_discovery":"At ambient pressure, bulk Nb0.07-CVS breaks time-reversal symmetry (the symmetry between forward and backward time evolution, whose breaking reveals itself in spontaneous static internal fields) below T* = 40 K, while its 2×2 chiral charge order sets in at TCO = 58 K; low-energy muon-spin rotation shows the magnetic signal begins at TCO near the surface, within about 20 nm, and is roughly twice as strong as in the bulk. Nb doping raises Tc from 2.5 K to 4.4 K, and hydrostatic pressure further enhances Tc to about 7 K and doubles the superfluid density. Above a critical pressure pcr ≈ 0.85 GPa, the superconducting state shows a nodeless (fully gapped) pairing symmetry and, from zero-field muon-spin rotation, weak internal fields appear only below Tc, indicating broken time-reversal symmetry in the superconducting state itself. The authors interpret the combination of a full gap and broken time-reversal symmetry as compatible with a chiral dx2-y2 + idxy or px + ipy pairing state, and they note the same TRS-breaking response below Tc in undoped CsV3Sb5 at 1.78 GPa and in Ta-doped CsV3Sb5 with suppressed charge order.","pith_inferences":["The depth-dependent decoupling suggests a natural experiment: map T* as a function of muon implantation depth at several Nb dopings; if the surface-bulk difference tracks charge-order strength, the near-surface TRS signal is likely charge-order driven, while the bulk 40 K signal may be a separate order.","If the pressure-induced TRS-breaking superconducting state is generic, compounds with fully suppressed charge order (Ta-doped and K, Rb, Cs compounds under pressure) should all show the same weak internal fields below Tc; a systematic comparison of their field strengths would test whether the pairing state is universal.","A testable extension of the Tc-superfluid-density scaling is to measure λ−2 at pressures up to 2.2 GPa; if the linear relation persists beyond the first-order jump, the enhancement would look like a continuous evolution of pairing strength rather than a simple consequence of charge-order suppression.","Because the assignment of the TRS signal to superconductivity rests on charge order being fully suppressed at 0.85 GPa, a direct high-pressure structural probe of Nb0.07-CVS would either validate or correct that assignment."],"forward_implications":["If the central claim holds, pressure is a clean switch between two broken-symmetry states in the same material: chiral charge order at ambient pressure and TRS-breaking (likely chiral) superconductivity above about 0.85 GPa.","The nodeless gap plus TRS breaking points to a chiral pairing state (dx2-y2+idxy or px+ipy), which would make Nb0.07-CVS a concrete platform for studying chiral or topological superconductivity.","The near-linear scaling between Tc and superfluid density across pressure suggests a common unconventional pairing mechanism shared by the kagome AV3Sb5 family.","The surface-bulk difference (T* = 40 K bulk versus TCO = 58 K surface) implies that surface-sensitive probes can overestimate the TRS-breaking onset, so reported onset temperatures should be accompanied by depth information."],"supporting_citations":[{"why":"Establishes Nb0.07-CVS as the current Nb-doping limit, reports the nodeless ARPES superconducting gap, and supplies the crystal-growth and previous characterization basis for this study.","marker":"[19]"},{"why":"Documents TRS breaking and its two-step onset in undoped CsV3Sb5 and supplies the high-field muSR approach used to confirm the magnetic origin of the relaxation increase.","marker":"[10]"},{"why":"Shows hydrostatic pressure suppresses charge order in CsV3Sb5 and establishes the pressure-dependent gap and TRS-breaking superconducting response used as a comparison at 1.78 GPa.","marker":"[12]"},{"why":"Demonstrates surface-enhanced TRS breaking in RbV3Sb5 and provides the depth-resolved low-energy muSR framework that the present surface-bulk study extends to Nb-doped CsV3Sb5.","marker":"[13]"},{"why":"Reports the two-step zero-field muon relaxation in undoped CsV3Sb5 and the TRS-breaking response below Tc that serves as a benchmark for interpreting the Nb0.07-CVS data.","marker":"[28]"},{"why":"Supplies the canonical case of spontaneous TRS breaking in Sr2RuO4 that motivates reading the zero-field relaxation increase below Tc as evidence of broken time-reversal symmetry in the superconducting state.","marker":"[33]"}],"fun_headline_variants":["Pressurized kagome superconductor breaks time-reversal symmetry","Chiral order gives way to TRS-breaking superconductivity at 0.85 GPa","At 0.85 GPa, kagome's chiral order yields to TRS-breaking superconductivity","Nodeless pairing with broken time-reversal in pressure-tuned kagome","Low-pressure switch to time-reversal-breaking superconductor in kagome"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that hydrostatic pressure above about 0.85 GPa fully suppresses charge order in Nb0.07-CVS, so the time-reversal-symmetry-breaking fields seen below Tc are intrinsic to the superconducting state and not remnants of the charge-ordered normal state; the paper states this assumption explicitly in the Discussion when interpreting the pressure data.","fun_headline_variants_meta":{"raw":{"variants":["Pressurized kagome superconductor breaks time-reversal symmetry","Chiral order gives way to TRS-breaking superconductivity at 0.85 GPa","At 0.85 GPa, kagome's chiral order yields to TRS-breaking superconductivity","Nodeless pairing with broken time-reversal in pressure-tuned kagome","Low-pressure switch to time-reversal-breaking superconductor in kagome"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.001388,"raw_usage":{"total_tokens":5769,"prompt_tokens":1248,"completion_tokens":4521,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":864,"completion_tokens_details":{"reasoning_tokens":4412}},"tokens_in":864,"tokens_out":4521,"duration_ms":30502,"temperature":1.0,"reasoning_tokens":4412,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-12T10:55:19.638959+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Look for the 2×2 charge-order superstructure or its ~20 meV gap under pressure: if x-ray scattering, STM, or transport measurements at 1.2 GPa show that charge order persists or is only partially suppressed, then the TRS-breaking signal below Tc cannot be cleanly assigned to superconductivity. Alternatively, measure zero-field µSR in a Nb-doped sample with charge order fully suppressed chemically; the absence of any internal-field increase below Tc would directly contradict the claim.","supporting_citations":[{"cited_title":"Zhong , author J","cited_arxiv_id":null,"evidence_quote":"Establishes Nb0.07-CVS as the current Nb-doping limit, reports the nodeless ARPES superconducting gap, and supplies the crystal-growth and previous characterization basis for this study."},{"cited_title":"Gupta , author D","cited_arxiv_id":null,"evidence_quote":"Shows hydrostatic pressure suppresses charge order in CsV3Sb5 and establishes the pressure-dependent gap and TRS-breaking superconducting response used as a comparison at 1.78 GPa."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the canonical case of spontaneous TRS breaking in Sr2RuO4 that motivates reading the zero-field relaxation increase below Tc as evidence of broken time-reversal symmetry in the superconducting state."}],"review_version":1}