{"id":"45985962-1202-4b75-9ebc-0fd1e3661ec6","arxiv_id":"2607.07341","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"unknown","formal_verification":"none","parameter_count":4,"one_line_summary":"Pressure drives Y-kapellasite toward a spin-liquid phase by suppressing J' through nonlinear Cu–O–Cu angle dependence, with hydrogen geometry strongly modulating exchange couplings.","lead":"DFT calculations show that hydrostatic pressure suppresses one of three magnetic exchange couplings in Y-kapellasite, pushing the kagome antiferromagnet toward a spin-liquid regime. The result gives a microscopic mechanism for pressure-induced frustration and highlights how hydrogen atom positions control the couplings.","discovery_kind":"unclear","skeptic_critique":{"model":"glm-5.2","headline":"The U-freezing assumption is the right concern, but the supplemental data partially mitigates it: the qualitative trajectory toward the SL region is robust across U = 4–10 eV, weakening the concern's impact on the central claim.","rationale":"The reader identified the correct load-bearing concern: the U-freezing assumption. This is the single most important methodological choice that could affect the central claim, and it is unverified by experiment. The CONDITIONAL verdict with MODERATE confidence is appropriate. However, I would note that the concern is somewhat less severe than it first appears, because the supplemental material (Tables S7–S12) demonstrates U-robustness of the qualitative trend across a wide U range (4–10 eV). The direction of motion on the phase diagram (toward the SL region) does not reverse for any U value tested. What is U-sensitive is the precise location and the pressure threshold for reaching the SL boundary — quantities the authors themselves flag as uncertain. The other caveats the reader notes (classical vs. quantum phase diagram, system not yet at the SL boundary at 7.9 GPa) are real but are acknowledged limitations rather than load-bearing weaknesses in the argument. The hydrogen-geometry analysis is a genuine contribution that strengthens the paper's credibility, as it identifies and quantifies a previously underappreciated source of uncertainty in exchange-coupling calculations for hydroxide-bridged systems. The paper does not overclaim: it says pressure drives 'toward' the SL region, not 'into' it, and explicitly states that pressures above 10 GPa would be needed within the classical diagram. This measured framing is consistent with the evidence presented. No verdict adjustment is needed.","tokens_in":16878,"tokens_out":2983,"duration_ms":122207,"concrete_test":"Recompute the phase-diagram trajectory (J′/J9, J/J9) at each pressure point using U values that would reproduce a range of hypothetical pressure-dependent θCW values (e.g., θCW = -70, -85, -100, -115, -130 K at 7.9 GPa, interpolated linearly from ambient). If the trajectory still moves monotonically toward the SL region for all assumed θCW profiles, the U-freezing concern is fully mitigated for the qualitative claim. If any plausible θCW profile reverses the direction of motion on the phase diagram, the concern becomes load-bearing for the central claim.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The reader correctly identifies the most load-bearing concern: U is calibrated to reproduce the ambient-pressure θCW = -100 K (yielding U = 9.27 eV at 293 K) and then held fixed across all pressures because the pressure dependence of θCW is unknown. This means the absolute coupling values and the exact trajectory on the phase diagram depend on an unverified assumption. At fixed U = 9.27 eV, the computed |θCW| drifts from 100 K at 0 GPa to 86.6 K at 7.9 GPa (Table S4), indicating that the frozen U is not self-consistent with the pressure-evolved structure. If the true experimental θCW under pressure were substantially different (e.g., 70 K or 120 K), the required U would shift, and the absolute couplings would change. However, the supplemental data provides important mitigating evidence: Tables S7 and S9 show the full U-dependence at 0 GPa and 7.9 GPa. At both pressures, the qualitative trend — J9 suppressed relative to J and J′, with J′/J9 and J/J9 both increasing — persists across U = 4–10 eV. For instance, J/J9 crosses unity at 7.9 GPa for all U values shown. This means the central qualitative claim (pressure drives toward the SL region) is U-robust, even though the precise location on the phase diagram and the pressure at which the SL boundary is crossed are U-sensitive. The concern therefore lands on the precision of the trajectory, not on the direction of movement. This is consistent with the authors' own acknowledgment that 'pressures above 10 GPa are likely required within the classical phase diagram' — the exact threshold is uncertain, but the trend is not.","agreement_with_reader":"agree"},"referee_report":{"model":"glm-5.2","summary":"This manuscript presents ab initio DFT calculations (FPLO, GGA+U, TEMA) of the pressure-dependent magnetic exchange couplings in Y-kapellasite Y3Cu9(OH)19Cl8, a distorted kagome antiferromagnet. Using experimentally determined crystal structures at pressures up to 7.9 GPa (293 K) and 7.0 GPa (3 K), the authors find that hydrostatic pressure primarily suppresses J9 while leaving J and J' approximately constant, driving the system toward the spin-liquid region of the classical kagome phase diagram. The Hubbard U is calibrated to reproduce the ambient-pressure Curie-Weiss temperature θCW = -100 K and then held fixed across all pressures. The authors additionally investigate the influence of hydrogen positions (O-H bond length and out-of-plane angle τ) on the exchange couplings, finding significant sensitivity that helps reconcile discrepancies with prior DFT studies. The central qualitative claim—that pressure drives Y-kapellasite toward the spin-liquid regime by suppressing J9—is supported by the data, though the precision of the trajectory on the phase diagram depends on the unverified assumption of pressure-independent U.","tokens_in":17690,"tokens_out":1239,"duration_ms":384456,"significance":"The paper addresses a timely question: the microscopic mechanism behind pressure-induced suppression of magnetic order in Y-kapellasite, recently reported experimentally. The DFT methodology is standard and well-converged, and the eight-coupling sanity check (Tables S7–S12) confirming three-coupling dominance under pressure is a valuable verification. The systematic study of hydrogen geometry effects on exchange couplings is a genuine contribution, as hydrogen positions are notoriously difficult to determine experimentally and their impact on superexchange in hydroxide-bridged systems is underappreciated. The cubic fit to the Cu-O-Cu angle dependence (Fig. 5) improves on the linear approximation used in prior experimental work. Reproducible data is provided via Zenodo and CCDC deposition. The qualitative conclusion (pressure drives toward SL region) is shown to be robust across U = 4–10 eV in the supplemental data, which strengthens the central claim beyond the specific U-calibration choice.","major_comments":[{"comment":"§III.A and Supplemental §IV: The central methodological choice is calibrating U to reproduce θCW = -100 K at ambient pressure and then holding U fixed across all pressures. This creates a partial circularity: θCW is computed from the couplings via the mean-field formula θCW = -(1/3)ΣJi (Supplemental §IV), and U is chosen so that the couplings reproduce the experimental θCW. Under pressure, the computed |θCW| drifts from 100 K to 86.6 K at 7.9 GPa (Table S4), indicating the frozen U is not self-consistent with the pressure-evolved structure. The authors acknowledge this ('the pressure dependence of the Curie temperature remains undetermined'), but the manuscript would benefit from a more explicit discussion of how sensitive the phase-diagram trajectory is to this assumption. The supplemental U-scan data (Tables S7, S9) partially mitigates this concern: the qualitative trend (J9 suppressed","section":null}],"minor_comments":[{"comment":"Table S12: The J9 values at U = 7 eV (80.7 K) and U = 9 eV (41.2 K) appear anomalously low compared to the corresponding full-eight-coupling values in Table S11 (179.6 K and 140.7 K respectively) and compared to the monotonic trend seen in all other tables. This is likely a typographical error. Please correct.","section":null},{"comment":"Notation: The coupling labeled J9 in the main text and J7 in several supplemental tables (S1, S4–S12) should be unified. The Hamiltonian in §III.A uses J9, but the supplemental tables switch to J7, which creates confusion.","section":null},{"comment":"§III.B, Table I: The caption states the structures have 'almost similar Cu-O-Cu angles but different τ,' but Table S1 shows ϕ_J9 differs by 0.5° between Olex2 and Jana2020. This is small but should be acknowledged as a confound in the disentanglement.","section":null},{"comment":"Fig. 5: The cubic fit is described as better capturing both AFM and FM regimes, but no functional form or fitting parameters are given. Including the fit equation and R² would make the comparison with the linear model of Ref. [40] more quantitative.","section":null},{"comment":"§IV: The statement 'pressures above 10 GPa are likely required within the classical phase diagram' is an important caveat but appears only in the Discussion. Consider mentioning this limitation earlier, when the phase-diagram trajectory is first shown in Fig. 2(b).","section":null},{"comment":"Ref. [56]: The journal name is given as 'Physical Review Retters' — should be 'Physical Review Letters'.","section":null}],"recommendation":"minor_revision","confidential_remarks":"The reader's concern about U-freezing is valid but, upon examining the supplemental U-scan data, the qualitative claim is robust across U = 4–10 eV. The concern therefore affects the precision of the phase-diagram trajectory rather than the direction of movement. This is appropriately a minor revision issue. The authors should be asked to make the U-robustness argument more explicit in the main text, as it is currently buried in the supplemental tables. The Table S12 anomaly should be checked carefully — if it is not a typo, it would indicate a fitting instability that needs investigation."},"author_rebuttal":{"model":"glm-5.2","summary":"We thank the referee for a careful and constructive report. The referee correctly identifies the one substantive methodological issue in our work: the partial circularity introduced by calibrating U to the ambient-pressure θCW and then holding it fixed under pressure. We agree that this deserves a more explicit discussion and will revise the manuscript accordingly. Below we address the comment point by point.","responses":[{"response":"We agree with the referee that the partial circularity in the U-calibration procedure and the sensitivity of the phase-diagram trajectory to the frozen-U assumption deserve a more explicit and quantitative discussion than what is currently in the manuscript. We will revise the manuscript to incorporate the following points. First, we will add a paragraph in Section III.A (or the Discussion) that explicitly states the circularity: U is tuned so that the ambient-pressure couplings reproduce the experimental θCW via the mean-field relation, and this U is then held fixed because no experimental θCW(P) is available. The drift of the computed |θCW| from 100 K to 86.6 K at 7.9 GPa (Table S4) is a direct consequence of this choice and reflects the fact that U would need to increase modestly under pressure to maintain self-consistency—if one assumes θCW remains at -100 K. Second, and more importantly, we will make explicit use of the U-scan data already present in the Supplemental Material (Tables S7–S12) to demonstrate the robustness of the qualitative conclusion. Specifically, at 7.9 GPa, the full eight-coupling calculations at U = 4, 5, 7, and 9 eV (Table S9) all show J9 suppressed relative to J, with J/J9 > 1, placing the system on the same side of the phase-diagram boundary as at the calibrated U. The same holds at 3.6 GPa and 3 K (Table S11). The ratios J'/J9 and J/J9 shift quantitatively with U, but the direction of the pressure-induced trajectory—toward the spin-liquid region—does not reverse for any U in the physically reasonable range of 4–10 eV. We will state this explicitly and, if space permits, include a small figure or table summarizing the phase-diagram coordinates (J'/J9, J/J9) at ambient and highest pressure for representative U values, to make the robustness直观","revision_made":"yes","referee_comment":"§III.A and Supplemental §IV: The central methodological choice is calibrating U to reproduce θCW = -100 K at ambient pressure and then holding U fixed across all pressures. This creates a partial circularity: θCW is computed from the couplings via the mean-field formula θCW = -(1/3)ΣJi (Supplemental §IV), and U is chosen so that the couplings reproduce the experimental θCW. Under pressure, the computed |θCW| drifts from 100 K to 86.6 K at 7.9 GPa (Table S4), indicating the frozen U is not self-consistent with the pressure-evolved structure. The authors acknowledge this ('the pressure dependence of the Curie temperature remains undetermined'), but the manuscript would benefit from a more explicit discussion of how sensitive the phase-diagram trajectory is to this assumption. The supplemental U-scan data (Tables S7, S9) partially mitigates this concern: the qualitative trend (J9 suppressed"}],"tokens_in":16563,"tokens_out":810,"duration_ms":27920,"standing_objections":[]},"desk_editor":{"model":"glm-5.2","letter":"The main result is that hydrostatic pressure suppresses J9 in Y-kapellasite while leaving J and J' roughly constant, driving the system toward the spin-liquid region of the classical kagome phase diagram. The cubic (nonlinear) Cu–O–Cu angle dependence and the systematic hydrogen-position scan are genuinely new and useful. The eight-coupling sanity check across pressures (Tables S7–S12) confirming that only three couplings matter is solid, reproducible work. Data and structures are deposited on Zenodo and CCDC. This is a competent DFT study that extends prior ambient-pressure calculations (Ref. [35]) and the linear-angle analysis of Ref. [40] in a meaningful way. The hydrogen-geometry analysis — showing that both the out-of-plane angle τ and the O–H bond length materially affect exchange couplings — is probably the most broadly useful contribution, since it applies to any hydroxide-bridged kagome magnet, not just Y-kapellasite. The Olex2 vs. Jana2020 comparison making this point is clean. The soft spot is the U calibration. U is fitted to reproduce θCW = −100 K at ambient pressure (yielding U ≈ 9.3 eV) and then frozen across all pressures because the pressure dependence of θCW is unknown. At fixed U, the computed |θCW| drifts from 100 K to 86.6 K at 7.9 GPa (Table S4), so the frozen U is not self-consistent with the evolved structure. If the true experimental θCW under pressure differed substantially, the absolute couplings and the exact trajectory on the phase diagram would shift. However — and this is the important mitigating point — the supplemental U-scan (Tables S7, S9) shows that the qualitative trend (J9 suppressed, J/J9 increasing, crossing unity at 7.9 GPa for all U values tested from 4–10 eV) is U-robust. So the concern lands on the precision of the trajectory, not the direction. The authors acknowledge that pressures above 10 GPa are likely needed within the classical phase diagram, and that a quantum treatment could shift boundaries. Both caveats are fairly stated. The classical-vs.-quantum gap between the phase diagram used and the experimental claim of a fluctuating ground state is real but not a defect of this paper — it is a limitation of the field. This paper is for people working on frustrated kagome magnets and pressure-tuned exchange interactions. It deserves a serious referee who should push the authors to state more explicitly how sensitive the SL-boundary crossing pressure is to U, and whether any existing experimental constraint on θCW under pressure could be brought to bear.","headline":"Pressure-dependent DFT on Y-kapellasite: qualitative trend toward spin-liquid region is robust, absolute couplings depend on an untested U-freezing assumption.","tokens_in":18007,"tokens_out":650,"would_cite":true,"duration_ms":76657,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":["75.10.Jm","75.30.Et","71.15.Mb"],"model":"glm-5.2","headline":"Pressure steers kagome magnet toward spin liquid by bending one bond","keywords":["kagome antiferromagnet","spin liquid","Y-kapellasite","hydrostatic pressure","magnetic exchange coupling","superexchange","Cu-O-Cu bond angle","hydrogen geometry"],"falsifier":"Measurement of the Curie-Weiss temperature under pressure that reveals substantial U-dependence, or neutron diffraction determination of hydrogen positions under pressure that contradicts the assumed O–H geometry, could shift the quantitative location of Y-kapellasite on the phase diagram.","tokens_in":16969,"feed_emoji":"🧲","tokens_out":839,"duration_ms":106741,"temperature":0.7,"pith_summary":"This paper uses density functional theory calculations on experimentally measured crystal structures of Y-kapellasite (Y3Cu9(OH)19Cl8), a distorted kagome antiferromagnet, to show that hydrostatic pressure suppresses one of the three dominant magnetic exchange couplings (J9) while leaving the other two (J' and J) nearly unchanged. This selective suppression increases the ratios J'/J9 and J/J9, moving the system toward the spin-liquid region of the classical kagome phase diagram. The mechanism is geometric: pressure alters the Cu–O–Cu bond angle that mediates superexchange, and the dominant coupling depends nonlinearly (cubic rather than linear) on this angle. The authors further show that hydrogen atom positions — both the O–H bond length and the out-of-plane angle — significantly affect the absolute values of the exchange couplings, enough to account for discrepancies with prior theoretical work, though the pressure-driven trend toward the spin-liquid regime is robust against hydrogen-position uncertainty.","feed_headline":"Pressure bends one bond, steers kagome magnet toward spin liquid","feed_subtitle":"DFT shows hydrostatic pressure selectively suppresses a single exchange coupling in Y-kapellasite, pushing it toward a spin-liquid regime — ","key_machinery":"Three nearest-neighbor Heisenberg exchange couplings (J', J9, J) on a distorted kagome lattice, mediated by hydroxide (Cu–O–Cu) superexchange paths. The Cu–O–Cubond angle controls the couplings via a nonlinear (cubic) dependence. Hydrogen positions (O–H bond length and out-of-plane angle τ) provide a secondary tuning of the couplings. The classical J1–J2–J3 kagome phase diagram (with J'=J1, J9=J2, J=J3) features a spin-liquid region whose boundaries depend on the ratios of these three couplings.","core_discovery":"Pressure suppresses the exchange coupling J9 in Y-kapellasite because it modifies the Cu–O–Cu bond angle, and this coupling depends cubically (not linearly, as previously assumed) on that angle. The result is that J'/J9 and J/J9 both increase with pressure, pushing the system toward the spin-liquid region of the kagome phase diagram. Hydrogen geometry — specifically the O–H bond length and out-of-plane angle — is a second, previously underappreciated control on the absolute coupling values, with each hydrogen position mainly affecting its corresponding exchange path.","pith_inferences":[],"forward_implications":[],"fun_headline_variants":["Pressure tunes kagome magnet toward spin liquid via cubic bond-angle dependence","J9 suppression under pressure drives Y-kapellasite toward kagome spin liquid","Cu-O-Cu angle controls J9 cubically, steering Y-kapellasite to spin liquid","Y-kapellasite approaches spin liquid as pressure suppresses dominant J9","Hydrogen geometry and bond angle jointly tune kagome exchange couplings"],"cache_read_input_tokens":0,"weakest_assumption_plain":"The Hubbard U parameter is calibrated to reproduce the ambient-pressure Curie-Weiss temperature of -100 K and then held fixed across all pressures, because no experimental measurement of the pressure dependence of that temperature exists. If U changes substantially under pressure, the absolute coupling values and the system's exact position on the phase diagram could shift.","fun_headline_variants_meta":{"raw":{"variants":["Pressure tunes kagome magnet toward spin liquid via cubic bond-angle dependence","J9 suppression under pressure drives Y-kapellasite toward kagome spin liquid","Cu-O-Cu angle controls J9 cubically, steering Y-kapellasite to spin liquid","Y-kapellasite approaches spin liquid as pressure suppresses dominant J9","Hydrogen geometry and bond angle jointly tune kagome exchange couplings","Bond-angle nonlinearity explains pressure-driven frustration in Y-kapellasite","DFT maps pressure-driven J9 suppression to cubic Cu-O-Cu angle dependence","Hydrogen positions add a second lever on kagome exchange in Y-kapellasite"]},"model":"glm-5.2","effort":"low","cost_usd":0.0,"raw_usage":{"total_tokens":1289,"prompt_tokens":474,"completion_tokens":815,"prompt_tokens_details":null},"tokens_in":474,"tokens_out":815,"duration_ms":35404,"temperature":1.0,"reasoning_tokens":620,"cache_read_input_tokens":0,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-07-09T13:44:27.269645+00:00","model_set":{"reader":"glm-5.2"},"falsifier":"Measurement of the Curie-Weiss temperature under pressure that reveals substantial U-dependence, or neutron diffraction determination of hydrogen positions under pressure that contradicts the assumed O–H geometry, could shift the quantitative location of Y-kapellasite on the phase diagram.","supporting_citations":[],"review_version":1}