{"id":"4b8b3fc9-69e2-43ad-84ca-8bf670d7e97c","arxiv_id":"2504.17470","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":6,"one_line_summary":"The new triangular-lattice compounds Rb3Yb(VO4)2 and Cs3Yb(VO4)2 show no magnetic order or spin freezing down to 0.1 K, consistent with a quantum-disordered, spin-liquid-like ground state.","lead":"This paper reports two new ytterbium-based crystals that form perfect triangular magnetic layers and show no sign of magnetic ordering or freezing down to very low temperatures (0.1 K). The results add clean, disorder-free candidates to the search for quantum spin liquids, exotic states where electron spins stay fluid-like instead of freezing into patterns.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Persistent spin dynamics is inferred, not directly measured: below 1.8 K no ac susceptibility or muSR data exclude spin freezing, so the disordered ground state could still be a spin glass rather than a QSL.","rationale":"The observational core is genuinely strong: susceptibility shows no transition or freezing to 1.8 K, specific heat shows no lambda anomaly to ~0.1 K, and neutron diffraction shows no magnetic Bragg peaks to 97 mK. The paper also honestly lists alternative states in Sec. IV, including gapped and gapless QSL, quantum dipole state, and valence-bond solid, which supports its credibility. My concern is not that the no-long-range-order claim is weak, but that the specific 'persistent spin dynamics' element of the central claim goes one step beyond the data. The only spin-freezing discriminator used by the authors, ZFC/FC susceptibility, stops at 1.8 K. Below that temperature, the observed Cp upturn and the field-dependent Schottky parameter n<1 at low fields are equally compatible with a spin-glass or random-singlet ground state. This is closely related to the reader's weakest assumption about structural disorder, but it is distinct: even a structurally perfect crystal with undetected magnetic disorder or with intrinsic randomness could in principle freeze. The missing direct dynamical probe is therefore a concrete and testable gap. The reader's CONDITIONAL verdict already accounts for this uncertainty, so I do not recommend changing it; instead I identify muSR as the single experimental check that would settle whether the persistent-dynamics claim survives.","tokens_in":18243,"tokens_out":8413,"duration_ms":94046,"concrete_test":"Perform zero-field and weak longitudinal-field muon spin relaxation (muSR) on powder Rb3Yb(VO4)2 down to at least 50 mK, tracking the relaxation rate lambda(T) and any appearance of a static Kubo-Toyabe component. Persistent spin dynamics in a QSL gives a relaxation rate that remains finite and slowly varying as T tends to zero, with no static field distribution. A spin-glass or random-singlet ground state would show a growing static fraction or a nonvanishing 'tail' in the asymmetry below the freezing temperature. This directly tests the persistent-spin-dynamics assertion without relying on ZFC/FC data that stop at 1.8 K.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim requires persistent spin dynamics, not merely the absence of long-range order. The only direct spin-freezing probe is zero-field-cooled/field-cooled dc susceptibility at 0.01 T, collected down to 1.8 K (Sec. III.B, Fig. 2). Below 1.8 K the evidence consists of specific heat (Sec. III.C) and neutron diffraction (Sec. III.D), and neither cleanly distinguishes a spin liquid from a spin glass. A canonical spin glass produces no magnetic Bragg peaks and only a broad or absent anomaly in Cp, while the observed zero-field Cp upturn below ~1 K plus field-dependent Schottky fits with n<1 at low fields (insets of Fig. 4) are also compatible with frozen or randomly coupled moments. The paper argues against spin freezing using the absence of ZFC/FC bifurcation, but that test is silent below 1.8 K, which is far above the 0.1 K temperature on which the 'quantum disordered ground state' claim rests. Thus the statement 'persistent spin dynamics ... reminiscent of QSL behaviors' (Sec. V) is underdetermined by the presented data. A spin-glass ground state would satisfy all the reported susceptibility, specific heat, and neutron results except the as-yet-unmeasured freezing signature.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports the synthesis and characterization of two new triangular-lattice ytterbium compounds, Rb3Yb(VO4)2 and Cs3Yb(VO4)2, which are positional analogues of the previously reported K3Yb(VO4)2. Using powder X-ray diffraction, dc magnetization, specific heat down to ~0.1 K, and inelastic neutron scattering on the Rb compound down to 97 mK, the authors find no evidence of long-range magnetic order or a lambda-type specific-heat anomaly. The low-temperature susceptibility is Curie-Weiss-like with antiferromagnetic Theta_CW values of -1.40 K (Rb) and -0.43 K (Cs); the magnetic specific heat is dominated by a field-dependent Schottky anomaly; and the INS data show no clear inelastic magnetic signal. The authors conclude that both compounds host a quantum disordered ground state with persistent spin dynamics, reminiscent of a quantum spin liquid, and emphasize the absence of detectable structural disorder as a key advantage over systems such as YbMgGaO4.","tokens_in":18578,"tokens_out":3794,"duration_ms":40969,"significance":"If the central claim holds, these compounds would be valuable additions to the short list of structurally clean, triangular-lattice Jeff=1/2 antiferromagnets that remain disordered to temperatures well below the exchange scale. The cross-consistency of the Landé g factors extracted from magnetization (2.57-2.62), Schottky fits (2.39-2.45), and saturation moments (2.60-2.62) is a genuine strength, as is the care taken to measure specific heat to 0.1 K and to compare with nonmagnetic Lu analogues. The paper is also honest in Sec. IV in listing several possible ground states. However, the central claim of 'persistent spin dynamics' is currently inferred rather than directly demonstrated, and the disorder-free claim rests on laboratory powder XRD with occupancies fixed at unity. The manuscript would be strengthened substantially by more direct probes of spin freezing and of structural disorder.","major_comments":[{"comment":"The INS measurement at Ei=3.69 meV with HWHM resolution of 0.05 meV cannot exclude magnetic spectral weight below roughly 0.2 meV, especially in a powder-averaged measurement where such weight is convoluted with the strong elastic signal. The authors themselves invoke Ce2Zr2O7, where the spectral weight lies predominantly below 0.2 meV, as a precedent for weak-interaction systems. Therefore the statement in Sec. III.D that the data indicate 'the absence of gapless magnetic excitations' overstates what the measurement can establish; the data are equally consistent with a low-energy continuum or with a small spin gap below the resolution limit. This distinction matters because the gapped-versus-gapless discussion in Sec. IV hinges on it.","section":"Sec. III.D and Sec. IV, item 2"},{"comment":"The claim of 'persistent spin fluctuations' or 'no spin freezing' is underdetermined by the presented data. The only direct freezing probe is the ZFC/FC susceptibility comparison, which is reported down to 1.8 K only (Fig. 2), whereas the quantum-disordered-ground-state claim rests on behavior down to ~0.1 K. No ac susceptibility, muon-spin rotation, or NMR data are presented below 1.8 K. A canonical spin glass can show no magnetic Bragg peaks, no lambda anomaly, a broad low-temperature specific-heat upturn, and a field-dependent Schottky-like contribution, so the specific heat and neutron diffraction data do not cleanly distinguish a spin glass from a spin liquid. The abstract and Sec. V should either soften the 'persistent spin dynamics' language or present a direct low-temperature probe of spin freezing.","section":"Sec. III.B, Sec. V"},{"comment":"The claim that these are 'disorder-free' systems is load-bearing for the framing of the paper, because it is used to distinguish the new compounds from YbMgGaO4 and herbertsmithite. However, the evidence is a Rietveld refinement of laboratory powder X-ray diffraction in which all occupancies were fixed at unity, and the scan range extends only to 80 degrees in 2theta. This procedure cannot rule out stacking faults, subtle site mixing, or incoherent intergrowths that would not appear as distinct Bragg peaks. No diffuse scattering, pair-distribution-function analysis, or local structural probe is provided. If undetected disorder is present, the low-temperature specific-heat upturn and the absence of magnetic order would also be consistent with random-singlet or cluster-spin-glass behavior. The authors should either provide stronger structural evidence or explicitly restrict the 'disorder-free' claim to the limits of the XRD measurement.","section":"Sec. III.A, Table I"}],"minor_comments":[{"comment":"There is a typo in the caption: 'repectively' should be 'respectively'.","section":"Fig. 2 caption"},{"comment":"The text twice uses 'Dybye model'; this should be 'Debye model'.","section":"Sec. III.C"},{"comment":"The header of Table I reads 'A3Yb(VO4)3', but the compounds are A3Yb(VO4)2; correct this typo.","section":"Table I"},{"comment":"The goodness-of-fit values reported in the text (chi2 = 4.353 and 3.654) differ from those listed in Table I (4.4 and 3.7); the authors should make the rounding consistent and state whether chi2 is the reduced chi-squared.","section":"Table I and Sec. III.A"},{"comment":"No uncertainties are given for the fitted Curie-Weiss temperatures, effective moments, or chi0 values. Error bars would help the reader assess whether the differences between the two compounds are significant.","section":"Table II and Sec. III.B"},{"comment":"In the discussion of the INS data, the phrase 'no magnetic signals such as diffuse scattering or well-defined spin waves are captured below 2 meV' is somewhat ambiguous because the spectrum shown in Fig. 5(a) has strong scattering near zero energy; it would be clearer to state explicitly which energy and Q ranges were used to define the background and to show a constant-Q or integrated-intensity comparison with the high-temperature data.","section":"Sec. III.D"}],"recommendation":"major_revision","confidential_remarks":"The experimental work appears careful and the raw data are mutually consistent in showing the absence of static magnetic order down to ~0.1 K. My main concern is the gap between the evidence and the 'persistent spin dynamics' / QSL-reminiscent conclusion, which requires either additional measurements or a more cautious framing. I would support publication after the authors address the INS-resolution issue, the spin-freezing distinction, and the overstatement of the disorder-free claim."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Quick take: this is a solid synthetic and characterization paper. Ma et al. have made two new Yb-based triangular-lattice antiferromagnets, Rb3Yb(VO4)2 and Cs3Yb(VO4)2, and shown with susceptibility, specific heat, and neutron diffraction that neither orders magnetically down to ~0.1 K. That's a real result. The compounds are direct alkali substitutions of K3Yb(VO4)2 and Rb3Yb(PO4)2, so the novelty is incremental, but the sub-Kelvin specific heat and 97 mK neutron data are new and cleanly executed. The cross-consistency of g factors from magnetization, saturation, and Schottky fits (2.4–2.6) is reassuring.\n\nThe main soft spot is exactly where the stress-test lands. The paper's abstract and summary say 'persistent spin dynamics' and 'reminiscent of QSL behaviors,' but nothing in the data directly measures spin dynamics. ZFC/FC susceptibility only goes down to 1.8 K. Below that, the absence of magnetic Bragg peaks and a lambda anomaly in Cp cannot distinguish a quantum spin liquid from a spin glass. A canonical spin glass would look the same in all reported measurements. So the central interpretive claim is underdetermined. The paper itself is more careful in the Discussion, where it lists gapped QSL, gapless QSL, quantum dipole, and VBS as possibilities — but the abstract overreaches.\n\nSecond soft spot: the 'disorder-free' framing rests on lab powder XRD with occupancies fixed at unity. There's no diffuse scattering, no PDF, no NMR or muSR. Given the YbMgGaO4 history, where disorder turned a QSL candidate into a spin glass, this is the right thing to probe. It's a limitation, not a fatal flaw: the two compounds are new and the structural chemistry is plausible.\n\nThe fitted parameters lack error bars, and the INS on a powder at Ei=3.69 meV with HWHM 0.05 meV can't exclude spectral weight below ~0.2 meV. Minor, but worth flagging.\n\nOverall: the experimental core is sound and the paper is honest about alternatives. It should go to peer review. A careful referee should push the authors to either soften the 'persistent spin dynamics' language or add ac susceptibility, muSR, or NMR to back it up.","headline":"New Yb triangular-lattice compounds with clean sub-Kelvin data and no order down to 0.1 K, but the 'QSL-like persistent dynamics' claim rides on an exclusion of spin freezing that the data only support above 1.8 K.","tokens_in":19152,"tokens_out":1885,"would_cite":true,"duration_ms":17897,"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":"Two new ytterbium triangular-lattice antiferromagnets remain magnetically disordered down to at least 0.1 K.","keywords":["triangular lattice antiferromagnet","quantum spin liquid","ytterbium Kramers doublet","magnetic frustration","Schottky anomaly","inelastic neutron scattering","Rb3Yb(VO4)2","Cs3Yb(VO4)2"],"falsifier":"Muon spin rotation or NMR measurements below 0.1 K that detect static internal fields or a spin-freezing transition, or elastic neutron diffraction below 97 mK that reveals magnetic Bragg peaks, would refute the persistent-spin-dynamics claim; a diffuse-scattering or pair-distribution-function study revealing cation site mixing in the Yb layers would refute the disorder-free premise.","tokens_in":18052,"feed_emoji":"🧲","tokens_out":16038,"duration_ms":143015,"temperature":0.7,"pith_summary":"This paper reports two new ytterbium-based triangular-lattice antiferromagnets, Rb3Yb(VO4)2 and Cs3Yb(VO4)2, and claims that both remain magnetically disordered to the lowest temperatures measured: no long-range order, no spin freezing, and no magnetic Bragg peaks at 97 mK in the Rb compound. The evidence combines powder X-ray diffraction showing a clean triangular lattice with no detectable site disorder, magnetic susceptibility with antiferromagnetic Curie-Weiss temperatures near $-1.4$ K and $-0.4$ K and no zero-field-cooling/field-cooling split, specific heat with only a field-tuned Schottky anomaly, and inelastic neutron scattering with no magnetic Bragg peaks or low-energy magnetic excitations. If the claim is right, these compounds offer a relatively disorder-free setting in which to study quantum-disordered magnetism on a triangular lattice, a state that in the most interesting reading is a quantum spin liquid. The paper lists several possible ground states, including gapped or gapless spin liquid, quantum dipole state, and valence-bond solid.","feed_headline":"Ytterbium triangular magnets stay disordered to 0.1 K","feed_subtitle":"Neutrons, heat capacity, and susceptibility all point to a quantum-spin-liquid-like ground state","key_machinery":"The load-bearing object is the Yb3+ Kramers doublet with effective spin $J_{\\mathrm{eff}}=1/2$ at low temperature, arranged on edge-sharing YbO6 octahedra in a perfect triangular lattice with space group P-3m1 and AAA stacking. The argument is carried by three measurement identities: the low-temperature Curie-Weiss fit $\\chi=\\chi_0+C/(T-\\Theta_{\\mathrm{CW}})$, which fixes the antiferromagnetic interaction scale; the two-level Schottky function for magnetic specific heat, whose gap $\\Delta$ grows linearly with field (giving $g$ about 2.4-2.6) and whose free-spin fraction saturates to 1 above about 3 T; and the elastic/inelastic neutron-scattering comparison at 97 mK versus 60 K, which shows no magnetic Bragg peaks and no low-energy magnetic scattering. Together these convert the absence of a transition into positive evidence for persistent spin dynamics rather than a conventional ordered or frozen state.","core_discovery":"The central discovery claim is that Rb3Yb(VO4)2 and Cs3Yb(VO4)2 host effective spin-1/2 Yb3+ moments on perfect triangular layers and settle into a quantum disordered ground state with persistent spin dynamics. The paper supports this with four concordant observations: susceptibility shows neither a transition nor spin freezing down to 1.8 K; specific heat shows no $\\lambda$ anomaly down to about 0.1 K, only a two-level Schottky anomaly whose peak moves to higher temperature with applied field and whose entropy recovers to $R\\ln2$ once the field exceeds about 1 T; inelastic neutron scattering on the Rb compound at 97 mK shows no magnetic Bragg peaks and no magnetic excitation signal below 2 meV; and the Curie-Weiss fit places the dominant exchange scale at about 1.4 K for the Rb compound, an order of magnitude above the lowest measured temperature. The paper concludes that the absence of order and freezing, together with persistent fluctuation signatures, points to a quantum disordered ground state reminiscent of quantum spin liquid behavior, without claiming that the smoking-gun evidence for a spin liquid has been obtained.","pith_inferences":["Beyond the paper's explicit conclusions, the missing low-temperature entropy (about 28-33% of $R\\ln2$ released above 0.1 K) implies that a gapless spin liquid should show a specific-heat anomaly or $T$-linear term below 0.1 K; measuring to 10 mK would distinguish a tiny gap from a gapless spectrum.","The absence of INS signal below 2 meV may be a resolution or energy-window effect rather than evidence of a gap, since the exchange scale is only about 1 K; higher-resolution cold-neutron or field-dependent spectroscopy could test whether the spectral weight sits below 0.2 meV.","If the disorder-free premise survives local probes, these compounds become a test bed for anisotropic spin-1/2 triangular-lattice models with easy-plane exchange and dipolar couplings.","The similar behavior of the Rb and Cs compounds despite a roughly threefold difference in $|\\Theta_{\\mathrm{CW}}|$ suggests the disordered state is robust within this family; replacing Yb with another rare earth would test whether it is specific to the Yb Kramers doublet."],"forward_implications":["If the ground state is indeed quantum disordered, Rb3Yb(VO4)2 and Cs3Yb(VO4)2 become cleaner triangular-lattice spin-1/2 platforms than YbMgGaO4, because the structural-disorder ambiguity that complicated that compound is absent according to the paper's XRD evidence.","The absence of magnetic Bragg peaks and of low-energy magnetic scattering at 97 mK sets an upper bound on any ordered moment and on the spectral weight of gapless excitations in the Rb compound, constraining candidate spin-liquid theories.","The field response, a Schottky gap linear in field and entropy recovery to $R\\ln2$ above about 1 T, implies that modest fields fully polarize the two-level system, which the paper notes could be useful for adiabatic demagnetization refrigeration at sub-kelvin temperatures.","The systematic K-to-Rb-to-Cs substitution enlarges the interlayer distance and lowers the magnitude of the Curie-Weiss temperature, so the family provides a tuning axis between more two-dimensional and weaker-coupled versions of the same triangular magnet.","Because the paper explicitly leaves open four possible ground states (gapped spin liquid, gapless spin liquid, quantum dipole state, valence-bond solid), the right consequence is not that a spin liquid is proven but that an intrinsic disordered state is established and its exact nature is reduced to identifiable alternatives."],"supporting_citations":[{"why":"First synthesis of the K3RE(VO4)2 glaserite family; establishes the chemistry and the perfect triangular YbO6 layers from which the new compounds are derived.","marker":"43"},{"why":"Parent-compound study of K3Yb(VO4)2; supplies the structural model for Rietveld refinement and the effective-spin-1/2 easy-plane anisotropic quantum-spin-liquid context that motivates replacing K with Rb and Cs.","marker":"44"},{"why":"YbMgGaO4 susceptibility and single-crystal studies that provide the comparison for ytterbium triangular-lattice behavior and define the structural-disorder problem the paper aims to avoid.","marker":"25,26"},{"why":"Supplies the interlayer-to-intralayer distance benchmark of Na2BaCo(PO4)2, a known two-dimensional triangular spin-liquid candidate, used to argue the new compounds are quasi-two-dimensional.","marker":"48"},{"why":"Provides the two-level Schottky function and Curie-Weiss analysis used for the specific-heat and susceptibility fits in a similar ytterbium triangular antiferromagnet.","marker":"51"},{"why":"Ultralow-temperature specific-heat work on KBaYb(BO3)2 supporting the assignment of the field-dependent broad peak in these compounds to a Schottky anomaly.","marker":"52"},{"why":"Weak-exchange pyrochlore example showing that a small Curie-Weiss temperature can shift magnetic spectral weight below the inelastic-neutron-scattering window, used to interpret the absence of scattering in Rb3Yb(VO4)2.","marker":"62"}],"fun_headline_variants":["Clean Yb triangular magnets stay disordered to 0.1 K","No magnetic order in Yb compounds down to 0.1 K","Quantum spin liquid clues in disorder-free Yb layers","Yb triangular lattices: persistent dynamics, zero order"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The central claim depends on the assumption that the crystals are truly disorder-free, which is supported only by powder X-ray diffraction with occupancies fixed at unity and an ionic-radius argument; if undetected site mixing or stacking faults exist, the low-temperature absence of order could be a disorder-driven spin-glass or singlet state rather than an intrinsic quantum disordered ground state.","fun_headline_variants_meta":{"raw":{"variants":["Clean Yb triangular magnets stay disordered to 0.1 K","No magnetic order in Yb compounds down to 0.1 K","Quantum spin liquid clues in disorder-free Yb layers","Yb triangular lattices: persistent dynamics, zero order"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.00018,"raw_usage":{"total_tokens":1399,"prompt_tokens":1138,"completion_tokens":261,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":754,"completion_tokens_details":{"reasoning_tokens":190}},"tokens_in":754,"tokens_out":261,"duration_ms":3248,"temperature":1.0,"reasoning_tokens":190,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-16T10:40:23.114702+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Muon spin rotation or NMR measurements below 0.1 K that detect static internal fields or a spin-freezing transition, or elastic neutron diffraction below 97 mK that reveals magnetic Bragg peaks, would refute the persistent-spin-dynamics claim; a diffuse-scattering or pair-distribution-function study revealing cation site mixing in the Yb layers would refute the disorder-free premise.","supporting_citations":[{"cited_title":"\\ Kimani , author Lindsey \\ Thompson , author Whitney \\ Snider , author Colin D","cited_arxiv_id":null,"evidence_quote":"First synthesis of the K3RE(VO4)2 glaserite family; establishes the chemistry and the perfect triangular YbO6 layers from which the new compounds are derived."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Parent-compound study of K3Yb(VO4)2; supplies the structural model for Rietveld refinement and the effective-spin-1/2 easy-plane anisotropic quantum-spin-liquid context that motivates replacing K with Rb and Cs."},{"cited_title":"Somesh , author S","cited_arxiv_id":null,"evidence_quote":"Provides the two-level Schottky function and Curie-Weiss analysis used for the specific-heat and susceptibility fits in a similar ytterbium triangular antiferromagnet."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Ultralow-temperature specific-heat work on KBaYb(BO3)2 supporting the assignment of the field-dependent broad peak in these compounds to a Schottky anomaly."},{"cited_title":"\\ Tam , author Chien-Lung \\ Huang , author Kalyan \\ Sasmal , author Devashibhai T","cited_arxiv_id":null,"evidence_quote":"Weak-exchange pyrochlore example showing that a small Curie-Weiss temperature can shift magnetic spectral weight below the inelastic-neutron-scattering window, used to interpret the absence of scattering in Rb3Yb(VO4)2."}],"review_version":1}