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REVIEW 3 major objections 5 minor 38 references

Pressure-Induced Stacking Disorder and Suppression of Long-Range Sm-type Order in Medium-Entropy Rare-Earth Alloys

T0 review · 3 major / 5 minor · reviewed 2026-08-12 · deepseek-v4-flash

Pith's one-line read Two medium-entropy rare-earth alloys, TbHoEr and TbHoDy, transform directly from hcp to dhcp under pressure without forming a resolved bulk Sm-type phase, passing instead through a stacking-disordered state.

desk verdict Solid first look at pressure-driven stacking in ternary RE-MEAs, but the headline claim of Sm-type suppression rests on absence-of-evidence and needs a quantitative sensitivity bound before it fully lands. read the letter →

arxiv 2608.08351 v1 pith:GDUOIWQG submitted 2026-08-08 cond-mat.mtrl-sci

classification cond-mat.mtrl-sci
keywords medium-entropyalloysrare-earthhigh-pressurephasetransitionsstackingdisorderSm-typehcp-to-dhcptransformationsynchrotronX-raydiffractionlanthanidestructuralevolution
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper claims that the pressure-driven structural evolution of two ternary rare-earth alloys, TbHoEr and TbHoDy, follows a different path from their constituent lanthanide metals: instead of forming the intermediate Sm-type (9-layer rhombohedral) phase, both alloys go directly from the hexagonal close-packed (hcp) structure to the double hexagonal close-packed (dhcp) structure. During the transformation, two-dimensional X-ray diffraction images show streak-like diffuse scattering, and line-width analysis gives stacking coherence lengths of about 1–3 nm, comparable to or shorter than one 9-layer Sm-type repeat. The paper interprets these signatures as evidence that the transformation proceeds through a stacking-disordered close-packed state rather than a well-ordered bulk Sm-type phase. The results matter because they show that moderate configurational disorder can reroute the stacking-sequence transformations that govern close-packed lanthanides under extreme conditions.

What carries the argument

The central object is the close-packed stacking sequence of the rare-earth lattice and its pressure-driven reorganization. The hcp ABAB stacking changes to the dhcp ABAC stacking, whereas a hypothetical Sm-type phase would require coherent nine-layer (9R) periodic order. Two experimental signatures of stacking disorder carry the argument: streak-like diffuse intensity in two-dimensional diffraction images, and the broadening of the dhcp (103) reflection, which has a c-axis index and is therefore sensitive to stacking faults, compared with the in-plane (100) reflection. Applying a Scherrer-type relation to the (103) width gives apparent coherence lengths of about 1.5–2.3 nm for TbHoEr and 1–3 nm for TbHoDy in the transition region, against a nine-layer Sm-type repeat of roughly 2.5 nm, showing that the stacking coherence is insufficient to support long-range 9R order.

What would settle it

Compress TbHoEr or TbHoDy on a fine pressure grid under hydrostatic conditions and look for the systematic set of sharp reflections predicted for a 9-layer 9R or Sm-type structure; alternatively, decompress samples from the transition region and examine them with transmission electron microscopy for 9R stacking domains. Observation of those reflections or domains at any pressure would refute the suppression claim.

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Extended reading notes

Core claim

On the paper's own terms, the central discovery is that medium-entropy alloying suppresses long-range Sm-type order in compressed rare-earth alloys. In TbHoEr and TbHoDy, synchrotron X-ray diffraction shows only an hcp-to-dhcp transformation, beginning near 10 GPa in TbHoEr and near 7.5 GPa in TbHoDy, with no systematic set of sharp reflections from a bulk 9R/alpha-Sm-type phase. TbHoEr compressed non-hydrostatically to 70 GPa additionally shows a rhombohedral hR24 phase. The two-dimensional diffraction images develop streak-like diffuse intensity in the transition region, and Scherrer-type analysis of the dhcp (103) width yields apparent coherent domain sizes of roughly 1–3 nm, comparable to or smaller than one nine-layer Sm-type repeat. The paper concludes that the transformation goes through a stacking-disordered state and attributes the suppression to configurational disorder, local lattice distortion, stacking-fault energetics, and transformation kinetics acting together.

Load-bearing premise

The central claim rests on the diffraction experiments being able to see a bulk Sm-type phase if it had formed; if coarse grain statistics, preferred orientation, or a narrow pressure window hid its reflections, the hcp-to-dhcp-only assignment would be wrong.

Editorial extensions

If this is right

  • Both TbHoEr and TbHoDy undergo a continuous, stacking-sequence-style transformation with no volume collapse, and the fitted bulk moduli rise modestly from hcp (about 38–42 GPa) to dhcp (about 43–46 GPa).
  • Composition shifts the transformation onset: TbHoDy, with the larger Dy atom, begins transforming near 7.5 GPa, below TbHoEr's onset near 10 GPa.
  • Extended compression of TbHoEr to 70 GPa reveals a further rhombohedral hR24 phase, giving the sequence hcp to dhcp to hR24.
  • Because configurational disorder and local strain are proposed to suppress the 9R polytype, medium-entropy alloying becomes a way to control which close-packed stacking pathway a rare-earth system follows under pressure.
  • The diffuse scattering and short coherence lengths imply that intermediate states are structurally faulted, so bulk thermodynamic polytype stability alone is insufficient to predict the transformation path.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If stacking disorder is the route, then the hcp-to-dhcp transformation should be describable by a faulted-layer model, such as random intergrowth of AB and ABC or ACB fragments; simulating the resulting diffraction and matching the streak profile would quantitatively test the stacking-disorder interpretation.
  • The appearance of hR24 only in the non-hydrostatic run leaves open whether that phase is intrinsic to TbHoEr or stress-stabilized; reproducing it under a hydrostatic medium above about 50 GPa would settle that question.
  • The paper's comparison with HoDyYGdTb, where a Sm-type phase was reported, suggests configurational entropy alone is not the controlling variable; systematically varying size mismatch while keeping the configurational entropy fixed could isolate the role of local lattice distortion.
  • If suppression of 9R order is kinetic, pressure ramp rate should matter: very slow compression or annealing near the transition pressure might allow a Sm-type phase to nucleate even in these alloys, and a rate-dependent diffraction experiment could distinguish thermodynamic suppression from kinetic bypass.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 5 minor

Summary. The paper reports synchrotron X-ray diffraction measurements on two equiatomic ternary rare-earth medium-entropy alloys, TbHoEr and TbHoDy, compressed in diamond anvil cells. It finds that both transform from hcp to dhcp, at about 10 GPa and 7.5 GPa respectively, without a well-resolved Sm-type (9R) intermediate phase. 2D diffraction images show streak-like diffuse intensity in the transition region, which the authors interpret as stacking disorder. In one non-hydrostatic TbHoEr run to 70 GPa, a rhombohedral hR24 phase is reported. The authors propose that configurational disorder, local lattice distortion, stacking-fault energetics, and transformation kinetics suppress long-range Sm-type order.

Significance. If correct, the result would show that medium-entropy alloying can change the pressure-driven stacking sequence in heavy rare earths, an important design consideration for rare-earth magnetocaloric and structural materials. The paper's strengths include homogeneous EDS characterization, consistent hcp-to-dhcp indexing in two alloys and multiple pressure media, and clear presentation of equation-of-state fits. However, the central claim rests on non-observation of a 9R phase; without quantitative sensitivity limits or a stacking-fault model, the suppression conclusion remains suggestive rather than established.

major comments (3)
  1. [Section 3B, Fig. 6] The conclusion that no bulk Sm-type phase forms is an absence-of-evidence claim. The comparison with calculated Sm-type reflection positions is qualitative; no two-phase (hcp + dhcp) refinement including an R-3m 9R phase is reported, and no minimum-detectable volume fraction is given. Please report, for example, Rwp for hcp + dhcp versus hcp + dhcp + 9R models at 12.0 GPa and a detection-limit estimate based on structure factors and background, or otherwise provide a quantitative upper bound on the Sm-type fraction that could be present but unresolved.
  2. [Eq. (1), Section 3D] The Scherrer analysis is explicitly a lower-bound estimate because the measured FWHM contains strain, pressure-gradient, and deviatoric-stress broadening. The statement that Lc of about 1-3 nm is 'comparable to or smaller than' the 2.5 nm Sm-type repeat therefore cannot rule out 9R coherence; a lower bound cannot establish that the stacking coherence is insufficient for nine-layer order. A strain-separated size-strain analysis (for example, Williamson-Hall) or a Lele-type stacking-fault probability fit to the (103)/(100) profiles is needed to support that specific claim.
  3. [Section 3B, Fig. 8] The hR24 phase is identified only in the non-hydrostatic run, using Le Bail rather than full Rietveld refinement. Given the known effect of deviatoric stress on phase stability (references [33,34]), the unqualified statement in the abstract that compression to 70 GPa 'reveals a high-pressure rhombohedral hR24 phase' overstates the evidence; please qualify this phase as tentative or provide corroborating hydrostatic or quasi-hydrostatic data.
minor comments (5)
  1. [Section 1] The phrase 'lanthanides phase' should read 'lanthanide phase'.
  2. [Section 3D] The sentence 'for both alloys, for both alloys' contains a duplicated phrase and should be edited.
  3. [Section 3B, Fig. 6 caption] 'The asterisk marks Neon peak' should use lowercase 'neon peak'.
  4. [Section 2C] Please state explicitly which beamline and wavelength were used for each of the three TbHoEr pressure environments; the text and figure captions can be cross-checked, but a single consolidated statement would improve clarity.
  5. [Section 3B] For the hR24 assignment, a brief statement of the Le Bail fit quality (for example, a residual or goodness-of-fit value) would help the reader judge the indexing reliability.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity; the central claim rests on direct diffraction observations with explicitly acknowledged limitations.

full rationale

The paper's derivation chain is observational and self-contained. The hcp-to-dhcp assignment is made by unconstrained indexing and Le Bail refinement against standard space groups (Section 3B), not by feeding the desired conclusion into the analysis. The claimed suppression of Sm-type order is a null result supported by the absence of a systematic set of 9R reflections (Sections 3B and 3D) and by direct observation of streak-like diffuse scattering in the 2D images (Fig. 11). No fitted parameter is renamed as a prediction; the Birch-Murnaghan equation-of-state parameters, Scherrer domain sizes, and peak-width ratios are reported as characterizations, not as evidence derived from the Sm-type hypothesis. The Scherrer analysis is explicitly labeled a lower-bound, semi-quantitative consistency check: the paper states 'no instrumental-broadening correction was applied' and that 'these values should be regarded as semi-quantitative supporting evidence, not as rigorous stacking-fault densities.' The only self-citations (refs. [28,29] for elemental Tb and Er) are prior experimental measurements external to this paper's fitted values and are corroborated by independent refs. [27,30,31]; they supply contextual comparison, not a load-bearing derivation. No equation in the paper reduces the conclusion to its inputs. The main vulnerability—lack of a quantitative sensitivity limit for detecting a small-volume Sm-type phase, or a full two-phase refinement—is a robustness-of-evidence concern, not circularity.

Assumptions & free parameters 6 free parameters · 4 assumptions · 0 invented entities

The central claim is an absence-of-evidence argument about a phase that was not observed. The paper's free parameters are limited to standard equation-of-state fits. No new particles, forces, or entities are introduced. The main burden lies in the assumptions that the diffraction experiment would have detected a Sm-type phase and that diffuse scattering is attributable to stacking disorder.

free parameters (6)
  • Bulk modulus K0 (hcp, TbHoEr) = 41.3 GPa (averaged over neon and silicone-oil media)
    Third-order Birch-Murnaghan fit to pressure-volume data. Supports compressibility interpretation, not the stacking-order claim.
  • Bulk modulus K0 (dhcp, TbHoEr) = 45.8 GPa
    Third-order Birch-Murnaghan fit to P-V data in the dhcp phase.
  • Bulk modulus K0 (hR24, TbHoEr) = 52.0 GPa (non-hydrostatic run)
    Third-order Birch-Murnaghan fit to P-V data for the hR24 phase.
  • Bulk modulus K0 (hcp, TbHoDy) = 38.0 GPa
    Third-order Birch-Murnaghan fit to P-V data for TbHoDy.
  • Bulk modulus K0 (dhcp, TbHoDy) = 43.1 GPa
    Third-order Birch-Murnaghan fit to P-V data for the dhcp phase.
  • Pressure derivative K0' = Fixed to 3.8-4.0 in hydrostatic fits; fitted to 3.0-3.6 in non-hydrostatic fit
    Used in the Birch-Murnaghan equation of state. Values are chosen or fitted to stabilize the fit over limited pressure ranges.
assumptions (4)
  • standard math Standard diffraction theory and Le Bail refinements correctly assign observed peaks to hcp, dhcp, and hR24 structures.
    Used throughout Section 3B and 3C for phase identification. The analysis relies on conventional crystallographic software and methods.
  • domain assumption The absence of a systematic set of strong Sm-type reflections in the 1D patterns and 2D images means no bulk Sm-type phase formed.
    Entered in Section 3B and 3D. The conclusion relies on detection sensitivity and grain statistics being adequate to reveal a bulk 9R phase if present.
  • domain assumption Streak-like diffuse intensity and enhanced (103) peak broadening are caused by stacking disorder rather than grain statistics, strain, or pressure gradients.
    Section 3D discusses and partially rules out alternatives, but the interpretation remains qualitative and is not quantified against a texture model.
  • domain assumption The hR24 assignment from the non-hydrostatic run is correct despite deviatoric stress broadening.
    Section 3B presents a Le Bail fit at 60.3 GPa, but non-hydrostatic data make the indexing less certain than under hydrostatic conditions.

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Pith. "Pith review of Pressure-Induced Stacking Disorder and Suppression of Long-Range Sm-type Order in Medium-Entropy Rare-Earth Alloys." pith.science (2026). https://pith.science/paper/GDUOIWQG

@misc{pith2026260808351,
  author       = {Pith},
  title        = {Pith review of: Pressure-Induced Stacking Disorder and Suppression of Long-Range Sm-type Order in Medium-Entropy Rare-Earth Alloys},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/GDUOIWQG}},
  note         = {Machine review of arXiv:2608.08351}
}
abstract

Rare-earth medium-entropy alloys provide a platform for investigating how chemical disorder modifies the well-established pressure-induced structural evolution of close-packed $4f$ lanthanides. Here, we study TbHoEr and TbHoDy using synchrotron X-ray diffraction in diamond anvil cells. Both alloys transform from the ambient hexagonal close-packed (hcp) structure to a double hexagonal close-packed (dhcp) phase, while no well-resolved bulk Sm-type intermediate phase is observed. For TbHoEr, compression to 70 GPa further reveals a high-pressure rhombohedral hR24 phase. Unlike the constituent heavy lanthanides, however, both alloys bypass the intermediate Sm-type phase. Two-dimensional diffraction images further reveal streak-like diffuse scattering in the transition region, indicating stacking disorder and limited stacking coherence along the close-packed direction. These observations indicate that the transformation proceeds through a stacking-disordered close-packed state rather than through a well-ordered bulk Sm-type phase. We propose that configurational disorder, local lattice distortion, stacking-fault energetics, and transformation kinetics collectively suppress the development of long-range Sm-type order. The results demonstrate that medium-entropy alloying can fundamentally modify pressure-induced stacking pathways in rare-earth materials under extreme conditions.

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Works this paper leans on

38 extracted references · 38 canonical work pages

  1. [1]

    Pecharsky V K and Gschneidner Jr K A 1997 Giant Magnetocaloric Effect in Gd5(Si2Ge2) Phys. Rev. Lett. 78 4494–7

  2. [2]

    Jr K A G and Pecharsky V K 2000 Magnetocaloric Materials Annu. Rev. Mater. Res. 30 387–429

  3. [3]

    Gschneidner K A, Pecharsky V K and Tsokol A O 2005 Recent developments in magnetocaloric materials Rep. Prog. Phys. 68 1479

  4. [4]

    Franco V, Blázquez J S, Ingale B and Conde A 2012 The Magnetocaloric Effect and Magnetic Refrigeration Near Room Temperature: Materials and Models Annu. Rev. Mater. Res. 42 305–42

  5. [5]

    Gutfleisch O, Willard M A, Brück E, Chen C H, Sankar S G and Liu J P 2011 Magnetic Materials and Devices for the 21st Century: Stronger, Lighter, and More Energy Efficient Adv. Mater. 23 821–42

  6. [6]

    Coey J M D 2012 Permanent magnets: Plugging the gap Scr. Mater. 67 524–9

  7. [7]

    Ghorbani Y, Ilankoon I M S K, Dushyantha N and Nwaila G T 2025 Rare earth permanent magnets for the green energy transition: Bottlenecks, current developments and cleaner production solutions Resour. Conserv. Recycl. 212 107966

  8. [8]

    Cantor B, Chang I T H, Knight P and Vincent A J B 2004 Microstructural development in equiatomic multicomponent alloys Mater. Sci. Eng. A 375–377 213–8

Show all 38 references
  1. [9]

    125 481–9 28

    Yuan Y, Wu Y, Tong X, Zhang H, Wang H, Liu X J, Ma L, Suo H L and Lu Z P 2017 Rare-earth high-entropy alloys with giant magnetocaloric effect Acta Mater. 125 481–9 28

  2. [10]

    Tracy C L, Park S, Rittman D R, Zinkle S J, Bei H, Lang M, Ewing R C and Mao W L 2017 High pressure synthesis of a hexagonal close-packed phase of the high-entropy alloy CrMnFeCoNi Nat. Commun. 8 15634

  3. [11]

    Law J Y and Franco V 2023 Review on magnetocaloric high-entropy alloys: Design and analysis methods J. Mater. Res. 38 37–51

  4. [12]

    Lužnik J, Koželj P, Vrtnik S, Jelen A, Jagličić Z, Meden A, Feuerbacher M and Dolinšek J 2015 Complex magnetism of Ho-Dy-Y-Gd-Tb hexagonal high-entropy alloy Phys. Rev. B 92 224201

  5. [13]

    Lu S F, Ma L, Rao G H, Wang J, Du Y S, Li L, Zhao J T, Zhong X C and Liu Z W 2021 Magnetocaloric effect of high-entropy rare-earth alloy GdTbHoErY J. Mater. Sci. Mater. Electron. 32 10919–26

  6. [14]

    Alloys Compd

    Wang L, Lu Z, Guo H, Wu Y, Zhang Y, Zhao R, Jiang S, Liu X, Wang H, Fu Z, Zhao J, Ma D and Lu Z 2023 Multi-principal rare-earth Gd-Tb-Dy-Ho-Er alloys with high magnetocaloric performance near room temperature J. Alloys Compd. 960 170901

  7. [15]

    Pajerowski D M, Ostrowski K J, de La Cruz C, Tong X, Yuan Y, Wu Y and Lu Z 2019 Magnetic structure of ternary rare-earth alloy Ho1/3Tb1/3Er1/3 J. Magn. Magn. Mater. 469 315–22

  8. [16]

    13 095323

    Nakamura Y, Takeshita K, Nishizaki T and Kitagawa J 2023 High-entropy effect at rare-earth site in DyNi AIP Adv. 13 095323

  9. [17]

    Shen G and Mao H K 2017 High-pressure studies with x-rays using diamond anvil cells Rep. Prog. Phys. 80 016101

  10. [18]

    Mao H-K, Chen X-J, Ding Y, Li B and Wang L 2018 Solids, liquids, and gases under high pressure Rev. Mod. Phys. 90 015007

  11. [19]

    Status Solidi RRL – Rapid Res

    Wei B, Lin L, Zhang J, Zhan Z, Cheng Z and Jiang J 2024 In Situ Measurement Techniques Using Diamond Anvil Cell at High Pressure–Temperature Conditions: A Review Phys. Status Solidi RRL – Rapid Res. Lett. 18 2300469

  12. [20]

    Mao H K, Xu J and Bell P M 1986 Calibration of the ruby pressure gauge to 800 kbar under quasi- hydrostatic conditions J. Geophys. Res. Solid Earth 91 4673–6

  13. [21]

    Torikachvili M S, Kim S K, Colombier E, Bud’ko S L and Canfield P C 2015 Solidification and loss of hydrostaticity in liquid media used for pressure measurements Rev. Sci. Instrum. 86 123904

  14. [22]

    Dewaele A, Loubeyre P and Mezouar M 2004 Equations of state of six metals above 94 GPa Phys. Rev. B 70 094112

  15. [23]

    Prescher C and Prakapenka V B 2015 DIOPTAS : a program for reduction of two-dimensional X- ray diffraction data and data exploration High Press. Res. 35 223–30

  16. [24]

    Für Krist

    Petříček V, Palatinus L, Plášil J and Dušek M 2023 Jana2020 – a new version of the crystallographic computing system Jana Z. Für Krist. - Cryst. Mater. 238 271–82

  17. [25]

    Toby B H and Von Dreele R B 2013 GSAS-II: the genesis of a modern open-source all purpose crystallography software package J. Appl. Crystallogr. 46 544–9 29

  18. [26]

    Für Krist

    Angel R J, Alvaro M and Gonzalez-Platas J 2014 EosFit7c and a Fortran module (library) for equation of state calculations Z. Für Krist. - Cryst. Mater. 229 405–19

  19. [27]

    Yu P F, Zhang L J, Ning J L, Ma M Z, Zhang X Y, Li Y C, Liaw P K, Li G and Liu R P 2017 Pressure-induced phase transitions in HoDyYGdTb high-entropy alloy Mater. Lett. 196 137–40

  20. [28]

    Clay M P, Sereika R, Bi W and Vohra Y K 2023 Magnetic ordering in terbium at high pressures and low temperatures J. Magn. Magn. Mater. 580 170935

  21. [29]

    Clay M P, Sereika R, Higgins M K, Dos Santos A M, Molaison J J and Vohra Y K 2024 High pressure neutron diffraction study of magnetic ordering in erbium J. Magn. Magn. Mater. 598 172066

  22. [30]

    Pardo-Sainz M, Cova F, Rodríguez-Velamazán J A, Puente-Orench I, Kousaka Y, Mito M and Campo J 2023 Revisiting the magnetic structure of Holmium at high pressure by using neutron diffraction Sci. Rep. 13 12168

  23. [31]

    Mito M, Kimura Y and Campo J 2024 High pressure studies of the T − P phase diagrams of erbium and thulium up to 30 GPa by using ac magnetization experiments Phys. Rev. B 109 064414

  24. [32]

    Samudrala G K and Vohra Y K 2013 Structural Properties of Lanthanides at Ultra High Pressure Handbook on the Physics and Chemistry of Rare Earths vol 43 (Elsevier) pp 275–319

  25. [33]

    Zhang F, Lou H, Chen S, Chen X, Zeng Z, Yan J, Zhao W, Wu Y, Lu Z and Zeng Q 2018 Effects of non-hydrostaticity and grain size on the pressure-induced phase transition of the CoCrFeMnNi high-entropy alloy J. Appl. Phys. 124 115901

  26. [34]

    Zhang F, Lou H, Cheng B, Zeng Z and Zeng Q 2019 High-Pressure Induced Phase Transitions in High-Entropy Alloys: A Review Entropy 21 239

  27. [35]

    or f.c.c

    Lele S 1969 X-ray diffraction by stacking faults in h.c.p. or f.c.c. crystals Acta Crystallogr. Sect. A 25 551–2

  28. [36]

    Zhang H, McGuire M A, May A F, Chao H-Y, Zheng Q, Chi M, Sales B C, Mandrus D G, Nagler S E, Miao H, Ye F and Yan J 2024 Stacking disorder and thermal transport properties of α − RuCl 3 Phys. Rev. Mater. 8 014402

  29. [37]

    Scherrer P 1918 Bestimmung der Größe und der inneren Struktur von Kolloidteilchen mittels Röntgenstrahlen Nachrichten Von Ges. Wiss. Zu Gött. Math.-Phys. Kl. 1918 98–100

  30. [38]

    Patterson A L 1939 The Scherrer Formula for X-Ray Particle Size Determination Phys. Rev. 56 978–82

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Reviewed August 12, 2026 · model on record in the stance chip above.