XY magnetism, Kitaev exchange, and long-range frustration in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>J</mml:mi><mml:mi>eff</mml:mi></mml:msub><mml:mo>=</mml:mo><mml:mfrac><mml:mn>1</mml:mn><mml:mn>2</mml:mn></mml:mfrac></mml:math> honeycomb cobaltates
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Bibliographic record
Abstract
The quest for Kitaev quantum spin liquids has led to great interest in honeycomb quantum magnets with strong spin-orbit coupling. It has been recently proposed that even Mott insulators with $3d$ transition-metal ions, having nominally weak spin-orbit coupling, can realize such exotic physics. Motivated by this, we study the rhombohedral honeycomb cobaltates ${\mathrm{CoTiO}}_{3},$ ${\mathrm{BaCo}}_{2}{({\mathrm{PO}}_{4})}_{2}$, and ${\mathrm{BaCo}}_{2}{({\mathrm{AsO}}_{4})}_{2}$, using ab initio density-functional theory, which takes into account realistic crystal-field distortions and chemical information, in conjunction with exact diagonalization numerics. We show that these ${\mathrm{Co}}^{2+}$ magnets host ${J}_{\mathrm{eff}}=1/2$ local moments with highly anisotropic $g$ factors, and we extract their full spin Hamiltonians including longer-range and anisotropic exchange couplings. For ${\mathrm{CoTiO}}_{3}$, we find a nearest-neighbor easy-plane ferromagnetic $XXZ$ model with additional bond-dependent anisotropies and interlayer exchange, which supports three-dimensional Dirac nodal line magnons. In contrast, for ${\mathrm{BaCo}}_{2}{({\mathrm{PO}}_{4})}_{2}$ and ${\mathrm{BaCo}}_{2}{({\mathrm{AsO}}_{4})}_{2}$, we find a strongly suppressed interlayer coupling, and significant frustration from additional third-neighbor antiferromagnetic exchange mediated by P or As. Such bond-anisotropic ${J}_{1}\text{\ensuremath{-}}{J}_{3}$ spin models can support collinear zigzag or coplanar spiral ground states. We discuss their dynamical spin correlations which reveal a gapped Goldstone mode and argue that the effective parameters of the pseudospin-$1/2$ models in these two materials may be strongly renormalized by coupling to a low-energy spin exciton. Our results call for reexamining proposals for realizing Kitaev spin liquids in the honeycomb cobaltates.
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Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.002 | 0.001 |
| Meta-epidemiology (narrow) | 0.002 | 0.003 |
| Meta-epidemiology (broad) | 0.001 | 0.003 |
| Bibliometrics | 0.000 | 0.003 |
| Science and technology studies | 0.002 | 0.002 |
| Scholarly communication | 0.002 | 0.002 |
| Open science | 0.003 | 0.003 |
| Research integrity | 0.001 | 0.003 |
| Insufficient payload (model declined to judge) | 0.024 | 0.005 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from it