Interplay of broken symmetry and delocalized excitations in the insulating state of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>1</mml:mn><mml:mi>T</mml:mi></mml:mrow><mml:mo>−</mml:mo><mml:mrow><mml:mi>Ta</mml:mi><mml:msub><mml:mi mathvariant="normal">S</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>
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Bibliographic record
Abstract
Coexistence of localized and extended excitations is central to the macroscopic properties of correlated materials. For $5d$ transition-metal compounds, electron correlations alone generally do not lead to a metal-insulator (Mott) transition, with insulating behavior usually resulting from their coupling with magnetic ordering and/or structural distortions. $1T\ensuremath{-}\mathrm{Ta}{\mathrm{S}}_{2}$ is a prototypical example of such correlated insulating behavior, with a high-symmetry metallic phase transforming into a distorted, charge-density wave (CDW) insulating state at low temperatures. The nature of the insulating phase as well as the existence and relevance of the localized electron physics remains debated. We resolve this standing controversy in $1T\ensuremath{-}\mathrm{Ta}{\mathrm{S}}_{2}$ combining resonant inelastic x-ray spectroscopy and first-principles calculations. We observe five electronic excitations arising from the interband transitions of the Ta $5d$ orbitals and the S $3p$ ligand state, with none of the excitations on the order of the Mott gap. These excitations cannot be explained within the framework of standard multiplet calculations that assume a localized wave function, but instead, are captured by a band-theory framework accounting for the low symmetry of the crystal field in the CDW state. Our findings suggest that the electronic properties of $1T\ensuremath{-}\mathrm{Ta}{\mathrm{S}}_{2}$ are dominated in the visible by both plasmonic quasiparticles and interband transitions, with no resonance associated with a putative Mott transition observed in the 0--3 eV energy range. Our discovery provides insights into the electron localization and the Mott vs band-insulator debate in $1T\ensuremath{-}\mathrm{Ta}{\mathrm{S}}_{2}$ and other transition-metal materials.
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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 | 0.004 | 0.006 |
| Meta-epidemiology (narrow) | 0.001 | 0.002 |
| Meta-epidemiology (broad) | 0.001 | 0.003 |
| Bibliometrics | 0.001 | 0.003 |
| Science and technology studies | 0.002 | 0.003 |
| Scholarly communication | 0.001 | 0.002 |
| Open science | 0.004 | 0.003 |
| Research integrity | 0.001 | 0.002 |
| Insufficient payload (model declined to judge) | 0.197 | 0.002 |
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