Orbital fluctuations and spin-orbital-lattice coupling in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>Bi</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mi>Fe</mml:mi> <mml:mn>4</mml:mn> </mml:msub> <mml:msub> <mml:mi mathvariant="normal">O</mml:mi> <mml:mn>9</mml:mn> </mml:msub> </mml:mrow> </mml:math>
Bibliographic record
Abstract
Magnetic frustrations and degeneracies profoundly affect ground-state magnetic properties emerging from competing exchange interactions. Controlling such frustrations using orbital and phonon engineering via the Kugel-Khomskii-type (KK-type) interactions has recently enabled the orbital enhancement of magnetoelectric (ME) coupling. Using combined spectroscopic techniques and first-principle simulations, here we demonstrate that the magnetically frustrated Cairo lattice, ${\mathrm{Bi}}_{2}{\mathrm{Fe}}_{4}{\mathrm{O}}_{9}$, exhibits a strong KK-type interaction resulting in a coupled spin-orbital phase below 1.8 times the N\'eel temperature (${T}_{\mathrm{N}}\ensuremath{\sim}245\phantom{\rule{0.16em}{0ex}}\mathrm{K}$). We observe an order of magnitude change in phonon linewidths that is not explainable considering spin-phonon coupling channels alone. Instead, the observed change is reminiscent of orbitally active materials, which we explicitly confirm by measuring the $T$ dependence of low-energy orbital excitations. We further find that ${\mathrm{Bi}}_{2}{\mathrm{Fe}}_{4}{\mathrm{O}}_{9}$ harbors an unstable polar mode, driving the lattice to a symmetry-lowered ferroelectric (FE) phase below ${T}_{\mathrm{N}}$, in line with the previously reported hysteresis in polarization. Nonetheless, the FE phase leads to extremely small calculated superlattice Bragg peak intensities that are yet to be experimentally confirmed. Moreover, thermal conductivity measurements do not show any measurable effect of KK-type interactions on thermal transport across ${T}_{\mathrm{N}}$. However, we observe a repeatable anomaly at $\ensuremath{\sim}57\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ appearing only in the heating cycle, which co-occurs with the $\ensuremath{\sim}400\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ broad continuum observed in Raman measurements. The observed KK-type interaction in ${\mathrm{Bi}}_{2}{\mathrm{Fe}}_{4}{\mathrm{O}}_{9}$ provides an opportunity for orbital enhancement of ME coupling by phonon control of superexchange interactions.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.023 | 0.001 |
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 itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".