Phonons reflect dynamic spin-state order in LaCoO$_3$

TL;DR

This study reveals that in LaCoO$_3$, phonon anomalies at specific temperatures and wave vectors are linked to dynamic spin-state fluctuations, providing momentum-resolved evidence of high-spin and low-spin state correlations.

Contribution

It demonstrates the connection between phonon softening and dynamic spin-state order in LaCoO$_3$, supported by inelastic scattering and ab-initio calculations.

Findings

Anomalous phonon softening occurs between 100K and 550K.

Phonon anomalies are localized at the wave vector associated with spin-state order.

Results provide momentum-resolved evidence of dynamic high-spin and low-spin correlations.

Abstract

We investigate lattice dynamics in LaCoO$_3$ using inelastic neutron and x-ray scattering over $T = 2\mbox{-}650\,\mathrm{K}$, spanning the spin-state crossover at $T_{1} \approx 100\,\mathrm{K}$ and the insulator--metal transition at $T_{2} \approx 550\,\mathrm{K}$. Comparison with quasi-harmonic $ab-initio$ lattice-dynamical calculations helps reveal anomalous softening of a $\approx 10\,\mathrm{meV}$ oxygen phonon, confined to the temperature interval $T_{1} \leq T \leq T_{2}$ and localized in momentum space at $\boldsymbol{q}_{\mathrm{SSO}} = \left( \frac{1}{2},\frac{1}{2},\frac{1}{2} \right)_{c}$. This wave vector corresponds to the spin-state ordering originally proposed by Goodenough [J. Phys. Chem. Solids 6, 287-297 (1958)]. Our results therefore provide momentum-resolved evidence for dynamic correlations of high-spin and low-spin Co$^{3+}$ states in LaCoO$_{3}$, linking spin-state fluctuations to anomalous phonon renormalization.