Type: Article
Publication Date: 2005-11-01
Citations: 35
DOI: https://doi.org/10.1103/physrevb.72.174405
We have investigated the phonon and the magnetic excitations in ${\mathrm{LaCoO}}_{3}$ by inelastic neutron scattering measurements. The acoustic phonon dispersions show some characteristic features of the folded Brillouin zone (BZ) for the rhombohedrally distorted perovskite structure containing two chemical formula units of ${\mathrm{LaCoO}}_{3}$ in the unit cell. We observed two transverse optical (TO) phonon branches along $(\ensuremath{\delta}\phantom{\rule{0.3em}{0ex}}\ensuremath{\delta}\phantom{\rule{0.3em}{0ex}}\ensuremath{\delta})$, consistent with previously reported Raman active ${E}_{g}$ modes which show remarkable softening associated with the spin-state transition [Ishikawa et al., Phys. Rev. Lett. 93, 136401 (2004)]. We found that the softening takes place in the TO mode over the whole BZ. In contrast, the acoustic phonons show no anomalous softening associated with the spin-state transition. The low-energy paramagnetic scattering at 8 K is weak, increasing towards a maximum at $E\ensuremath{\gtrsim}15\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, consistent with excitation of the nonmagnetic low-spin to magnetic intermediate-spin state of ${\mathrm{Co}}^{3+}$ ions.