  We have investigated the phonon and the magnetic excitations in LaCoO3 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 LaCoO3 in the unit cell. We observed two transverse optical (TO)
phonon branches along (delta, delta, delta), consistent with previously
reported Raman active Eg modes which show remarkable softening associated with
the spin-state transition [Ishikawa et al., (Phys. Rev. Lett. 93 (2004)
136401.)]. 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 > 15 meV,
consistent with excitation of the nonmagnetic low-spin to magnetic
intermediate-spin state of Co 3+ ions.
