Phonon study by neutron scattering on the orbital-order of the intermediate-spin-state of Co^<3+> and the spin-state transition

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Co^<3+> ions in LaCoO_3 exhibit spin-state transition from the low-spin (LS, S=0) to an intermediate-spin (IS, S=1) state around 100 K. The purpose of the present study, described in the proposal, was to clarify the orbital-order of the Jahn-Tellar active Co^<3+> in the IS state through phonons observed by neutron scattering. The neutron measurement was completed in 2005, and the study was extended to the ultrasonic measurement to investigate lattice anomalies associated with the spin-state transition.In the neutron study, the acoustic (A) and two transverse optical (TO) modes along [δ,δ,δ] were observed. We found that the TO modes show remarkable softening over the whole Brillouin zone associated with the spin-state transition. The softening is most pronounced at the R-point of the pseudo-cubic reciprocal lattice, and the result at the R-point is consistent with the E_g modes observed in the Raman spectroscopy. In contrast, the acoustic phonons both in the LA and TA modes show no anom … More alous softening associated with the spin-state transition.In the ultrasonic measurements, a remarkable slowing down of the LA sound velocity was observed associated with the spin-state transition around 100 K. The slowing down is interpreted as due to the response of the spin-state population to the sound pressure. Above 150 K, the sound velocity depends on the sound frequency. The relaxation rate of the lattice evaluated based on the Debye model has an activation-type temperature dependence with the attempt frequency of 187x10^6 s^<-1> and the energy of 11 meV. This slow lattice relaxation is attributed to the "slow" response of the spin-state population to the sound in the temperature region where the orbital-order of the IS state Co atoms is established since the response is suppressed to the rate of the orbital-order fluctuation.In summary, the present study first reveals the orbital fluctuation of the Co atoms in IS state, and also the strong coupling of the spin-state and the lattice. Less