Multi-spin-state dynamics during insulator-metal crossover in LaCoO

Doi, Atsunori*; Fujioka, Jun*; Fukuda, Tatsuo  ; Tsutsui, Satoshi*; Okuyama, Daisuke*; Taguchi, Yasujiro*; Arima, Takahisa*; Baron, A. Q. R.*; Tokura, Yoshinori*

The phonon dynamics of spin-state crossover for perovskite LaCoO was investigated using infrared and inelastic X-ray spectroscopy. We observed that the Co-O bond stretching phonons couples to the thermally excited spin-state. The momentum dispersion of the optical mode is anomalously dispersionless (less than 3 meV), and this supports the spin-state disproportionation scenario. The steep enhancement of spin-state fluctuation compared to the charge gap suggests the density dependent spin state disproportionation; a simple low spin (LS)/high spin (HS)- or LS/intermediate spin (IS)-state disproportionation is realized at the initial stage with dilute HS- (or IS-) state density, while the complex spin-state disproportionation involving all of the LS-, IS- and HS-state is formed when the density of HS- (or IS-) state is enhanced.