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Ultrafast Spectral Weight Transfer in RBaCo2O6-delta (R=Sm, Gd, and Tb): A Role of Electronic Correlation in Photoinduced Phase Transition

We performed femtosecond reflection spectroscopy on a series of perovskite-type cobalt oxide $R$BaCo$_2$O$_{6-δ}$ ($R$=Sm, Gd, and Tb) crystals, in which the electronic transfer was controlled by $R$. The transient reflectivity and the optical conductivity ($σ^{\rm PI}(ω)$) obtained by Kramers-Kronig analysis showed an ultrafast change within a time resolution ($\approx 150$ fs) at room temperature and the appearance of signals of a hidden state different from the high temperature metallic state. The transferred spectral weight in $σ^{\rm PI}(ω)$ upon photoexcitation sensitively depended on the $R$-species, indicating an important role of electronic correlation in the photoexcited state.