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$_{6-\delta}$ ($R$=Sm, Gd, and Tb) crystals, in which the electronic transfer was controlled by $R$.   The transient reflectivity and the optical conductivity ($\sigma^{\rm PI}(\omega)$) 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 $\sigma^{\rm PI}(\omega)$ upon photoexcitation sensitively depended on the $R$-species, indicating an important role of electronic correlation in the photoexcited state. 
Y. Okimoto, T. Miyata, M.S. Endo, M. Kurashima, K. Onda, T. Ishikawa,   S. Koshihara, M. Lorenc, E. Collet, H. Cailleau, and...
cond-mat.str-el