Novel Photo-excited State and Femtosecond Dynamics in Perovskite-type Cobalt Oxides

We performed femtosecond reflection spectroscopy on a series of perovskite-type cobalt oxide, R BaCo 2 O 6- d ( R = Sm, Gd, and Tb). The transient reflectivity as well as the optical conductivity just after photoirradiation shows ultrafast change within the time resolution (ca. 120 fs) at room temperature, implying appearance of a hidden state different from the high temperature phase. The transferred spectral weight in the optical conductivity by the photoexcitation sensitively depends on the R -species. i.e ., transfer of the d electron. Recent theoretical treatment which quantitatively succeeded in reproducing the transfer dependence of the excited state indicates that the photoirradiation causes locally ferromag-netic state via double-exchange interaction between the injected hole and spins of Co ion, which can be viewed as a novel example of photoinduced phase transition.