Picosecond dynamics in layered cobalt perovskites studied by time-resolved Raman spectroscopy

Picosecond dynamics of the photoexcited charge-ordered perovskite-type cobalt oxides La 1.5 Ca 0.5 CoO 4 and La 1.5 Sr 0.5 CoO 4 were investigated based on time-resolved Raman and reflection spectroscopy. These cobalt oxides show strong Raman scattering at around 700cm -1 , originating from the breathing vibration mode of the CoO 6 octahedron coupled with the checkerboard-type charge ordering of Co 2+ and Co 3+ . We performed time-resolved Raman and reflection spectroscopy by photoirradiating with picosecond pulses at 400 nm, exciting the charge transfer (CT) transition from the O 2p site to the Co 3d site. As a result of the excitation, the Raman intensity of the breathing mode was suppressed by 10% on a similar time scale in the reflectance change without changing the frequency of the mode, directly indicating ultrafast suppression of the order parameter of the charge-ordered state by CT transition.