부산대학교 도서관

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 $700~\mathrm{cm}^{-1}$, 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.