Ultrafast Real Space Dynamics of Photoexcited State in a Layered Perovskite-Type Spin Crossover Oxide La1.5Sr0.5CoO4

作者：Okimoto Yoichi^*; Egawa Tsugumi; Fukaya Ryo; Matsubara Yoshitaka; Yamada Yuki; Yamaya Nao; Ishikawa Tadahiko; Onda Ken; Koshihara Shin ya; Taniguchi Hiroki; Itoh Mitsuru; Isayama Akira; Sasagawa Takao

来源：Journal of the Physical Society of Japan, 2013, 82(7): 074721.

DOI：10.7566/JPSJ.82.074721

• femtosecond spectroscopy
• spin crossover
• photoinduced phase transition

Ultrafast optical responses were investigated using a fs laser system on a layered perovskite-type cobalt oxide, La1.5Sr0.5CoO4 with a checkerboard-type charge ordering. To investigate the spatial variation of the photoexcited state, we measured the transient changes of three independent optical quantities, i.e., transmittance, reflectivity, and backside reflectivity, as well as their fluence dependence at 0.25 eV. Using the numerical calculations based on Maxwell%26apos;s equations, we calculated not only the dielectric constant but also the decay length of the photoexcited region (L) and analyzed the real-space dynamics of the photoexcited area in terms of the fluence and time dependence of L. With the above information on the time dependence of L and Kramers-Kronig analysis, we numerically calculated the transient optical conductivity spectra, which showed instantaneous formation of a polaron-like absorption peak in the mid-infrared region. A 10 fs pump-probe reflection measurement revealed that the photoexcited state occurs within the time resolution (approximate to 10 fs). The time profile of the reflectivity is accompanied with an oscillation assigned as a breathing phonon mode in the CoO2 layer observed in Raman spectroscopy. Analyses of the decay time and the frequency revealed that the photoirradiation melted the charge ordering in La1.5Sr0.5CoO4.

• 出版日期2013-7