Optical Study of Metal-Insulator Transition in
SmBaCo₂O_6-δ Single Crystal

Abstract

The optical reflectivity spectrum and its temperature dependence were measured on a cleaved (001) surface of SmBaCo₂O_5.6. With an increase in temperature, the reflectivity below 1eV increases up to the metal-insulator transition temperature, T_MI=360K, and saturates above T_MI. The spectrum of optical conductivity σ(ω) shows large variation in temperature which accompanies the transfer of spectral weight between a broad peak around 3eV, which can be assigned to the O_2p–Co_3d charge-transfer excitation, and the incoherent excitation of carriers below 1eV. The transferred spectral weight is as large as 0.17 per Co ion, which implies that the effective mass of an electron, m^*, can be estimated to be 6m₀. The optical gap of 0.2eV at 20K is consistent with the resistivity measurement.