Optical study of metal-insulator transition in SmBaCo2O6-δ single crystal

The optical reflectivity spectrum and its temperature dependence were measured on a cleaved (001) surface of SmBaCo2O5.6. With an increase in temperature, the reflectivity below 1 eV increases up to the metal-insulator transition temperature, T-MI = 360 K, and saturates above T-MI. The spectrum of optical conductivity sigma(omega) shows large variation in temperature which accompanies the transfer of spectral weight between a broad peak around 3 eV; which can be assigned to the O-2p-CO3d charge-transfer excitation, and the incoherent excitation of carriers below 1 eV, 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 6m(0). The optical gap of 0.2 eV at 20 K is consistent with the resistivity measurement.