Optical investigation of the metal-insulator transition regime in single-domain crystals of YBa2Cu3Ox

Abstract We have determined the polarized reflectance spectra of YBa 2 Cu 3 O x crystals with single-domain (001) surfaces. The oxygen content of the crystals was varied between the superconducting limit x = 7 and the semiconducting limit x = 6. By fitting a Lorentz—Drude model as well as by performing a Kramers—Kronig transformation the spectral distribution of the in-plane tensor components of the cnergy loss function and of the conductivity function were obtained. Using the in-plane anisotropy of the optical properties we were able to separate the contribution of the CuO layers and of the CuO chains to the electronic transport properties. An analysis of the free-carrier concentration shows that there is no sharp metal—insulator transition if x is varied from x = 7 to x = 6. Rather, we find a broad transition regime extending from x = 6.5 to x = 6, where different consecutive localization processes of the free carriers are observed.