Localization of charge carriers in the c-axis electronic transport of YBa2Cu3Oχ

Reflectance measurements with E c on a YBa₂Cu₃O_x single crystal where the oxygen content was varied between x = 6.8 and X = 6.98 are presented in the far infrared range (5 rmmeV < ω < 100 meV) at temperatures between 10 K and 300 K. Performing a Kramers-Kronig transformation we derived the optical conductivity δ _c ( ω). An analysis of the spectral weight shows that the main contribution of the electronic transport is due to localized carriers. With decreasing the oxygen content, the spectral weight of free carriers decreases whereas the spectral weight of localized carriers is enhanced.