LOCALIZATION OF CHARGE-CARRIERS IN THE C-AXIS ELECTRONIC TRANSPORT OFYBA2CU3OX

Reflectance measurements with E parallel to c on a YBa2Cu3Ox single cr ystal where the oxygen content was varied between x = 6.8 and x = 6.98 are presented in the far infrared range (5 meV < (h) over bar omega < 100 meV) at temperatures between 10 K and 300 K. Performing a Kramers -Kronig transformation we derived the optical conductivity sigma(paral lel to c)((h) over bar omega). An analysis of the spectral weight show s that the main contribution of the electronic transport is due to loc alized carriers. With decreasing the oxygen content, the spectral weig ht of free carriers decreases whereas the spectral weight of localized carriers is enhanced.