Changes in Optical Absorption with Carrier Density in High T sub c Superconductors

We have measured the reflectivity of crystal plates of Ba sub 2 YCu sub 3 O sub x , for x between 6 and 7, for temperatures, T, between 10K and 300K, and for frequencies, nu, between 30 and 16000 cm sup -1 (0.004 to 2 eV). We find that the conductivity spectrum as a whole can be separated into a T-dependent part at low nu and an essentially T-independent part at nu above about 430 cm sup -1 (50 meV). Both parts of the spectrum increase with x, but in different ways, suggesting that the low nu part is due to free carriers with density varying like (x-6.5), and the nigh nu part is due to non-conducting excitations.