Development of the optical conductivity with doping in single-domain YBa2Cu3O6+x.

Optical studies of single-domain crystals of YBa{sub 2}Cu{sub 3}O{sub 6+{ital x}} allow us to separate the CuO{sub 2}-plane and CuO-chain contributions to the optical conductivity as a function of doping between 0.025 and 5.5 eV. We find several interesting consequences of doping on the optical conductivity of YBa{sub 2}Cu{sub 3}O{sub 6+{ital x}}: the 4.1-eV transition in tetragonal YBa{sub 2}Cu{sub 3}O{sub 6.1} is split into {ital a}- and {ital b}-axis-polarized components by CuO-chain formation; oscillator strength associated with the charge-transfer band of insulating YBa{sub 2}Cu{sub 3}O{sub 6.1} is redistributed to low frequencies; and the low-frequency conductivity in the CuO{sub 2} planes evolves into a non-Drude response comprised of a single resolvable component. The CuO-chain conductivity, by contrast, is dominated by a midinfrared absorption band.