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

Abstract

Optical studies of single-domain crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$ allow us to separate the ${\mathrm{CuO}}_2$-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 ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$: the 4.1-eV transition in tetragonal ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.1}$ is split into a- and b-axis-polarized components by CuO-chain formation; oscillator strength associated with the charge-transfer band of insulating ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.1}$ is redistributed to low frequencies; and the low-frequency conductivity in the ${\mathrm{CuO}}_2$ 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.