Photoconductivity in insulating ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$: From Mott-Hubbard insulator to Fermi glass via oxygen doping

Photoconductivity, ${\mathrm{\sigma }}_{\mathrm{ph}}$(ω), and optical conductivity, σ(ω), are compared for insulating ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$ (x<0.4) in the photon energy range from 0.6 to 3.3 eV. With x≊0, there is an energy gap with weak spectral features at 1.5 and 2.1 eV, in addition to the well-known 1.75 and 2.7 eV bands. The coincidence between ${\mathrm{\sigma }}_{\mathrm{ph}}$(ω) and σ(ω) at the band edge implies the photogeneration of separated charge carriers; no significant exciton binding energy is observed. The spectral gap in stoichiometric ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_6$ is consistent with the electronic structure of a Mott-Hubbard insulator with a well-defined energy gap between the filled O 2p band and the empty Cu 3d upper Hubbard band. The 1.5-eV feature determines the lowest-energy interband transition. Oxygen doping into the O(1) sites results in a major change in electronic structure. For x≊0.3, the absorption observed throughout the infrared has no counterpart in ${\mathrm{\sigma }}_{\mathrm{ph}}$(ω); the photoconductivity turns on near 2 eV. In addition, thermally activated behavior is observed for the 1.75-eV band in ${\mathrm{\sigma }}_{\mathrm{ph}}$(ω). We conclude that upon doping, the states involved in transitions below 2 eV become localized. The data imply that the random distribution of oxygen ions at O(1) sites causes a change of electronic structure from a Mott-Hubbard insulator with a well-defined interband charge-transfer energy gap (at x=0) to a Fermi glass (at x≊0.3).