Photoconductivity in insulating YBa2Cu3O6+x: From Mott-Hubbard insulator to Fermi glass via oxygen doping.

Abstract

Photoconductivity, ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{ph}}$(\ensuremath{\omega}), and optical conductivity, \ensuremath{\sigma}(\ensuremath{\omega}), are compared for insulating ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$ (x0.4) in the photon energy range from 0.6 to 3.3 eV. With x\ensuremath{\approxeq}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 ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{ph}}$(\ensuremath{\omega}) and \ensuremath{\sigma}(\ensuremath{\omega}) 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\ensuremath{\approxeq}0.3, the absorption observed throughout the infrared has no counterpart in ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{ph}}$(\ensuremath{\omega}); the photoconductivity turns on near 2 eV. In addition, thermally activated behavior is observed for the 1.75-eV band in ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{ph}}$(\ensuremath{\omega}). 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\ensuremath{\approxeq}0.3).