Photoconductivity in Insulating YBa2Cu3O6+x: From Mott-Hubbard Insulator to Fermi Glass via Oxygen Doping

Abstract

Photoconductivity, σph(ω), and optical conductivity, σ(ω), are compared for insulating YBa2Cu3O6+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 σph(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 YBa2Cu3O6 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 σph(ω); the photoconductivity turns on near 2 eV. In addition, thermally activated behavior is observed for the 1.75-eV band in σph(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).