Electronic-Structure of 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) for ħω = 0.6−3.3 eV. With x≈0, there is an energy gap with weak spectral features at 1.5 eV and 2.1 eV, in addition to the well-known 1.75 eV and 2.6 eV bands. The σph(ω) and σ(ω) coincide at the band edge; no significant exciton binding energy is observed. The spectral gap is consistent with the electronic structure of a Mott-Hubbard insulator with a charge transfer (CT) gap between the 0 2p band and the Cu 3d upper Hubbard band. For x≈0.3, the broad sub-gap absorption induced by oxygen doping has no counterpart in σph(ω); σph(ω) 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. Random distribution of oxygen-ions at 0(1) sites introduces disorder and causes a change of electronic structure from a Mott-Hubbard insulator with a CT energy gap (at x = 0) to a Fermi glass (at x≈0.3).