Electronic structure of insulating YBa 2Cu 3O 6+ x: From Mott-Hubbard insulator to Fermi glass via Oxygen doping

Abstract

Photoconductivity, σ ph(ω), and optical conductivity, σ(ω), are compared for insulating YBa 2Cu 3O 6+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 σ ph(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 σ ph(ω); σ ph(ω) turns on near 2 eV. In addition, thermally-activated behavior is observed for the 1.75 eV band in σ ph(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).