ELECTRONIC-STRUCTURE OF INSULATING YBA2CU3O6- FROM MOTT-HUBBARD INSULATOR TO FERMI GLASS VIA OXYGEN DOPING(X )

Photoconductivity, sigma(ph)(omega), and optical conductivity, sigma(o mega), are compared for insulating YBa2Cu36+x (x<0.4) for HomegaBAR=0. 6-3.3 eV. With x almost-equal-to 0, there is an energy gap with weak s pectral features at 1.5 eV and 2.1 eV, in addition to the well-known 1 .75 eV and 2.6 eV bands. The sigma(ph)(omega) and sigma(omega) coincid e at the band edge; no significant exciton binding energy is observed. The spectral gap is consistent with the electronic structure of a Mot t-Hubbard insulator with a charge transfer (CT) gap between the 0 2p b and and the Cu 3d upper Hubbard band. For x almost-equal-to 0.3, the b road sub-gap absorption induced by oxygen doping has no counterpart in sigma(ph)(omega); sigma(ph)(omega) turns on near 2 eV. In addition, t hermally-activated behavior is observed for the 1.75 eV band in sigma( ph)(omega). We conclude that upon doping, the states involved in trans itions below 2 eV become localized. Random distribution of oxygen-ions at O(1) site-s 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 almost-equal-to 0.3).