  We recently demonstrated that the superconductor-to-insulator transition
induced by ionic liquid gating of the high temperature superconductor YBa2Cu3O7
(YBCO) is accompanied by a deoxygenation of the sample [Perez-Munoz et al.,
PNAS 114, 215 (2017)]. DFT calculations helped establish that the pronounced
changes in the spectral features of the Cu K-edge absorption spectra measured
in situ during the gating experiment arise from a decrease of the Cu
coordination within the CuO chains. In this work, we provide a detailed
analysis of the electronic structure origin of the changes in the spectra
resulting from three different types of doping: i) the formation of oxygen
vacancies within the CuO chains, ii) the formation of oxygen vacancies within
the CuO2 planes and iii) the electrostatic doping. For each case, three
stoichiometries are studied and compared to the stoichiometric YBa2Cu3O7, i.e
YBa2Cu3O6.75, YBa2Cu3O6.50 and YBa2Cu3O6.25. Computed vacancy formation
energies further support the chain-vacancy mechanism. In the case of doping by
vacancies within the chains, we study the effect of oxygen ordering on the
spectral features and we clarify the connection between the polarization of the
x-rays and this doping mechanism. Finally, the inclusion of the Hubbard U
correction on the computed spectra for antiferromagnetic YBa2Cu3O6.25 is
discussed.
