Prediction and Direct Observation of Apical Oxygen Vacancies in YBCO

Prior study of the high-temperature superconductor YBa2Cu3O7-x (YBCO) has revealed the fundamental role of oxygen vacancies in controlling the material’s superconductivity. At low oxygenation (x > 0.5), the vacancies are ordered in the CuO chains, and nearly all studies of the vacancies in YBCO, even for high oxygenation (x » 0) have focused on the chain vacancy for this reason. However, optimally doped YBCO (x = 0.06) has relatively few vacancies, and these do not form ordered patterns, which are easy to detect using diffraction techniques. In this work, we use first-principles calculations and scanning transmission electron microscopy (STEM) to investigate vacancy formation in optimally doped YBCO. Our calculations show that at optimal doping, vacancy formation at the apical site is equally favorable to chain vacancy formation. To confirm this prediction, we use the atomic resolution of STEM to provide direct observation of these apical vacancies under certain experimental conditions. We also characterize the effects of apical vacancies on YBCO’s electronic properties, using the calculated density of electronic states and experimental electron energy loss spectroscopy.