In operando evidence of deoxygenation in ionic liquid gating of YBa ₂ Cu ₃ O _7-X

Significance In this work, we investigate the origin of the charge induced in a high-temperature superconducting cuprate film, incorporated in an electric double-layer transistor. Using X-ray spectroscopic measurements made while operating the transistor, together with simulations based on density functional theory, we find that the accumulated charge in the cuprate is due to the depletion of oxygen from specific sites in its unit cell. These results constitute direct evidence of the microscopic mechanism of charge modification. The systematic control of the oxygen content in complex oxides such as the cuprates allows for the study of complex phase diagrams and opens up a route for the design of new complex oxide compounds and devices with improved functionalities.