Local strain-driven migration of oxygen vacancies to apical sites in YBa₂Cu₃O_7-x

It is well known that in the high-temperature superconductor YBa₂Cu₃O_7-x (YBCO), oxygen vacancies (V_O) control the carrier concentration, its critical current density and transition temperature. In this work, it is revealed that V_O also allows the accommodation of local strain fields caused by large-scale defects within the crystal. We show that the nanoscale strain associated with Y₂Ba₄Cu₈O₁₆ (Y124) intergrowths - that are common defects in YBCO - strongly affect the venue and concentration of V_O. Local probe measurements in conjunction with density-functional-theory calculations indicate a strain-driven reordering of V_O from the commonly observed CuO chains towards the bridging apical sites located in the BaO plane and bind directly to the superconducting CuO₂ planes. Our findings have strong implications on the physical properties of the YBCO, as the presence of apical V_O alters the transfer of carriers to the CuO₂ planes, confirmed by changes in the Cu and O core-loss edge probed using electron energy loss spectroscopy, and creates structural changes that affect the Cu-O bonds in the superconducting planes. In addition, the revelation of apical V_O also has implications on modulating critical current densities and enhancing vortex pinning.