2, Washington University in St. Louis, Saint Louis, Washington, United States
3, Escola Universitària Salesiana de Sarrià (EUSS), Barcelona, Barcelona, Spain
Controlling the oxygen content (δ) is a key issue in the fabrication of YBa2Cu3O7-δ (YBCO) films since subtle variations in δ lead to large structural distortions and alter the number of available carriers within the superconducting CuO2 planes, which ultimately affect the superconducting properties . It is commonly accepted that oxygen vacancies are located within the CuO-chains; however, in this talk, we will present results challenging this hypothesis. We have used aberration-corrected scanning transmission electron microscopy (STEM) to show a periodic and uneven distribution of Oxygen vacancies located at the BaO planes, especially in the proximity of Y2Ba4Cu8-γO16-d (Y124) intergrowths, the most common defect in YBCO. The presence of these Oxygen vacancies triggers structural changes in the superconducting planes reducing the symmetries of the YBa2Cu3O7-x unit cell: the Oapical-Cu spacing increases as well as the O-Cu-O angle of the CuO2 planes. Our results are supported by first-principles density-functional theory calculations, which show comparable formation energy of oxygen vacancies in Cu-O chains and in Ba-O planes. Additionally, previous results showed that Y248 intergrowths comes along with Cu and Oxygen vacancies buried within the very same planar defect, which in turn induce a dilute ferromagnetism in the superconducting state of the YBCO . We will discuss the implications of all the observed distortions on the YBCO superconducting properties.
 Jorgensen J D, Veal B W, Paulikas A P, Nowicki L J, Crabtree G W, Claus H and Kwok W K 1990 Structural properties of oxygen-deficient YBa2Cu3O7- Phys. Rev. B 41 1863–77
 Gazquez J, Guzman R, Mishra R, Bartolomé E, Salafranca J, Magén C, Varela M, Coll M, Palau A, Valvidares S M, Gargiani P, Pellegrin E, Herrero-Martin J, Pennycook S J, Pantelides S T, Puig T and Obradors X 2016 Emerging Diluted Ferromagnetism in High- T c Superconductors Driven by Point Defect Clusters Adv. Sci. 3 1500295
Authors acknowledge the MICIN (NANOSELECT, CSD2007-00041 and MAT2014-51778-C2-1-R), Generalitat de Catalunya (2014SGR 753 and Xarmae), and the EU (EU-FP7 NMP-LA-2012-280432 EUROTAPES project, Cost Action MP1201 and CA16218 NANOCOHYBRI). They also acknowledge MINECO for the Center of Excellence award Severo Ochoa (SEV-2015-0496).