Bernat Mundet1 Jaume Gázquez1 Roger Guzman1 Steven Hartman2 Guangfu Luo2 Rohan Mishra2 Elena Bartolome3 Mariona Coll1 Anna Palau1 Teresa Puig1 Xavier Obradors1

1, Institute of Material Science of Barcelona (ICMAB), Bellaterra, Barcelona, Spain
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 [1]. 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 [2]. We will discuss the implications of all the observed distortions on the YBCO superconducting properties.

[1] 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
[2] 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).