Ms. Lucía Iglesias (PhD Student)
The control and tune of the cationic and anionic defects formation is a novel route to discover new functional properties in transition metal oxides in general, and particularly in SrTiO3 . STO is a diamagnetic quantum paraelectric insulator, in which oxygen vacancies are typical anionic defects. Due to the donor character of the VO and the very large electron mobilities of these defects, even the slightest concentration of vacancies produces a measurable electrical conductivity. Thus, the presence of VO plays a very important role in the transport properties of STO both in the form of thin films and interfaces, such as the well-studied STO/LaAlO3 interface . Additionally, this heterostructures are subjected to epitaxial strain, which has been shown a remarkable effect in the oxygen vacancy formation enthalpy. For this reason, it is necessary to perform a systematic study of the VO formation energy in STO as a function of strain and understand the influence of these anionic defects in the transport properties of STO. Moreover, the cationic defects, namely strontium vacancies (VSr), also could play a crucial role in the structural properties of the STO thin films , producing a characteristic rotation pattern of the TiO6 octahedra to accommodate the coherent epitaxial strain. In this work, we present a complete thermodynamic study of VO formation in e-doped thin films grown by Pulsed Laser Deposition. Hall effect measurements confirmed that each VO is doubly ionized and donates two electrons to the conduction band of STO. Furthermore, our results demonstrate that both, compressive and tensile strain, leads to a decrease in the VO formation enthalpy, in good agreement with the reported ab-initio calculations . On the other hand, we determined the existence of a different rotation pattern of the TiO6 octahedra of the thin films under compressive or tensile strain, which is related with the unintentional presence of the strontium vacancies in the STO thin films. Additionally, we show the possibility to manipulate the oxygen vacancies by applying an external electric field with an Atomic Force Microscopy tip. We demonstrate a change in the local volume and surface potential of the sample associated to the accumulation of oxygen vacancies. The application of a negative (positive) voltage causes a local expansion (contraction) in the surface of the sample about a half or one unit cell height, as can be observed in Figure 1. We also determine the diffusion coefficient of the Vo after removing the electric field as a function of strain, proving that the mechanical effect is stable for hours as well as reversible . References  S. V. Kalinin, N. A. Spaldin, Science 341 (2013) 858.  A. Ohtomo, H.Y. Hwang, Nature 427 (2004) 123.  A. Sarantopoulos, E. Ferreiro-Vila, V. Pardo, C. Magén, M. H. Aguirre, F. Rivadulla, Physical Review Letters 115 (2015) 166801.  S. -Y. Choi, S. -D. Kim, M. Choi, H. -S. Lee, J. Ryu, N. Shibata, T. Mizoguchi, E. Tochigi, T. Yamamoto, S. -J. L. Kang, Y. Ikuhara, Nano Letters 15 (2015) 4129.  L. Iglesias, A. Sarantopoulos, C. Magén, F. Rivadulla, Physical Review B 95 (2017) 165138.
Ms. Lucía Iglesias (PhD Student)
Mr. Alexandros Sarantopoulos (University of Santiago de Compostela) Prof. Francisco Rivadulla (University of Santiago de Compostela)