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UNderstanding, descRibing And Visualizing Electronic charge in noveL oxide heteroStructures

Periodic Reporting for period 1 - UNRAVELS (UNderstanding, descRibing And Visualizing Electronic charge in noveL oxide heteroStructures)

Reporting period: 2015-05-01 to 2017-04-30

In recent years with the improving of experimental techniques in thin film deposition and with the possibility to measure, visualize and control at atomic scale the electron charge, Transition Metal Oxides and their heterostructures revealed to be promising building blocks for revolutionary oxide electronics (oxitronics). The properties of the interface region between two oxides, determined by the coupling of the properties of each oxide, are at the origin of different microscopic mechanisms: charge fluctuations, disproportionation, symmetry breaking, spin frustration. These atomic scale interactions drive the macroscopic behavior of the heterostructures in a way to generate new and yet unpredicted properties:magnetic/ferroelectric polarization,H-Tc superconductivity, fast metal-insulator phase transitions, domain walls,multiferroicity. The aim of this project is to use ab initio parameter-free calculations based on DFT methodologies to investigate the electronic structure and the spectroscopic properties of oxides side-by-side with the experiment. Ab initio techniques are essential to investigate and predict correlation features at the atomistic level, evaluating the role of geometry reconstruction at the interface, predicting the charge redistribution, calculating the electronic structure across the phase transition and the band offsets at the interfaces.I have studied how strain and structural and chemical defects and the presence of interfaces tends to alter the charge localization and the magnetic configuration with respect the pristine oxides systems. I showed how different geometric reconstructions due to epitaxial strain and defects affect the charge occupation of orbitals and the electronic structure in the atomic layers surroundings the interface and deeper into the substrate. The electronic structure and the magnetic configuration can be affected by the number of atomic layers constituting the heterostructure and eventually by the presence of different capping thin films at the surface. The systems subdue an iso-structural electronic phase transition from insulating to metallic configuration. Also the magnetic structure is affected by the presence of different magnetic atoms with a mixed valence configuration. Thanks to DFT simulations we have demonstrated for some paradigmatic oxides and their heterostuctures (I.e: SrTiO3-LaAlO3, LaMnO3-LaNiO3) which are the mechanisms occurring at the atomic scale that permit to control their electronic and magnetic properties. DFT simulations helped to guide the experiments in the proper realization of such complex heterostructures in order to achieve a wide range of functionalities to implement into innovating and faster technological devices.
Scientific part.
Cerium oxides structures.
I studied the electronic correlation features and magnetism in diluted semiconductor structures of CeO2 and Ce2O3 with magnetic impurities in the bulk and at the surface. I investigated how the electronic correlation changes in presence of surfaces and interfaces. This approach has permitted to describe how the charge varies on Ce and O atoms as a function of external constraints related to geometry and defects (magnetic impurities, oxygen vacancies).

In this work I have studied super-lattices of LaMnO3 and LaNiO3. I have been interested in describing by first principles methods the magnetic exchange at the interface of LMO-LNO super-lattices. I have studied different magnetic and structural configurations in order to point out the more stable configuration and to derive the magnetic exchange energy to jump from on magnetic structure to another. Work in collaboration with the J.M. Triscone experimental group in Geneve.

Another type of interesting super-lattice is represented by heterostructure of
I studied the the formation and the control of the 2D electronic gas at the interface between the two band insulator SrTiO3/LaAlO3 with a metallic capping at the outermost surface. I have studied the role of interface terminations and of the structural reconstruction with the presence of oxygen vacancies in order to discuss the origin of the charges at the interface and of the level of electronic correlation. This work is in collaboration with Manuel Bibes’ group in Thales in charge of the characterization and transport measurements.

Bismuth and Yttrium Iron garnet (BIG and YIG)
I have studied the effect of stoichiometry and the role of epitaxial strain on the structural, electronic and magnetic properties of BIG and YIG. To the best of our knowledge, this work is the first ab initio study that aims to predict and correctly characterize the electronic structure in Bi-Y iron garnets in a direct relation with experimental measurements.
This project has permitted to advance in the ab initio prediction of the properties of oxides and to reinforce the connection among different communities working on this field at Paris-Saclay University. Ab initio methods have become a robust methodology for materials science inside the Laboratoire de Physique des Solides. Such state-of-the-art expertise constitutes a solid foundation for further developments in controlling the physics of oxides and for novel advanced applications.
Thanks to a direct interaction with experimentalist, the project helped me in developing and strengthen competences in oxides physics. On the experiment side, considering preliminary predictive ab initio results, the STEM group aims to study the electronic and optical excitations in oxides as a function of temperature and strain with the new ultimate generation Chromatem microscope.The training received, together with the related EU mobility, has permitted noticeably the grow the opportunities of my professional maturity. The multidisciplinary character of the project has favored the collaboration with various scientists, attending to international conferences and courses with the aim of enhancing his visibility in the scientific community. I have boosted my expertise demonstrating enough ability to engage in research, teaching in a French university, management, promoting common strategies to reinforce the interaction between experimental groups and theoretical groups, to use my communicating skills to organize and promote outreach activities in science for non-scientific public and high school students. Moreover such expertise help me in improving my rate of success in public selections processes to find a permanent position in academia.