X-ray diffraction ; surface structures ; interface structures ; complex metal oxides ; thin film heterostructures ; perovskites
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This document describes a research proposal for funding by the Swiss National Science Foundation of a postdoctoral position over two years. On the one hand, it details the development of phase-retrieval direct-methods (‘DIMES’) algorithms and multiwavelength anomalous scattering techniques to recover the phases in structural studies of thin films and interfaces using synchrotron-based surface x-ray diffraction (SXRD); and on the other, the investigation of the physical properties of heteroepitaxial thin films of nickelate perovskites from a structural basis, exploiting the above phasing methods. The project is a natural progression of the success in these fields within the group of the author over the last four years. The proposed work is divided into two aspects. First, the entire pipeline for treatment of SXRD data, from raw data acquisition, through data mining, and finally structural determination, will be streamlined and optimized. This involves (a) identifying the best conditions for recording SXRD signal using the new high-brilliance undulatorsource; (b) improving and automating the structure factor extraction method via an automated cluster-identification approach; (c) quantitatively analyzing the impact of missing data (caused by limitations in the experimental geometry) and the error associated with the extracted data. This is paramount to the successful implementation of DIMES; (d) extensions to DIMES in the form of innovative and enhanced projections tested via a detailed study of DIMES with complementary experimental and simulated datasets; and (e) the development of methods to solve reconstructed thin films using direct methods and multiwavelength anomalous SXRD.The second aspect of the project is the study of the physics of rare-earth perovskite nickelates RNiO3 from a structural perspective. This complex system will act as an ideal testbed for the first part of the project. The RNiO3 perovskites are an exciting family with potential technological applications, exhibiting a sharp metal-insulator (MI) transition with resistance increases of several orders of magnitude. LaNiO3 has proved to be the exception, remaining metallic at all temperatures in its bulk form. However, a weak MI-transition in LaNiO3 was recently observed when grown as a thin film up to a critical thickness of 8 u.c. (unit cells). Our preliminary results, in collaboration with the University of Geneva, who have provided high-quality samples, have demonstrated a probable oxygen octahedra rotation structural variation across the MI transition. Rotations of this nature are well known in bulk perovskites, as recently reported by May et al., in thicker films of LaNiO3.The synchrotron studies will be complemented by experimental techniques, including x-ray photoelectron spectroscopy, atomic-force microscopy, and conductivity measurements; and also by density functional theory (DFT) calculations in a collaborative exercise within the Paul Scherrer Institut.