Project

Back to overview

Direct Methods Applied to Oxide Heterostructures

English title Direct Methods Applied to Oxide Heterostructures
Applicant Willmott Philip
Number 137507
Funding scheme Project funding (Div. I-III)
Research institution Paul Scherrer Institut
Institution of higher education Paul Scherrer Institute - PSI
Main discipline Condensed Matter Physics
Start/End 01.01.2012 - 31.12.2013
Approved amount 209'050.00
Show all

Keywords (6)

X-ray diffraction ; surface structures ; interface structures ; complex metal oxides ; thin film heterostructures ; perovskites

Lay Summary (English)

Lead
Lay summary

X-ray diffraction (XRD) techniques are uniquely capable of determining the structures of crystalline materials with an accuracy of the order of picometers or better. However, the main obstacle to determining structures using XRD is the so-called "phase problem" - XRD patterns provide the intensities of diffraction spots, but the relative phases are inaccessible. In order to retrieve the phases, traditionally a model is proposed, and the structure modified until a self-consistent solution is found. Although this approach, coupled with sound stereochemical constraints, can lead to an unambiguous solution for simple structures, increasingly it fails for more complex systems - the starting model is insufficiently close to reality that this iterative procedure can remove the structure out of a local minimum towards the real solution.

New techniques, however, based on direct methods, are model-free and can potentially overcome the problem of being trapped in a local minimum. This project is concerned with developing such methods in the technique of surface x-ray diffraction (SXRD), used to determine the structures of crystalline surfaces and interfaces.

Direct link to Lay Summary Last update: 21.02.2013

Responsible applicant and co-applicants

Employees

Name Institute

Publications

Publication
Chemistry and structure of homoepitaxial SrTiO3 films and their influence on oxide-heterostructure interfaces
M. L. Reinle-Schmitt C. Cancellieri A. Cavallaro G. F. Harrington S. J. Leake et al. (2014), Chemistry and structure of homoepitaxial SrTiO3 films and their influence on oxide-heterostructure interfaces, in Nanoscale , 6, 2598-2602.
Moire beating in graphene on Ru(0001)
M. Iannuzzi I. Kalichava H. Ma S. J. Leake et al. (2013), Moire beating in graphene on Ru(0001), in Physcial Review B, 88, 125433.
Atomic imaging and direct phase retrieval using anomalous surface x-ray diffraction
Pauli S. A., Leake S. J., Bjoerck M., Willmott P. R. (2012), Atomic imaging and direct phase retrieval using anomalous surface x-ray diffraction, in JOURNAL OF PHYSICS-CONDENSED MATTER, 24(30), 305002:1-305002:7.
Cluster method for analysing surface X-ray diffraction data sets using area detectors
S. J. Leake M. L. Schmitt I. Kalichava S. A. Pauli P. R. Willmott, Cluster method for analysing surface X-ray diffraction data sets using area detectors, in Jounal of Applied Crystallography, 47, 207-214.

Collaboration

Group / person Country
Types of collaboration
Professor Philipp Aebi/University of Neuchatel Switzerland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
J.-M. Triscone/Physics Department, University of Geneva Switzerland (Europe)
- in-depth/constructive exchanges on approaches, methods or results

Associated projects

Number Title Start Funding scheme
117615 Interfacial properties of perovskite thin films 01.10.2007 Project funding (Div. I-III)
126901 The physics of conducting interfaces in insulating heterostructures 01.05.2010 Project funding (Div. I-III)

Abstract

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.
-