Collective dynamics and pair correlations in atomic systems

Project Description:

Knowledge of atomic structure and interatomic interactions in materials is a prerequisite for understanding of their physical properties. One of the long-standing fundamental challenges in experimental condensed matter physics is investigation of the structure of non-periodic materials (e.g. glasses, liquids, nanoparticles), since widely used x-ray diffraction is not well-suited for non-periodic systems. At the same time, X-ray absorption spectroscopy (XAS) is a technique sensitive to the local structure (short-range order) and since 1970s has been a powerful tool to study non-periodic systems. However, accuracy (and precision) of structural information obtained from XAS crucially depends on an ability to adequately describe local atomic dynamics (LAD). Hence, if LAD can be suitably parametrised, then the key structural parameters (interatomic distances, number of nearest neighbours etc.) can be extracted with much improved accuracy. Conversely, if the structure is known, then such parametrisation can be used to study interatomic interactions (atomic dynamics). Today, description of LAD in XAS is still based on Debye and Einstein models which fail to adequately describe atomic vibrations beyond the first coordination shell and to account for anharmonic effects. Resolving these issues would create a step change in modern XAS analysis of non-periodic (and crystalline) systems. Most significantly, it will finally make structure of and atomic dynamics in liquids - a frontier of the modern fundamental condensed matter science - accessible for accurate analysis by XAS technique. The aim of this project is to develop a methodology for parametrising local atomic pair correlations in extended x-ray absorption fine structure (EXAFS) based on shortest-graph interpolation approach. This will be achieved by integrating the shortest-graph method into theoretical EXAFS calculations and testing the method against experimental data on number of reference systems ranging from metals, to semiconductors and finally on liquids. 

The chief outcome of this project will be methodology for calculations of dynamic disorder in XAS that will finally take into account distance-dependence of MSRDs and anharmonic effects. 

J. Chem Phys. 143 (3), (2015) 034506 

Phys. Rev. B, 65, (2002) 172104

Requirements:

An interest in the field of condensed matter physics

A good first degree in Physics/Chemistry/Materials Science

SPA Academics: Andrei Sapelkin