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Electronic structure of low-dimensional topological matter

Project summary:
  • A Materials Physics Doctorate PhD project
  • Supervisor: Dr. Gavin Bell
  • Feedback supervisor: Dr. Neil Wilson
  • Supporting supervisor (modelling): Dr. Peter Brommer
  • Mainly experimental but with modelling input using existing DFT codes
  • Involves laboratory photoemission (XPS, ARPES) and inverse photoemission (IPES) plus synchrotron radiation photoemission at Diamond Light Source (UK), Soleil (Paris), Elettra (Trieste)
  • Topological material thin films supplied by Warwick (Bell group), Oxford (Prof. Thorsten Hesdejal), Cambridge (Prof. Crispin Barnes)
  • Also study 2D materials (Dr. Neil Wilson, Warwick) and opportunity to visit Fudan University, Shanghai

R4000 and ARPES data

Left: the Scienta R4000 ARPES system at Warwick (electron energy and momentum are measured in the shiny hemisphere on the left). Right: synchrotron ARPES data (yellow-red) and DFT effective band structure (green dashes) for valence band structure of graphene on Cu(100). The pattern is essentially a slice through graphene's Dirac cone.

Topological materials have unusual intrinsic properties arising from fundamental symmetries. For example, topological insulators (TIs) have a bulk band gap but conducting surface states arising from strong spin-orbit coupling. These differ from regular surface states in that they are protected by time-reversal symmetry: they always exist despite surface defects and can support dissipationless and spin-momentum locked transport. These remarkable properties could be exploited in advanced low-power spintronic devices and quantum computation. However, for TIs and related materials (e.g. topological Weyl semimetals) to be used in real device structures, it will be necessary to grown thin-film material by methods such as molecular beam epitaxy (MBE). We are part of a collaboration with the universities of Cambridge and Oxford, and with Diamond Light Source, aiming to do just this. In Warwick we are growing toplogical materials such as TaAs (a topological Weyl semimetal) and SmB6 (a topological Kondo insulator) using a dedicated mini-MBE system.

A critical aspect of this work is to measure the electronic structure of thin-film topological materials. This can be done directly using angle-resolved photoemission spectroscopy (ARPES). We have an advanced lab-based ARPES system (based on a Scienta R4000 2D-dispersing electron energy analyser) and use several synchrotron radiation sources (Diamond Light Source, UK; Soleil, France; Elettra, Italy) to perform ARPES. Your project will be based on characterising thin-film topological materials by ARPES.Our MBE-grown samples could be transferred under ultra-high vacuum (UHV) to the ARPES system allowing for analysis of pristine material without surface degradation. We want to develop similar methods for UHV transfer of material between Oxford, Diamond and Warwick. At Diamond, beamline I05 has the capability to grow materials in situ using a dedicated mini-MBE system. Beamline I09 has the capability to perform simulatenous hard X-ray photoemission spectroscopy (HAXPES) and standard PES on a single point on the sample. For granular or non-uniform samples, we also use nano-spot ARPES, where a sub-micron beam focus allows imaging of the electronic structure and measurements on a single small area.

As well as experiments, you will simulate ARPES and XPS data using existing density functional theory (DFT) packages such as CASTEP and SPRKKR. One particular goal is to simulate chemical shifts in core-level X-ray photoelectron spectroscopy (XPS). Dr. Peter Brommer (Centre for Predictive Modelling, School of Engineering) has recently received a Royal Society grant supporting a dedicated cluster for these calculations. He also recently implemented a band-structure-unfolding algorithm for the CASTEP code allowing effective band structures from large supercells to be visualised. Dr. Brommer will provide additional support and supervision for CASTEP work. You will also have the chance to attend hands-on workshops for DFT organised by, e.g. the European Psi-K Network.

You will also have the opportunity to travel to Fudan University, Shanghai, where we collaborate on TI materials, and to get involved in ARPES work on novel 2D materials heterostructures led by Dr. Neil Wilson with Dr. Nick Hine (Warwick Physics).

The project offers a capable student many opportunities to develop expertise in very widely-used photoemission techniques as well as the modelling prowess to exploit the data fully. The research area is very much cutting-edge with plenty of scope for high impact publications and international conference presentations. Extensive travel and collaboration with leading UK groups and European synchrotron beamlines are integral to the project.