PhD Project Outline
Topological Kondo insulators: from surfaces to epitaxy to spintronics
Summary
You will join a team of experimental and theoretical physicists investigating topological
Kondo insulator (TKI) materials. Our goals are to understand the surfaces of bulk TKI
materials such as SmB6, perform epitaxial growth of TKIs, and produce hybrid semiconductor
/ TKI / ferromagnet heterostructures to investigate new paradigms in spintronic device
structures. The work will be based in Warwick but we collaborate closely with the Universities
of Cambridge and York, with Toshiba Research Europe Ltd. and with Diamond Light Source.
Background
Topological insulators are materials with low bulk conductivity which support conducting
symmetry-protected surface states. TKI materials also have strong electron correlations
leading to Kondo insulator behaviour (a band gap opening at low temperature due to
conduction electron / f-electron hybridization). Samarium hexaboride, SmB6, is the canonical
example of a TKI material. As well as the intrinsic interest of topological insulators in terms of
the role of symmetry in condensed matter physics, they are also beginning to attract attention
as potential spintronic materials. This is because the protected surface states are both spinpolarised and not subject to ordinary spin-flip scattering (due to spin-momentum locking). For
spintronic applications it is necessary to use thin-film material, ideally showing crystal and
chemical compatibility with ferromagnetic and semiconducting materials so that high quality
epitaxy can be achieved. SmB6 fulfils these criteria. However, there is much debate over the
exact nature of the SmB6 surface (relaxation, reconstruction and polarity) and epitaxy has not
been demonstrated. We have high quality SmB6 bulk crystals showing Kondo insulator
properties and are beginning a programme of surface studies and epitaxial growth. The figure
shows some example data. Our aim is to develop spintronic device structures involving this
and other TKI materials.
PhD project
You will join the Surface, Interface & Thin Film Group at Warwick, a predominantly
experimental group investigating advanced materials in thin-film form in particular with
surface-specific techniques. We boast several epitaxy systems (MBE, PLD, CVD) and a wide
range of state-of-the-art surface science tools (XPS, UPS, ARPES, LEED, RHEED, STM,
AFM). The group works closely with colleagues in Condensed Matter Physics at Warwick
allowing PhD students to use advanced X-ray diffraction, magnetometry and microscopy
facilities, e.g. a double aberration-corrected Jeol ARM200-F transmission electron microscope.
We support our experimental work with density functional theory (DFT) calculations mainly
using the CASTEP and SPR-KKR codes.
Externally, you will collaborate with Diamond Light Source (especially beamline I07,
Surface and Interface Diffraction), other synchrotron radiation sources, and neutron facilities
such as ILL and ISIS (for polarised neutron reflectometry). This project will also involve the
University of Cambridge (Thin Film Magnetism Group at the Cavendish Laboratory),
University of York (Department of Physics and York-Jeol Nanocentre) and Toshiba Research
Europe Ltd., especially as we move towards spintronic device structures.
The project would suit students interested in mainly experimental research on advanced
materials, spintronics, nano-physics or surface science. The balance between different aspects
of the work can be tailored (e.g. microscopy vs. synchrotron radiation-based techniques),
although the project requires work on epitaxial growth.
Figure: (a) powder XRD showing high crystalline quality and phase-purity of our SmB6 bulk crystals;
(b) temperature dependence of the resistivity of SmB6, showing rapid onset of Kondo insulator state
and saturation due to surface conductivity; (c) LEED pattern at 80 eV from bulk SmB6 with surface
approx. 15° misaligned from (001) after polishing, chemical cleaning and UHV annealing; (d)
calculated [01L] CTRs for Sm terminated SmB6(001) with different inward relaxations of the outer Sm
layer (black to red curves 0 – 6%). Clear signatures are readily detectable in SXRD for relaxations
smaller than those predicted by DFT for SmB6 and LaB6 (up to 9%).
Teaching and support
You will be able to benefit from graduate-level teaching through the Midlands Physics
Alliance Graduate School (MPAGS) as well as specialised workshops such as those run by the
Psi-K Network on advanced DFT applications or by synchrotron radiation and neutron
facilities. We also offer the possibility of an industrial placement (extending your PhD
correspondingly) and extensive training and portfolio-building in transferable skills, teaching
and research, appropriate for both academic and industrial careers post-PhD.
Financial Support: Scholarships
There are several full or partial scholarships available for financial support. For further
details please contact the Physics postgraduate admissions team and/or Dr. Gavin Bell
(Surface, Interface & Thin Film Group). You will be able to supplement grants or scholarships
by paid teaching duties in Physics. Options include:
 Doctoral training grant (normally UK students only, EU students may be elibigle)
 Chancellor’s International Scholarship (overseas students: highly competitive)
 China Scholarships Council (Chinese students only)
 Overseas national government scholarships (we would be happy to support
applications e.g. for Commonwealth scholarships)