Topological Materials: Half Heusler Alloys

Lead Research Organisation: University of Leeds
Department Name: Physics and Astronomy

Abstract

Recent years have seen an explosion in the interest in materials where a large spin-orbit interaction can lead to a non-trivial topological order of the band structure. The most widely studied class of such materials are the topological insulators (TIs), such as, for example, Bi2Se3 or Bi2Te3. TIs have a large band gap in the bulk of the material and gapless conducting surface states which are protected by a combination of time reversal symmetry and spin-orbit interactions. These surface states typically have a single Dirac cone-like dispersion relationship and locking of the electron spin to the momentum, resulting in a number of unique phenomena. In addition to the intrinsic scientific interest, TIs have been suggested for application in Quantum computing - where the protected nature of the surface states might lead to significantly longer de-coherence times for qubits, or the exploitation of exotic particles such as Majorana fermions - or in spintronics - where the spin locking of the surface states could be used in spin filtering.
Although the binary alloys have received the most attention to date, their comparative difficulty to fabricate in thin film form, volatility and toxicity make them unattractive for developing applications. In the last few years there has been growing interest in a class of ternary alloys, the Heuslers and half -Heuslers for these applications. This project will aim to develop a growth capability of such materials and eventually integrate them with magnetic and superconducting materials. To grow the materials we will adapt an existing ultra-high vacuum chamber to be able to co-deposit the ternary alloy by dc magnetron sputtering onto heated substrates, allowing a full control over the material composition and growth process. Initial characterisation will be by local facilities in Leeds, such as electron microscopy, X-ray diffraction and electron transport measurements, although eventually, successful materials will need to be measured at central facilities such as the new ARPES beamline at the Diamond light source.
The successful applicant will have a background in physics or materials science and a strong interest in experimental work. Experience with thin film growth and characterisation or electron microscopy would be highly beneficial.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509681/1 01/10/2016 30/09/2021
1807027 Studentship EP/N509681/1 01/10/2016 31/03/2020 Matthew Thomas Vaughan
 
Description During the course of this research it was noted that superconducting bismuth nickel bilayers will anneal at room temperature over a few days allowing for bismuth and nickel to diffuse into one another, this allowed the creation of an already known superconducting alloy of NiBi3. These Bi/Ni bilayers were of interest to the academic community as a possible option for novel types of superconductivity, but this annealing process at such low temperatures was not previously seen and a road block to studying completely clean interfaces that many groups weren't unaware of.


We successfully grew half Heusler alloys of YPtSb and YbPtBi which are candidates for topological insulators and weyl semimetals respectively. These were grown on c-plane sapphire substrates by DC magnetron sputtering produced a preferred [111] orientation and epitaxial growth. Cross sectional Transmission Electron Microscopy (TEM) was also done on the YPtSb thin films (And is scheduled for the YbPtBi too) relieving a well order system that grows as neat layers.

YPtSb as thin films show a weak semiconductor like behaviour with a carrier density much lower than that of normal metals (Metals~10^23cm^-3 vs. YPtSb~10^20cm^-3). Although this is still higher than desired and the mobility is too low to observe oscillation is the magnetoresistance.

YbPtBi in literature as far as I can see has only been studied in bulk grown crystals and never as thin films. The thin films we have grown are well order to the substrate in the same way YPtSb. One charatieristic feature of weyl semimetals in the chiral anomly, which is a change in the sign of the magnetoresistance. We have seen some features that are similar to this chiral anomly but not all of them.
Exploitation Route For the bismuth nickel bilayers this work should help other groups refine their sample preparation, storage and measurement processes to avoid annealing their films, and on the other side allow groups to intentionally anneal them to study BiNi3 is this way.

For the Half Heulser alloys this proves that it is possible to grow high quality epitaxial crystals on c-plane sapphire, with the possiblity to extent this method onto other half Heuslers not grown in this work. If the process is refined more to the point of consistent films with topological properties then sputtering is generally an easier method than MBE or thermal evaporation, and is also more industrially compatible.
Sectors Electronics