Topological Materials: Half Heusler Alloys

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


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.


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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 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 is a roadblock to studying completely clean interfaces that many groups were not unaware of. We preformed Polarized Neutron Reflectometry (PNR) on the Bi/Ni bilayers at the ISIS beamline, there we measure the thin film structure as the sample was annealed at 70C over 1 hour. This work again shows the diffusion and formation of NiBi3 at the interface and the loss of the ferromagnetic Ni. These PNR runs were in parallel with measurements of the superconducting transition temperature and magnetization. This work has since been published as a preprint article and currently in production for the physical review research journal.

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 and YbPtBi thin films.

The structure of the YPtSb thin films was of high quality. X-ray diffraction (XRD) measures an epitaxial matching to the substrate and induced strain as was intended. From TEM images we saw neat ordered layer from the substrate to the surface with only a few nm of disorder at the top. The chemical composition was close to the 1:1:1 ratio. YPtSb grown as thin films shows a weak semiconductor like behavior 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. Very low temperature measurements at 250mK did not show signs of superconductivity only a very small dip in resistivity, the dip may be from a magnetic ordering as is suggested in other half Heusler alloys but there is no evidence for this.

YbPtBi in literature has only been studied in bulk grown crystals and never as thin films. The thin films we have grown are well ordered to the substrate in the same way YPtSb. SEM/TEM images of the thin films show a triangular island from the effect of the hexagonal symmetry of the sapphire substrate and the epitaxial matching. These triangle islands are in issue for electrical transport measurements as they course erratic current paths. One characteristic feature of Weyl semimetals is the chiral anomaly, which manifests as a negative longitudinal magnetoresistance (LMR). We have fitted the LMR to the chiral anomaly temperature dependence and found an excellent fit to theory. This is strong evidence for the chiral anomaly, unfortunately, the erratic current path also causes a negative magnetoresistance in other orientations where it might be expected to be positive. We have also observed a large hysteresis at very low temperatures of 250mK, the shape of the magnetoresistance at this temperature could be described but a ferromagnetic transition. Overall there is strong evidence of interesting physics within our samples but is obscured but the formation fo islands.
Exploitation Route For the bismuth nickel bilayers, this work will help other groups refine their sample preparation, storage, and measurement processes to avoid annealing their films, and as well allow groups to intentionally anneal them to study Bi3Ni is this way. Although Bi3Ni is a known BCS superconductor, there is still the presence of high SOC and residual magnetic moment that can lead to interesting types of superconductivity.

For the Half Heulser alloys, this proves that it is possible to grow high-quality epitaxial crystals on c-plane sapphire, with the possibility to extend this method onto other half Heuslers not grown in this work. If the process is refined more to the point of consistent continuous films with topological properties then sputtering is generally an easier method than MBE or thermal evaporation, and is also more industrially compatible. Also proving the viability of thin films, in general, allows for the study of nano-device using typical cleanroom fabrication techniques.
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