Angle-resolved photoemission spectroscopy of delafossites

Named in honour of the French crystallographer Gabriel Delafosse, the delafossites are layered materials with triangular in-plane lattices. They can exist in many chemical combinations, and display a wide range of physical properties. Our current research focuses on the metallic delafossite oxides such as PdCoO2, PtCoO2, PdCrO2 and PdRhO2. These show some of the most spectacular properties of any known metals, with extremely high electrical conductivity which can even exceed that of noble metals, and whose origin is not yet very well understood. There is good evidence that this electrical transport may take them into a ‘hydrodynamic’ regime that has been discussed theoretically for over fifty years but not previously observed in a bulk material [1]. Since they are layered compounds, the delafossites are well suited to study with modern surface spectroscopies such as angle resolved photoelectron spectroscopy (ARPES).

A number of experiments have demonstrated the potential of the delafossites to become ‘benchmark’ metals for modern photoemission studies. The electronic states originating from the bulk of the material give resolution-limited photoemission spectra that extend from the Fermi level to over 0.5 eV below it [2]. The unusual absence of energy-dependent spectral broadening, pointing to suppressed electronic interactions, is still not well understood, and its investigation will form part of this project. Arguably even more interesting is the opportunity to study the interplay between the bulk states originating from Pd or Pt and surface states which can have either Pd/Pt character or come from the transition metals Co or Cr, or Rh, depending on details of the sample being studied. Again, this is an opportunity to use the delafossites to study entirely new physics. For example, one of our recent graduates, Veronika Sunko, made a breakthrough in understanding the origin of so-called Rashba spin texturing at surfaces and interfaces [3]. The insight that led to this came from analysing the experimental data that she and other members of our collaboration had taken on Co/Rh surface states of PtCoO2, and PdCoO2, and the correctness of the model was confirmed by follow-up work on PdRhO2. Our project is a collaboration between the groups of Phil King in St Andrews and Andy Mackenzie at MPI-CPfS Dresden. The successful applicant will perform work in Dresden and St Andrews, and also at international synchrotron facilities with photoemission beam lines.

The information obtained from these ARPES experiments will be complementary to results obtained by scanning-tunneling spectroscopy, and if an exceptionally well-qualified applicant were interested in a project combining the two techniques, we would be happy to discuss it. Theoretical collaboration with the group of Helge Rosner is also a possibility.

Example of an ARPES measurement on the delafossite material PdCoO2

[1] P.J.W. Moll, P. Kushwaha, N. Nandi, B. Schmidt, A.P. Mackenzie
Evidence for hydrodynamic electron flow in PdCoO2
Science Magazine, 351(6277), 1061–1064 (2016)
[2] P. Kushwaha, V. Sunko, P.J.W. Moll, L. Bawden, J.M. Riley, N. Nandi, H. Rosner, M.P. Schmidt, F. Arnold, E. Hassinger, T.K. Kim, M. Hoesch, A.P. Mackenzie, and P.D.C. King
Nearly free electrons in a 5d delafossite oxide metal
Science Advances 1, 1500692 (2015)
[3] V. Sunko, H. Rosner, P. Kushwaha, L. Bawden, O.J. Clark, J.M. Riley, D. Kasinathan, M.W. Haverkort, T.K. Kim, M. Hoesch, J. Fujii, I. Vobornik, A.P. Mackenzie, and P.D.C. King
Maximal Rashba-like spin splitting via kinetic-energy-coupled inversion-symmetry breaking
Nature 549, 492 (2017)

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