Phil King1

1, School of Physics and Astronomy, University of St Andrews, St Andrews, , United Kingdom

The ABO2 family of delafossite oxide metals can be considered as natural superlattice structures comprising high-conductivity metal layers separated by insulating transition-metal oxide building blocks. I will present our angle-resolved photoemission (ARPES) measurements from several members of this series. In PdCrO2, the oxide layer is an antiferromagnetically-ordered Mott insulator, where we find that a coupling between the metallic and insulating subsystems renders photoemission sensitive to the spin susceptibility of the Mott layer [1]. In PdCoO2 and PtCoO2, the oxide layer is a band insulator, but this becomes hole-doped at the surface due to an electronic reconstruction driven by a polar surface charge. We demonstrate how the resulting CoO2 surface supports metallic states which host a surprisingly-large Rashba-like spin-orbit splitting, resulting from a structural configuration which ensures a large energy scale associated with inversion symmetry breaking at this surface [2]. For the Pd-terminated surface, we find how an electron self-doping, again driven by the surface polarity, mediates a Stoner transition to itinerant ferromagnetism [3], in contrast to the non-magnetic nearly-free electron-like character of the Pd-derived states in the bulk [4]. Together, these results indicate the wide range of materials properties that can be stabilized in delafossites, as well as at their surfaces and interfaces.

Key collaborators on this work include Veronika Sunko (St Andrews and Max-Planck Institute for Chemical Physics of Solids, Dresden), Federico Mazzola (StA), Helge Rosner, Seunghyum Khim, Pallavi Kushwaha, and Andy Mackenzie (MPI-CPFS), Sota Kitamura and Takashi Oka (MPI-PKS), and Leonid Pourovskii and Antoine Georges (College de France).

[1] Sunko et al., arXiv:1809.08972 (2018)
[2] Sunko et al., Nature 549 (2017) 492
[3] Mazzola et al., arXiv:1710.05392 (2017)
[4] Kushwaha, Sunko et al., Science Advances 1 (2015) e1500692