Delafossite (ABO₂) is a correlated oxide material family whose structure comprises layers of triangular A lattice stacked alternately with those of transition metal oxide (BO₂) octahedra.  Such configuration leads to highly anisotropic transport properties, meanwhile offers an ideal platform for studying the interplay between the itinerant and strong correlated electrons present within the two different types of layers respectively  [1].  Among all the studied delafossite materials, PdCoO₂ and PdCrO₂ have recently attracted the most attention. In today’s talk, I will discuss our scanning tunnelling microscopy (STM) findings obtained from these two compounds  [2,3]:

First, I will present our quasiparticle interference (QPI) data obtained from the Pd- and CoO₂- terminated surfaces of PdCoO₂, interfacial systems hosting Stoner’s instability induced ferromagnetic  [4] and inversion symmetry breaking (ISB) dominated Giant Rashba-like spin-split surface states respectively  [5].  Detailed analysis on the QPI data will be discussed.

Then, I will present our very recent findings on the CrO₂ terminated surface of PdCrO₂, an antiferromagnetic metal.  CrO₂ layers inside the bulk are Mott-insulating.  When a CrO₂ surface layer is exposed to the vacuum, intrinsic polar catastrophe results in substantial doping of the surface layer.   However, from experiments we find that the surface retains an insulating character, with a modified electronic structure and the development of a short-range ordered state with distinct spatial periodicity.

Last, I will briefly discuss our observations on the Pd-terminated surface of CrO₂.

Chi Ming Yim is a tenure-track associate professor of Tsung Dao Lee Institute, Shanghai Jiao Tong University, focusing on studying the ground-state properties of strongly correlated electronic materials and unconventional superconductors using low temperature scanning tunnelling microscopy and spectroscopy.  Chi Ming received his PhD degree in Chemistry from University College London (UK) in 2012.  Prior to joining TDLI, he carried out postdoctoral research at UCL (2012-2016) then at University of St Andrews (UK) between 2016 and 2020.  Till now, Chi Ming has published 34 papers in peer-review journals including Nature Communications, Science Advances, PNAS and PRL.  A full list of his publications can be found at: https://web.tdli.sjtu.edu.cn/cmyim/.

[1] A. P. Mackenzie, The Properties of Ultrapure Delafossite Metals, Rep. Prog. Phys. 80, 032501 (2017).
[2] C. M. Yim, D. Chakraborti, L. C. Rhodes, S. Khim, A. P. Mackenzie, and P. Wahl, Quasiparticle Interference and Quantum Confinement in a Correlated Rashba Spin-Split 2D Electron Liquid, Sci. Adv. 7, eabd7361 (2021).
[3] C. M. Yim et al., Avoided Metallicity in a Hole-Doped Mott Insulator on a Triangular Lattice, arXiv:2311.17139.
[4] F. Mazzola et al., Itinerant Ferromagnetism of the Pd-Terminated Polar Surface of PdCoO2, Proc. Natl. Acad. Sci. 115, 12956 (2018).
[5] V. Sunko et al., Maximal Rashba-like Spin Splitting via Kinetic-Energy-Coupled Inversion-Symmetry Breaking, Nature 549, 492 (2017).

Please contact phweb@ust.hk should you have questions about the talk.