Studying the electronic structure of the polar surfaces of delafossites using μ-ARPES

Abstract

This thesis presents the results of my angle-resolved photoemission (ARPES) study of the delafossite oxides PdCoO₂, PtCoO₂ and PdCrO₂ as well as delafossite-like AgCrSe₂. When cleaving the sample, their layered ABX₂ structure results in distinct surface terminations that are spatially distributed across the sample. These are either electron (A-termination) or hole doped (BX₂-termination) with respect to the bulk, resulting in their markedly different surface electronic structures. These electronic surface structures host a delicate interplay between the spin-, charge-, and orbital-degrees of freedom, driving the formation of new phases. The understanding of this interplay is the central concept of this thesis. Probing defined surface domains has often been challenging due to the beam spot of the incoming light being larger than a single domain. Here, I show that by using μ-ARPES, where the beam spot is focused to 4 μm, it is possible to probe the electronic structure of a distinct surface termination and study the extent of spatial variations in the electronic structure across the sample. Identifying areas of pristine terminations greatly increased the effective resolution, enabling the detailed study of the surface electronic structure. I show that PdCrO₂ undergoes a charge-order driven surface reconstruction that protects the insulating nature of the CrO₂ states at the surface. For PdCoO₂, I demonstrate strong electron-phonon coupling on both the CoO₂- and Pd-terminated surfaces, with an unusually strong coupling on the Pd-termination driving the formation of two distinct polaron modes. I conclude by presenting my study on non-centrosymmetric delafossite-like AgCrSe₂. Here, a delicate interplay between bulk and surface inversion symmetry breaking results in a Rashba-type splitting of the states derived from different magnetic domains. I will also revise the role of the subsurface layer for these systems which has thus far been widely neglected but is crucial to achieve an accurate description of the underlying physics.