Title: Understanding the role of interlayer charge transfer and polaron forma4ons on the surface electronic structure of metallic delafossite oxides.

Abstract: 

The charge-compensa4on of the permanent dipole moment of a polar surface structure has been known to drive the forma4on of novel surface sates. The delafossite oxides present an exci4ng material class in which to study these effects. Here, I present our angle-resolved photoemission (ARPES) study of the metallic delafossites PdCoO2 and PtCoO2. When cleaving the sample, their layered ABO2 structure results in dis4nct surface termina4ons that are spa4ally distributed across the sample. These surfaces are either electron (A-termina4on) or hole doped (BO2-termina4on) with respect to the bulk1. This results in markedly different surface electronic structures, which exhibit a delicate interplay between the spin-, charge-, and orbital-degrees of freedom resul4ng in a maximal Rashba-type spin spliLng in the electronic structure of the CoO2-terminated surface2 and i4nerant ferromagne4sm on the Pd-terminated surface3,4, which is absent in the bulk. However, probing regions of defined surface termina4ons has to date been challenging for these samples 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 well-defined surface termina4on and study the extent of spa4al varia4ons in the electronic structure across the samples. Iden4fying areas of pris4ne termina4ons greatly increases the effec4ve resolu4on of our experiment, enabling a detailed study of the fermiology of both Pd and Pt based samples. This analysis reveals strong electron-phonon coupling on their CoO2-terminated surfaces, as well as a crucial role of the sub-surface layer which thus far has been widely neglected. I further show signatures of an unusually strong coupling on the Pd-terminated surface of PdCoO2, which drives the forma4on of two dis4nct polaron modes. These polaron states can only be observed on a Pd-terminated surface but are absent in the electronic structure of the CoO2-termina4on. Furthermore, a strong 4me dependence of their coupling strength can be observed in our data thus making PdCoO2 a model system in which to study the forma4on and tunability of polaron states on these polar surfaces.

1A. P. Mackenzie, Rep. Prog. Phys. 80, 032501 (2017). 2V. Sunko et al., Nature 549, 492–496 (2017).
3F. Mazzola et al., PNAS 115(51)12956-12960 (2018). 4F. Mazzola et al., npj Quantum Materials 7, 20 (2022).