Microscopic observation of coherent 2D metallic behavior on the Pd-terminated surface of metallic delafossite PdCrO₂

The extraordinarily strong in-plane conductivity of metallic delafossites such as PdCoO₂ and PdCrO₂ and their possible origins have been subjects of considerable attention. We performed Scanning Tunneling Microscopy/Spectroscopy (STM/STS) study on the cleaved surfaces of single crystal PdCrO₂. The CrO₂-terminated surface shows a 0.5 eV gap, which is consistent with Mott insulating state of CrO₂ layer. The Pd-terminated surface has metallic density of states and shows hexagon-shaped phase domains of Pd atoms displaced from their bulk-like Oxygen(O) on-top sites to O-O bridge sites. On the Pd-terminated surface, we observed incommensurate quasiparticle interferences with long coherence lengths due to Pd 5s band in the empty states, whereas strongly localized 4d electronic states on individual Pd atoms in the filled states. The extraordinarily long coherence length of the empty-state quasiparticle interferences that extends beyond multiple phase boundaries on Pd-terminated surface may be attributed to the coupling to the underlying CrO₂ layer and is consistent with the unusually high in-plane conductivity in this class of materials.