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Abstract:
Single-crystal delafossite PdCoO2 is known to have an extremely low intrinsic impurity concentration of ∼0.001, demonstrating extraordinarily high conductivity with a mean free path of ∼20 μm at low temperatures. However, when grown as thin films, the resistivity at room temperature increases by a factor of 3-80 times, depending on the film thickness. Using scanning transmission electron microscopy, we identify different classes of defects for the single crystal vs epitaxial thin film. The dominant defect for single-crystal PdCoO2 is found to be ribbonlike defects. For the thin films, we identify different types of defects arising in epitaxial thin films mainly due to substrate termination that disrupt the lateral connectivity of the conducting planes. Our results are consistent with the high conductivity of single crystals and increased electrical resistivity of the thin films compared to that of single crystals, suggesting that selecting a proper substrate, improving surface quality, and reducing the step density are the keys to enhance the film quality for utilizing PdCoO2 as a platform for future applications. © 2022 American Physical Society.
