Metal-insulator transition in Pd_1-xCu_xCrO₂ delafossite thin films

ABO₂ delafossite oxides possess a layered crystal structure, providing a highly conducting 2D channel for electronic conduction with a large mean free path. The overall conductivity is governed by the choice of A-site elements. For instance, Cu- and Ag-based delafossites are insulating or semiconducting, while Pd- and Pt-based delafossites are metallic. In addition, the B-site cation can be chosen to make the material either magnetic (Cr, Fe) or non-magnetic (Co, Rh). However, the research on delafossite thin films has been limited due to the difficulty in synthesizing high-quality epitaxial thin films. We recently discovered the use of CuCrO₂ as a buffer layer critically enables the epitaxy of PdCrO₂ thin films. In this study, we further attempted to synthesize Pd_1-xCu_xCrO₂ thin films using pulsed laser epitaxy (PLE) to understand the electronic structure evolution, yielding a metal-insulator transition. We conducted a systematic analysis of the structure and electronic structure as well as transport property of A-site-doped delafossite thin films across various Cu doping levels. Our observations of Cu concentration-dependent electronic structures in transport and optical conductivity measurements suggest a clear transition of the band structure in the Pd_1-xCu_xCrO₂ system. We also examined the materials to check the viability of inducing cation-ordered structure that may lead to a topological band structure, and the overall result will be systematically compared to explain the challenges associated with the precision control of cation ordering. Our findings underscore the potential and challenges for manipulating the band structure of delafossite oxide thin films.