Tuning transport properties via rare-earth doping and epitaxial strain in Sr₂IrO₄ thin films

Sr₂IrO₄ is predicted to be a candidate for high-temperature superconductivity upon carrier doping, whereas extensive research has proved it challenging to obtain a metallic phase in this compound, especially in thin films. Here, we explore the impact of stoichiometry on the crystallinity, as well as carrier doping and epitaxial strain on the transport properties of Sr_2−xNd_xIrO₄ thin films. Via fine tuning the stoichiometry, the crystallinity of Sr₂IrO₄ films can be greatly enhanced, as indicated by the minimized systematic deviations of the x-ray diffraction peak positions. As the cation doping level increases, the resistivity of Sr_2−xNd_xIrO₄ films decreases, but it remains semiconducting even at a high level of x = 0.4 where the resistivity has dropped by three orders of magnitude. By further applying compressive epitaxial strain, the Sr_1.8Nd_0.2IrO₄ films exhibit a metallic-like behavior with an upturn at low temperature. Our finding reveals a promising combination of electron doping and compressive strain to narrow the band gap and tune the transport properties of iridate films for potential superconductivity.