Interfacial Oxygen-Driven Charge Localization and Plasmon Excitation in Unconventional Superconductors

Charge localization is critical to the control of charge dynamics in systems such as perovskite solar cells, organic-, and nanostructure-based photovoltaics. However, the precise control of charge localization via electronic transport or defect engineering is challenging due to the complexity in reaction pathways and environmental factors. Here, charge localization in optimal-doped La_1.85 Sr_0.15 CuO₄ thin-film on SrTiO₃ substrate (LSCO/STO) is investigated, and also a high-energy plasmon is observed. Charge localization manifests as a near-infrared mid-gap state in LSCO/STO. This is ascribed to the interfacial hybridization between the Ti3d-orbitals of the substrate and O2p-orbitals of the film. The interfacial effect leads to significant changes in the many-body correlations and local-field effect. The local-field effect results in an inhomogeneous charge distribution, and due to perturbation by an external field, the high polarizability of this nonuniform charge system eventually generates the high-energy plasmon. Transformation of the electronic correlations in LSCO/STO is further demonstrated via temperature-dependent spectral-weight transfer. This study of charge localization in cuprates and interfacial hybridization provides important clues to their electronic structures and superconductive properties.