Low-dimensional electronic state at the surface of a transparent conductive oxide

Materials that blend physical properties that are usually mutually exclusive could facilitate devices with novel functionalities. For example, the doped perovskite alkaline earth stannates BaSnO₃ and SrSnO₃ show the intriguing combination of high light transparency and high electrical conductivity. Understanding such emergent physics requires deep insight into the materials’ electronic structures. Moreover, the band structure at the surfaces of those materials can deviate significantly from their bulk counterparts, thereby unlocking novel physical phenomena. Employing angle-resolved photoemission spectroscopy and ab initio calculations, we reveal the existence of a 2-dimensional metallic state at the SnO₂-terminated surface of 1% La-doped BaSnO₃ thin films. The observed surface state is characterized by a distinct carrier density and a lower effective mass compared to the bulk conduction band, of about 0.12m_e. These particular surface state properties place BaSnO₃ among the materials suitable for engineering highly conductive transition metal oxide heterostructures.