Tunable Mottness in correlated oxide monolayer with epitaxial strain

 The electron occupancy among d orbitals determines various functionalities in transition metal oxides. Especially, in correlated materials with enough Coulomb interaction, such effect will help to modulate correlated phases, such as Mott phase. Engineering orbital degeneracy lifting is one of the efficient ways to control the electron filling. For example, electrons in d orbitals of perovskite structures can be redistributed into t_2g and e_g orbitals due to crystal field splitting. Similarly, epitaxial strain can give additional anisotropic crystal field among t_2g orbitals. However, tailoring the magnitude of orbital degeneracy lifting and observing related correlated phenomena has rarely been realized.

 SrRuO₃ monolayer film can be a good system for investigating such orbital anisotropic effect. It provides various correlated phases with moderate Coulomb interaction and large crystal field splitting among t_2g levels. Here, we controlled electron filling among d orbitals and controlled Mottness via applying epitaxial strain. The electronic structures are investigated by angle resolved photoemission and optical spectroscopy. We show metal to Mott insulator transition with tetragonal crystal field effect. Our work will provide a platform for correlated physics as well as applications for electronic devices with oxide monolayer.