Tuning orbital-selective phase transitions in a two-dimensional Hund's correlated system

For describing the novel quantum phases in multi-orbital materials, Hund’s rule coupling (J) has attracted much attention. Various intriguing phases can occur by roles of J depending on the orbital occupancy. However, experimental confirmation of the orbital occupancy dependency has been challenging since controlling the orbital degrees of freedom normally accompanies chemical inhomogeneities. In this study, we demonstrate a method to investigate the role of orbital occupancy in J related phenomena without inducing inhomogeneities. By growing SrRuO₃monolayers on various substrates with symmetry-preserving interlayers, we gradually tune the crystal field splitting and thus the orbital degeneracy of the Ru t_2g orbitals. It effectively varies the orbital occupancies. Via in-situ angle-resolved photoemission spectroscopy, we observe a metal-insulator transition (MIT). The MIT occurs with orbital differentiation: concurrent opening of a band insulating gap in the d_xy band and a Mott gap in the d_xz/yz bands. Our study provides an effective experimental method for investigation of orbital-selective phenomena in multi-orbital materials.