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

Abstract

Hund's rule coupling (J) has attracted much attention recently for its role in the description of the novel quantum phases of multi-orbital materials. Depending on the orbital occupancy, J can lead to various intriguing phases. However, experimental confirmation of the orbital occupancy dependency has been difficult as controlling the orbital degrees of freedom normally accompanies chemical inhomogeneities. Here, we demonstrate a method to investigate the role of orbital occupancy in J related phenomena without inducing inhomogeneities. By growing SrRuO3 monolayers 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 of two-dimensional (2D) ruthenates. Via in-situ angle-resolved photoemission spectroscopy, we observe a progressive metal-insulator transition (MIT). It is found that 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. Hund's coupling, or the intra-atomic exchange, can drive novel quantum phases in multi-orbital systems, but this requires precise control of orbital occupancy. Ko et al. report an orbital-selective metal-to-insulator transition driven by Hund & PRIME;s physics via symmetry-preserving strain tuning in monolayer SrRuO3.