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

TL;DR

This study demonstrates a method to control orbital occupancy in 2D ruthenates, revealing how Hund's coupling influences orbital-selective metal-insulator transitions without chemical inhomogeneity.

Contribution

It introduces a substrate-based approach to tune orbital degeneracy and investigate orbital-selective phase transitions in multi-orbital systems.

Findings

Observation of a progressive metal-insulator transition with orbital differentiation.

Concurrent opening of band and Mott gaps in different orbitals.

Effective experimental method for studying orbital-selective phenomena.

Abstract

Hund's rule coupling ($\textit{J}$) 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, $\textit{J}$ 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 $\textit{J}$ related phenomena without inducing inhomogeneities. By growing SrRuO$_3$ monolayers on various substrates with symmetry-preserving interlayers, we gradually tune the crystal field splitting and thus the orbital degeneracy of the Ru $\textit{t$_2$$_g$}$ orbitals. It effectively varies the orbital occupancies of two-dimensional (2D) ruthenates. Via in-situ angle-resolved photoemission spectroscopy, we observe a progressive metal-insulator transition (MIT). It is found that the MIT occurs with orbital differentiation: concurrent opening of a band insulating gap in the $\textit{d$_x$$_y$}$ band and a Mott gap in the $\textit{d$_x$$_z$$_/$$_y$$_z$}$ bands. Our study provides an effective experimental method for investigation of orbital-selective phenomena in multi-orbital materials.