Observation of a novel orbital selective Mott transition in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$

TL;DR

This paper reports the first experimental observation of an orbital selective Mott transition in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$, where some orbitals become localized while others remain itinerant, revealing complex electron behavior.

Contribution

It provides the first direct evidence of an orbital selective Mott transition in a transition metal oxide using ARPES, highlighting the roles of interorbital transfer and orbital degeneracy.

Findings

Fermi surface associated with $d_{xy}$ is missing due to localization.

Two dispersing bands observed for $d_{yz}$ and $d_{zx}$ orbitals.

Theoretical calculations identify key mechanisms driving the OSMT.

Abstract

Electrons in a simple correlated system behave either as itinerant charge carriers or as localized moments. However, there is growing evidence for the coexistence of itinerant electrons and local moments in transition metals with nearly degenerate $d$-orbitals. It demands one or more selective electron orbitals undergo the Mott transition while the others remain itinerant. Here we report the first observation of such an orbital selective Mott transition (OSMT) in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ by angle-resolved photoemission spectroscopy (ARPES). While we observed two sets of dispersing bands and Fermi surface associated with the doubly-degenerate $d_{yz}$ and $d_{zx}$ orbitals, the Fermi surface associated with the wider $d_{xy}$ band is missing, a consequence of selective Mott localization. Our theoretical calculations demonstrate that this novel OSMT is mainly driven by the combined effects of interorbital carrier transfer, superlattice potential, and orbital degeneracy, whereas the bandwidth difference plays a less important role.