Spatial inhomogeneity and the metal-insulator transition in Ca$_3$(Ru$_{1-x}$Ti$_x$)$_2$O$_7$

TL;DR

This study investigates the unusual metal-insulator transition in Ca$_3$Ru$_2$O$_7$ caused by Ti doping, highlighting the roles of electronic correlations and spatial inhomogeneity in driving site-selective Mott criticality.

Contribution

It reveals how electronic correlations and spatial inhomogeneity cooperate to induce a site-selective Mott transition in doped Ca$_3$Ru$_2$O$_7$, a novel insight into the MIT mechanism.

Findings

Electronic correlations and inhomogeneity drive the MIT.

Site-selective Mott criticality emerges in the doped compound.

Competing orbital-ordering tendencies influence the transition.

Abstract

Turning a pristine Mott insulator into a correlated metal by chemical doping is a common procedure in strongly correlated materials physics, e.g. underlying the phenomenology of high-$T_c$ cuprates. The ruthenate bilayer compound Ca$_3$Ru$_2$O$_7$ is a prominent example of a reversed case, namely a correlated metal at stoichiometry that realizes a transition into an insulating state via Ti doping. We here investigate this puzzling metal-insulator transition (MIT) by first-principles many-body theory and elucidate a challenging interplay between electronic correlations and symmetry breakings on the Ru sublattice. While average effects on the Ca$_3$Ru$_2$O$_7$ crystal structure are still relevant, key to the MIT is the cooperation of electronic correlations with the spatial inhomogeneity in the defect regime. Together they give rise to the emergence of site-selective Mott criticality and competing orbital-ordering tendencies.