Lattice Energetics and Correlation-Driven Metal-Insulator Transitions: The Case of Ca$_2$RuO$_4$

TL;DR

This paper combines density functional, dynamical mean field, and Landau-theory methods to analyze the energetics and structural effects driving the Mott metal-insulator transition in Ca$_2$RuO$_4$, highlighting the importance of lattice-electronic coupling.

Contribution

It develops a Landau-theory free energy model that integrates electronic energetics with lattice distortions and strain effects, applied specifically to Ca$_2$RuO$_4$.

Findings

Lattice energy change is comparable to electronic energy change across the transition.

The transition is strongly first order and highly pressure-sensitive.

Epitaxial constraints can suppress the metal-insulator transition.

Abstract

This Letter uses density functional, dynamical mean field, and Landau-theory methods to elucidate the interplay of electronic and structural energetics in the Mott metal-insulator transition. A Landau-theory free energy is presented that incorporates the electronic energetics, the coupling of the electronic state to local distortions and the coupling of local distortions to long-wavelength strains. The theory is applied to Ca$_2$RuO$_4$. The change in lattice energy across the metal-insulator transition is comparable to the change in electronic energy. Important consequences are a strongly first order transition, a sensitive dependence of the phase boundary on pressure and that the geometrical constraints on in-plane lattice parameter associated with epitaxial growth on a substrate typically change the lattice energetics enough to eliminate the metal-insulator transition entirely.