Nature of the Mott transition in Ca2RuO4

TL;DR

This study reveals that the temperature-induced Mott transition in Ca2RuO4 is driven by structural changes linked to orbital order, with specific orbital and electronic behaviors identified in metallic and insulating phases.

Contribution

The paper demonstrates how structural transitions induce the Mott transition in Ca2RuO4 and clarifies the role of orbital order and Coulomb interactions using advanced theoretical methods.

Findings

Mott transition driven by structural change from L-Pbca to S-Pbca phase.

Orbital polarization is negligible in the metallic phase.

Lower energy orbital (~xy) is fully occupied in the insulating phase.

Abstract

We study the origin of the temperature-induced Mott transition in Ca2RuO4. As a method we use the local-density approximation+dynamical mean-field theory. We show the following. (i) The Mott transition is driven by the change in structure from long to short c-axis layered perovskite (L-Pbca to S-Pbca); it occurs together with orbital order, which follows, rather than produces, the structural transition. (ii) In the metallic L-Pbca phase the orbital polarization is ~0. (iii) In the insulating S-Pbca phase the lower energy orbital, ~xy, is full. (iv) The spin-flip and pair-hopping Coulomb terms reduce the effective masses in the metallic phase. Our results indicate that a similar scenario applies to Ca_{2-x}Sr_xRuO_4 (x<0.2). In the metallic x< 0.5 structures electrons are progressively transferred to the xz/yz bands with increasing x, however we find no orbital-selective Mott transition down to ~300 K.