Orbital Dependent Phase Control in Ca2-xSrxRuO4

TL;DR

This study uses first-principles calculations to explore how orbital states and their occupations evolve in Ca$_{2-x}$Sr$_x$RuO$_4$, revealing orbital ordering and electronic structure changes across different magnetic phases.

Contribution

It provides new insights into orbital-dependent phase control in Ca$_{2-x}$Sr$_x$RuO$_4$, highlighting the role of 2D crystal structure effects without Jahn-Teller distortions.

Findings

Identification of ferro-orbital ordering in the AF regime

Explanation of optical spectra and X-ray absorption data

Description of pseudo-gap formation and magnetic moments at x=0.5

Abstract

We present first-principles studies on the orbital states of the layered perovskites Ca$_{2-x}$Sr$_x$RuO$_4$. The crossover from antiferromagnetic (AF) Mott insulator for $x < 0.2$ to nearly ferromagnetic (FM) metal at $x=0.5$ is characterized by the systematic change of the $xy$ orbital occupation. For the AF side ($x < 0.2$), we present firm evidence for the $xy$ ferro-orbital ordering. It is found that the degeneracy of $t_{2g}$ (or $e_g$) states is lifted robustly due to the two-dimensional (2D) crystal-structure, even without the Jahn-Teller distortion of RuO$_6$. This effect dominates, and the cooperative occupation of $xy$ orbital is concluded. In contrast to recent proposals, the resulting electronic structure explains well both the observed X-ray absorption spectra and the double peak structure of optical conductivity. For the FM side ($x=0.5$), however, the $xy$ orbital with half filling opens a pseudo-gap in the FM state and contributes to the spin $S$=1/2 moment (rather than $S$=1 for $x$=0.0 case) dominantly, while $yz,zx$ states are itinerant with very small spin polarization, explaining the recent neutron data consistently.