Orbital Dependent Phase Control in Ca2-xSrxRuO4

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.

17 pages, 5 figures