Orbital Ordering and Spin-Ladder Formation in ${\mathrm{La}}_2{\mathrm{RuO}}_5$

The semiconductor-semiconductor transition of $\mathrm{L}{\mathrm{a}}_2\mathrm{Ru}{\mathrm{O}}_5$ is studied by means of augmented spherical wave electronic structure calculations as based on density-functional theory and the local density approximation. This transition has lately been reported to lead to orbital ordering and a quenching of the local spin magnetic moment. Our results hint towards an orbital ordering scenario which, markedly different from the previously proposed scheme, preserves the local $S=1$ moment at the Ru sites in the low-temperature phase. The unusual magnetic behavior is interpreted by the formation of spin ladders, which result from the structural changes occurring at the transition and are characterized by antiferromagnetic coupling along the rungs.

Figure 1

Crystal structure of HT ${\mathrm{La}}_2{\mathrm{RuO}}_5$ viewed along the $c$ direction. La, Ru, and O atoms are displayed as large, medium, and small circles, respectively.

Figure 2

Partial DOS as resulting from the HT (left panel) and the LT (right panel) crystal structure.

Figure 3

Integrated partial DOS (NOS) of Ru $4d$ $t_{2g}$ states as resulting from the HT (left panel) and the LT (right panel) crystal structure. Results for the Ru(1) atom of the LT phase are very similar to those for atom Ru(2), which are shown here. Orbitals refer to the rotated coordinate system depicted in Fig. 1. The arrows indicate the centers of gravity of the respective bands.

Figure 4

Crystal and spin structure of LT ${\mathrm{La}}_2{\mathrm{RuO}}_5$ viewed along the $c$ direction. La, Ru, and O atoms are displayed as large, medium, and small circles, respectively.