Orbitally driven spin-singlet dimerization in $S$=1 La$_{4}$Ru$_{2}$O$_{10}$

TL;DR

This study reveals that in La$_{4}$Ru$_{2}$O$_{10}$, orbital ordering drives the formation of spin-singlet dimers in an $S$=1 system, despite the Ru$^{4+}$ ions remaining in the $S$=1 state across the transition.

Contribution

The paper demonstrates that orbital physics, rather than crystal field effects, induces spin-singlet dimerization in a quasi 2D $S$=1 material, supported by spectroscopy and band structure calculations.

Findings

Ru$^{4+}$ ions stay in $S$=1 state across the transition.

Orbital ordering causes anisotropic antiferromagnetic exchange.

Spin-singlet dimers form due to orbital-driven interactions.

Abstract

Using x-ray absorption spectroscopy at the Ru-$L_{2,3}$ edge we reveal that the Ru$^{4+}$ ions remain in the $S$=1 spin state across the rare 4d-orbital ordering transition and spin-gap formation. We find using local spin density approximation + Hubbard U (LSDA+U) band structure calculations that the crystal fields in the low temperature phase are not strong enough to stabilize the $S$=0 state. Instead, we identify a distinct orbital ordering with a significant anisotropy of the antiferromagnetic exchange couplings. We conclude that La$_{4}$Ru$_{2}$O$_{10}$ appears to be a novel material in which the orbital physics drives the formation of spin-singlet dimers in a quasi 2-dimensional $S$=1 system.