Orbital ordering transition in Ca$_2$RuO$_4$ observed with resonant x-ray diffraction

TL;DR

This study uses resonant x-ray diffraction to reveal both magnetic and orbital ordering transitions in Ca$_2$RuO$_4$, including a newly observed orbital order transition at 260 K, challenging some existing theoretical models.

Contribution

It provides experimental evidence of orbital ordering in Ca$_2$RuO$_4$ and identifies a new phase transition at 260 K, expanding understanding of its electronic phases.

Findings

Antiferromagnetic transition at 110 K confirmed.

Orbital ordering transition observed around 260 K.

Propagation vector of orbital order differs from some theoretical predictions.

Abstract

Resonant x-ray diffraction performed at the $\rm L_{II}$ and $\rm L_{III}$ absorption edges of Ru has been used to investigate the magnetic and orbital ordering in Ca$_2$RuO$_4$ single crystals. A large resonant enhancement due to electric dipole $2p\to 4d$ transitions is observed at the wave-vector characteristic of antiferromagnetic ordering. Besides the previously known antiferromagnetic phase transition at $\rm T_{N}=110$ K, an additional phase transition, between two paramagnetic phases, is observed around 260 K. Based on the polarization and azimuthal angle dependence of the diffraction signal, this transition can be attributed to orbital ordering of the Ru $t_{2g}$ electrons. The propagation vector of the orbital order is inconsistent with some theoretical predictions for the orbital state of Ca$_2$RuO$_4$.