Dimerized Excitations in La$_{4}$Ru$_{2}$O$_{10}$

The interplay between orbital, spin and charge degrees of freedom is at the forefront of condensed matter physics. The discovery of orbital ordering in La$_{4}$Ru$_{2}$O$_{10}$ [1] offers a unique opportunity to study orbital phenomena in a 4d transition metal oxide. La$_{4}$Ru$_{2}$O$_{10}$ undergoes a structural phase transition at $\sim $158K, below which there is a spin gap of $\sim $40 meV caused by Ru-O-Ru dimerization. We have performed single crystal time-of-flight neutron scattering measurements on the ARCS spectrometer at the SNS in ``sweep'' mode, a new technique in which neutron events are measured during continuous sample rotation, in order to quickly and efficiently map out four-dimensional volumes of S(q,w). With this method, we measured the structure factors and full dispersion of the singlet-triplet excitations as a function of \textbf{Q} and w. A model of the orbital ordering producing the magnetic excitations and the event-based technique used for their measurement will be discussed.\\[4pt] [1] P. Khalifah, et al., Science \textbf{297},2237 (2002)