Evidence for high temperature orbital fluctuations in La$_{4}$Ru$_{2}$O$_{10}$

Among ruthenates, the compound La$_{4}$Ru$_{2}$O$_{10}$ is unique in having a complete orbital ordering transition. Upon cooling below $T_{OO}$ = 160K, the magnetism of this compound is abruptly quenched. Initial crystallographic studies of this phase [1] showed that strong distortions in the Ru-O bond lengths appear only below $T_{OO}$. We have found distinctly different thermal conductivities of single-crystal La$_{4}$Ru$_{2}$O$_{10}$ above and below $T_{OO}$. At high temperatures, this material unexpectedly behaved as a thermal glass, providing strong evidence for dynamic orbital fluctuations in La$_{4}$Ru$_{2}$O$_{10}$. This is supported by a detailed analysis of the atomic displacement parameters (ADPs) of this material. We will discuss our current model for orbital ordering in La$_{4}$Ru$_{2}$O$_{10}$, the differences in the local and average structure of this material, and the reasons for the strong magnetoelastic coupling in this phase. [1] P. Khalifah, R. Osborn, Q. Huang, H. W. Zandbergen, R. Jin, Y. Liu, D. Mandrus, and R. J. Cava. ``Orbital ordering transition in La$_{4}$Ru$_{2}$O$_{10}$'', \textit{Science}, \textbf{297}, 2237-40 (2002).