P. Khalifah,^12* R. Osborn,³ Q. Huang,⁴⁵ H. W. Zandbergen,²⁶ R. Jin,⁷ Y. Liu,⁷ D. Mandrus,⁸ R. J. Cava¹²

We report experimental evidence for a full orbital ordering transition in the two-dimensional lanthanum ruthenate La₄Ru₂O₁₀. The observable consequences of this orbital ordering include the loss of the Ru local moment, a structural distortion which partitions Ru-O bonds into axially oriented short and long sets, a sharp jump in electrical resistivity, and the opening of a spin gap that is visible in neutron scattering experiments. This is a rare example of a discrete orbital ordering transition in a 4d transition metal oxide and demonstrates that orbital effects can have an influence on the properties of layered ruthenates, a family of compounds that notably includes the p-wave superconductor Sr₂RuO₄ and the field-tuned quantum critical metamagnet Sr₃Ru₂O₇.

¹ Department of Chemistry,
² Princeton Materials Institute, Princeton University, Princeton, NJ 08540, USA.
³ Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA.
⁴ NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
⁵ Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742, USA.
⁶ National Center for High Resolution Electron Microscopy, Technical University of Delft, Rotterdamsweg 137, 2628AL Delft, The Netherlands.
⁷ Department of Physics, The Pennsylvania State University, University Park, PA 16802, USA.
⁸ Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.
^*   To whom correspondence should be addressed. E-mail: kpete{at}ornl.gov

   Present address: Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.