Evidence for a strong coupling transition in Y$_2$Ru$_2$O$_7$

Y$_2$Ru$_2$O$_7$ adopts the pyrochlore structure with Ru occupying the octahedral sites. Magnetization and specific heat measurements indicate that there is a magnetic phase transition at T= 78 K. Diffraction experiments have shown that the Ru-sublattice orders in a long range ordered q=0 structure where the total spin vanishes on each tetrahedron. We have performedneutron inelastic scattering experiments in order to investigate this magnetic phase transition. These experiments show what I would call a strong coupling transition within this material. There are rearrangements in the excitation spectrum at T= 78 K to energies that correspond to room temperature without an apparent change in the wave vector dependence. Light scattering experiments indicate that magneto-elastic effects are important in allowing this phase transition to proceed. From previous work on spinel’s it has been suggested that this type of transition should be considered the 3 D analogue of a spin-peierls transition in a cooperative spin system [1]. This research was funded by the U.S. Department of Energy, under Grant No. DE-FG02-02ER45983, and by the National Science Foundation, under Grant No. NSF-DMR-0103858. [1]S.-H. Lee et al., Phys. Rev. Lett. 84, 3718 (2000).