Spin excitations in the non-magnetic ground state of In2Ru2O7

Correlated transition-metal oxides with orbital degeneracy such as ruthenates with d4 (S = 1) configuration offer a plethora of unconventional electronic states. Combined with geometrical frustration, such materials may display exotic spin-orbital ordering phenomena. We discovered a new pyrochlore ruthenate In2Ru2O7, which undergoes a magnetic transition at ~ 220 K with a sharp drop of magnetic susceptibility to nearly zero simultaneously with a structural transition. This behaviour may point to a spin-singlet formation accompanying orbital order as observed in other ruthenates. muSR data excludes the presence of long-range antiferromagnetic order. In order to evidence a spin-singlet ground state, an observation of an excitation from a spin-singlet to spin-triplet is significant. We propose to perform inelastic neutron scattering on Merlin to detect the possible spin-gap of In2Ru2O7.