Search for hidden orbital currents and observation of an activated ring of magnetic scattering in the heavy fermion superconductor URu2Si2

We have performed neutron-scattering experiments on the heavy fermion superconductor URu2Si2 to search for the orbital currents predicted to exist in the ordered phase below TN=17.5 K which result in a ring in momentum space. Elastic scans in the (H, K, 0) and (H, 0, L) planes revealed no such order parameter at low temperatures. This shows that any orbital current formation is quite small and less than our detection limit for a ring of scattering of 0.06(1)μB (albeit somewhat greater than the size of the predicted moment of ∼0.03μB). On heating, however, we find that a ring of quasielastic scattering forms in the (H, K, 0) plane centered at an incommensurate radius τ=0.4 from the (1, 0, 0) antiferromagnetic (AF) Bragg position. The intensity at a point on the ring, (1.4, 0, 0), is thermally activated below TN with a characteristic energy scale of Δ=110 K∼6TN. This is the coherence temperature, and it is much higher than the spin-wave energy for the selected momentum. We believe that the incommensurate spin fluctuations compete with the AF spin fluctuations, drive the transition to a disordered magnetic state above TN, and contribute to the formation of the heavy fermion state.