Nature of the order parameter in the heavy-fermion systemURu2Si2

Abstract
The weak order parameter observed below the 17.5 K transition in UrU2 Si2 has been variously attributed to magnetic dipoles, quadrupoles, or octupoles. By polarized neutron scattering we show that the order parameter breaks time reversal invariance and so cannot be quadrupolar or a spin nematic. Of the five dipolar and octupolar order parameters that are then allowed by symmetry, polarization analysis reveals that only the spin dipole, M2, orders at the transition.