SPIN-WAVE COLLAPSE AND INCOMMENSURATE FLUCTUATIONS IN URU2SI2

To test if the T(N) = 17.7 K transition in URu2Si2 is driven by a dive rgence of a magnetic order parameter we performed high-resolution neut ron scattering. At the ordering wave vector the spin-wave energy colla psed. and the susceptibility diverged as T(N) was approached. This con firms that the order parameter is the magnetic dipole, as shown by rec ent symmetry arguments and polarized neutron experiments [1]. We also observe incommensurate fluctuations, suggesting that competing tempera ture-dependent interactions may influence this weak-moment transition.