Evidence for a hidden order parameter in URu2Si2

We have measured the field dependence of the ordered antiferromagnetic moment and the magnetic excitations in URu2Si2 for fields up to 17 T applied along c axis, using neutron scattering. The decrease of the magnetic intensity of the tiny moment m with increasing field does not follow a simple power law, but shows a clear inflection point, suggesting that the moment m is connected to a hidden order parameter Psi. The magnetic excitation gap at Q = (1, 0, 0) increases continuously with increasing field, while that at Q = (1, 4, 0, 0) is nearly constant. (C) 2003 Published by Elsevier B.V.