Hidden order in URu 2 Si 2

Neutron-scattering and specific-heat measurements of the heavy-fermion superconductor URu2Si2 under hydrostatic pressure and with Rh-doping, UðRu0:98Rh0:02Þ2Si2; show the existence of two magnetic phase transitions. At T m � 17: 5K ; a second-order phase transition with strong anomalies in the specific heat and other macroscopic and transport properties are accompanied by an antiferromagnetically ordered dipolar moment of only 0:03mB: At T MoT m under pressure, pX5 kbar; or small Rh doping, a first-order phase transition gives rise to small anomalies in the specific heat and a large ordered momentof � 0:3mB; but with the same magnetic structure. The results can be understood in terms of a hidden order parameter c; which is linearly coupled to the ordered moment m. Itfollows t m and c have the same symmetry and hence both break time-reversal symmetry. r 2005 Elsevier B.V. All rights reserved.