Hidden order in URu2Si2

Neutron-scattering and specific-heat measurements of the heavy-fermion superconductor URu2Si2 under hydrostatic pressure and with Rh-doping, U(Ru0.98Rh0.02)(2)Si-2, show the existence of two magnetic phase transitions. At T-m similar to 17.5 K, 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 0.03 mu(B). At T-M < T-m under pressure, p >= 5 kbar, or small Rh doping, a first-order phase transition gives rise to small anomalies in the specific heat and a large ordered moment of similar to 0.3 mu(B), but with the same magnetic structure. The results can be understood in terms of a hidden order parameter psi, which is linearly coupled to the ordered moment m. It follows that m and psi have the same symmetry and hence both break time-reversal symmetry. (c) 2005 Elsevier B.V. All rights reserved.