Hidden order in URu2Si2

Neutron-scattering and specific-heat measurements of the heavy-fermion superconductor URu 2 Si 2 under hydrostatic pressure and with Rh-doping, U(Ru 0.98 Rh 0.02 ) 2 Si 2 , show the existence of two magnetic phase transitions. At T m ∼ 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 0.03μ 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 ∼ 0.3μ B , but with the same magnetic structure. The results can be understood in terms of a hidden order parameter ψ, which is linearly coupled to the ordered moment m. It follows that m and ψ have the same symmetry and hence both break time-reversal symmetry.