Hidden order in URu2Si2URu2Si2

Neutron-scattering and specific-heat measurements of the heavy-fermion superconductor URu2Si2URu2Si2 under hydrostatic pressure and with Rh-doping, U(Ru0.98Rh0.02)2Si2U(Ru0.98Rh0.02)2Si2, show the existence of two magnetic phase transitions. At Tm∼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 0.03μB0.03μB. At TMmagnetic structure. The results can be understood in terms of a hidden order parameter ψψ, which is linearly coupled to the ordered moment m. It follows that m and ψψ have the same symmetry and hence both break time-reversal symmetry.