Comparison of the hidden order transition in URu2Si2 to the λ-transition in 4He

The low -temperature states of ambient URu2Si2 and superfluid 4He are both characterized by momentumdependent energy gaps between the ground and excited states. This behavior weakly persists even above the transition temperatures but becomes overdamped (ungapped) because of the number of excitations present at elevated temperature. We show that, akin to the normal-fluid-to-superfluid transition in 4He, the hidden order (HO) transition in URu2Si2 can be understood by a change of the ungapped excitations to the gapped, elementary excitations (EE) of the unknown ordered state. These underdamped EEs reflect the basic character and order parameters of the different phase transitions. This view accounts for the full amount of entropy released in these transitions, the jumps in the resistivity and thermal conductivity directly below the transition, as well as the reduction of the Fermi surface. We argue that the behavior in the HO phase is that of a gas of weakly interacting excitations from charge density wave or crystal -field states in a similar manner to that of the phonon-roton excitations of the superfluid 4He phase. We discuss the influence of applying pressure and magnetic fields within this scenario and the role of the small moment antiferromagnetic clustering in the hidden order phase.