Uncovering the hidden order in URu2Si2: Identification of Fermi surface   instability and gapping
  Spontaneous, collective ordering of electronic degrees of freedom leads to second-order phase transitions that are characterized by an order parameter. The notion "hidden order" (HO) has recently been used for a variety of materials where a clear phase transition occurs to a phase without a known order parameter. The prototype example is the heavy-fermion compound URu2Si2 where a mysterious HO transition occurs at 17.5K. For more than twenty years this system has been studied theoretically and experimentally without a firm grasp of the underlying physics. Using state-of-the-art density-functional theory calculations, we provide here a microscopic explanation for the HO. We identify the Fermi surface "hot spots" where degeneracy induces a Fermi surface instability and quantify how symmetry breaking lifts the degeneracy, causing a surprisingly large Fermi surface gapping. As mechanism for the HO we propose spontaneous symmetry breaking through collective antiferromagnetic moment excitations. 
S. Elgazzar, J. Rusz, M. Amft, P. M. Oppeneer, J. A. Mydosh