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.