The compound URu2Si2 is a heavy-fermion compound with a linear T term in the low temperatures electronic specific heat with a coefficient of 155 mJ/mole/K2. At T=1.5 K the system undergoes a transition to a superconducting state but exhibits another transition at the higher temperature of T=17.5 K. The 17.5 K transition is marked by a large lambda-like anomaly in the specific heat and was originally thought to be a second-order transition to a spin density wave state. The change in entropy associated with the specific heat anomaly is of the order of 0.3 kB ln(2). Also, at the transition, the system loses 90% of the carriers and develops a gap of the order of 7 meV over about 60% of the Fermi-surface. However, neutron scattering experiments have demonstrated that spin density wave order is absent below the 17.5 K transition and, likewise, x-ray scattering experiments have indicated the absence of charge or orbital density wave ordering. Despite over 30 years of intensive study, the nature of the ordering is still unknown. Hence, the transition is now known as the Hidden Order transition.
We propose that the 17.5 K transition is due to a combined spin and orbital ordering which can only be probed by measurements that are both charge and spin sensitive. We have developed a model in which the Hund’s rule exchange interaction stabilizes this novel state. Furthermore, we show that the material breaks spin-rotational invariance at low temperatures, but does not develop a static magnetic moment. In particular, the model shows that the magnetic susceptibility becomes anisotropic below the ordering temperature. Our calculations are compared with the results of the magnetic torque measurements of Okazaki et al. that found the four-fold symmetry of the magnetic susceptibility in the a-b plane is broken as the susceptibility develops a two-fold axis below the transition.