Signatures of broken spin-rotational invariance in the “Hidden Ordered” compound URu₂Si₂?

We briefly review the status of research on the “Hidden Ordered” compound . We focus on evidence which indicates that the compound undergoes an electronic structural change from body-centred tetragonal to simple tetragonal, but which apparently does not involve any atomic displacements. The fourfold rotational invariance under rotations in the a–b plane may also be spontaneously broken by the transition. We suggest that the electronic system may be modelled by an undercompensated Anderson Lattice Model, and that the transition to the “Hidden Ordered” state is driven by nested interband correlations. We show that the model exhibits a delicate competition between the “Hidden Order” and a Neel antiferromagnetic phase, and that it might also describe the first-order transition in found by applying pressure. Furthermore, we show that the broken spin-rotational invariance of the novel phase becomes apparent when magnetic fields are applied.