Unusual magnetic field-dependence of a possible hidden order phase

Abstract URu₂Si₂ exhibits a second-order phase transition at 17.5 K. Initially it was thought that the transition was to a spin density wave phase, however, subsequent measurements do not support this assignment. Despite the unknown nature of the order parameter, many experimental results can be described in terms of the formation of a generic density wave. Here, we report calculations on an unusual phase of the underscreened Anderson lattice model, the so called spin-dependent inter-orbital density wave that has been proposed as describing the “hidden order” phase of URu₂Si₂. We determine the effects of an applied magnetic field. Since the order parameter describes an ordering in the x–y plane, the electronic properties of the system are anisotropic below the critical temperature T_HO. We show that the magnetic susceptibility becomes anisotropic below T_HO. Furthermore, for fields applied along a spontaneously chosen hard axis, T_HO decreases towards zero and that the HO transition changes from second order to first order at a large value of the magnetic field. Also, we find that the system undergoes a cascade of field-induced Lifshitz transitions and also show how these properties originate from the dependence of the quasi-particle bands on the orientation of the applied field. The good qualitative agreement with experimental findings provides strong support for the proposed description of the HO phase as a spin-dependent inter-orbital density wave phase.