Temperature- and field-dependent measurements of the Hall effect of pure and 4% Rh-doped ${\mathrm{URu}}_2{\mathrm{Si}}_2$ reveal low density ($0.03\mathrm{\text{hole}}/\mathrm{U}$) high mobility carriers to be unique to the “hidden order“ phase and consistent with an itinerant density-wave order parameter. The Fermi surface undergoes a series of abrupt changes as the magnetic field is increased. When combined with existing de Haas–van Alphen data, the Hall data expose a strong interplay between the stability of the “hidden order,” the degree of polarization of the Fermi liquid, and the Fermi surface topology.

• Received 20 September 2006

DOI:https://doi.org/10.1103/PhysRevLett.98.016401