Interplay between Fermi surface topology and ordering in URu$_{2}$Si$_2$   revealed through abrupt Hall coefficient changes in strong magnetic fields
  Temperature- and field-dependent measurements of the Hall effect of pure and 4 % Rh-doped URu$_{2}$Si$_{2}$ reveal low density (0.03 hole/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. 
Y. S. Oh, Kee Hoon Kim, P. A. Sharma, N. Harrison, H. Amitsuka, and J.   A. Mydosh