Uncovering the Hidden Order in URu2Si2 by Impurity Doping

We report the use of impurities to probe the hidden order parameter of the strongly correlated metal URu_2Si_2 below the transition temperature T_0 ~ 17.5 K. The nature of this order parameter has eluded researchers for more than two decades, but is accompanied by the development of a partial gap in the single particle density of states that can be detected through measurements of the electronic specific heat and nuclear spin-lattice relaxation rate. We find that impurities in the hidden order phase give rise to local patches of antiferromagnetism. An analysis of the coupling between the antiferromagnetism and the hidden order reveals that the former is not a competing order parameter but rather a parasitic effect of the latter.