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Evolution of Heterogeneous Antiferromagnetic State in URu2Si2: Study of Hydrostatic-Pressure, Uniaxial-Stress and Rh-Dope Effects

We have investigated the nature of the competition between hidden order and antiferromagnetic (AF) order in URu_2Si_2 by performing the neutron scattering experiments under hydrostatic-pressure P, uniaxial-stress sigma, and Rh-substitution conditions. Hidden order observed at ambient pressure in pure URu_2Si_2 is found to be replaced by the AF order by applying P, sigma along the tetragonal basal plane, and by doping Rh. We discuss these experimental results on the basis of the crystalline strain calculations, and suggest that this phase transition is generated by the 0.1% increase of the tetragonal c/a ratio. We have also found that the magnetic excitation observed in the hidden order phase vanishes in the AF phase. We show that this variation can be qualitatively explained by assuming the hidden order parameter to be quadrupole.