Pressure-temperature phase diagram of the heavy-electron superconductor URu₂ Si₂

The pressure-temperature phase diagram of the heavy-electron superconductor URu₂ Si₂ has been reinvestigated by AC-susceptibility and elastic neutron-scattering (NS) measurements performed on a small single-crystalline rod (2 mm in diameter, 6 mm in length) in a Cu-Be clamp-type high-pressure cell (P < 1.1 GPa). At ambient pressure, this sample shows the weakest antiferromagnetic (AF) Bragg reflections reported so far, corresponding to the volume-averaged staggered moment of μ_o ∼ 0.011 μ_B / U. Under applied pressure, the AF scattering intensity exhibits a sharp increase at P ∼ 0.7 GPa at low temperatures. The saturation value of the AF scattering intensity above 0.7 GPa corresponds to μ_o ∼ 0.41 μ_B / U, which is in good agreement with that (∼ 0.39 μ_B / U) observed above 1.5 GPa in our previous NS measurements. The superconductivity is dramatically suppressed by the evolution of AF phase, indicating that the superconducting state coexists only with the hidden order phase. The presence of parasitic ferro- and/or antiferromagnetic phases with transition temperatures T₁^{{star, open}} = 120 (5), T₂^{{star, open}} = 36 (3) and T₃^{{star, open}} = 16.5 (5) K and their relationship to the low-T ordered phases are also discussed.