Superconductivity and Magnetic Ordering in the Heavy Fermion System URu₂Si₂

The tetragonal (1-2-2) intermetallic compound URu₂Si₂ represents the only heavy-electron material [γ(T > 18 K) ≈ 180 mJ/mol · K²] in which both antiferromagnetic (T_N ≃ 17.5 K) and superconducting (T_c ≃ 1.2 K) transitions occur. At low temperatures (T < T_c) there is a coexistence of these two ordered states. The purpose of this paper is to present the basic experimental features of URu₂Si₂ including the results of specific-heat, magnetization, susceptibility, critical-field, high-field magnetization, (magneto-) resistivity, pressure dependence, thermal expansion, neutron-scattering, NMR, Hall-effect, electron spectroscopy, and μSR. These properties suggest that the antiferromagnetic transition, i.e., the alternate plane order of unusually small 0.03μ_B/U moments along the c-axis, opens up an energy gap on some portions of the Fermi surface. The remaining nongapped electrons are able at a lower temperature to become superconducting. Hence it would seem that both the magnetism and the superconductivity are carried by the same hybridized 5f-U electrons.