Magnetic and electronic properties of URu₂Si₂ revealed by comparison with nonmagnetic references ThRu₂Si₂ and LaRu₂Si₂

We have carried out nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on ThRu₂Si₂ and LaRu₂Si₂, which are the nonmagnetic references of the intriguing heavy fermion URu₂Si₂. The comparisons of URu₂Si₂ with the reference materials allow us to analyze the already known NMR and NQR data on URu₂Si₂ phenomenologically and semiquantitatively. The study of ¹⁰¹Ru-NQR frequency suggests the relatively close electronic configuration of URu₂Si₂, including the valence of the actinide ion, to that of the tetravalent ThRu₂Si₂ at high temperatures, as well as the delocalization of 5 f electrons at low temperatures. Ising-like spin fluctuations along the c-axis were brought to light by ²⁹Si-NMR data in the so-called hidden order phase of URu₂Si₂. The unique magnetic property is plausibly associated with the mechanism of the unconventional superconductivity.