¹⁰¹Ru NQR and hidden order in URu₂Si₂

The still unclarified hidden order phase below T₀ = 17.5 K in the heavy-electron system URu₂Si₂ has been studied by ¹⁰¹Ru NQR on a single crystal under H_ext = 0 and ambient P. Considering an invariant form of the quadrupole coupling between a ¹⁰¹Ru nucleus and U 5f electrons, which is compatible with the symmetry of the U-Ru bonds, it is shown that U 5f quadrupoles can be probed by ¹⁰¹Ru NQR and that the singlet ground-state models for the (5f) ² configuration of U can naturally explain the observed large T-dependence of the NQR frequency ¹⁰¹ν_2Q(T) above T₀. ¹⁰¹ν_2Q(T) shows a slight but significant increase below T_Q ≅ 13.5 K. Combining with general considerations on the effects on ¹⁰¹Ru NQR of a possible symmetry-breaking electric field gradient (EFG) or hyperfine field due to hidden order, we discuss that the increase can be interpreted as arising from symmetry-breaking EFG components of V_yz and/or V_zx due to quadrupole order. In this interpretation the onset of the quadrupole order is retarded by about 4 K below T₀, which is discussed in connection with related measurements.