101Ru NQR and hidden order in URu2Si2

The still unclarified hidden order phase below To = 17.5 K in the heavy-electron system URu2Si2 has been studied by Ru-101 NQR on a single crystal under H-ext = 0 and ambient P. Considering an invariant form of the quadrupole coupling between a Ru-101 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 Ru-101 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 (101)v(2Q)(T) above T-o. (101)v(2Q)(T) shows a slight but significant increase below TQ congruent to 13.5 K. Combining with general considerations on the effects on Ru-101 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 T-o. which is discussed in connection with related measurements.