¹⁰¹Ru NQR and Hidden Order in URu₂Si₂

Abstract

The still unclarified hidden order phase below T 0 =17.5 K in the heavy-electron system URu 2 Si 2 has been studied by 101 Ru NQR on a single crystal under H ext =0 and ambient P . Considering an invariant form of the quadrupole coupling between a 101 Ru nucleus and U 5 f electrons, which is compatible with the symmetry of the U–Ru bonds, it is shown that U 5 f quadrupoles can be probed by 101 Ru NQR and that the singlet ground-state models for the (5 f ) 2 configuration of U can naturally explain the observed large T -dependence of the NQR frequency 101 ν 2Q ( T ) above T 0 . 101 ν 2Q ( T ) shows a slight but significant increase below \(T_{\text{Q}}\cong13.5\) K. Combining with general considerations on the effects on 101 Ru 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 y z and/or V z x due to quadrupole order. In this interpretation the onset of the quadrupole order is retarded by about 4 K below T 0 , which is discussed in connection with related measurements.