Local moments in the heterogeneous electronic state of Cd-substituted CeCoIn₅: NQR relaxation rates

We have used nuclear quadrupole resonance (NQR) to probe microscopically the response of a prototypical quantum critical metal CeCoIn₅ to substitutions of small amounts of Cd for In. Approximately half of the Cd substituents induce local Ce moments in their close proximity, as observed by site-dependent longitudinal nuclear spin relaxation rates 1/T₁. To reaffirm that localized f moments are induced around the Cd substituents, we find a Gaussian spin-echo decay rate 1/T_2G of transverse nuclear spin relaxation. Further, $T_{1}T/T_{2\text{G}}^2$ for the NQR subpeak is found to be proportional to temperatures, again indicating local moments fluctuations around the Cd substituents, while that for the NQR main peak shows a T^0.7-dependence. The latter temperature dependence is close to 0.75 in pure CeCoIn₅ and indicates that the bulk electronic state is located close to a two dimensional quantum critical instability. (paper)