Anomalous curie response of impurities in quantum-critical spin-1/2 heisenberg antiferromagnets

We consider a magnetic impurity in two different S = 1/2 Heisenberg bilayer antiferromagnets at their respective critical interlayer couplings separating Neel and disordered ground states. We calculate the impurity susceptibility using a quantum Monte Carlo method. With intralayer couplings in only one of the layers (Kondo lattice), we observe an anomalous Curie constant C-*, as predicted on the basis of field-theoretical work [S. Sachdev et al., Science 286, 2479 (1999)]. The value C-* = 0.262 +/- 0.002 is larger than the normal Curie constant C = S(S+1)/3. Our low-temperature results for a symmetric bilayer are consistent with a universal C-*.