We consider a magnetic impurity in two different $S=1/2$ Heisenberg bilayer antiferromagnets at their respective critical interlayer couplings separating Néel 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 $C^{*}=0.262\pm 0.002$ 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 $C^{*}$.

• Received 17 January 2007

DOI:https://doi.org/10.1103/PhysRevLett.99.027205