Quantum impurity in a nearly critical two-dimensional antiferromagnet

The spin dynamics of an arbitrary Localized impurity in an insulating two-d imensional antiferromagnet, across the host transition from a paramagnet wi th a spin gap to a Neel state, is described. The impurity spin susceptibili ty has a Curie-like divergence at the quantum-critical coupling, but with a universal effective spin that is neither an integer nor a half-odd integer . In the Neel state, the transverse impurity susceptibility is a universal number divided by the host spin stiffness (which determines the energy cost to slow twists in the orientation of the Neel order). These and numerous o ther results for the thermodynamics, Knight shift, and magnon damping have important applications in experiments on Layered transition metal oxides.