2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/20958We present a new formulation of the theory of an arbitrary quantum impurity in an antiferromagnet, using the O(3) non-linear sigma model. We obtain the low temperature expansion for the impurity spin susceptibilities of antiferromagnets with magnetic long-range order in the ground state. We also consider the bulk quantum phase transition in d=2 to the gapped paramagnet (d is the spatial dimension): the impurity is described solely by a topological Berry phase term which is an exactly marginal perturbation to the critical theory. The physical properties of the quantum impurity near criticality are obtained by an expansion in (d-1).14 pages, 7 figures; (v2) added refStrongly Correlated ElectronsSuperconductivityHigh Energy Physics - Theorytext