Local impurities in 2D quantum antiferromagnets
  The two-dimensional quantum Heisenberg antiferromagnet at zero temperature with two kinds of locally frustrating defects - isolated ferromagnetic bonds and impurity spins, is studied. For a quantum spin, coupled antiferromagnetically to the two sublattices, we find a local ground state transition. In the case of a ferromagnetic bond, the local magnetization is found to remain finite even for strong coupling, contrary to previous results, derived within the linear spin-wave approximation. Our results are based on an analytic approach, suitable to treat strong defect-environment interaction, as well as exact diagonalization studies. Technical details and some extension of this work can be found in cond-mat/9712281. 
V.N. Kotov, J. Oitmaa, and O. Sushkov (UNSW)