  The study of impurities in low dimensional antiferromagnets has been a very
active field in magnetism ever since the discovery of high temperature
superconductivity. One of the most dramatic effects is the appearance of large
Knight shifts in a long range around non-magnetic impurities in an
antiferromagnetic background. The dependence of the Knight shifts on distance
and temperature visualizes the correlations in the system. In this work we
present the results for Knight shifts around a single vacancy in the one and
two dimensional Heisenberg model.
