Local enhancement of antiferromagnetic correlations by nonmagnetic impurities

The local enhancement of antiferromagnetic correlations near vacancies observed in a variety of spin systems is analyzed in a single framework. Variational calculations suggest that the resonating-valence-bond character of the spin correlations at short distances is responsible for the enhancement. Numerical results for uniform spin chains, with and without frustration, dimerized chains, ladders, and two dimensional clusters are in agreement with our conjecture. This shore distance phenomenon occurs independently of the long distance behavior of the spin correlations in the undoped system. Experimental predictions for a variety of compounds are briefly discussed.