Vortex-enhanced alternating order around impurities in antiferromagnets

It has been recently pointed out that the presence of a nonmagnetic impurity in a Heisenberg antiferromagnet generates an alternating order of the surrounding spins which is independent of temperature in a wide range. Quantum Monte Carlo simulations in the two-dimensional $S=1/2$ case confirmed this picture, but showed a counterintuitive enhancement of the alternating order around the Kosterlitz-Thouless transition. We propose here an explanation in terms of the effect of vortex excitations.