Exotic vortex effect on the alternating order around impurities in two-dimensional antiferromagnets

It is well known that a magnetic field applied onto a Heisenberg antiferromagnet acts as an effective easy-plane anisotropy leading, in the case of two dimensions, to a Berezinskii-Kosterlitz-Thouless transition at a field-dependent temperature. If a nonmagnetic impurity is introduced in the lattice, an island of finite staggered magnetization develops, for which a recently proposed effective model predicts a universal shape that is almost independent of temperature and whose amplitude decays exponentially with the distance from the impurity. This picture was confirmed for the two-dimensional spin-1/2 case by quantum Monte Carlo simulations, whose outcomes, however, display also a counterintuitive effect: as the temperature approaches the critical value, the field-induced staggered magnetization surprisingly increases its range, which reaches a maximum before definitely dropping to zero in the disordered phase. In this paper an explanation of such a strange behavior is given as the result of vortex excitation. (C) 2009 American Institute of Physics.