Edge effects in the two-dimensional spin-1/2 Heisenberg antiferromagnet
  We use quantum Monte Carlo simulations to study effects of free edges in the two-dimensional spin-1/2 Heisenberg antiferromagnet. We find that the magnetic response of an edge is smaller than the bulk susceptibility. This counter-intuitive quantum effect can be traced to enhanced antiferromagnetic nearest-neighbor spin correlations, i.e., tendency to local singlet formation, at and close to the edge. These correlations form a comb-like pattern, which can be reproduced with a simple variational valence-bond state. We also study rough edges, and find that these instead significantly enhance the susceptibility, due to local sublattice imbalance impeding singlet formation. 
Kaj H. Hoglund and Anders W. Sandvik