Site-dilution-induced antiferromagnetic long-range order in a two-dimensional spin-gapped Heisenberg antiferromagnet

Effects of the site dilution on spin-gapped Heisenberg antiferromagnets with S=1/2 and S=1 on a square lattice of coupled dimerized chains arc investigated by means of the quantum Monte Carlo method. It is found that effective magnetic moments induced around the diluted sites exhibit the antiferromagnetic long-range order in the medium of spin-singlet pairs. Their microscopic structure is examined in detail and important roles of higher dimensionality other than one in the phenomenon are discussed.