  Bond-dilution effects on the ground state of the square-lattice
antiferromagnetic Heisenberg model, consisting of coupled bond-alternating
chains, are investigated by means of the quantum Monte Carlo simulation. It is
found that, when the ground state of the non-diluted system is a non-magnetic
state with a finite spin gap, a sufficiently weak bond dilution induces a
disordered state with a mid gap in the original spin gap, and under a further
stronger bond dilution an antiferromagnetic long-range order emerges. While the
site-dilution-induced long-range order is induced by an infinitesimal
concentration of dilution, there exists a finite critical concentration in the
case of bond dilution. We argue that this essential difference is due to the
occurrence of two types of effective interactions between induced magnetic
moments in the case of bond dilution, and that the antiferromagnetic
long-range-ordered phase does not appear until the magnitudes of the two
interactions become comparable.
