Bond-Dilution Effects on Two-Dimensional Spin-Gapped Heisenberg Antiferromagnets

Bond-dilution effects on spin-1/2 spin-gapped Heisenberg antiferromagnets of coupled alternating chains on a square lattice are investigated by means of the quantum Monte Carlo method. It is found that, in contrast with the site-diluted system having an infinitesimal critical concentration, the bond-diluted system has a finite critical concentration of diluted bonds, $x_c$, above which the system is in an antiferromagnetic (AF) long-range ordered phase. In the disordered phase below $x_c$, plausibly in the concentration region significantly less than $x_c$, the system has a spin gap due to singlet pairs of induced magnetic moments reformed by the AF interactions through the two-dimensional shortest paths.