Spin gap and antiferromagnetic fluctuation in spin ladder system doped with nonmagnetic impurities

Effects of nonmagnetic impurity doping in a spin ladder system with a spin gap are investigated by the exact diagonalization and the variational Monte Carlo calculations. Small amount of impurity doping generates a nearly free spin bound to each impurity. These nearly free spins interact each other only weakly giving rise to gapless spin excitations. Another important aspect is that short-ranged antiferromagnetic correlation is enhanced especially around the impurity sites. We propose a possible scenario for this drastic change in the spin gapped ladder system. We also compare with the experimental results of Zn-doped SrCu₂O₃.