Effects of Nonmagnetic Impurity Doping on Spin Ladder System

Effects of nonmagnetic impurity doping on an antiferromagnetic spin-1/2 Heisenberg ladder system are studied by the quantum Monte Carlo method. A single nonmagnetic impurity induces a localized spin-1/2 moment accompanied by static and enhanced antiferromagnetic correlations around it. A small and finite concentration of impurities induces a remarkable change of magnetic and thermodynamic properties with gapless excitations. It also shows rather sharp but continuous crossover around a impurity concentration of about 4%. Above the crossover concentration, all the spins are strongly coupled, participating in the enhanced and rather uniform power-law decay of the antiferromagnetic correlation. Below the crossover concentration, each impurity forms an antiferromagnetically correlated cluster only weakly coupled each other. For random distribution of impurities, large Curie-like susceptibility accompanied with small residual entropy is obtained at low temperatures, in agreement with recent experimental observation in Zn-doped SrCu₂O₃. The temperature dependence of antiferromagnetic susceptibility shows power-law-like but weaker divergence than the single chain antiferromagnet in the temperature range studied.