Effects of Nonmagnetic Impurity Doping on Spin Ladder System

Abstract

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. 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 the 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, each impurity forms an antiferromagnetic 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 $SrCu_{2}O_{3}$.