Impurity Effect on Spin Ladder System

Effects of nonmagnetic impurity doping in a spin ladder system with a spin gap are investigated by exact diagonalization andvariational Monte Carlo calculations.Substantial changes in macroscopic properties such as enhancements in spin correlations and magnetic susceptibilities are observed in the low impurity concentration region,which are due to an increase of low-energy states. These results suggestthat a small but finite amount of nonmagnetic impurity dopingcauses reduction or disappearance of the spin gap.This qualitatively explains the experimental resultsfor Zn-doped SrCu₂O₃ in which a small amount of doping results in disappearance of the spin gap.We propose a possible scenario for this marked changeas a quantum phase transitionin a spin-gap ladder system due to spinon doping effects.