Effects of nonmagnetic impurity doping in a spin ladder system with a spin gap are investigated by exact diagonalization and variational 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 suggest that a small but finite amount of nonmagnetic impurity doping causes reduction or disappearance of the spin gap. This qualitatively explains the experimental results for 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 change as a quantum phase transition in a spin-gap ladder system due to spinon doping effects.
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