Impurity-Induced Phase Transitions in Low-Dimensional Magnets

1 Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan 2 Institute for Solid State Physics, University of Tokyo, Chiba 277-8581, Japan We study the impurity-induced phase transitions in a quasi-one-dimensional Heisenberg antiferromagnet doped with magnetic spin-1/2 impurities and non-magnetic ones. The impurity-induced transition temperature determined by the quantum Monte Carlo method with the continuous-time loop algorithm is monotonically increasing as a function of the magnitude of the impurity spin. To these results, we give discussions based on the valence-bond solid-like picture for the pure system and the inspection of the local magnetic structure around the impurities.