2 5 D ec 1 99 8 Temperature dependence of local states due to S = 1 / 2 impurities and their correlation in a S = 1 Heisenberg chain

We study the temperature dependence of the low temperature spin configurations, investigating the magnetization profile of the local states due to the impurities and the two point correlation function centered in one of the impurities. This correlation is found to be weak against temperature effects although the magnetization profile in the triplet state is visible up to higher temperatures. Here we introduce a loop cluster quantumMonte-Carlo method with a fixed magnetization Mz in order to study the correlations in the ground state of a given value of Mz. From the population distribution of magnetization, the very small energy gap between the quasi degenerate states due to the impurities is obtained.