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 low-temperature spin configurations, investigating the magnetization profile of the local states due to the imp urities and the two point correlation function centered on one of the impur ities. This correlation is found to be weak against increases in temperatur e, although the magnetization profile in the triplet state is visible up to higher temperatures. Here we introduce a loop cluster quantum Monte Carlo method with a fixed magnetization M-z in order to study the correlations in the ground state with a given value of M-z. From the population distributi on of magnetization, a very small energy gap between the quasidegenerate st ates due to the impurities is obtained. [S0163-1829(99)05521-6].