Quantum Monte Carlo Simulations of Bosonic and Fermionic Impurities in a Two-Dimensional Hard-Core Boson System

A two-dimensional lattice hard-core boson system with a small fraction of bosonic or fermionic impurities is studied. The impurity hopping and interactions are identical to those of the dominant bosons, so that effects due to quantum statistics can be isolated. A quantum Monte Carlo scheme is developed in which bosonic paths are sampled and the impurities are introduced at the level of summing over permutation cycles. For both types of impurities, an anomaly in the effective impurity interaction energy is found at the Kosterlitz-Thouless temperature; it changes from attractive for T>T_KT to repulsive for T<T_KT, suggesting that impurities strongly affect the vortex physics.