  A two-dimensional lattice hard-core boson system with a small fraction of bosonic or fermionic impurity particles is studied. The impurities have the same hopping and interactions as the dominant bosons and their effects are solely due to quantum statistics. Quantum Monte Carlo simulations are carried out in which paths of the dominant boson species are sampled and a summation is performed over all second-species paths compatible with the permutation cycles. Both kinds of impurities reduce modestly and equally the Kosterliz-Thouless superfluid transition temperature. However, the effective impurity interactions are found to be qualitatively different at lower temperatures; fermions are repulsive and further suppress superfluidity at low temperatures. 
Quantum Monte Carlo simulations of bosonic and fermionic impurities in a   two-dimensional hard-core boson system