Excitations of liquid 4He in disorder

The dynamic structure factor. S(q, omega) for. a model of superfluid He-4 in disorder is evaluated using Path Integral Monte Carlo and Maximum Entropy methods. Disorder is represented by randomly distributed static impurities interacting,vith the He-4 atoms via a simple attractive potential. The potential is parameterized to yield different values of the variance and correlation length of the resulting disordering environment. New weight in S(q, omega) at low omega (i.e., low energy excitations) is induced by disorder, as predicted in some previous calculations, and S(q, omega) is broadened. Assuming that S(q, omega) in disorder is dominated by a single peak, just as in the pure superfluid, the darn suggest that the peak position is shifted by the disorder from the bulk value with a unique q dependence. The static structure factor S(q) is reduced at all wave vectors with increased impurity concentration.