Bosons in a random potential: evidence for new low energy excitations

We study the T = 0 critical properties of the superfluid-insulator transition in 2D hard core bose systems with disorder. Using quantum Monte Carlo simulations and finite size scaling on up to 64x64 size lattices we find the dynamical exponent z = 0.5 ± 0.05 and the compressibility κ is finite at the transition. These conclusion differ from the existing scaling theory that argues for z = 2 based on a finite κ at the transition. We also find that in the presence of disorder, the boson model and the quantum rotor model are in different universality classes. Our results are suggestive of new low lying collective excitations in the disordered system that are modified from usual phonons. Such a conjecture is further supported by the anomalous temperature dependence of the specific heat deep within the superfluid phase which deviates from the expected Cv ∼ T2 behavior for phonons. (orig.)