Possibility of direct Mott insulator-to-superfluid transitions in weakly disordered boson systems

We study the zero-temperature phase transitions of a two-dimensional disordered boson Hubbard model at incommensurate boson densities. Via matrix diagonalization and quantum Monte Carlo simulations, we construct the phase diagram and evaluate the correlation length exponent n . In the presence of weak disorder, we obtain n50.560.1, the same value as that in the pure model, near the tip of a Mott insulator lobe, using the dynamical critical exponent z52. As the strength of disorder is increased beyond a certain value, however, the value of n is found to change to 0.960.1. This result strongly suggests that there exist direct Mott insulatorto-superfluid transitions around the tip of a Mott insulator lobe in the weak disorder regime. @S0163-1829~99!08513-6#