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

We study the zero-temperature phase transitions of a two-dimensional disord ered boson Hubbard model at incommensurate boson densities. Via matrix diag onalization and quantum Monte Carlo simulations, we construct the phase dia gram and evaluate the correlation length exponent v. In the presence of wea k disorder, we obtain v = 0.5 +/- 0.1, the same value as that in the pure m odel, near the tip of a Mott insulator lobe, using the dynamical critical e xponent z = 2. As the strength of disorder is increased beyond a certain va lue, however, the value of v is found to change to 0.9 +/- 0.1. This result strongly suggests that there exist direct Mott insulator-to-superfluid tra nsitions around the tip of a Mott insulator lobe: in the weak disorder regi me. [S0163-1829(99)08513-6].