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

TL;DR

This study investigates phase transitions in a disordered two-dimensional boson Hubbard model, revealing that weak disorder allows direct Mott insulator-to-superfluid transitions, with critical exponents indicating a change in universality class as disorder increases.

Contribution

It demonstrates the possibility of direct Mott insulator-to-superfluid transitions in weakly disordered systems, supported by numerical simulations and critical exponent analysis.

Findings

Critical exponent nu=0.5 near pure model

Nu changes to 0.9 with increased disorder

Direct transitions occur around the Mott lobe tip in weak disorder regime

Abstract

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 nu. In the presence of weak disorder, we obtain nu=0.5 +- 0.1, the same value as that in the pure model, near the tip of a Mott insulator lobe, using the dynamical critical exponent z=2. As the strength of disorder is increased beyond a certain value, however, the value of nu is found to change to 0.9 +- 0.1. This result strongly suggests that there exist direct Mott insulator-to-superfluid transitions around the tip of a Mott insulator lobe in the weak disorder regime.