Universal conductivity in the two-dimensional boson Hubbard model.

We use Quantum Monte Carlo to evaluate the conductivity σ of the 2–dimensional disordered boson Hubbard model at the superfluid-bose glass phase boundary. At the critical point for particle density ρ = 0.5, we find σ c = (0.45 + 0.07)σ Q , where σ Q = e 2 * /h from a finite size scaling analysis of the superfluid density. We obtain σ c = (0.47 + 0.08)σ Q from a direct calculation of the current–current correlation function. Simulations at the critical points for other particle densities, ρ = 0.75 and 1.0, give similar values for σ. We discuss possible origins of the difference in this value from that recently obtained by other numerical approaches.