Quantum Monte Carlo study of hard-core bosons in Creutz ladder with zero flux

The quantum phase of hard-core bosons in Creutz ladder with zero flux is studied. For a specific regime of the parameters (t_x=t_p, t_y<0), the exact ground-state is found analytically, which is a dimerized insulator with one electron bound in each rung of the ladder. For the case t_x,t_y,t_p>0, the system is exactly studied using quantum Monte Carlo (QMC) method without a sign problem. It is found that the system is a Mott insulator for small t_p and a quantum phase transition to a superfluid phase is driven by increasing t_p. The critical t_p^c is determined precisely by a scaling analysis. Since it is possible that the Creutz ladder is realized experimentally, the theoretical results are interesting to the cold-atom experiments.