Quantum phases of the biased two-chain-coupled Bose-Hubbard Ladder

We investigate the quantum phases of bosons in a two-chain-coupled ladder. This bosonic ladder is generally in a biased configuration, meaning that the two chains of the ladder can have dramatically different on-site interactions and potential energies. Adopting the numerical density-matrix renormalization-group method, we analyze the phase transitions in various parameter spaces. We find signatures of both insulating-to-superfluid and superfluid-to-insulating quantum phase transitions as the interchain tunnelling is increased. Interestingly, tunning the interaction to some intermediate values, the system can exhibit a reentrant quantum phase transition between insulating and superfluid phases. We show that for infinite interaction bias, the model is amenable to some analytical treatments, whose prediction about the phase boundary is in great agreement with the numerical results. We finally clarify some critical parameters which separate the system into regimes with distinct phase behaviours, and briefly compare typical properties of the biased and unbiased bosonic ladder systems. Our work enriches the Bose-Hubbard physics.