Possible Exciton Bose Liquid in a Hard-Core Boson Ring Model

We present a quantum Monte Carlo study of a hard-core boson model with ring-only exchanges on a square lattice, where a K-1 term acts on 1 x 1 plaquettes and a K-2 term acts on 1 x 2 and 2 x 1 plaquettes. At half-filling, the phase diagram reveals charge density wave for small K-2, valence bond solid for intermediate K-2, and possibly for large K-2 the novel exciton Bose liquid (EBL) phase first proposed by Paramekanti et al [Phys. Rev. B 66, 054526 (2002)]. Away from half-filling, the EBL phase is present already for intermediate K-2 and remains stable for a range of densities below 1/2 before phase separation sets in at lower densities.