We investigate quantum phase transitions occurring in a system of strongly interacting ultracold bosons in a 1D optical lattice. After discussing the commensurate-incommensurate transition, we focus on the phases appearing at incommensurate filling. We find a rich phase diagram, with superfluid, supersolid and solid (kink-lattice) phases. Supersolids generally appear in theoretical studies of systems with long-range interactions; our results break this paradigm and show that they may also emerge in models including only short-range (contact) interactions, provided that quantum fluctuations are properly taken into account.
PACS numbers:The rapid progress in trapping and cooling atoms has rendered the study of "tailor-made" low-dimensional (D) systems [1] experimentally accessible. Both the dimensionality and the interactions can be controlled, allowing great flexibility in realizing almost arbitrary stronglycorrelated physical systems. A superfluid-Mott insulator (SF-MI) quantum phase transition, driven by increasing the potential depth of the optical lattice (and hence the relative strength of interactions) beyond a critical value, has been observed for bosons loaded into an optical lattice in 3D [2], 2D [3], and 1D [4]. In addition, the Tonks-Girardeau gas, where bosons avoid spatial overlap and acquire fermionic properties due to strong repulsive interactions, has been experimentally realized in 1D [5].Recently, a new type of quantum phase transition was observed in 1D in the very strongly interacting regime: for an arbitrarily weak optical lattice potential commensurate with the atomic density of the Bose gas, a quantum phase transition into an insulating, gapped state, was observed, with the atoms pinned at the lattice minima [6]. Theoretical studies of 1D systems based on the sine-Gordon model indeed predict that above a critical interaction strength, the SF should become a MI even for a vanishingly weak optical lattice [7].