Duality, magnetic space group and their applications to quantum phases and phase transitions on bipartite lattices in several experimental systems

摘要

By using a dual vortex method, we study phases such as superfluid, solids, supersolids and quantum phase transitions in a unified scheme in extended boson Hubbard models at and slightly away from half filling on bipartite optical lattices such as honeycomb and square lattice. We also map out its global phase diagram at T = 0 of chemical potential versus the ratio of kinetic energy over the interaction. We stress the importance of the self-consistence condition on the saddle point structure of the dual gauge fields in the translational symmetry breaking insulating sides, especially in the charge density wave side. We find that in the translational symmetry breaking side, different kinds of supersolids are generic possible states slightly away from half filling. We propose a new kind of supersolid: valence bond supersolid (VB-SS). In this VB-SS, the density fluctuation at any site is very large indicating its superfluid nature, but the boson kinetic energies on bonds between two sites are given and break the lattice translational symmetries indicating its valence bound nature. We show that the quantum phase transitions from solids to supersolids driven by a chemical potential are in the same universality class as that from a Mott insulator to a superfluid, therefore have exact exponents z = 2, nu = 1/2, eta = 0 with a logarithmic correction. Comparisons with previous quantum Monte Carlo (QMC) simulations on a square lattice are made. Implications on possible future QMC simulations in both bipartite lattices are given. All these phases and phase transitions can be potentially realized in ultra-cold atoms loaded on optical bipartite lattices. Then we apply our results to investigate the reentrant "superfluid" in a narrow region of coverages in the second layer of He-4 adsorbed on graphite and the low temperature phase diagram of hydrogen physisorbed on krypton-preplated graphite (H-2/Kr/graphite) near half filling. We suggest that He-4 and H-2 lattice supersolids maybe responsible for the experimental signals in the two systems. Finally, we suggest Cooper supersolid is repressible for the phase diagram of La2-x BaxCuO4 near x = 1/8.