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Journal of Physics B Atomic Molecular and Optical Physics

Phase diagram of trapped bosons in a kagome lattice—application of inhomogeneous mean field theory

Apurba Barman, Saurabh Basu

Published Jan 8, 2014• 2 citations
Publisher

Abstract

Motivated by the possibility of obtaining a trimerized optical lattice, such as a kagome lattice experimentally by employing counter propagating laser beams, we investigate the phase diagram of correlated bosons in a kagome lattice in the presence of a harmonic confinement. To study the phase diagram, we compute the order parameters of an extended Bose–Hubbard model which is solved via the standard mean field technique in the presence of a trapping potential. It is emphasized that such trapping effects (together with the density exchange term) lead to the necessity of considering the detailed geometry of the lattice, and thus the measured quantities appeal to a particular geometry, kagome lattice being the case here. The phase diagram thus obtained yields co-existence of different compressible and incompressible phases. There are distinct signatures of superfluid, insulating, density ordered and supersolid phases as one moves towards the edge of the lattice starting from the centre of the confining potential. A comparison of the results obtained for the order parameters corresponding to other geometries, such as square and honeycomb lattices are presented. Relevant to the time-of-flight experiments, the order parameters have been computed in the momentum space, which corroborate the existence of different phases for a kagome lattice.

DOI
10.1088/0953-4075/47/2/025302

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Motivated by the possibility of obtaining a trimerized optical lattice, such as a kagome lattice experimentally by employing counter propagating laser beams, we investigate the phase diagram of correlated bosons in a kagome lattice in the presence of a harmonic confinement. To study the phase diagram, we compute the order parameters of an extended Bose–Hubbard model which is solved via the standard mean field technique in the presence of a trapping potential. It is emphasized that such trapping effects (together with the density exchange term) lead to the necessity of considering the detailed geometry of the lattice, and thus the measured quantities appeal to a particular geometry, kagome lattice being the case here. The phase diagram thus obtained yields co-existence of different compressible and incompressible phases. There are distinct signatures of superfluid, insulating, density ordered and supersolid phases as one moves towards the edge of the lattice starting from the centre of the confining potential. A comparison of the results obtained for the order parameters corresponding to other geometries, such as square and honeycomb lattices are presented. Relevant to the time-of-flight experiments, the order parameters have been computed in the momentum space, which corroborate the existence of different phases for a kagome lattice.
We investigate the phase diagram of interacting bosons in a tripartite lattice in two dimensions. An analytic computation of the phase diagram in the parameter space defined by the on-site boson–boson repulsion parameter and the chemical potential is done via a second-order strong coupling perturbation theory on the extended Bose–Hubbard model, which is hence supplemented by numerical mean-field calculations. Interesting results emerge in the form of exotic density ordered phases with one-third, two-third and unity filling of bosons per trimer. The pattern repeats as the chemical potential is increased. Further regions of supersolid and superfluid phases are noted in the numerical study of the mean-field model where the phase diagram shows a transition from compressible to insulating phases.
We present an extensive study of Mott insulator (MI) and superfluid (SF) shells in Bose-Hubbard (BH) models for bosons in optical lattices with harmonic traps. For this we apply the inhomogeneous mean-field theory developed by Sheshadri et al. [Phys. Rev. Lett. 75, 4075 (1995)]. Our results for the BH model with one type of spinless bosons agree quantitatively with quantum Monte Carlo simulations. Our approach is numerically less intensive than such simulations, so we are able to perform calculations on experimentally realistic, large three-dimensional systems, explore a wide range of parameter values, and make direct contact with a variety of experimental measurements. We also extend our inhomogeneous mean-field theory to study BH models with harmonic traps and (a) two species of bosons or (b) spin-1 bosons. With two species of bosons, we obtain rich phase diagrams with a variety of SF and MI phases and associated shells when we include a quadratic confining potential. For the spin-1 BH model, we show, in a representative case, that the system can display alternating shells of polar SF and MI phases, and we make interesting predictions for experiments in such systems.
Ultracold atomic gases in optical lattices provide an unique framework to study quantum phenomena in strongly correlated systems. In addition to the precise control over all relevant parameters in the experiment, these experiments can be mapped to a fundamental model from solid-state physics. For the bosonic version of this model, the so-called Bose-Hubbard model, a phase transition from a superfluid to a Mott insulator was theoretically predicted and later experimentally observed in an ultracold gas of Rb-87 atoms in three-dimensional as well as in one-dimensional optical lattices. Apart from homogeneous optical lattices one can introduce more complex lattice topologies such as two-color superlattices which give rise to a rich phase diagram including more exotic phases like the Bose-glass. We employ various powerful many-body techniques like exact diagonalization in complete and truncated Hilbert spaces and the Density-Matrix Renormalization Group (DMRG) algorithm to study the phase diagrams of the one-dimensional Bose-Hubbard and the one-dimensional Bose-Fermi-Hubbard Hamiltonian. Most theoretical studies of these systems discuss the phase diagrams with respect to the generic parameters of the Hubbard model. These Hubbard parameters, however, depend non-trivially on the control parameters used in experiments. The focus of this work is on the ab-initio calculation of the phase diagram of ultracold Rb-87 in one-dimensional optical superlattices starting directly from the experimental setup. To this end, we first employ band-structure calculations to extract the Hubbard parameters from the experimental parameters. Then, we use state-of-the-art DMRG techniques to solve the many-body problem for realistic particle numbers and lattice sizes that occur in experiments. Our results show that by using the intensities of the two laser fields forming the two-color superlattice as control parameters while keeping all other experimental parameters fixed, it is possible to access all relevant quantum phases of the system. Furthermore, we found out that the critical values of the laser intensities for the different phase transitions depend strongly on a third parameter that has to be included for a realistic description of the experiment. This third parameter is the strength of a harmonic trapping potential which accounts for the Gaussian shape of the laser fields and an additional magnetic potential used to confine the atoms in the center of the trap.
Abstract We develop an inhomogeneous mean‐field theory for the extended Bose‐Hubbard model with a quadratic, confining potential. In the absence of this potential, our mean‐field theory yields the phase diagram of the homogeneous extended Bose‐Hubbard model. This phase diagram shows a superfluid (SF) phase and lobes of Mott‐insulator (MI), density‐wave (DW), and supersolid (SS) phases in the plane of the chemical potential µ and on‐site repulsion U; we present phase diagrams for representative values of V, the repulsive energy for bosons on nearest‐neighbor sites. We demonstrate that, when the confining potential is present, superfluid and density‐wave order parameters are nonuniform; in particular, we obtain, for a few representative values of parameters, spherical shells of SF, MI, DW, and SS phases. We explore the implications of our study for experiments on cold‐atom dipolar condensates in optical lattices in a confining potential.
The Bose-Hubbard model describes the physics of a system of bosonic ultracold atoms in an optical lattice, in which a phase transition is present between a superfluid phase and a Mott insulator one. \nThe exact solution of this Hamiltonian is only feasible to find the ground-state of small systems, while other techniques (as mean-field schemes or quantum Monte Carlo) are necessary to study systems of larger size. \n \nAs a first application, we study the trapped model - relevant for the comparison with current experiments - through an inhomogeneous mean-field scheme. We describe some signatures of the phase crossover between superfluid and Mott insulator. In particular, the visibility of the quasimomentum distribution shows some kinks as a function of the lattice depth; we describe these features and we link them with the ones observed in other works in the literature. \n \nAs a second application, we use quantum Monte Carlo techniques to study the one-dimensional Bose-Hubbard model with long-range interactions and we focus on the appearance of the Haldane insulating phase, distinguishable from the Mott one through the presence of non-local hidden order. \nNon-local correlation functions are also used to describe the difference between the superfluid phase and the Mott insulator one.
We study the Bose-Hubbard model using the finite size density matrix renormalization group method. We obtain a complete phase diagram for a system in the presence of a harmonic trap and compare it with that of the homogeneous system. The superfluid to the Mott-insulator phase transition is investigated using different experimental signatures of these phases in quantities such as momentum distribution, visibility, condensate fraction, and the total number of bosons at a particular density. The relationships between the various experimental signatures and the phase diagram are highlighted.