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arXiv:0903.4058

Interaction Effects on Wannier Functions of a Bose-Einstein Condensate in an Optical Lattice and Implications for Bose-Hubbard Model

Zhaoxin Liang, Hu, B. B., Biao Wu

Published Mar 24, 2009• 0 citations
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Abstract

We show that one can properly take into account of the interaction effects and construct a set of orthonormal Wannier functions for a Bose-Einstein condensate in an optical lattice. These interaction-dependent Wannier functions are used to compute the tunneling rate $J$ and the on-site repulsion $U$ in the Bose-Hubbard model. Both parameters are found to be substantially different from ones calculated with the single-particle Wannier functions. Our numerical results of $U$ are found in good agreement with the measured on-site energy in a recent experiment [Campbell {\it et al.} Science {\bf 314}, 281 (2006)].

DOI
10.48550/arxiv.0903.4058

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We show that one can properly take into account of the interaction effects and construct a set of orthonormal Wannier functions for a Bose-Einstein condensate in an optical lattice. These interaction-dependent Wannier functions are used to compute the tunneling rate JJJ and the on-site repulsion UUU in the Bose-Hubbard model. Both parameters are found to be substantially different from ones calculated with the single-particle Wannier functions. Our numerical results of UUU are found in good agreement with the measured on-site energy in a recent experiment [Campbell {\it et al.} Science {\bf 314}, 281 (2006)].
We study the effects of repulsive on-site interactions on the broadening of the localized Wannier functions used for calculating the parameters to describe ultracold atoms in optical lattices. For this, we replace the common single-particle Wannier functions, which do not contain any information about the interactions, by two-particle Wannier functions obtained from an exact solution which takes the interactions into account. We then use these interaction-dependent basis functions to calculate the Bose-Hubbard model parameters, showing that they are substantially different both at low and high lattice depths from the ones calculated using single-particle Wannier functions. Our results suggest that density effects are not negligible for many parameter ranges and need to be taken into account in metrology experiments.
We have numerically calculated the single-band Wannier functions for interacting Bose gases in optical lattices with a self-consistent approach. We find that the Wannier functions are broadened by repulsive interaction. The tunneling parameter JJJ and the on-site interaction UUU computed with the broadened Wannier functions are found to change significantly with the number of atoms per site. Our theory can explain the nonuniform atomic clock shift observed in Campbell et al., Science 313, 649 (2006).
Superfluid to Mott-insulator transitions in atomic Bose–Einstein condensates (BEC) in optical lattices are investigated for the case of number of atoms per site larger than one. To account for mean field repulsion between the atoms in each well, we construct an orthogonal set of Wannier functions. The resulting hopping amplitude and on-site interaction may be substantially different from those calculated with single-atom Wannier functions. As illustrations of the approach, we consider lattices of various dimensionality and different mean occupations. We find that in three-dimensional optical lattices the correction to the critical lattice depth is significant enough to be measured experimentally even for a small number of atoms. Finally, we discuss the validity of the single-band model.
We construct the effective lowest-band Bose-Hubbard model incorporating interaction-induced on-site correlations. The model is based on ladder operators for local correlated states, which deviate from the usual Wannier creation and annihilation, allowing for a systematic construction of the most appropriate single-band low-energy description in the form of the extended Bose-Hubbard model. A formulation of this model in terms of ladder operators not only naturally contains the previously found effective multibody interactions, but also contains multibody-induced single-particle tunneling, pair tunneling, and nearest-neighbor interaction processes of higher orders. An alternative description of the same model can be formulated in terms of occupation-dependent Bose-Hubbard parameters. These multiparticle effects can be enhanced using Feshbach resonances, leading to corrections which are well within experimental reach and of significance to the phase diagram of ultracold bosonic atoms in an optical lattice. We analyze the energy-reduction mechanism of interacting atoms on a local lattice site and show that this cannot be explained only by a spatial broadening of Wannier orbitals on a single-particle level, which neglects correlations.
In systems with periodic potential fields, building relatively local Wannier functions can significantly simplify the Hamiltonian and enhance our understanding of the system's ground state and dynamic properties. In this work, we improve the current method of building the Wannier functions of ultracold atomic systems, including the case in the presence or absence of interactions. In noninteracting systems, we propose a method to directly obtain the real-valued maximally localized Wannier functions (MLWFs) by using real-valued eigenstates, and verify the effectiveness of this method in a two-dimensional (2D) degenerate system. In interacting systems, we obtain the effect of high-energy bands on the lowest-energy band by using the accurate calculation results of the two-particle system. In the two-particle system, we consider the effect of the entanglement between the particles and obtain the optimal two-particle Wannier functions. These Wannier functions are then further utilized to obtain the parameters of the extended Bose-Hubbard model. The effectiveness of the method is verified by taking a one-dimensional (1D) system with contact interaction as an example. In the three-particle and four-particle systems, compared calculation results with the original system and the unmodified two-band Bose-Hubbard model, we find that the effective Hamiltonian is more accurate than the unmodified two-band model. This verifies the effectiveness of our method, and the parameters obtained can reflect the original system well, which provides an effective method for accurate modeling of interacting systems.