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arXiv:0904.0905 · Physical Review B

Process-chain approach to the Bose-Hubbard model: Ground-state properties and phase diagram

Niklas Teichmann, Dennis Hinrichs, Martin Holthaus, André Eckardt

Published Jun 12, 2009• 57 citations

Abstract

We carry out a perturbative analysis, of high order in the tunneling parameter, of the ground state of the homogeneous Bose-Hubbard model in the Mott insulator phase. This is made possible by a diagrammatic process-chain approach, derived from Kato's representation of the many-body perturbation series, which can be implemented numerically in a straightforward manner. We compute ground-state energies, atom-atom correlation functions, density-density correlations, and occupation number fluctuations, for one-, two-, and three-dimensional lattices with arbitrary integer filling. A phenomenological scaling behavior is found which renders the data almost independent of the filling factor. In addition, the process-chain approach is employed for calculating the boundary between the Mott insulator phase and the superfluid phase with high accuracy. We also consider systems with dimensionalities $d>3$, thus monitoring the approach to the mean-field limit. The versatility of the method suggests further applications to other systems which are less well understood.

DOI: 10.1103/physrevb.79.224515

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We carry out a perturbative analysis, of high order in the tunneling parameter, of the ground state of the homogeneous Bose-Hubbard model in the Mott insulator phase. This is made possible by a diagrammatic process-chain approach, derived from Kato's representation of the many-body perturbation series, which can be implemented numerically in a straightforward manner. We compute ground-state energies, atom-atom correlation functions, density-density correlations, and occupation number fluctuations, for one-, two-, and three-dimensional lattices with arbitrary integer filling. A phenomenological scaling behavior is found which renders the data almost independent of the filling factor. In addition, the process-chain approach is employed for calculating the boundary between the Mott insulator phase and the superfluid phase with high accuracy. We also consider systems with dimensionalities

d>3

, thus monitoring the approach to the mean-field limit. The versatility of the method suggests further applications to other systems which are less well understood.

We study the transition from a Mott insulator to a superfluid in the Bose-Hubbard model at zero temperature for different lattice types. Converting Kato's perturbation series into an efficient numerical algorithm capable of reaching high orders, we obtain accurate critical parameters for any filling factor g and dimensionality d. Furthermore we clarify the g- and the d-dependence of the critical value and devise a method for extracting the critical value from the correlation function.

The one-dimensional Bose-Hubbard model at a unit filling factor is studied by means of a very high-order symbolic perturbative expansion. Analytical expressions are derived for the ground-state quantities such as energy per site, variance of on-site occupation, and correlation functions: $⟨{\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{a}}_{j}^{\ifmmode\dagger\else\textdagger\fi{}}{\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{a}}_{j+r}⟩$ and $⟨{\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{n}}_{j}{\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{n}}_{j+r}⟩$. These findings are compared to numerics and good agreement is found in the Mott insulator phase. Our results provide analytical approximations to important observables in the Mott phase, and are also of direct relevance to future experiments with ultracold atomic gases placed in optical lattices. We also discuss the symmetry of the Bose-Hubbard model associated with the sign change of the tunneling coupling.

We employ a high-order perturbative expansion to characterize the ground state of the Mott phase of the one-dimensional Bose–Hubbard model. We compute for different integer filling factors the energy per lattice site, the two-point and density–density correlations, and expectation values of powers of the on-site number operator determining the local atom number fluctuations (variance, skewness, kurtosis). We compare these expansions to numerical simulations of the infinite-size system to determine their range of applicability. We also discuss a new sum rule for the density–density correlations that can be used in both equilibrium and non-equilibrium systems.

We study the transition from a Mott insulator to a superfluid in both the two- and the three-dimensional Bose-Hubbard model at zero temperature, employing the method of the effective potential. Converting Kato's perturbation series into an algorithm capable of reaching high orders, we obtain accurate critical parameters for any integer filling factor. Our technique allows us to monitor both the approach to the mean-field limit by considering spatial dimensionalities

d>3

and to the quantum rotor limit of high filling, which refers to an array of Josephson junctions.

Abstract We analyse the distinction between the three different ground states presented by a system of spinless bosons with short-range interactions submitted to a random potential using the disordered Bose–Hubbard model. The criteria for identifying the superfluid, the Mott-insulator, and the Bose-glass phases at finite temperatures are discussed for small values of the kinetic energy associated with the tunnelling of particles between potential wells. Field theoretical considerations are applied in order to construct a diagrammatic hopping expansion to the finite-temperature Green’s function. By performing a summation of subsets of diagrams we are able to find the condition to the long-range correlations which leads to the phase boundary between superfluid and insulating phases. The perturbative expression to the local correlations allows us to calculate an approximation to the single-particle density of states of low-energy excitations in the presence of small hopping, which characterizes unambiguously the distinction between the Mott-insulator and the Bose-glass phases. We obtain the phase diagram for bounded on-site disorder. It is demonstrated that our analysis is capable of going beyond the mean-field theory results for the classification of these different ground states.