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arXiv:cond-mat/0510341 · Physical Review B

Phase diagram of the Bose-Hubbard model with T3 symmetry

Matteo Rizzi, V. Cataudella, Rosario Fazio

Published Apr 14, 2006• 90 citations
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Abstract

We study the quantum phase transition between the insulating and the globally coherent superfluid phases in the Bose-Hubbard model with ${\mathcal{T}}_{3}$ structure, the ``dice lattice.'' Even in the absence of any frustration the superfluid phase is characterized by modulation of the order parameter on the different sublattices of the ${\mathcal{T}}_{3}$ structure. The zero-temperature critical point as a function of magnetic field shows the characteristic ``butterfly'' form. At full frustration the superfluid region is strongly suppressed. In addition, due to the existence of the Aharonov-Bohm cages at $f=1∕2$, we find some evidence for the existence of an intermediate insulating phase characterized by a zero superfluid stiffness but finite compressibility. In this intermediate phase bosons are localized due to the external frustration and the topology of the ${\mathcal{T}}_{3}$ lattice. We name this new phase the Aharonov-Bohm insulator. In the presence of charge frustration the phase diagram acquires the typical lobe structure. The form and hierarchy of the Mott insulating states with fractional fillings are dictated by the particular topology of the ${\mathcal{T}}_{3}$ lattice. The results presented were obtained by a variety of analytical methods: mean-field and variational techniques to approach the phase boundary from the superconducting side and a strongly coupled expansion appropriate for the Mott insulating region. In addition we performed quantum Monte Carlo simulations of the corresponding $(2+1)$-dimensional $XY$ model to corroborate the analytical calculations with a more accurate quantitative analysis. We finally discuss experimental realization of the ${\mathcal{T}}_{3}$ lattice both with optical lattices and with Josephson junction arrays.

DOI
10.1103/physrevb.73.144511

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We study the quantum phase transition between the insulating and the globally coherent superfluid phases in the Bose-Hubbard model with T3{\mathcal{T}}_{3}T3​ structure, the dice lattice.'' Even in the absence of any frustration the superfluid phase is characterized by modulation of the order parameter on the different sublattices of the ${\mathcal{T}}_{3}$ structure. The zero-temperature critical point as a function of magnetic field shows the characteristic butterfly'' form. At full frustration the superfluid region is strongly suppressed. In addition, due to the existence of the Aharonov-Bohm cages at f=1∕2f=1∕2f=1∕2, we find some evidence for the existence of an intermediate insulating phase characterized by a zero superfluid stiffness but finite compressibility. In this intermediate phase bosons are localized due to the external frustration and the topology of the T3{\mathcal{T}}_{3}T3​ lattice. We name this new phase the Aharonov-Bohm insulator. In the presence of charge frustration the phase diagram acquires the typical lobe structure. The form and hierarchy of the Mott insulating states with fractional fillings are dictated by the particular topology of the T3{\mathcal{T}}_{3}T3​ lattice. The results presented were obtained by a variety of analytical methods: mean-field and variational techniques to approach the phase boundary from the superconducting side and a strongly coupled expansion appropriate for the Mott insulating region. In addition we performed quantum Monte Carlo simulations of the corresponding (2+1)(2+1)(2+1)-dimensional XYXYXY model to corroborate the analytical calculations with a more accurate quantitative analysis. We finally discuss experimental realization of the T3{\mathcal{T}}_{3}T3​ lattice both with optical lattices and with Josephson junction arrays.
We present a semiclassical perspective of the topological Bose-Hubbard model recently studied by quantum Monte Carlo (QMC) simulation on the honeycomb lattice [Phys. Rev. B 93, 121401(R) (2016)]. The three insulating phases uncovered by QMC are as follows: a superfluid, a Mott phase, and a charge-density-wave insulating phase. We show that this model maps to a spin-1/21/21/2 quantum XY model with competing magnetic fields on the two honeycomb sublattices. We give an explicit analytical origin of the quantum phase diagram using mean-field theory. Basically, the competition between the sublattice magnetic fields leads to three distinct phases uncovered by QMC. We find that the mean-field phase diagram captures a large regime of the quantum phase diagram. Furthermore, due to the mapping from bosons to spin variables and vice versa, the bosonic excitations of the Bose-Hubbard model correspond to the magnon excitations of the quantum spin system. We corroborate this assertion by computing the magnon bulk bands and the Berry curvatures and we find that the Berry curvatures have the same trends as the QMC result.
We obtain the complete phase diagram of the hardcore Bose-Hubbard model in the presence of a period-two superlattice in two and three dimensions. First we acquire the phase boundaries between the superfluid phase and the `trivial' insulating phases of the model (the completely-empty and completely-filled lattices) analytically. Next, the boundary between the superfluid phase and the half-filled Mott-insulating phase is obtained numerically, using the stochastic series expansion (SSE) algorithm followed by finite-size scaling. We also compare our numerical results against the predictions of several approximation schemes, including two mean-field approaches and a fourth-order strong-coupling expansion (SCE), where we show that the latter method in particular is successful in producing an accurate picture of the phase diagram. Finally, we examine the extent to which several approximation schemes, such as the random phase approximation and the strong-coupling expansion, give an accurate description of the momentum distribution of the bosons inside the insulating phases.
The phase diagrams and phase transitions of bosons with short-ranged repulsive interactions moving in periodic and/or random external potentials at zero temperature are investigated with emphasis on the superfluid-insulator transition induced by varying a parameter such as the density. Bosons in periodic potentials (e.g., on a lattice) at T=0 exhibit two types of phases: a superfluid phase and Mott insulating phases characterized by integer (or commensurate) boson densities, by the existence of a gap for particle-hole excitations, and by zero compressibility. Generically, the superfluid onset transition in d dimensions from a Mott insulator to superfluidity is ``ideal,'' or mean field in character, but at special multicritical points with particle-hole symmetry it is in the universality class of the (d+1)-dimensional XY model. In the presence of disorder, a third, ``Bose glass'' phase exists. This phase is insulating because of the localization effects of the randomness and analogous to the Fermi glass phase of interacting fermions in a strongly disordered potential.The Bose glass phase is characterized by a finite compressibility, no gap, but an infinite superfluid susceptibility. In the presence of disorder the transition to superfluidity is argued to occur only from the Bose glass phase, and never directly from the Mott insulator. This zero-temperature superfluid-insulator transition is studied via generalizations of the Josephson scaling relation for the superfluid density at the ordinary \ensuremath{\lambda} transition, highlighting the crucial role of quantum fluctuations. The transition is found to have a dynamic critical exponent z exactly equal to d and correlation length and order-parameter correlation exponents \ensuremath{\nu} and \ensuremath{\eta} which satisfy the bounds \ensuremath{\nu}\ensuremath{\ge}2/d and \ensuremath{\eta}\ensuremath{\le}2-d, respectively. It is argued that the superfluid-insulator transition in the presence of disorder may have an upper critical dimension dc{d}_{c}dc​ which is infinite, but a perturbative renormalization-group calculation wherein the critical exponents have mean-field values for weak disorder above d=4 is also discussed. Many of these conclusions are verified by explicit calculations on a model of one-dimensional bosons in the presence of both random and periodic potentials. The general results are applied to experiments on 4He^{4}\mathrm{He}4He absorbed in porous media such as Vycor. Some measurable properties of the superfluid onset are predicted exactly [e.g., the exponent x relating the \ensuremath{\lambda} transition temperature to the zero-temperature superfluid density is found to be d/2(d-1)], while stringent bounds are placed on others. Analysis of preliminary data is consistent with these predictions.
We report results of quantum Monte Carlo simulations of the Bose-Hubbard model in three dimensions. Critical parameters for the superfluid-to-Mott-insulator transition are determined with significantly higher accuracy than has been done in the past. In particular, the position of the critical point at filling factor n=1n=1n=1 is found to be at (U∕t)c=29.34(2){(U∕t)}_{\mathrm{c}}=29.34(2)(U∕t)c​=29.34(2), and the insulating gap $\ensuremath{\Delta}$ is measured with accuracy of a few percent of the hopping amplitude ttt. We obtain the effective mass of particle and hole excitations in the insulating state---with explicit demonstration of the emerging particle-hole symmetry and relativistic dispersion law at the transition tip---along with the sound velocity in the strongly correlated superfluid phase. These parameters are the necessary ingredients to perform analytic estimates of the low temperature $(T⪡\ensuremath{\Delta})$ thermodynamics in macroscopic samples. We present accurate thermodynamic curves, including these for specific heat and entropy, for typical insulating (U∕t=40)(U∕t=40)(U∕t=40) and superfluid (t∕U=0.0385)(t∕U=0.0385)(t∕U=0.0385) phases. Our data can serve as a basis for accurate experimental thermometry, and a guide for appropriate initial conditions if one attempts to use interacting bosons in quantum information processing.
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