Bosons In Lattices Research Articles

Probing correlated phases of bosons in optical lattices via trap squeezing

We theoretically analyze the response properties of ultracold bosons in optical lattices to the static variation of the trapping potential. We show that, upon an increase of such a potential (trap squeezing), the density variations in a central region, with linear size of ≲10 wavelengths, reflect that of the bulk system upon changing the chemical potential: hence measuring the density variations gives direct access to the bulk compressibility. When combined with standard time-of-flight measurements, this approach has the potential of unambiguously detecting the appearance of the most fundamental phases realized by bosons in optical lattices, with or without further external potentials: superfluid, Mott insulator, band insulator and Bose glass.

Cavity-enhanced detection of magnetic order in lattice spin models

We develop a general scheme for detecting spin correlations inside a two-component lattice gas of bosonic atoms, stimulated by the recent theoretical and experimental advances on analogous systems for a single component quantum gas. Within a linearized theory for the transmission spectra of the cavity mode field, different magnetic phases of a two-component (spin $1∕2$) lattice bosons become clearly distinguishable. In the Mott-insulating (MI) state with unit filling for the two-component lattice bosons, three different phases: antiferromagnetic, ferromagnetic, and the $XY$ phases are found to be associated with drastically different cavity photon numbers. Our suggested study can be straightforwardly implemented with current cold atom experiments.

Quantum phase diagram of bosons in optical lattices

We work out two different analytical methods for calculating the boundary of the Mott-insulator--superfluid (MI-SF) quantum phase transition for scalar bosons in cubic optical lattices of arbitrary dimension at zero temperature which improve upon the seminal mean-field result. The first one is a variational method, which is inspired by variational perturbation theory, whereas the second one is based on the field-theoretic concept of effective potential. Within both analytical approaches we achieve a considerable improvement of the location of the MI-SF quantum phase transition for the first Mott lobe in excellent agreement with recent numerical results from quantum Monte Carlo simulations in two and three dimensions. Thus, our analytical results for the whole quantum phase diagram can be regarded as being essentially exact for all practical purposes.

Effective action approach for quantum phase transitions in bosonic lattices

Based on standard field-theoretic considerations, we develop an effective action approach for investigating quantum phase transitions in lattice Bose systems at arbitrary temperature. We begin by adding to the Hamiltonian of interest a symmetry breaking source term. Using time-dependent perturbation theory, we then expand the grand-canonical free energy as a double power series in both the tunneling and the source term. From here, an order parameter field is introduced in the standard way and the underlying effective action is derived via a Legendre transformation. Determining the Ginzburg-Landau expansion to first order in the tunneling term, expressions for the Mott insulator-superfluid phase boundary, condensate density, average particle number, and compressibility are derived and analyzed in detail. Additionally, excitation spectra in the ordered phase are found by considering both longitudinal and transverse variations of the order parameter. Finally, these results are applied to the concrete case of the Bose-Hubbard Hamiltonian on a three-dimensional cubic lattice, and compared with the corresponding results from mean-field theory. Although both approaches yield the same Mott insulator–superfluid phase boundary to first order in the tunneling, the predictions of our effective action theory turn out to be superior to the mean-field results deeper into the superfluid phase.

Phase diagram of one-dimensional hard-core bosons with three-body interactions

We determine the phase diagram of a one-dimensional system of hard-core lattice bosons interacting via repulsive three-body interactions by analytic methods and extensive quantum Monte Carlo simulations. Such three-body interactions can be derived from a microscopic theory for polar molecules trapped in an optical lattice. Depending on the strength of the interactions and the particle density, we find superfluid and solid phases, the latter appearing at an unconventional filling of the lattice and displaying a coexistence of charge-density wave and bond orders.

Cooling ultracold bosons in optical lattices by spectral transform

It is shown theoretically how to directly obtain the energy distribution of a weakly interacting gas of bosons confined in an optical lattice in the tight-binding limit. This is accomplished by adding a linear potential to a suitably prepared lattice, and allowing the gas to evolve under the influence of the total potential. After a prescribed time, a spectral transform is effected where each (highly nonlocal) energy state is transformed into a distinct site of the lattice, thus allowing the energy distribution to be (nondestructively) imaged in real space. Evolving for twice the time returns the atoms to their initial state. The results suggest efficient methods to both measure the temperature in situ, as well as to cool atoms within the lattice: after applying the spectral transform one simply needs to remove atoms from all but a few lattice sites. Using exact numerical calculations, the effects of interactions and errors in the application of the lattice are examined.

Infinite Cycles in Boson Lattice Models

We study the relationship between long cycles and Bose-Einstein condensation (BEC) in the case of several models. A convenient expression for the density of particles on cycles of length $q$ is obtained, in terms of $q$ unsymmetrised particles coupled with a boson field. Using this formulation we reproduce known results on the Ideal Bose Gas, Mean-Field and Perturbed Mean-Field Models, where the condensate density exactly equals the long cycle density. Then we consider the Infinite-Range-Hopping Bose-Hubbard model, with and without hard-cores. For the hard-core case, we find we can disregard the hopping contribution of the q unsymmetrised particles to obtain an exact expression for the density of particles on long cycles. It is shown that only the cycle of length one contributes to the cycle density. We conclude that while the existence of a non-zero long cycle density coincides with the occurrence of Bose-Einstein condensation, the respective densities are not necessarily equal. For the case of a finite on-site repulsion, we obtain an expression for the cycle density by neglecting the $q$ unsymmetrised hop. Inspired by the Approximating Hamiltonian method we conjecture a simplified expression for the short cycle density as a ratio of single-site partition functions. In the absence of condensation we prove that this simplification is exact and find that in this case the long-cycle density vanishes. In the presence of condensation we justify this simplification when a gauge-symmetry breaking term is introduced in the Hamiltonian. Assuming our conjecture is correct, we compare numerically the long-cycle density with the condensate and again find that though they coexist, in general they are not equal.

Josephson physics mediated by the Mott insulating phase

We investigate the static and dynamic properties of bosonic lattice systems in which condensed and Mott insulating phases coexist due to the presence of a spatially varying potential. We formulate a description of these inhomogeneous systems and calculate the bulk energy at and near equilibrium. We derive the explicit form of the Josephson coupling between disjoint superfluid regions separated by Mott insulating regions. We obtain detailed estimates for the case of alternating superfluid and Mott insulating spherical shells in a radially symmetric parabolically confined cold atom system.

Ultracold dipolar gas in an optical lattice: The fate of metastable states

We study the physics of ultracold dipolar bosons in optical lattices. We show that dipole-dipole interactions lead to the appearance of many insulating metastable states. We study the stability and lifetime of these states using a generalization of the instanton theory. We investigate also possibilities to prepare, control and manipulate these states using time dependent superlattice modifications and modulations. We show that the transfer from one metastable configuration to another necessarily occurs via superfluid states, but can be controlled fully on the quantum level. We show how the metastable states can be created in the presence of the harmonic trap. Our findings open the way toward applications of the metastable states as quantum memories.

Mott-Hubbard transition of bosons in optical lattices with three-body interactions

In this paper, the quantum phase transition between the superfluid state and the Mott-insulator state is studied based on an extended Bose-Hubbard model with two- and three-body on-site interactions. By employing the mean-field approximation we find the extension of the insulating ``lobes'' and the existence of a fixed point in three-dimensional phase space. We investigate the link between experimental parameters and theoretical variables. The possibility to observe our results through some experimental effects in optically trapped Bose-Einstein condensates is also discussed.

Quantum Monte Carlo Simulations of Bosonic and Fermionic Impurities in a Two-Dimensional Hard-Core Boson System

A two-dimensional lattice hard-core boson system with a small fraction of bosonic or fermionic impurities is studied. The impurity hopping and interactions are identical to those of the dominant bosons, so that effects due to quantum statistics can be isolated. A quantum Monte Carlo scheme is developed in which bosonic paths are sampled and the impurities are introduced at the level of summing over permutation cycles. For both types of impurities, an anomaly in the effective impurity interaction energy is found at the Kosterlitz-Thouless temperature; it changes from attractive for T>T(KT) to repulsive for T<T(KT), suggesting that impurities strongly affect the vortex physics.

Adiabatic cooling and heating of cold bosons in three-dimensional optical lattices and the superfluid-normal phase transition

We investigate effects of the adiabatic ramp-up of optical lattices to the temperatures of Bose gases. Applying the mean-field approximation to the three-dimensional Bose-Hubbard model at finite temperatures, we calculate the isentropic curves in the phase diagram against the temperatures and the lattice height. The adiabatic cooling or heating through superfluid-normal phase transition is clearly understood. It is found that the cooling occurs in superfluid phase while the heating occurs in normal phase and the efficiency of adiabatic cooling or heating is higher at higher temperatures. This behavior can be understood from the lattice-height dependence of dispersion relation in each phase. Furthermore, the connection of the adiabatic cooling or heating between the cases with and without the trap potential is addressed.

Minimal model for disorder-induced missing moment of inertia in solidH4e

The absence of a missing moment inertia in clean solid $^{4}\text{H}\text{e}$ suggests that the minimal experimentally relevant model is the one in which disorder induces superfluidity in a bosonic lattice. To this end, we explore the relevance of the disordered Bose-Hubbard model in this context. We posit that a clean array of $^{4}\text{H}\text{e}$ atoms is a self-generated Mott insulator; that is, the $^{4}\text{H}\text{e}$ atoms constitute the lattice as well as the ``charge carriers.'' With this assumption, we are able to interpret the textbook defect-driven supersolids as excitations of either the lower or the upper Hubbard bands. In the experiments at hand, disorder induces the closing of the Mott gap through the generation of midgap localized states at the chemical potential. Depending on the magnitude of the disorder, we find that the destruction of the Mott state takes place for $d+z&gt;4$ either through a Bose-glass phase (strong disorder) or through a direct transition to a superfluid (weak disorder). For $d+z&lt;4$, disorder is always relevant. The critical value of the disorder that separates these two regimes is shown to be a function of the boson filling, interaction, and the momentum cutoff. We apply our work to the experimentally observed enhancement $^{3}\text{H}\text{e}$ impurities have on the onset temperature for the missing moment of inertia. We find quantitative agreement with experimental trends.

Effects of frustration on the anisotropic triangular lattice bosons

We study the extended hard-core Bose-Hubbard model with nearest-neighbor interactions on a triangular lattice with spatial anisotropy by quantum Monte Carlo simulations. The effects of the geometric frustration or the anisotropic strength, represented by the ratio ${t}^{\ensuremath{'}}/t$ between the intrachain nearest-neighbor hopping ${t}^{\ensuremath{'}}$ and the interchain hopping $t$, are studied. We find that for small values of anisotropy ratio ${t}^{\ensuremath{'}}/t$, a solid state emerges at $\ensuremath{\rho}=1/2$ and a supersolid is unstable toward phase separation; for large values of anisotropy ratio ${t}^{\ensuremath{'}}/t$, solid states emerge at $\ensuremath{\rho}=1/3$, 2/3, and a supersolid phase is stable in the region $\ensuremath{\rho}l2/3$; for intermediate values of anisotropy ratio ${t}^{\ensuremath{'}}/t$, three kinds of solid states at fillings $\ensuremath{\rho}=1/3$, 1/2, and $\ensuremath{\rho}=2/3$ coexist and no stable supersolid phase is observed. At half filling, we find that with increasing frustration ${t}^{\ensuremath{'}}/t$, the $\ensuremath{\rho}=1/2$ solid transitions indirectly into the supersolid via an intervening superfluid.

Spectral weight redistribution in strongly correlated bosons in optical lattices

We calculate the single-particle spectral function for the one-band Bose-Hubbard model within the random-phase approximation (RPA). In the strongly correlated superfluid, in addition to the gapless phonon excitations, we find extra gapped modes, which become particularly relevant near the superfluid-Mott quantum phase transition (QPT). The strength in one of the gapped modes, a precursor of the Mott phase, grows as the QPT is approached and evolves into a hole (particle) excitation in the Mott insulator depending on whether the chemical potential $\ensuremath{\mu}$ is above (below) the tip of the lobe. The sound velocity $c$ of the Goldstone modes remains finite when the transition is approached at constant density; otherwise, it vanishes at the transition. It agrees well with Bogoliubov theory except close to the transition. We also calculate the spatial correlations for bosons in an inhomogeneous trapping potential creating alternating shells of Mott insulator and superfluid. Finally, we discuss the capability of the RPA to correctly account for quantum fluctuations in the vicinity of the QPT.

Stability of Superflow for Ultracold Fermions in Optical Lattices

We investigate the stability of superflow of paired fermions in an optical lattice. We show that there are two distinct dynamical instabilities which limit the superflow in this system. One dynamical instability occurs when the superfluid stiffness becomes negative; this evolves, with increasing pairing interaction, from the fermion pair breaking instability to the well-known dynamical instability of lattice bosons. The second, more interesting, dynamical instability is marked by the emergence of a transient atom density wave. Both dynamical instabilities can be experimentally accessed by tuning the pairing interaction and the fermion density.

Rise and fall of hidden string order of lattice bosons

We investigate the ground-state properties of a newly discovered phase of one-dimensional lattice bosons with extended interactions [E. G. Dalla Torre et al., Phys. Rev. Lett. 97, 260401 (2006)]. The new phase, termed the Haldane insulator in analogy with the gapped phase of spin-1 chains, is characterized by a nonlocal order parameter, which can only be written as an infinite string in terms of the bosonic densities. We show that the string order can nevertheless be probed with physical fields that couple locally, via the effect those fields have on the quantum phase transitions separating the exotic phase from the conventional Mott and density wave phases. Using a field theoretical analysis, we show that a perturbation that breaks lattice inversion symmetry gaps the critical point separating the Mott and Haldane phases and eliminates the sharp distinction between them. This is remarkable given that neither of these phases involves broken inversion symmetry. We also investigate the evolution of the phase diagram with the tunable coupling between parallel chains in an optical lattice setup. We find that interchain tunneling destroys the direct phase transition between the Mott and Haldane insulators by establishing an intermediate superfluid phase. On the other hand, coupling the chains only by weak repulsive interactions does not modify the structure of the phase diagram. The theoretical predictions are confirmed with numerical calculations using the density matrix renormalization group.

Mixtures of Strongly Interacting Bosons in Optical Lattices

We investigate the properties of strongly interacting heteronuclear boson-boson mixtures loaded in realistic optical lattices, with particular emphasis on the physics of interfaces. In particular, we numerically reproduce the recent experimental observation that the addition of a small fraction of 41K induces a significant loss of coherence in 87Rb, providing a simple explanation. We then investigate the robustness against the inhomogeneity typical of realistic experimental realizations of the glassy quantum emulsions recently predicted to occur in strongly interacting boson-boson mixtures on ideal homogeneous lattices.

Sharp peaks in the momentum distribution of bosons in optical lattices in the normal state

Cold atoms in optical lattices provide a unique laboratory for investigating quantum phase transitions between strongly correlated superfluid and Mott insulator phases1,2. One of the major bottlenecks in the analysis of experiments is a clear set of criteria for identifying the superfluid phase3. A ‘sharp’ interference pattern in time-of-flight experiments has been widely adopted as a signature of superfluidity4,5,6,7,8. Here, we show that sharp peaks are not a reliable diagnostic of superfluidity. Using large-scale quantum Monte Carlo simulations of the Bose–Hubbard model in three dimensions with up to N=1.4×104 particles, we calculate the momentum distribution n(k) as a function of temperature T and t/U, the ratio of hopping to the onsite repulsion. We find that even above the transition temperature Tc where both superfluid density and condensate fraction vanish, the interference pattern can nevertheless have sharp peaks riding over a broad background. We identify the true signature of the superfluid and give a deeper understanding of why such sharp peaks appear in the normal state.

A t–J1–J2 model of spin-1 bosons in optical lattices

A t–J1–J2 model is constructed to describe the spin-1 bosons in opticallattices in the strong correlation limit. In the parameter region ofJ1<J2, with the slave-boson-mean-field approach, it is found that two kinds of condensed phases mayexist: a condensate with spin singlet pairs and a condensate with ferro-quadruple long rangeorder coexisting with singlet pairs. A first-order quantum phase transition occurs at the pointJ1 = 0. The finite-temperature phase transition in a cubic optical lattice was also discussed.