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arXiv:0708.2789 · Physical Review A

Collective excitations of trapped one-dimensional dipolar quantum gases

P. Pedri, S. De Palo, E. Orignac, R. Citro, Maria Luisa Chiofalo

Published Jan 29, 2008• 44 citations
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Abstract

We calculate the excitation modes of a one-dimensional (1D) dipolar quantum gas confined in a harmonic trap with frequency ${\ensuremath{\omega}}_{0}$ and predict how the frequency of the breathing $n=2$ mode characterizes the interaction strength evolving from the Tonks-Girardeau value ${\ensuremath{\omega}}_{2}=2{\ensuremath{\omega}}_{0}$ to the quasi-ordered, superstrongly interacting value ${\ensuremath{\omega}}_{2}=\sqrt{5}{\ensuremath{\omega}}_{0}$. Our predictions are obtained within a hydrodynamic Luttinger-liquid theory after applying the local density approximation to the equation of state for the homogeneous dipolar gas, which are in turn determined from reptation quantum Monte Carlo simulations. They are shown to be in quite accurate agreement with the results of a sum-rule approach. These effects can be observed in current experiments, revealing the Luttinger-liquid nature of 1D dipolar Bose gases.

DOI
10.1103/physreva.77.015601

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We calculate the excitation modes of a one-dimensional (1D) dipolar quantum gas confined in a harmonic trap with frequency \ensuremathω0{\ensuremath{\omega}}_{0}\ensuremathω0​ and predict how the frequency of the breathing n=2n=2n=2 mode characterizes the interaction strength evolving from the Tonks-Girardeau value \ensuremathω2=2\ensuremathω0{\ensuremath{\omega}}_{2}=2{\ensuremath{\omega}}_{0}\ensuremathω2​=2\ensuremathω0​ to the quasi-ordered, superstrongly interacting value \ensuremathω2=5\ensuremathω0{\ensuremath{\omega}}_{2}=\sqrt{5}{\ensuremath{\omega}}_{0}\ensuremathω2​=5​\ensuremathω0​. Our predictions are obtained within a hydrodynamic Luttinger-liquid theory after applying the local density approximation to the equation of state for the homogeneous dipolar gas, which are in turn determined from reptation quantum Monte Carlo simulations. They are shown to be in quite accurate agreement with the results of a sum-rule approach. These effects can be observed in current experiments, revealing the Luttinger-liquid nature of 1D dipolar Bose gases.
Ultracold bosonic and fermionic quantum gases confined to quasi-one-dimensional (1D) geometry are promising candidates for probing fundamental concepts of Luttinger liquid (LL) physics. They can also be exploited for devising applications in quantum information processing and precision measurements. Here, we focus on 1D dipolar Bose gases, where evidence of super-strong coupling behavior has been demonstrated by analyzing the low-energy static and dynamical structures of the fluid at zero temperature by a combined reptation quantum Monte Carlo (RQMC) and bosonization approach. Fingerprints of LL behavior emerge in the whole crossover from the already strongly interacting Tonks–Girardeau at low density to a dipolar density wave regime at high density. We have also shown that a LL framework can be effectively set up and utilized to describe this strongly correlated crossover physics in the case of confined 1D geometries after using the results for the homogeneous system in LL hydrodynamic equations within a local density approximation. This leads to the prediction of observable quantities such as the frequencies of the collective modes of the trapped dipolar gas under the more realistic conditions that could be found in ongoing experiments. The present paper provides a description of the theoretical framework in which the above results have been worked out, making available all the detailed derivations of the hydrodynamic Luttinger equations for the inhomogeneous trapped gas and of the correlation functions for the homogeneous system.
We theoretically investigate the ground-state properties and breathing-mode collective excitations of three-dimensional dipolar Bose gases in anisotropic harmonic traps incorporating quantum fluctuations. Combining a Gaussian variational ansatz with a non-perturbative sum-rule analysis, we derive explicit analytical expressions for both axial and radial breathing-mode frequencies, which are validated by numerical solutions of the time-dependent extended Gross-Pitaevskii equation. Our theoretical predictions show excellent agreement with existing experimental data for 166^{166}166Er and 162^{162}162Dy gases. By constructing comprehensive phase diagrams across the parameter space of the sss-wave scattering length, atom number, and trap aspect ratio, we reveal both discontinuous first-order phase transitions and smooth crossovers between the dilute Bose-Einstein condensate and dense quantum droplet phases. We confirm that the enhanced incompressibility induced by quantum fluctuations significantly elevates the breathing-mode frequencies in the droplet phase compared to conventional weakly interacting Bose gases. Furthermore, the system undergoes a phase transition and a crossover over the scattering length under the quasi-two-dimensional and quasi-one-dimensional confinements, characterized by discontinuous jumps and continuous crossovers in peak density and atomic cloud sizes, respectively. Our work offers a rigorous and highly accurate framework to characterize collective excitations in dipolar quantum gases, providing quantitative insights for forthcoming ultracold atom experiments in lanthanide atoms and polar molecules.
By using a sum-rule approach we investigate the transition between the hydrodynamic and the collisionless regime of the collective modes in a one-dimensional (1D) harmonically trapped Bose gas. Both the weakly interacting gas and the Tonks-Girardeau limits are considered. We predict that the excitation of the dipole compression mode is characterized in the high-temperature collisionless regime by a beating signal of two different frequencies (${\ensuremath{\omega}}_{z}$ and $3{\ensuremath{\omega}}_{z}$), while in the high-temperature collisional regime, the excitation consists of a single frequency ($\sqrt{7}{\ensuremath{\omega}}_{z}$). This behavior differs from the case of the lowest breathing mode whose excitation consists of a single frequency ($2{\ensuremath{\omega}}_{z}$) in both regimes. Our predictions for the dipole compression mode open promising perspectives for the experimental investigation of collisional effects in 1D configurations.
We calculate the breathing mode frequency ω\omegaω in a one-dimensional Bose gas confined to a harmonic trap of frequency ωz\omega_zωz​. We predict a smooth crossover from Thomas-Fermi Bose-Einstein condensate (BEC) regime (ω=3ωz\omega = \sqrt{3} \omega_zω=3​ωz​) to Gaussian BEC regime (ω=2ωz\omega = 2 \omega_zω=2ωz​) using Hartree approximation and sum rules approach, adopted to small system sizes. Local density approximation (LDA) correctly captures the crossover from Tonks-Girardeau (ω=2ωz\omega = 2 \omega_zω=2ωz​) to Thomas-Fermi BEC (ω=3ωz\omega = \sqrt{3} \omega_zω=3​ωz​) regimes. Predictions from local density and Hartree approximations can be continuously matched for N>25N>25N>25 particles, providing a complete zero-temperature description for large number of particles and resulting in a non-monotonic reentrant behavior of the breathing frequency. For smaller number of particles (ranging from N=2N=2N=2 to N=25N=25N=25) we perform extensive diffusion Monte Carlo simulations. This permits us to obtain a complete picture, applicable to arbitrary number of particles and any repulsive interaction strength. We provide perturbative analysis for both weak and strong coupling regimes. We revisit Innsbruck group experiment [Science 325, 1224 (2009)], analyzing the measurements done in Thomas-Fermi - Gaussian BEC crossover, and demonstrate that the breathing frequency follows a reentrant behavior as a function of the interaction strength in full agreement with our theory.
A strongly correlated Luttinger-liquid behavior is found to emerge well beyond the Tonks-Girardeau (TG) regime in a one-dimensional Bose gas with dipolar repulsions at T=0T=0T=0, persisting for a wide range of densities. After combining reptation quantum Monte Carlo and bosonization techniques, we provide a unifying theory of the underlying crossover physics, evolving from the TG gas at low density into a classical quasiordered state at high density. The density dependent Luttinger parameters extracted from the numerical data provide all that is needed to determine the low-energy behavior from analytical expressions. Our quantitative predictions, in the whole crossover, for measurable quantities such as the structure factor and the momentum distribution, are estimated to be accessible in underway experiments with ultracold polar molecules.
We investigate the breathing mode and the stability of a quantum droplet in a tightly trapped one-dimensional dipolar gas of bosonic atoms. When the droplet with a flat-top density profile is formed, the breathing-mode frequency scales as the inverse of the number of atoms in the cloud. This is straightforwardly derived within a phenomenological hydrodynamical approach and confirmed using both a variational method based on a generalized Gross-Pitaevskii action functional and the sum-rule approach. We extend our analysis also to the presence of axial confinement showing the effect of the trap on the density profile and therefore on the breathing-mode frequency scaling. Our analysis confirms the stability of the quantum droplet against the particle emission when the flat-top density profile is observed. Our results can be used as a guide to the experimental investigations of collective modes to detect the formation of quantum droplets in quasi-one-dimensional dipolar gases.