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arXiv:2311.05361 · Comptes Rendus Mathématique

A Lower Bound on the Critical Momentum of an Impurity in a Bose–Einstein Condensate

Benjamin Hinrichs, Jonas Lampart

Published Nov 14, 2024• 2 citations
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Abstract

In the Bogoliubov–Fröhlich model, we prove that an impurity immersed in a Bose–Einstein condensate forms a stable quasi-particle when the total momentum is less than its mass times the speed of sound. The system thus exhibits superfluid behavior, as this quasi-particle does not experience friction. We do not assume any infrared or ultraviolet regularization of the model, which contains massless excitations and point-like interactions.

DOI
10.5802/crmath.652

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In the Bogoliubov–Fröhlich model, we prove that an impurity immersed in a Bose–Einstein condensate forms a stable quasi-particle when the total momentum is less than its mass times the speed of sound. The system thus exhibits superfluid behavior, as this quasi-particle does not experience friction. We do not assume any infrared or ultraviolet regularization of the model, which contains massless excitations and point-like interactions.
We analyze the many-body Hamiltonian describing a mobile impurity immersed in a Bose-Einstein condensate (BEC). Using exact unitary transformations and rigorous error estimates, we show the validity of the Bogoliubov-Fr\"ohlich Hamiltonian for the Bose polaron in the regime of moderately strong, repulsive interactions with a dilute BEC. Moreover, we calculate analytically the universal logarithmic correction to the ground state energy that arises from an impurity mediated phonon-phonon interaction.
We investigate the superfluid properties of a Bose–Einstein condensate (BEC) trapped in a one-dimensional periodic potential. We study, both analytically (in the tight binding limit) and numerically, the Bloch chemical potential, the Bloch energy and the Bogoliubov dispersion relation, and we introduce two different, density dependent, effective masses and group velocities. The Bogoliubov spectrum predicts the existence of sound waves, and the arising of energetic and dynamical instabilities at critical values of the BEC quasi-momentum which dramatically affect its coherence properties. We investigate the dependence of the dipole and Bloch oscillation frequencies in terms of an effective mass averaged over the density of the condensate. We illustrate our results with several animations obtained solving numerically the time-dependent Gross–Pitaevskii equation.
An investigation is performed using normal-mode analysis on a quantum mechanical cell model whose general features are representative of 4He at low temperatures. Both random-phase and self-consistent field approximations are invoked. A Bogoliubov-type excitation energy spectrum is derived for the interacting Bose system, and the amplitudes of the system quasi-particles are obtained; these quantities are expressed in terms of zero-momentum occupation number. A relationship giving the temperature-dependent fractional occupation of the single-particle zero-momentum level is established, and special attention is devoted to the temperature extreme near absolute zero. An estimate is made of the zero-temperature sound speed corresponding to the special case of phonon excitation given by the model in low momentum. It is shown that at a critical temperature for which the zero-momentum occupation is depleted, the quasi-particles indeed become normal Bose particles. In first approximation, this critical temperature is akin in form to that obtained in treatments of ideal boson systems.
Several aspects of superfluidity in an atomic Bose-Einstein Condensate are studied in a torus-type quasi-1D model. The critical velocity of the superfluid in this quasi-1D model is found to differ from the conventional concept of the limit of velocity for frictionless flow. This result is one of the unique features of the quasi-1D system. For a generalized discrete quasi-1D model, the stability of Bloch oscillations is studied. The quasi-particle spectrum of small oscillations is analyzed, which in turn gives the phase diagram of the stable-unstable region. The results are in good agreement with a recent experimental observation. Last, we consider an experiment proposal where a phase-imprinting technique is used to generate superfluid flow with fixed momentum or a vortex in atomic BEC. Our study shows that because of backflow generated by part of the imprinting potential, phase imprinting is not an ideal tool to generate a single-momentum superflow or single vortex. ^
A model for classical impurities moving in Bose-Einstein Condensate (BEC) is proposed in the framework of quantum field theory and is solved within the Bogoliubov approximation at zero temperature. Several formulae are obtained for physical quantities such as the occupation number, the dissipation, the free energy, and the depletion. To illustrate this model two examples are studied; an impurity moving with i) a constant velocity and ii) a constant acceleration. Landau's criterion for energy dissipation accompanying the motion of impurities due to creation of quasi-particles in a superfluid, which was proposed on the basis of purely phenomenological argument, is obtained in the frame work of an exact quantum mechanical model.