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Register a new userJosephson plasma oscillations and the Gross-Pitaevskii equation: Bogoliubov approach vs two-mode model
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We show that the Josephson plasma frequency for a condensate in a double-well potential, whose dynamics is described by the Gross-Pitaevskii (GP) equation, can be obtained with great precision by means of the usual Bogoliubov approach, whereas the two-mode model - commonly constructed by means of a linear combinations of the low-lying states of the GP equation - generally provides accurate results only for weak interactions. A proper two-mode model in terms of the Bogoliubov functions is also discussed, revealing that in general a two-mode approach is formally justified only for not too large interactions, even in the limit of very small amplitude oscillations. Here we consider specifically the case of a one-dimensional system, but the results are expected to be valid in arbitrary dimensions.
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We consider the two-dimensional Gross-Pitaevskii equation describing a Bose-Einstein condensate in an isotropic harmonic trap. In the small coupling regime, this equation is accurately approximated over long times by the corresponding nonlinear resonant system whose structure is determined by the fully resonant spectrum of the linearized problem. We focus on two types of consistent truncations of this resonant system: first, to sets of modes of fixed angular momentum, and second, to excited Landau levels. Each of these truncations admits a set of explicit analytic solutions with initial conditions parametrized by three complex numbers. Viewed in position space, the fixed angular momentum solutions describe modulated oscillations of dark rings, while the excited Landau level solutions describe modulated precession of small arrays of vortices and antivortices. We place our findings in the context of similar results for other spatially confined nonlinear Hamiltonian systems in recent literature.
We introduce 1D and 2D models of a degenerate bosonic gas composed of ions with positive and negative charges (cations and anions). The system may exist in the mean-field condensate state, enabling the competition of the Coulomb coupling, contact repulsion, and kinetic energy of the particles, provided that their effective mass is reduced by means of a lattice potential. The model combines the Gross-Pitaevskii (GP) equations for the two-component wave function of the cations and anions, coupled to the Poisson equation for the electrostatic potential mediating the Coulomb coupling. The contact interaction in the GP system can be derived, in the Thomas-Fermi approximation, from a system of three GP equations, which includes the wave function of heavy neutral atoms. In the system with fully repulsive contact interactions, we construct stable spatially periodic patterns (density waves, DWs). The transition to DWs is identified by analysis of the modulational instability of a uniformly mixed neutral state. The DW pattern, which represents the systems ground state (GS), is predicted by a variational approximation. In 2D, a stable pattern is produced too, with a quasi-1D shape. The 1D system with contact self-attraction in each component produces bright solitons of three types: neutral ones, with fully mixed components; dipoles, with the components separated by the inter-species contact repulsion; and quadrupoles, with a layer of one component sandwiched between side lobes formed by the other. The transition from the neutral solitons to dipoles is accurately modeled analytically. A chart of the GSs of the different types (neutral solitons, dipoles, or quadrupoles) is produced. Different soliton species do not coexist as stable states. Collisions between traveling solitons are elastic for dipole-dipole pairs, while dipole-antidipole ones merge into stable quadrupoles via multiple collisions.
We carry out extensive direct numerical simulations (DNSs) to investigate the interaction of active particles and fields in the two-dimensional (2D) Gross-Pitaevskii (GP) superfluid, in both simple and turbulent flows. The particles are active in the sense that they affect the superfluid even as they are affected by it. We tune the mass of the particles, which is an important control parameter. At the one-particle level, we show how light, neutral, and heavy particles move in the superfluid, when a constant external force acts on them; in particular, beyond a critical velocity, at which a vortex-antivortex pair is emitted, particle motion can be periodic or chaotic. We demonstrate that the interaction of a particle with vortices leads to dynamics that depends sensitively on the particle characteristics. We also demonstrate that assemblies of particles and vortices can have rich, and often turbulent spatiotemporal evolution. In particular, we consider the dynamics of the following illustrative initial configurations: (a) one particle placed in front of a translating vortex-antivortex pair; (b) two particles placed in front of a translating vortex-antivortex pair; (c) a single particle moving in the presence of counter-rotating vortex clusters; and (d) four particles in the presence of counter-rotating vortex clusters. We compare our work with earlier studies and examine its implications for recent experimental studies in superfluid Helium and Bose-Einstein condensates.
We consider an effective scaling approach for the free expansion of a one-dimensional quantum wave packet, consisting in a self-similar evolution to be satisfied on average, i.e. by integrating over the coordinates. A direct comparison with the solution of the Gross-Pitaevskii equation shows that the effective scaling reproduces with great accuracy the exact evolution - the actual wave function is reproduced with a fidelity close to unity - for arbitrary values of the interactions. This result represents a proof-of-concept of the effectiveness of the scaling ansatz, which has been used in different forms in the literature but never compared with the exact evolution.
We describe a method for evolving the projected Gross-Pitaevskii equation (PGPE) for an interacting Bose gas in a harmonic oscillator potential, with the inclusion of a long-range dipolar interaction. The central difficulty in solving this equation is the requirement that the field is restricted to a small set of prescribed modes that constitute the low energy c-field region of the system. We present a scheme, using a Hermite-polynomial based spectral representation, that precisely implements this mode restriction and allows an efficient and accurate solution of the dipolar PGPE. We introduce a set of auxiliary oscillator states to perform a Fourier transform necessary to evaluate the dipolar interaction in reciprocal space. We extensively characterize the accuracy of our approach, and derive Ehrenfest equations for the evolution of the angular momentum.
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