We propose a phenomenological equation for the vortex line density in rotating Bose-Einstein condensates as a function of the angular speed. This equation provides a simple description of the gross features of the increase in vortex number from the appearance of the first vortex to the theoretical rigid-body result for high vortex density, and allows one to compare with analogous situations in superfluid helium, after the suitable changes in the relevant parameters are made.
We propose a phenomenological equation for the vortex line density in rotating Bose-Einstein condensates as a function of the angular speed. This equation provides a simple description of the gross features of the increase in vortex number from the appearance of the first vortex to the theoretical rigid-body result for high vortex density, and allows one to compare with analogous situations in superfluid helium, after the suitable changes in the relevant parameters are made.
The paper describes several features of vortex filaments in a weakly non-ideal Bose-Einstein condensate (BEC) with a counterflow. The study focuses on a process of reconnection and equations of motion. A primary motivation of this study is a fact that quantum vortex dynamics for superfluid helium was obtained previously at a phenomenological level. But some critical problems can be solved through a study of a corresponding problem in BEC. By considering a vortex filament as a crossing line for surfaces where the real and imaginary parts of a macroscopic wave function ψ ( r , t ) turn into zero, we derived the equations of motion for a vortex filament. It is remarkable that the equation structure coincides completely with a structure of equation for vortex filament dynamics in superfluid helium that has been obtained from other (phenomenological) foundations.
A quantum liquid of an almost ideal Bose gas brought into rotation is investigated from a physical and mechanical point of view on the basis of the Gross–Pitaevskii (GP) equation by applying a quantum-mechanical scenario, i.e. the London scenario. This scenario allows a superfluid to have rotational states. Considering that the equation governs an interacting Bose gas, it is proposed that the GP equation admits rotational flows of a superfluid. This is carried out without incurring essential change of the equation. By this reformulation, a superfluid placed in a rotating vessel is able to have a solid body rotation with the same angular velocity as its container and also to have a meniscus approximated by a parabolic profile. The solid body rotation is accompanied by a density increase proportional to the square of its angular velocity. These are consistent with experimental observations. In addition, this formulation allows a vortex of quantized circulation with coaxial rotational core whose density does not necessarily vanish at its center.
The aim of this paper is to derive the equations of motion of quantum vortices in a weakly non-ideal Bose–Einstein condensate (BEC) at finite temperatures and in the presence of macroscopic superfluid and normal velocities. The motivation for this work is that the dynamics of quantum vortices in superfluid helium has been studied at the phenomenological level, and a number of issues require clarification, which can be addressed by studying the corresponding problem in BEC. In BEC, quantum vortices are located at points where the macroscopic wave function (MWF) ψ(r,T), which describes the BEC dynamics, vanishes ψ(r,T)=0. The intersection of the surfaces Reψ(r,T)=0 and Imψ(r,T)=0 forms a one-dimensional line, which is just the quantum vortex line. During the evolution of the MWF, the position of the zero surfaces changes, that is, the vortex lines move. Conversely, the motion of these vortex filaments determines the evolution of the MWF. Thus, there is a mutual correspondence, and the quantity ψ(r,T) can be considered as a functional of the entire vortex configuration, i.e., {s(ξ,t)}. This viewpoint allows us to derive the equations of motion for vortices from the Gross–Pitaevskii equation (GPE), which describes the dynamics of MWF. The key finding is the confirmation that the equation derived from the fundamental quantum description (the GPE) for a BEC has the same mathematical structure as the equation derived from macroscopic, phenomenological reasoning for superfluid helium.
Numerical simulations of vortex motion in a trapped Bose-Einstein condensate were performed by solving the two-dimensional Gross-Pitaevskii equation in the presence of a simple phenomenological model of interaction between the condensate and the finite temperature thermal cloud. The log (base e) of total energy, trap energy, quantum energy, kinetic energy, internal energy and z-component of the angular momentum vs time were compared with f(x)=a+bx for that time when the vortices come in in the condensate. The increasing/decay rate of these energies and L_{z} were studied as a function of dissipation.
A statement of the quantum turbulence problem in both a Bose – Einstein condensate (BEC) and superfluid helium is formulated. In superfluid helium use is made of a so-called vortex filament method, in which quantum vortices are represented by string-like objects, i.e. vortex lines. The dynamics of the vortex lines is determined by deterministic equations of motion, supplemented by random reconnections. Unlike He II, the laws of the dynamics of quantum vortices in BEC are based on the nonlinear Schrödinger equation. This makes it possible to obtain a microscopic description of the collision of vortices, the structure of a vortex filament, etc. A comparative analysis of these complementary approaches is carried out. It is shown that there are some features that do not automatically transfer the results obtained for BEC to vortices in He II and vice versa.
One of the key issues related to superfluidity is the existence of quantized vortices. Following recent experiments on Bose–Einstein condensates exhibiting vortices, we investigate the behavior of the wave function which solves the Gross–Pitaevskii equation. For a rotating Bose–Einstein condensate in a harmonic trap, we give a simplified expression of the Gross–Pitaevskii energy in the Thomas–Fermi regime, which only depends on the number and shape of the vortex lines. This allows us to study in detail the structure of the line of a single quantized vortex, which has a U or S shape. S type vortices exist for all values of the angular velocity Ω but are not minimizers of the energy while U vortices are minimizers and exist only for Ω sufficiently large. Finally, we study the drag created by the movement of a detuned laser beam in a condensate and the nucleation of vortices in the low density region close to the boundary layer of the cloud.