研究单个玻色-爱因斯坦凝聚体在双抛物面近似下的力行为，发现格鲁本 canonical 集合中 Robin 边界条件更优，而在 canonical 集合中存在相变，同时考虑了表面张力力和卡西米尔力。
摘要：
使用双抛物面近似法研究了单个玻色-爱因斯坦凝聚体在双板之间被限制时的力行为，证明在 grand canonical 集合中，Robin 边界条件的基态更优，而在 canonical 集合中，系统在小 L 区域从 Robin BC 到 Dirichlet BC 转变，而在大 L 区域则相反，且基态空间的相变是首order。同时详细考虑了在 canonical 和 grand canonical 集合中的表面张力力和卡西米尔力。
英文摘要：
Using double parabola approximation for a single Bose-Einstein condensate confined between double slabs we proved that in grand canonical ensemble (GCE) the ground state with Robin boundary condition (BC) is favored, whereas in canonical ensemble (CE) our system undergoes from ground state with Robin BC to the one with Dirichlet BC in small-$L$ region and vice versa for large-$L$ region and phase transition in space of the ground state is the first order. The surface tension force and Casimir force are also considered in both CE and GCE in detail.
The role of complex potentials in single-body Schrődinger equation has been studied intensively. We study the quantum coherence for degenerate Bose gases in complex potentials, when the exchange symmetry of identical bosons is considered. For initially independent Bose-Einstein condensates, it is shown that even very weak imaginary potential can induce perfect quantum coherence between different condensates. The scheme to observe imaginary potential induced quantum coherence is discussed.
Starting from the Bogoliubov diagonalization for the Hamiltonian of a weakly interacting Bose gas under the presence of a Bose-Enstein Condensate, we derive the kinetic equation for the Bogoliubov excitations. Without dropping any of the commutators, we find three collisional processes. One of them describes the $1\leftrightarrow2$ interactions between the condensate and the excited atoms. The other two describe the $2\leftrightarrow2$ and $1\leftrightarrow3$ interactions between the excited atoms themselves.
Ground state properties of trapped Bose condensate with repulsive interaction in non-homogeneous gravitational field are studied. Spatial structure of Bose condensate and its momentum distributions in 3-D anisotropic trap are considered by the solution of the modified non-linear Schrodinger equation. The results are compared with the corresponding properties of condensate in a harmonic trap without gravitational field.
The possibility of creating a Bose condensate of trapped atoms in a non-ground state is suggested. Such a nonequilibrium Bose condensate can be formed if one, first, obtains the conventional Bose condensate in the ground state and then transfers the condensed atoms to a non-ground state by means of a resonance pumping. The properties of ground and non-ground states are compared and plausible applications of such nonequilibrium condensates are discussed.
We show that spinor Bose-Einstein condensates not only have line-like vortex excitations, but in general also allow for point-like topological excitations, i.e., skyrmions. We discuss the static and dynamic properties of these skyrmions for spin-1/2 and ferromagnetic spin-1 Bose gases.
We show that a ``fragmented Bose condensate'' in which two or more distinct single-particle states are macroscopically occupied by the same species of boson is inherently unstable to the formation of a conventional Bose condensate whose macroscopically occupied state is a linear combination of the ``fragments'' with definite relative phases. A related analysis shows that a reproducible relative phase develops when two initially decoupled condensates are placed in contact.
The dynamical response of an atomic Bose-Einstein condensate manipulated by an integrated atom optics device such as a microtrap or a microfabricated waveguide is studied. We show that when the miniaturization of the device enforces a sufficiently high condensate density, three-body interactions lead to a spatial modulational instability that results in a fundamental limit on the coherent manipulation of Bose-Einstein condensates.
We suggest a practical way to estimate the condensate fraction of an interacting dilute Bose gas confined by an external harmonic potential as a function of temperature and scattering length. It shows that an increase of the scattering length produces an exponential decrease of condensate fraction.
We analytically examine the dynamics of quasi-particle modes occuring in a Bose-Einstein condensate which is subject to a weak acceleration. It is shown that the momentum of a quasi-particle mode is squeezed rather than accelerated.
Based on an analytic solution of the Gross-Pitaevskii equation in the large-condensate (Thomas-Fermi) limit we determine the structure of a stationary vortex in a Bose-Einstein condensate in a nonaxisymmetric rotating trap. The condensate velocity field has cylindrical symmetry only near the vortex core and becomes intrinsically anisotropic near the condensate boundaries. Rotating the anisotropic trap induces an additional irrotational velocity field even for a vortex-free condensate.
In this paper we investigate the macroscopic quantum tunnelling and the phase coherence property of the rotating Bose-Einstein condensates in both static and dynamic cases by using the mean field theory.
We find that the key features of the evolution and collapse of a trapped Bose condensate with negative scattering length are predetermined by the particle flux from the above-condensate cloud to the condensate and by 3-body recombination of Bose-condensed atoms. The collapse, starting once the number of Bose-condensed atoms reaches the critical value, ceases and turns to expansion when the density of the collapsing cloud becomes so high that the recombination losses dominate over attractive interparticle interaction. As a result, we obtain a sequence of collapses, each of them followed by dyna…
The system of dynamic equations for Bose-Einstein condensate at zero temperature with account of pair correlations is obtained. The spectrum of small oscillations of the condensate in a spatially homogeneous state is explored. It is shown that this spectrum has two branches: the sound wave branch and branch with an energy gap.
We study the drag on an object moving through a trapped Bose-Einstein condensate, and show that finite compressibility leads to a mutual drag, which is subsequently suppressed by the formation of a vortex ring.
We briefly discuss energy, normal modes and dynamics of a vortex in a trapped Bose-Einstein condensate. Theoretical results are compared with current experiments.
We consider two characteristic temperatures for a Bose-Einstein condensate, that are related to certain properties of the condensate statistics. We calculate them for an ideal gas confined in power-law traps and show that they approach the critical temperature in the limit of large number of particles. The considered characteristic temperatures can be useful in the studies of Bose-Einstein condensates of a finite number of atoms, indicating the point of a phase transition.
We analize the dynamics of two trapped interacting Bose-Einstein condensates and indentify two regimes for the evolution: the regime of slow periodic oscillations and the regime of strong non-linear mixing leading to the damping of the relative motion of the condensates. We compare our predictions with an experiment recently performed at JILA.
We investigate the propagation of zero sound in two-species interpenetrating Bose-Einstein condensates. In very elongated clouds, this propagation is shown to be essentially one-dimensional. We also present 1D numerical experiments that exhibit the existence of two different sound pulses with velocities consistent with the theoretical predictions.
We investigate energetic stability of vortices near the surface of a Bose-Einstein condensate. From an energy functional of a rotating Bose-Einstein condensate, written in terms of variables local to the surface, and a suitable trial wavefunction we calculate the energy of a moving vortex. The energetic stability of the vortex is investigated in terms of the rotation frequency of the confining potential. The critical frequency at which the vortices enter the condensate is calculated and compared with the experiment where a reasonable agreement is obtained.
The Bogoliubov theory of a uniform weakly-nonideal Bose-Einstein condensate leads to a divergent expression for the kinetic energy of atoms. However, the latter can be determined provided that the dependence of the scattering length on atomic mass is known. The explicit expressions are derived for the kinetic energy through parameters that specify interatomic interaction. The kinetic energy of condensate atoms noticeably exceeds the total energy. Numerical data are presented for Rb-87 and Na-23 condensates.
We study the process of photo-dissociation of a molecular Bose-Einstein condensate as a potential source of strongly correlated twin atomic beams. We show that the two beams can possess nearly perfect quantum squeezing in their relative numbers.
We solve the time-dependent Gross-Pitaevskii in 2-D to simulate the flow of an object through a dilute Bose condensate trapped in a harmonic well. We demonstrate vortex formation and analyze the process in terms of the accumulation of phase-slip and the evolution of the fluid velocity.
We solve the Gross-Pitaevskii equation to model the behaviour of a weakly-interacting Bose condensate. Solutions are presented for the eigenstates in one- and two-dimensional harmonic traps. We include the effect of gravity and coupling to a second condensate to simulate an output coupler for Bose condensed atoms, and find that the output pulse shape is in qualitative agreement with experiment. We also model flow under gravity through a constriction, demonstrating excitation and separation of eigenmodes in the condensate.
Fluctuations of the number of particles for the dilute interacting gas with Bose-Einstein condensate are considered. It is shown that in the Bogolubov theory these fluctuations are normal. The fluctuations of condensed as well as noncondensed particles are also normal both in canonical and grand canonical ensembles.
We study the process of scattering of two impurity atoms accompanied by generation of an elementary excitation in a surrounding Bose--Einstein condensate. This process, unlike the phonon generation by a {\it single} impurity atom, has no velocity threshold and can be regarded as a quantum acoustic analog of a bremsstrahlung in quantum electrodynamics.
Two independently prepared condensates can be combined into a single larger condensate by detection of their relative phase in an intereference measurement.
We present an elementary model of the collapses and revivals in the visibility of the interference between two atomic Bose-Einstein condensates. We obtain different predictions of the revival times whether we conserve or break atom number conservation from the outset. The validity of Bose-broken symmetry can be tested by observations of these collapses and revivals.
The Bose condensation of a stack of low dimensional disks which is composed of a noninteracting charged Bose gas in a uniform magnetic field is studied. A statistical approach with density of states at noninteger dimensions are applied for the system. The condensate fraction of the disk system in a uniform magnetic field is calculated. The stack of low dimensional charged Bose disks is found not to share the condensate behavior of the traditional BCS superfluids.
These notes discuss two aspects of the physics of atomic Bose-Einstein condensates: optical properties and spinor condensates. The first topic includes light scattering experiments which probe the excitations of a condensate in both the free-particle and phonon regime. At higher light intensity, a new form of superradiance and phase-coherent matter wave amplification were observed. We also discuss properties of spinor condensates and describe studies of ground--state spin domain structures and dynamical studies which revealed metastable excited states and quantum tunneling.
We derive the dynamic and thermodynamic stability conditions for dilute multicomponent Bose-Einstein condensates (BECs). These stability conditions, generalized for $n$-component BECs, are found to be equivalent and are shown to be consistent with the phase diagrams of two- and three-component condensates that are derived from energetic arguments.
A stack of disks with numerous holes composed of a non-interacting charged Bose gas is modeled as a low dimensional disk. The Bose condensation of the net-like disk system in a uniform magnetic field is studied. Calculation of the condensate fraction of the net-like disk system showed that there still exists a non-zero condensate fraction at low temperature and in weak field.
We have observed phase singularities due to vortex excitation in Bose-Einstein condensates. Vortices were created by moving a laser beam through a condensate. They were observed as dislocations in the interference fringes formed by the stirred condensate and a second unperturbed condensate. The velocity dependence for vortex excitation and the time scale for re-establishing a uniform phase across the condensate were determined.
Surface modes in a Bose-Einstein condensate of sodium atoms have been studied. We observed excitations of standing and rotating quadrupolar and octopolar modes. The modes were excited with high spatial and temporal resolution using the optical dipole force of a rapidly scanning laser beam. This novel technique is very flexible and should be useful for the study of rotating Bose-Einstein condensates and vortices.
It is found that combining an excitation-energy sum rule with Fetter's trial wave function gives almost exact low-lying collective-mode frequencies of a trapped Bose-Einstein condensate at zero temperature.
We investigate the fluctuations of the condensate in the ideal and weakly interacting Bose gases confined in a box of volume V within canonical ensemble. Canonical ensemble is developed to describe the behavior of the fluctuations when different methods of approximation to the weakly interacting Bose gases are used. Research shows that the fluctuations of the condensate exhibit anomalous behavior for the interacting Bose gas confined in a box.
In this article we study the formation of the two modes Bose-Einstein condensate and the correlation between them. We show that beyond the mean field approximation the dissociation of a molecular condensate due to the chemical potential curve crossing leads to the formation of two modes condensate. We also show that these two modes are correlated in a two mode squeezed state.
In this paper, we present a theoretical study of a Bose-Einstein condensate of interacting bosons in a quartic trap in one, two, and three dimensions. Using Thomas-Fermi approximation, suitably complemented by numerical solutions of the Gross-Pitaevskii equation, we study the ground sate condensate density profiles, the chemical potential, the effects of cross-terms in the quartic potential, temporal evolution of various energy components of the condensate, and width oscillations of the condensate. Results obtained are compared with corresponding results for a bose condensate in a harmonic con…
The Bogoliubov theory of weakly interacting bosons is generalized to Bose-Einstein condensates with internal degrees of freedom so that a single effective Hamiltonian produces various many-body ground states or metastable spin domains and the corresponding collective modes on an equal footing.
In this letter, atom optic techniques are proposed to control the excitation of a Bose-Einstein condensate in an atomic trap. We show that by employing the dipole potential induced by four highly detuned travelling-wave laser beams with appropriate phases and frequencies, one can coherently excite a trapped Bose-Einstein condensate composed of ultracold alkali atoms into a state rotating around the trap center. The connection to vortex states is discussed.
The theory of resonant generation of nonground-state Bose-Einstein condensates is extended to Bose-condensed systems at finite temperature. The generalization is based on the notion of representative statistical ensembles for Bose systems with broken global gauge symmetry. Self-consistent equations are derived describing an arbitrary nonequilibrium nonuniform Bose system. The notion of finite-temperature topological coherent modes, coexisting with a cloud of noncondensed atoms, is introduced. It is shown that resonant generation of these modes is feasible for a gas of trapped Bose atoms at fin…
The general system is given of nonlinear equations describing dissipationless evolution of the oscillating Bose-condensate. The relaxation of transverse oscillations of the condensate in a trap of the cylindric symmetry is considered. The evolution occurs due to parametric resonance coupling the transverse mode with the longitudinal ones. The nonlinear rescattering in the subsystem of discrete longitudinal modes results in suppression of the return of energy, yielding dissipationless nonmonotonic relaxation of transverse oscillations in the condensate.
The problem of the Bose-Einstein condensate preservation under thermofield and standard gauge-invariant perturbations is discussed. A new result on stability of the condensate under thermofield perturbations of a polynomial type is presented.
