arXiv subjects

R. J. Ballagh

Publications and source records attributed to R. J. Ballagh.

29 records · Page 2Linked to original sources

arXiv2001

Elementary excitation families and their frequency ordering in cylindrically symmetric Bose-Einstein condensates

We present a systematic classification of the elementary excitations of Bose-Einstein condensates in cylindrical traps in terms of their shapes. The classification generalizes the concept of families of excitations first identified by Hutchinson and Zaremba (1998) Phys. Rev. A 57 1280 by introducing a second classification number that allows all possible modes to be assigned to a family. We relate the energy ordering of the modes to their family classification, and provide a simple model which explains the relationship.

cond-mat↗

arXiv2001

Spatially selective Bragg scattering: a signature for vortices in Bose-Einstein condensates

We demonstrate that Bragg scattering from a condensate can be sensitive to the spatial phase distribution of the initial state. This allows preferential scattering from a selected spatial region, and provides a robust signature for a vortex state. We develop an analytic model which accurately describes this phenomenon and we give quantitative predictions for current experimental conditions.

cond-mat.soft↗

arXiv2000

Quantum kinetic theory VII: The influence of vapor dynamics on condensate growth

We extend earlier models of the growth of a Bose-Einstein condensate to include the full dynamical effects of the thermal cloud by numerically solving a modified quantum Boltzmann equation. We determine the regime in which the assumptions of the simple model are a reasonable approximation, and compare our new results with those that were earlier compared with experimental data. We find good agreement with our earlier modelling, except at higher condensate fractions, for which a significant speedup is found. We also investigate the effect of temperature on condensate growth, and find that this has a surprisingly small effect. The discrepancy between theory and experiment remains, since the speedup found in these computations does not occur in the parameter regime specified in the the experiment.

cond-mat↗

arXiv2000

Three-dimensional vortex dynamics in Bose-Einstein condensates

We simulate in the mean-field limit the effects of rotationally stirring a three-dimensional trapped Bose-Einstein condensate with a Gaussian laser beam. A single vortex cycling regime is found for a range of trap geometries, and is well described as coherent cycling between the ground and the first excited vortex states. The critical angular speed of stirring for vortex formation is quantitatively predicted by a simple model. We report preliminary results for the collisions of vortex lines, in which sections may be exchanged.

cond-mat↗

arXiv1999

Coherent Dynamics of Vortex Formation in Trapped Bose-Einstein Condensates

Simulations of a rotationally stirred condensate show that a regime of simple behaviour occurs in which a single vortex cycles in and out of the condensate. We present a simple quantitative model of this behaviour, which accurately describes the full vortex dynamics, including a critical angular speed of stirring for vortex formation. A method for experimentally preparing a condensate in a central vortex state is suggested.

cond-mat↗

arXiv1999

Formation of Ramsey Fringes in Double Bose-Einstein Condensates

Numerical simulations of Ramsey fringe formation in double Bose condensates are carried out in two spatial dimensions. The effects of meanfield nonlinearity and diffusion in the condensates give rise to new features in the Ramsey fringes, namely spatial variation and an asymmetry between positive and negative field detunings. We introduce the concept of an effective detuning, which for moderate interpulse times provides a qualitative understanding of the new features.

cond-mat↗

arXiv1999

Dressed States of a two component Bose-Einstein Condensate

A condensate with two internal states coupled by external electromagnetic radiation, is described by coupled Gross Pitaevskii equations, whose eigenstates are analogous to the dressed states of quantum optics. We solve for these eigenstates numerically in the case of one spatial dimension, and explore their properties as a function of system parameters. In contrast to the quantum optical case, the condensate dressed states exhibit spatial behaviour which depends on the system parameters, and can be manipulated by changing the cw external field.

cond-mat↗

arXiv1998

Quantum Kinetic Theory of Condensate Growth---Comparison of Experiment and Theory

In a major extension of our previous model (C.W. Gardiner, P. Zoller, R.J. Ballagh and M.J. Davis, Phys. Rev. Lett. 79, 1793 (1997)) of condensate growth, we take account of the evolution of the occupations of lower trap levels, and of the full Bose-Einstein formula for the occupations of higher trap levels. We find good agreement with experiment, especially at higher temperatures. We also confirm the picture of the ``kinetic'' region of evolution, introduced by Kagan et al, for the time up to the initiation of the condensate. The behavior after initiation essentially follows our original growth equation, but with a substantially increased rate coefficient W^{+}.

cond-mat↗

arXiv1998

Role of quasiparticles in the growth of a trapped Bose-Einstein condensate

A major extension of the model of condensate growth introduced by us is made to take account of the evolution of the occupations of lower trap levels (quasiparticles) by scattering processes, and of the full Bose-Einstein formula for the occupations of higher trap levels, which are assumed to have a time independent occupation. The principal effect is a speedup of the growth rate by somewhat less than an order of magnitude, the precise value depending on the the assumptions made on scattering and transition rates for the quasiparticle levels.

cond-mat↗

arXiv1997

Formation of fundamental structures in Bose-Einstein Condensates

The meanfield interaction in a Bose condensate provides a nonlinearity which can allow stable structures to exist in the meanfield wavefunction. We discuss a number of examples where condensates, modelled by the one dimensional Gross Pitaevskii equation, can produce gray solitons and we consider in detail the case of two identical condensates colliding in a harmonic trap. Solitons are shown to form from dark interference fringes when the soliton structure, constrained in a defined manner, has lower energy than the interference fringe and an analytic expression is given for this condition.

cond-mat.stat-mech↗

arXiv1997

Kinetics of Bose-Einstein Condensation in a Trap

The formation process of a Bose-Einstein condensate in a trap is described using a master equation based on quantum kinetic theory, which can be well approximated by a description using only the condensate mode in interaction with a thermalized bath of noncondensate atoms. A rate equation of the form n = 2W(n)[(1-exp((mu_n - mu)/kT))n + 1] is derived, in which the difference between the condensate chemical potential mu_n and the bath chemical potential mu gives the essential behavior. Solutions of this equation, in conjunction with the theoretical description of the process of evaporative cooling, give a characteristic latency period for condensate formation and appear to be consistent with the observed behavior of both rubidium and sodium condensate formation.

quant-ph↗