20 research outputs found
Bose-Einstein condensates with long-range dipolar interactions
Bose-Einstein condensation is a phase transition which atoms undergo when cooled near absolute zero temperature Since the theoretical prediction in 1924, and the spectacular experimental confirmation of Bose-Einstein condensation in 1995, a rich new field in physics has emerged studying ultracold degenerate quantum gases. Although these ultracold gases are very dilute, their properties are nevertheless strongly influenced by interatomic interactions. Usually, these interactions are dominated by short range, isotropic contact interactions. In contrast, the recently realised Bose-Einstein Condensate (BEC) of Chromium atoms contains long-range, anisotropic dipolar interactions leading to interesting new physics. In this graduation project, stationary states of such dipolar BECs in harmonic traps are investigated for various experimentally relevant parameters. Furthermore, the elementary excitations of the BEC are calculated, as well as its response to a rotating perturbation. Finally, some more advanced topics such as vortex interactions and condensate response to impurities are investigated. Bose-Einstein condensation is a phase transition which atoms undergo when cooled near absolute zero temperature Since the theoretical prediction in 1924, and the spectacular experimental confirmation of Bose-Einstein condensation in 1995, a rich new field in physics has emerged studying ultracold degenerate quantum gases. Although these ultracold gases are very dilute, their properties are nevertheless strongly influenced by interatomic interactions. Usually, these interactions are dominated by short range, isotropic contact interactions. In contrast, the recently realised Bose-Einstein Condensate (BEC) of Chromium atoms contains long-range, anisotropic dipolar interactions leading to interesting new physics. In this graduation project, stationary states of such dipolar BECs in harmonic traps are investigated for various experimentally relevant parameters. Furthermore, the elementary excitations of the BEC are calculated, as well as its response to a rotating perturbation. Finally, some more advanced topics such as vortex interactions and condensate response to impurities are investigated
Rydberg crystals, and how to make them in theory
Abstract only. Change of title
Self-consistent field theory of polarized BEC: dispersion of collective excitation
We suggest the construction of a set of the quantum hydrodynamics equations
for the Bose-Einstein condensate (BEC), where atoms have the electric dipole
moment. The contribution of the dipole-dipole interactions (DDI) to the Euler
equation is obtained. Quantum equations for the evolution of medium
polarization are derived. Developing mathematical method allows to study effect
of interactions on the evolution of polarization. The developing method can be
applied to various physical systems in which dynamics is affected by the DDI.
Derivation of Gross-Pitaevskii equation for polarized particles from the
quantum hydrodynamics is described. We showed that the Gross-Pitaevskii
equation appears at condition when all dipoles have the same direction which
does not change in time. Comparison of the equation of the electric dipole
evolution with the equation of the magnetization evolution is described.
Dispersion of the collective excitations in the dipolar BEC, either affected or
not affected by the uniform external electric field, is considered using our
method. We show that the evolution of polarization in the BEC leads to the
formation of a novel type of the collective excitations. Detailed description
of the dispersion of collective excitations is presented. We also consider the
process of wave generation in the polarized BEC by means of a monoenergetic
beam of neutral polarized particles. We compute the possibilities of the
generation of Bogoliubov and polarization modes by the dipole beam.Comment: 16 pages, 15 figures. arXiv admin note: substantial text overlap with
arXiv:1106.082
Texture and shape of two-dimensional domains of nematic liquid crystals
We present a generalized approach to compute the shape and internal structure of two-dimensional nematic domains. By using conformal mappings, we are able to compute the director field for a given domain shape that we choose from a rich class, which includes drops with large and small aspect ratios and sharp domain tips as well as smooth ones. Results are assembled in a phase diagram that for given domain size, surface tension, anchoring strength, and elastic constant shows the transitions from a homogeneous to a bipolar director field, from circular to elongated droplets, and from sharp to smooth domain tips. We find a previously unaccounted for regime, where the drop is nearly circular, the director field bipolar, and the tip rounded. We also find that bicircular director fields, with foci that lie outside the domain, provide a remarkably accurate description of the optimal director field for a large range of values of the various shape parameters