264 research outputs found
Vortex-lattice pinning in two-component Bose-Einstein condensates
We investigate the vortex-lattice structure for single- and two-component
Bose-Einstein condensates in the presence of an optical lattice, which acts as
a pinning potential for the vortices. The problem is considered in the
mean-field quantum-Hall regime, which is reached when the rotation frequency
of the condensate in a radially symmetric trap approaches the (radial)
trapping frequency and the interactions between the atoms are weak. We
determine the vortex-lattice phase diagram as a function of optical-lattice
strength and geometry. In the limit of strong pinning the vortices are always
pinned at the maxima of the optical-lattice potential, similar to the
slow-rotation case. At intermediate pinning strength, however, due to the
competition between interactions and pinning energy, a structure arises for the
two-component case where the vortices are pinned on lines of minimal potential
Thermotunnel refrigerator with vacuum/insulator tunnel barrier: A theoretical analysis
The authors use two insulator layers in thermotunnel refrigerator to modify
the shape of the tunneling barrier so that electrons with high kinetic energy
pass it with increased probability. Theoretical analysis show that the overall
tunneling current between the electrodes contains an increased number of high
kinetic energy electrons and a reduced number of low energy ones, leading to
high efficiency. The particular case of vacuum gap and solid insulator layer is
calculated using digital methods. Efficiency remains high in the wide range of
the emitter electric field. The cooling coefficient is found to be as high as
40%-50% in the wide range of the emitter electric field.Comment: 9 pages, 3 figure
Pinning and collective modes of a vortex lattice in a Bose-Einstein condensate
We consider the ground state of vortices in a rotating Bose-Einstein
condensate that is loaded in a corotating two-dimensional optical lattice. Due
to the competition between vortex interactions and their potential energy, the
vortices arrange themselves in various patterns, depending on the strength of
the optical potential and the vortex density. We outline a method to determine
the phase diagram for arbitrary vortex filling factor. Using this method, we
discuss several filling factors explicitly. For increasing strength of the
optical lattice, the system exhibits a transition from the unpinned hexagonal
lattice to a lattice structure where all the vortices are pinned by the optical
lattice. The geometry of this fully pinned vortex lattice depends on the
filling factor and is either square or triangular. For some filling factors
there is an intermediate half-pinned phase where only half of the vortices is
pinned. We also consider the case of a two-component Bose-Einstein condensate,
where the possible coexistence of the above-mentioned phases further enriches
the phase diagram. In addition, we calculate the dispersion of the low-lying
collective modes of the vortex lattice and find that, depending on the
structure of the ground state, they can be gapped or gapless. Moreover, in the
half-pinned and fully pinned phases, the collective mode dispersion is
anisotropic. Possible experiments to probe the collective mode spectrum, and in
particular the gap, are suggested.Comment: 29 pages, 4 figures, changes in section
Inductively coupled plasmas sustained by an internal oscillating current
A global electromagnetic model of an inductively coupled plasma sustained by an internal
oscillating current sheet in a cylindrical metal vessel is developed. The electromagnetic field
structure, profiles of the rf power transferred to the plasma electrons, electron/ion number density,
and working points of the discharge are studied, by invoking particle and power balance. It is
revealed that the internal rf current with spatially invariable phase significantly improves the radial
uniformity of the electromagnetic fields and the power density in the chamber as compared with
conventional plasma sources with external flat spiral inductive coils. This configuration offers the
possibility of controlling the rf power deposition in the azimuthal direction
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