14,820 research outputs found
Permanent-magnet atom chips for the study of long, thin atom clouds
Atom-chip technology can be used to confine atoms tightly using permanently magnetised videotape along with external magnetic fields. The one-dimensional (1D) gas regime can be realised and studied by trapping the atoms in high-aspect-ratio traps in which the radial motion of the system is confined to zero-point oscillation
Vortices in fermion droplets with repulsive dipole-dipole interactions
Vortices are found in a fermion system with repulsive dipole-dipole
interactions, trapped by a rotating quasi-two-dimensional harmonic oscillator
potential. Such systems have much in common with electrons in quantum dots,
where rotation is induced via an external magnetic field. In contrast to the
Coulomb interactions between electrons, the (externally tunable) anisotropy of
the dipole-dipole interaction breaks the rotational symmetry of the
Hamiltonian. This may cause the otherwise rotationally symmetric exact
wavefunction to reveal its internal structure more directly.Comment: 5 pages, 5 figure
Asymmetric magnetic reconnection with a flow shear and applications to the magnetopause
We perform a theoretical and numerical study of anti-parallel 2D magnetic
reconnection with asymmetries in the density and reconnecting magnetic field
strength in addition to a bulk flow shear across the reconnection site in the
plane of the reconnecting fields, which commonly occurs at planetary
magnetospheres. We predict the speed at which an isolated X-line is convected
by the flow, the reconnection rate, and the critical flow speed at which
reconnection no longer takes place for arbitrary reconnecting magnetic field
strengths, densities, and upstream flow speeds, and confirm the results with
two-fluid numerical simulations. The predictions and simulation results counter
the prevailing model of reconnection at Earth's dayside magnetopause which says
reconnection occurs with a stationary X-line for sub-Alfvenic magnetosheath
flow, reconnection occurs but the X-line convects for magnetosheath flows
between the Alfven speed and double the Alfven speed, and reconnection does not
occur for magnetosheath flows greater than double the Alfven speed. We find
that X-line motion is governed by momentum conservation from the upstream
flows, which are weighted differently in asymmetric systems, so the X-line
convects for generic conditions including sub-Alfvenic upstream speeds. For the
reconnection rate, while the cutoff condition for symmetric reconnection is
that the difference in flows on the two sides of the reconnection site is twice
the Alfven speed, we find asymmetries cause the cutoff speed for asymmetric
reconnection to be higher than twice the asymmetric form of the Alfven speed.
The results compare favorably with an observation of reconnection at Earth's
polar cusps during a period of northward interplanetary magnetic field, where
reconnection occurs despite the magnetosheath flow speed being more than twice
the magnetosheath Alfven speed, the previously proposed suppression condition.Comment: 46 pages, 7 figures, abstract abridged here, accepted to Journal of
Geophysical Research - Space Physic
The effect of multiple paternity on genetic diversity during and after colonisation
In metapopulations, genetic variation of local populations is influenced by
the genetic content of the founders, and of migrants following establishment.
We analyse the effect of multiple paternity on genetic diversity using a model
in which the highly promiscuous marine snail Littorina saxatilis expands from a
mainland to colonise initially empty islands of an archipelago. Migrant females
carry a large number of eggs fertilised by 1 - 10 mates. We quantify the
genetic diversity of the population in terms of its heterozygosity: initially
during the transient colonisation process, and at long times when the
population has reached an equilibrium state with migration. During
colonisation, multiple paternity increases the heterozygosity by 10 - 300 % in
comparison with the case of single paternity. The equilibrium state, by
contrast, is less strongly affected: multiple paternity gives rise to 10 - 50 %
higher heterozygosity compared with single paternity. Further we find that far
from the mainland, new mutations spreading from the mainland cause bursts of
high genetic diversity separated by long periods of low diversity. This effect
is boosted by multiple paternity. We conclude that multiple paternity
facilitates colonisation and maintenance of small populations, whether or not
this is the main cause for the evolution of extreme promiscuity in Littorina
saxatilis.Comment: 7 pages, 5 figures, electronic supplementary materia
Cold atoms in videotape micro-traps
We describe an array of microscopic atom traps formed by a pattern of
magnetisation on a piece of videotape. We describe the way in which cold atoms
are loaded into one of these micro-traps and how the trapped atom cloud is used
to explore the properties of the trap. Evaporative cooling in the micro-trap
down to a temperature of 1 microkelvin allows us to probe the smoothness of the
trapping potential and reveals some inhomogeneity produced by the magnetic
film. We discuss future prospects for atom chips based on microscopic
permanent-magnet structures.Comment: Submitted for EPJD topical issue "Atom chips: manipulating atoms and
molecules with microfabricated structures
A three-dimensional electrostatic actuator with a locking mechanism for a new generation of atom chips
A micromachined three-dimensional electrostatic actuator that is optimized for aligning and tuning optical microcavities on atom chips is presented. The design of the 3D actuator is outlined in detail, and its characteristics are verified by analytical calculations and finite element modelling. Furthermore, the fabrication process of the actuation device is described and preliminary fabrication results are shown. The actuation in the chip plane which is used for mirror positioning has a working envelope of 17.5 ?m. The design incorporates a unique locking mechanism which allows the out-of-plane actuation that is used for cavity tuning to be carried out once the in-plane actuation is completed. A maximum translation of 7 ?m can be achieved in the out-of-plane direction
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