3,574 research outputs found
Periodic shedding of vortex dipoles from a moving penetrable obstacle in a Bose-Einstein condensate
We investigate vortex shedding from a moving penetrable obstacle in a highly
oblate Bose-Einstein condensate. The penetrable obstacle is formed by a
repulsive Gaussian laser beam that has the potential barrier height lower than
the chemical potential of the condensate. The moving obstacle periodically
generates vortex dipoles and the vortex shedding frequency linearly
increases with the obstacle velocity as , where is a
critical velocity. Based on periodic shedding behavior, we demonstrate
deterministic generation of a single vortex dipole by applying a short linear
sweep of a laser beam. This method will allow further controlled vortex
experiments such as dipole-dipole collisions.Comment: 6 pages, 7 figure
Collisional Dynamics of Half-Quantum Vortices in a Spinor Bose-Einstein Condensate
We present an experimental study on the interaction and dynamics of
half-quantum vortices (HQVs) in an antiferromagnetic spinor Bose-Einstein
condensate. By exploiting the orbit motion of a vortex dipole in a trapped
condensate, we perform a collision experiment of two HQV pairs, and observe
that the scattering motions of the HQVs is consistent with the short-range
vortex interaction that arises from nonsingular magnetized vortex cores. We
also investigate the relaxation dynamics of turbulent condensates containing
many HQVs, and demonstrate that spin wave excitations are generated by the
collisional motions of the HQVs. The short-range vortex interaction and the
HQV-magnon coupling represent two characteristics of the HQV dynamics in the
spinor superfluid.Comment: 7 pages, 6 figure
Critical Velocity for Vortex Shedding in a Bose-Einstein Condensate
We present measurements of the critical velocity for vortex shedding in a
highly oblate Bose-Einstein condensate with a moving repulsive Gaussian laser
beam. As a function of the barrier height , the critical velocity
shows a dip structure having a minimum at , where is
the chemical potential of the condensate. At fixed , we
observe that the ratio of to the speed of sound monotonically
increases for decreasing , where is the beam width and
is the condensate healing length. The measured upper bound for
is about 0.4, which is in good agreement with theoretical predictions for a
two-dimensional superflow past a circular cylinder. We explain our results with
the density reduction effect of the soft boundary of the Gaussian obstacle,
based on the local Landau criterion for superfluidity.Comment: 5 pages, 4 figure
DPRS transformer - Dynamic pressure resistant system - Part I
In general, a transformer is designed and manufactured to operate under normal conditions. However, unexpected fault events occur due to various reasons in real-life substations. When such events do occur, an electric arc inside a transformer vaporizes the insulating oil, leading to a generation of very high expansion pressure. Once this pressure exceeds the designed threshold, the tank is then compromised, and oil starts to leak, becoming a potential cause of fire or explosion.
DPRS (Dynamic Pressure Resistant System) transformer has been developed to cope with such unexpected events. In general, a PRD (Pressure Relief Device) is installed on a transformer to stabilize the pressure inside the tank.
However, it requires a certain amount of time for this device to operate. DPRS transformer is designed to withstand the immediate pressure increase without severely damaging the tank (severe enough to cause an oil leak) until the PRD starts operating. Although not as much as to cause a leak, the tank will still be deformed as a result of the pressure increase. Then, insulating oil expanded by the arc is emitted safely through a designated path as the PRD starts to operate. DPRS transformer does not require additional equipment to prevent damage to the tank and is
also capable of preventing fire while maintaining a similar configuration to common transformers. Due to these merits, the global demand for DPRS transformers is steadily increasing. In this article, the DPRS transformer tank design procedure and tank deformation prediction technology are presented. Additionally, a brief introduction to the explosion-proof performance verification test is addressed
Generalized gravity model for human migration
The gravity model (GM) analogous to Newton's law of universal gravitation has
successfully described the flow between different spatial regions, such as
human migration, traffic flows, international economic trades, etc. This simple
but powerful approach relies only on the 'mass' factor represented by the scale
of the regions and the 'geometrical' factor represented by the geographical
distance. However, when the population has a subpopulation structure
distinguished by different attributes, the estimation of the flow solely from
the coarse-grained geographical factors in the GM causes the loss of
differential geographical information for each attribute. To exploit the full
information contained in the geographical information of subpopulation
structure, we generalize the GM for population flow by explicitly harnessing
the subpopulation properties characterized by both attributes and geography. As
a concrete example, we examine the marriage patterns between the bride and the
groom clans of Korea in the past. By exploiting more refined geographical and
clan information, our generalized GM properly describes the real data, a part
of which could not be explained by the conventional GM. Therefore, we would
like to emphasize the necessity of using our generalized version of the GM,
when the information on such nongeographical subpopulation structures is
available.Comment: 14 pages, 6 figures, 2 table
Relaxation of superfluid turbulence in highly oblate Bose-Einstein condensates
We investigate thermal relaxation of superfluid turbulence in a highly oblate
Bose-Einstein condensate. We generate turbulent flow in the condensate by
sweeping the center region of the condensate with a repulsive optical
potential. The turbulent condensate shows a spatially disordered distribution
of quantized vortices and the vortex number of the condensate exhibits
nonexponential decay behavior which we attribute to the vortex pair
annihilation. The vortex-antivortex collisions in the condensate are identified
with crescent-shaped, coalesced vortex cores. We observe that the
nonexponential decay of the vortex number is quantitatively well described by a
rate equation consisting of one-body and two-body decay terms. In our
measurement, we find that the local two-body decay rate is closely proportional
to , where is the temperature and is the chemical potential.Comment: 7 pages, 9 figure
Observation of a Geometric Hall Effect in a Spinor Bose-Einstein Condensate with a Skyrmion Spin Texture
For a spin-carrying particle moving in a spatially varying magnetic field,
effective electromagnetic forces can arise due to the geometric phase
associated with adiabatic spin rotation of the particle. We report the
observation of a geometric Hall effect in a spinor Bose-Einstein condensate
with a skyrmion spin texture. Under translational oscillations of the spin
texture, the condensate resonantly develops a circular motion in a harmonic
trap, demonstrating the existence of an effective Lorentz force. When the
condensate circulates, quantized vortices are nucleated in the boundary region
of the condensate and the vortex number increases over 100 without significant
heating. We attribute the vortex nucleation to the shearing effect of the
effective Lorentz force from the inhomogeneous effective magnetic field.Comment: 9 pages, 11 figure
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