3,545 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
Observation of wall-vortex composite defects in a spinor Bose-Einstein condensate
We report the observation of spin domain walls bounded by half-quantum
vortices (HQVs) in a spin-1 Bose-Einstein condensate with antiferromagnetic
interactions. A spinor condensate is initially prepared in the easy-plane polar
phase, and then, suddenly quenched into the easy-axis polar phase. Domain walls
are created via the spontaneous symmetry breaking in the phase
transition and the walls dynamically split into composite defects due to snake
instability. The end points of the defects are identified as HQVs for the polar
order parameter and the mass supercurrent in their proximity is demonstrated
using Bragg scattering. In a strong quench regime, we observe that singly
charged quantum vortices are formed with the relaxation of free wall-vortex
composite defects. Our results demonstrate a nucleation mechanism for composite
defects via phase transition dynamics.Comment: 10 pages, 11 figures, reference update
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
Observation of vortex-antivortex pairing in decaying 2D turbulence of a superfluid gas
In a two-dimensional (2D) classical fluid, a large-scale flow structure
emerges out of turbulence, which is known as the inverse energy cascade where
energy flows from small to large length scales. An interesting question is
whether this phenomenon can occur in a superfluid, which is inviscid and
irrotational by nature. Atomic Bose-Einstein condensates (BECs) of highly
oblate geometry provide an experimental venue for studying 2D superfluid
turbulence, but their full investigation has been hindered due to a lack of the
circulation sign information of individual quantum vortices in a turbulent
sample. Here, we demonstrate a vortex sign detection method by using Bragg
scattering, and we investigate decaying turbulence in a highly oblate BEC at
low temperatures, with our lowest being , where is the
superfluid critical temperature. We observe that weak spatial pairing between
vortices and antivortices develops in the turbulent BEC, which corresponds to
the vortex-dipole gas regime predicted for high dissipation. Our results
provide a direct quantitative marker for the survey of various 2D turbulence
regimes in the BEC system.Comment: 8 pages, 8 figure
2-D Non-Fickian Dispersion Model for the Initial Period of River Mixing
Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchive
Behavior of Longitudinal and Transverse Dispersion Coefficient in Three-Dimensional Open Channels
Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv
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