1,193 research outputs found
Role of thermal friction in relaxation of turbulent Bose-Einstein condensates
In recent experiments, the relaxation dynamics of highly oblate, turbulent
Bose-Einstein condensates (BECs) was investigated by measuring the vortex decay
rates in various sample conditions [Phys. Rev. A , 063627 (2014)] and,
separately, the thermal friction coefficient for vortex motion was
measured from the long-time evolution of a corotating vortex pair in a BEC
[Phys. Rev. A , 051601(R) (2015)]. We present a comparative analysis of
the experimental results, and find that the vortex decay rate is
almost linearly proportional to . We perform numerical simulations of
the time evolution of a turbulent BEC using a point-vortex model equipped with
longitudinal friction and vortex-antivortex pair annihilation, and observe that
the linear dependence of on is quantitatively accounted for
in the dissipative point-vortex model. The numerical simulations reveal that
thermal friction in the experiment was too strong to allow for the emergence of
a vortex-clustered state out of decaying turbulence.Comment: 7 pages, 5 figure
Metastable hard-axis polar state of a spinor Bose-Einstein condensate under a magnetic field gradient
We investigate the stability of a hard-axis polar state in a spin-1
antiferromagnetic Bose-Einstein condensate under a magnetic field gradient,
where the easy-plane spin anisotropy is controlled by a negative quadratic
Zeeman energy . In a uniform magnetic field, the axial polar state is
dynamically unstable and relaxes into the planar polar ground state. However,
under a field gradient , the excited spin state becomes metastable down to
a certain threshold and as decreases below , its intrinsic
dynamical instability is rapidly recalled. The incipient spin excitations in
the relaxation dynamics appear with stripe structures, indicating the
rotational symmetry breaking by the field gradient. We measure the dependences
of on and the sample size, and we find that is highly
sensitive to the field gradient in the vicinity of , exhibiting power-law
behavior of with . Our results
demonstrate the significance of the field gradient effect in the quantum
critical dynamics of spinor condensates.Comment: 8 pages, 7 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
Crossover from weak to strong quench in a spinor Bose-Einstein condensate
We investigate the early-time dynamics of a quasi-two-dimensional spin-1 antiferromagnetic Bose-Einstein condensate after a sudden quench from the easy-plane to the easy-axis polar phase. The postquench dynamics shows a crossover behavior as the quench strength (q) over tilde is increased, where (q) over tilde is defined as the ratio of the initial excitation energy per particle to the characteristic spin interaction energy. For a weak quench of (q) over tilde , the length scale of the initial spin excitations decreases, and we demonstrate that the long-wavelength instability is strongly suppressed for high (q) over tilde > 2. The observed crossover behavior is found to be consistent with the Bogoliubov description of the dynamic instability of the initial spinor condensate. ©2020 American Physical Society11sciescopu
AD-YOLO: You Look ONly Once in Training Multiple Sound Event Localization and Detection
Sound event localization and detection (SELD) combines the identification of
sound events with the corresponding directions of arrival (DOA). Recently,
event-oriented track output formats have been adopted to solve this problem;
however, they still have limited generalization toward real-world problems in
an unknown polyphony environment. To address the issue, we proposed an
angular-distance-based multiple SELD (AD-YOLO), which is an adaptation of the
"You Look Only Once" algorithm for SELD. The AD-YOLO format allows the model to
learn sound occurrences location-sensitively by assigning class responsibility
to DOA predictions. Hence, the format enables the model to handle the polyphony
problem, regardless of the number of sound overlaps. We evaluated AD-YOLO on
DCASE 2020-2022 challenge Task 3 datasets using four SELD objective metrics.
The experimental results show that AD-YOLO achieved outstanding performance
overall and also accomplished robustness in class-homogeneous polyphony
environments.Comment: 5 pages, 3 figures, accepted for publication in IEEE ICASSP 202
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