4,568 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
PERSPECTIVES OF WATER QUALITY IMPROVEMENT BY AQUATIC MACROPHYTES AS NUTRIENT SINK IN THE LAKE PALDANGHO, KOREA
Improvement of water quality by aquatic macrophytes was studied at the Lake Paldangho, a reservoir supplying the service water for 15 million inhabitants in the Capital of Korea. Annual production of aboveground of emergent plants having 1800 g DW/m² for Typha angustata and 1700g DW/m² for Zizania latifolia, are 15- to 30-folds as large as those of submersed plants having 110 g DW/m² for Ceratophyllum demersum and 60 g DW/m² for Hydrilla verticillata. Although concentrations of nitrogen and phosphorus in mg per g dry matter of emergent plants are small as a half of those of submersed ones, standing nutrients in g per m² of the former are 8- to 9-folds as large as that of the latter because of large production. Annual return of nutrients from the litters are also more in emergent plants than in submersed one. In the whole Lake the uptake amounts by macrophyte (sink) are 30.3 ton of nitrogen and 4.9 ton of phosphorus, and the return amounts from the litter (source) are 18.5 ton of nitrogen and 4.3 ton of phosphorus. Difference between the sink and the source, 11.8 ton for nitrogen and 0.6 ton for phosphorus, will remain in sediment of littoral zone. If aboveground of macrohytes are weeded out on time occurring the maximum production nutrients remaining will be reduced. To increase the annual production, repetitive harvesting of Zizania latifolia is effective.Article信州大学理学部付属諏訪臨湖実験所報告 7: 167-175(1991)departmental bulletin pape
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