Engineering researchers develop new explanation for formation of
vortices in 2D superfluid

Date:
September 21, 2021
Source:
Florida State University
Summary:
Researchers have new insight about the formation of vortices in
a type of quantum fluid, work that could help our comprehension
of the physics mystery of how vortex clusters form and provide
valuable understanding into the atmospheric swirling motion on
planets such as Earth and Jupiter.



FULL STORY ========================================================================== Researchers at the FAMU-FSU College of Engineering and the National High Magnetic Field Laboratory have new insight about the formation of vortices
in a type of quantum fluid, work that could help our comprehension of
the physics mystery of how vortex clusters form and provide valuable understanding into the atmospheric swirling motion on planets such as
Earth and Jupiter.


==========================================================================
The researchers revealed an alternative explanation for the formation
of so- called Onsager vortices in a two-dimensional superfluid, a type
of quantum fluid that can flow without friction. Their findings were
published in Physical Review Letters.

"This finding gives us a new explanation for how these vortices might form
and is an important step toward continuing to improve our understanding
of quantum physics," said Wei Guo, associate professor in mechanical engineering and lead investigator of the study.

In 1949, Nobel Laureate Lars Onsager proposed a simple theory relating
to our understanding of the swirling motion in turbulent 2D fluids,
which are fluids constrained to move in a two-dimensional space.

Onsager's theory says when energy is added constantly into a 2D turbulent
fluid with chaotic small swirls (formally called vortices), those
swirls rotating in the same direction would cluster to form large-scale persistent swirls when the energy is sufficiently high. These clusters
or large-scale swirls are known as "Onsager vortices." Jupiter's Great
Red Spot is a good example.

"Onsager's theory requires energy to be added to the 2D fluid," Guo said.

"However, recent publications revealed that in 2D Bose-Einstein
Condensates (BECs), Onsager vortices can appear spontaneously without
energy input. We set out to do more investigations into these surprising results." BEC is a state of matter where identical atoms or molecules
are confined by a laser or magnetic trap and cooled to near absolute
zero. The particles in the BEC state occupy the same quantum mechanical
energy level and can exhibit superfluid behaviors, such as flowing without
the apparent loss of kinetic energy and rotating around tiny hollow tubes called vortex tubes. With an appropriate trap, a BEC can be confined to
have a quasi-2D disk shape, that is, a shape with a small thickness but a
large radius. The vortex tubes in a 2D BEC appear like tiny point swirls,
which can serve as an ideal testbed for Onsager's theory.



==========================================================================
"A well-accepted explanation of the spontaneous appearance of Onsager
vortices in 2D disk BECs is the evaporative heating mechanism," Guo
said. "This mechanism refers to what physicists call vortex annihilation,
which is when a pair of vortices with opposite rotations merge and
disappear, just like a positive charge neutralizes with a negative
charge." "As the vortices annihilate, the energy per vortex increases,
which leads to the formation of Onsager vortices," said Toshiaki Kanai,
a graduate research student working with Guo at the National High Magnetic Field Laboratory.

However, in a numerical simulation performed by Kanai and Guo while
studying the turbulent swirling motion in 2D, with BECs confined on
the surface of a sphere, they found that Onsager vortices never formed
despite the annihilations of the vortex pairs. The researchers then
conducted a comparative study of the vortex dynamics in both disk BECs
and spherical shell BECs.

"We finally figured out that the spontaneous formation of Onsager vortices
is not due to the well-accepted evaporative heating mechanism," Guo
said. "The true mechanism is the exiting of the vortices from the 2D BEC boundary." The disk BEC has a circumference where the vortices can exit,
while the spherical shell BEC does not have any boundaries. Therefore,
Onsager vortices are observed only in the disk BECs but not in the
spherical BEC shells, according to the researchers.

"Identifying the true mechanism for the spontaneous formation of Onsager vortices in 2D BECs represents a major progress in our understanding
of 2D superfluid turbulence," Guo said. "Our findings about the point
vortices on a sphere may represent a conceptually interesting model of
a planetary atmosphere, and these findings can be tested experimentally
at NASA's space laboratory." NASA has recently installed a cold atom laboratory at the International Space Station, which has made it feasible
to produce BECs in spherical shell geometry. Future experimental studies
in zero gravity could help advance the science in this field.

This research was funded by a three-year National Science Foundation grant (Grant # DMR-2100790).

========================================================================== Story Source: Materials provided by Florida_State_University. Original
written by Trisha Radulovich. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Toshiaki Kanai, Wei Guo. True Mechanism of Spontaneous Order from
Turbulence in Two-Dimensional Superfluid Manifolds. Physical Review
Letters, 2021; 127 (9) DOI: 10.1103/PhysRevLett.127.095301 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210921154207.htm

--- up 2 weeks, 5 days, 8 hours, 25 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)