Research Interests
Superfluidity in two dimensions
Topological excitations and dynamics in spinor condensates
Fermionic superfluidity with strong
interactions
Atom interferometry/ atom chip experiments
Research Highlights
Half-Quantum
Vortices in an Antiferromagnetic Spinor Bose-Einstein Condensate
Physical
Review Letters 115, 015301 (2015)
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We report on the
observation of half-quantum vortices (HQVs) in the easy-plane polar phase
of an antiferromagnetic spinor Bose-Einstein condensate. Using in situ agnetization-sensitive imaging, we observe that pairs
of HQVs with opposite core magnetization are generated when singly charged
quantum vortices are injected into the condensate. The dynamics of HQV pair
formation is characterized by measuring the temporal evolutions of the pair
separation distance and the core magnetization, which reveals the
short-range nature of the repulsive interactions between the HQVs. We find
that spin fluctuations arising from thermal population of transverse magnon excitations do not significantly affect the HQV
pair formation dynamics. Our results demonstrate the instability of a
singly charged vortex in the antiferromagnetic spinor condensate.
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Bose-Einstein
condensation of 174 Ytterbium atoms has been observed on Nov 21st, 2014!!
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Relaxation
of superfluid turbulence in highly oblate Bose-Einstein condensates
Physical
Review A 90, 063627 (2014)
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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 T^2/\mu,
where T is the temperature and \mu is the chemical potential.
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Observation of a
Geometric Hall Effect in a Spinor BEC with a Skyrmion
Spin Texture
Physical Review
Letters 111, 245301 (2013)
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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.
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Observation of
Thermally Activated Vortex Pairs in a Quasi-2D Bose Gas
Physical Review
Letters 110, 175302 (2013)
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We measure the in-plane distribution of thermally activated
vortices in a trapped quasi-2D Bose gas, where we enhance the visibility of
density-depleted vortex cores by radially compressing the sample before
releasing the trap. The pairing of vortices is revealed by the two-vortex
spatial correlation function obtained from the vortex distribution. The
vortex density decreases gradually as temperature is lowered, and below a
certain temperature, a vortex-free region emerges in the center of the
sample. This shows the crossover from a Berezinskii-Kosterlitz-Thouless
phase containing vortex-pair excitations to a vortex-free Bose-Einstein
condensate in a finite-size 2D system.
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Probing Phase
Fluctuations in a 2D Degenerate Bose Gas by Free Expansion
Physical Review
Letters 109, 125301 (2012)
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We measure the power spectrum of the density distribution of
a freely expanding two-dimensional degenerate Bose gas, where irregular
density modulations gradually develop due to initial phase fluctuations in
the sample. The spectrum has an oscillatory shape, where the peak positions
are found to be independent of temperature and show scaling behavior in the
course of expansion. The relative intensity of phase fluctuations is
estimated from the normalized spectral peak strength and observed to
decrease at lower temperatures, confirming the thermal nature of the phase fluctuations.We investigate the relaxation dynamics of nonequilibrium states using the power spectrum. Free
vortices are observed with ring-shaped density ripples in a perturbed
sample after a long relaxation time.
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Observation of
Topologically Stable 2D Skyrmions in an
Antiferromagnetic BEC
Physical Review
Letters 108, 035301 (2012)
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We
present the creation and time evolution of two-dimensional Skyrmion excitations in an antiferromagnetic spinor
Bose-Einstein condensate. Using a spin rotation method, the Skyrmion spin textures were imprinted on a sodium
condensate in a polar phase, where the two-dimensional Skyrmion
is topologically protected. The Skyrmion was
observed to be stable on a short time scale of a few tens of ms but to dynamically deform its shape and eventually
decay to a uniform spin texture. The deformed spin textures reveal that the
decay dynamics involves breaking the polar phase inside the condensate
without having topological charge density flow through the boundary of the
finite-sized sample. We discuss the possible formation of half-quantum
vortices in the deformation process.
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Bose-Einstein
condensation of sodium atoms has been observed on June 16th, 2010!!
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First
quantum gas in Korea !!
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QGL@SNU, Seoul 151-747, Korea. Tel +82(2)880-4233
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