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
Observation
of von Kármán Vortex Street in an Atomic Superfluid Gas
Physical
Review Letters 117, 245301 (2016)


We report on the
experimental observation of vortex cluster shedding from a moving obstacle
in an oblate atomic BoseEinstein condensate. At low obstacle velocities v
above a critical value, vortex clusters consisting of two likesign
vortices are generated to form a regular configuration like a von Kármán
street, and as v is increased, the shedding pattern becomes irregular with many
different kinds of vortex clusters. In particular, we observe that the Stouhal number associated with the shedding frequency
exhibits saturation behavior with increasing v. The regulartoturbulent
transition of the vortex cluster shedding reveals remarkable similarities
between a superfluid and a classical viscous fluid. Our work opens a new
direction for experimental investigations of the superfluid Reynolds number
characterizing universal superfluid hydrodynamics.
Related
articles in Viewpoint in
Physics and Physics
Today

Collisional
Dynamics of HalfQuantum Vortices in a Spinor BoseEinstein Condensate
Physical
Review Letters 116, 185301 (2016)


We present an experimental
study on the interaction and dynamics of halfquantum vortices (HQVs) in an
antiferromagnetic spinor BoseEinstein condensate. By exploiting the orbit
motion of a vortex dipole in a trapped condensate, we perform a collision
experiment of two HQV pairs, and observe that the scattering motions of the
HQVs is consistent with the shortrange vortex interaction that arises from
nonsingular magnetized vortex cores. We also investigate the relaxation
dynamics of turbulent condensates containing many HQVs, and demonstrate
that spin wave excitations are generated by the collisional motions of the
HQVs. The shortrange vortex interaction and the HQVmagnon
coupling represent two characteristics of the HQV dynamics in the spinor
superfluid.

HalfQuantum
Vortices in an Antiferromagnetic Spinor BoseEinstein Condensate
Physical
Review Letters 115, 015301 (2015)


We report on the
observation of halfquantum vortices (HQVs) in the easyplane polar phase
of an antiferromagnetic spinor BoseEinstein condensate. Using in situ agnetizationsensitive 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
shortrange 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.

BoseEinstein
condensation of 174 Ytterbium atoms has been observed on Nov 21st, 2014!!


Relaxation
of superfluid turbulence in highly oblate BoseEinstein condensates
Physical
Review A 90, 063627 (2014)

We investigate thermal
relaxation of superfluid turbulence in a highly oblate BoseEinstein
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 vortexantivortex
collisions in the condensate are identified with crescentshaped, coalesced
vortex cores. We observe that the nonexponential
decay of the vortex number is quantitatively well described by a rate
equation consisting of onebody and twobody decay terms. In our
measurement, we find that the local twobody decay rate is closely
proportional to T^2/\mu,
where T is the temperature and \mu is the chemical
potential.

Observation of a
Geometric Hall Effect in a Spinor BEC with a Skyrmion
Spin Texture
Physical Review
Letters 111, 245301 (2013)


For a spincarrying 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 BoseEinstein
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.

Observation of
Thermally Activated Vortex Pairs in a Quasi2D Bose Gas
Physical Review
Letters 110, 175302 (2013)


We measure the inplane distribution of thermally activated
vortices in a trapped quasi2D Bose gas, where we enhance the visibility of
densitydepleted vortex cores by radially compressing the sample before
releasing the trap. The pairing of vortices is revealed by the twovortex
spatial correlation function obtained from the vortex distribution. The
vortex density decreases gradually as temperature is lowered, and below a
certain temperature, a vortexfree region emerges in the center of the
sample. This shows the crossover from a BerezinskiiKosterlitzThouless
phase containing vortexpair excitations to a vortexfree BoseEinstein
condensate in a finitesize 2D system.

Probing Phase
Fluctuations in a 2D Degenerate Bose Gas by Free Expansion
Physical Review
Letters 109, 125301 (2012)


We measure the power spectrum of the density distribution of
a freely expanding twodimensional 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 ringshaped density ripples in a perturbed
sample after a long relaxation time.

Observation of
Topologically Stable 2D Skyrmions in an
Antiferromagnetic BEC
Physical Review
Letters 108, 035301 (2012)


We
present the creation and time evolution of twodimensional Skyrmion excitations in an antiferromagnetic spinor
BoseEinstein condensate. Using a spin rotation method, the Skyrmion spin textures were imprinted on a sodium condensate
in a polar phase, where the twodimensional 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
finitesized sample. We discuss the possible formation of halfquantum
vortices in the deformation process.

BoseEinstein
condensation of sodium atoms has been observed on June 16th, 2010!!

First
quantum gas in Korea !!

QGL@SNU, Seoul 151747, Korea. Tel +82(2)8804233
