arXiv subjects

Yong-il Shin

Publications and source records attributed to Yong-il Shin.

At least 37 records · Page 2Linked to original sources

arXiv2021

Defect Saturation in a Rapidly Quenched Bose Gas

We investigate the saturation of defect density in an atomic Bose gas rapidly cooled into a superfluid phase. The number of quantum vortices, which are spontaneously created in the quenched gas, exhibits a Poissonian distribution not only for a slow quench in the Kibble-Zurek (KZ) scaling regime but also for a fast quench in which case the mean vortex number is saturated. This shows that the saturation is not caused by destructive vortex collisions, but by the early-time coarsening in an emerging condensate, which is further supported by the observation that the condensate growth lags the quenching in the saturation regime. Our results demonstrate that the defect saturation is an effect beyond the KZ mechanism, opening a path for studying critical phase transition dynamics using the defect number distribution.

cond-mat.quant-gas↗

arXiv2021

Large-area $^{87}$Rb Bose-Einstein condensate in a clipped-Gaussian optical dipole trap

We demonstrate a production of large-area $^{87}$Rb Bose-Einstein condensates (BECs) using a non-Gaussian optical dipole trap (ODT). The ODT is formed by focusing a symmetrically truncated Gaussian laser beam and it is shown that the beam clipping causes the trap geometry elongated and flattened along the beam axis direction. In the clipped-Gaussian ODT, an elongated, highly oblate BEC of $^{87}$Rb is generated with length and width of approximately $470~μ\textrm{m}$ and $130~μ\textrm{m}$, respectively, where the condensate healing length is estimated to be $ξ\approx 0.25~μ\textrm{m}$ at the trap center. The ODT is characterized to have a quartic trapping potential along the beam axis and the atom density of the condensate is uniform within 10% over $1000ξ$ in the central region. Finally, we discuss the prospect of conducting vortex shedding experiments using the elongated condensate.

cond-mat.quant-gas↗

arXiv2020

Topological Floquet engineering of a 1D optical lattice via resonantly shaking with two harmonic frequencies

We investigate the topological properties of a resonantly shaken one-dimensional optical lattice system, where the lattice position is periodically driven with two harmonic frequencies to generate one- and two-photon couplings between the two lowest orbitals. In a two-band approximation, we numerically show that degenerate edge states appear under a certain driving condition and that the corresponding topological phase is protected by the chiral symmetry of the periodically driven system. The system's micromotion is characterized with oscillating Zak phases and we find that the Zak phases are quantized only at the time when the chiral symmetry condition is explicitly satisfied. Finally, we describe the topological charge pumping effect which arises when the driving parameters are slowly modulated around a critical point, and investigate its adiabaticity for increasing the modulation frequency.

cond-mat.quant-gas↗

arXiv2020

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 post-quench dynamics shows a crossover behavior as the quench strength $\tilde{q}$ is increased, where $\tilde{q}$ is defined as the ratio of the initial excitation energy per particle to the characteristic spin interaction energy. For a weak quench of $\tilde{q}<1$, long-wavelength spin excitations are dominantly generated, leading to the formation of irregular spin domains. With increasing $\tilde{q}$, the length scale of the initial spin excitations decreases, and we demonstrate that the long-wavelength instability is strongly suppressed for high $\tilde{q}>2$. The observed crossover behavior is found to be consistent with the Bogoliubov description of the dynamic instability of the initial spinor condensate.

cond-mat.quant-gas↗

arXiv2019

Observation of Two Sound Modes in a Binary Superfluid Gas

We study the propagation of sound waves in a binary superfluid gas with two symmetric components. The binary superfluid is constituted using a Bose-Einstein condensate of $^{23}$Na in an equal mixture of two hyperfine ground states. Sound waves are excited in the condensate by applying a local spin-dependent perturbation with a focused laser beam. We identify two distinct sound modes, referred to as density sound and spin sound, where the densities of the two spin components oscillate in phase and out of phase, respectively. The observed sound propagation is explained well by the two-fluid hydrodynamics of the binary superfluid. The ratio of the two sound velocities is precisely measured with no need for absolute density calibration, and we find it in quantitatively good agreement with known interaction properties of the binary system.

cond-mat.quant-gas↗

arXiv2019

Observation of wall-vortex composite defects in a spinor Bose-Einstein condensate

We report the observation of spin domain walls bounded by half-quantum vortices (HQVs) in a spin-1 Bose-Einstein condensate with antiferromagnetic interactions. A spinor condensate is initially prepared in the easy-plane polar phase, and then, suddenly quenched into the easy-axis polar phase. Domain walls are created via the spontaneous $\mathbb{Z}_2$ symmetry breaking in the phase transition and the walls dynamically split into composite defects due to snake instability. The end points of the defects are identified as HQVs for the polar order parameter and the mass supercurrent in their proximity is demonstrated using Bragg scattering. In a strong quench regime, we observe that singly charged quantum vortices are formed with the relaxation of free wall-vortex composite defects. Our results demonstrate a nucleation mechanism for composite defects via phase transition dynamics.

cond-mat.quant-gas↗

arXiv2019

Topological Creutz Ladder in a Resonantly Shaken 1D Optical Lattice

We report the experimental realization of a topological Creutz ladder for ultracold fermionic atoms in a resonantly driven 1D optical lattice. The two-leg ladder consists of the two lowest orbital states of the optical lattice and the cross inter-leg links are generated via two-photon resonant coupling between the orbitals by periodic lattice shaking. The characteristic pseudo-spin winding in the topologically non-trivial bands of the ladder system is demonstrated using momentum-resolved Ramsey-type interferometric measurements. We discuss a two-tone driving method to extend the inter-leg link control and propose a topological charge pumping scheme for the Creutz ladder system.

cond-mat.quant-gas↗

arXiv2018

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 $q<0$. 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 $B'$, the excited spin state becomes metastable down to a certain threshold $q_{th}$ and as $q$ decreases below $q_{th}$, 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 $q_{th}$ on $B'$ and the sample size, and we find that $q_{th}$ is highly sensitive to the field gradient in the vicinity of $B'=0$, exhibiting power-law behavior of $|q_{th}|\propto B'^α$ with $α\sim 0.5$. Our results demonstrate the significance of the field gradient effect in the quantum critical dynamics of spinor condensates.

cond-mat.quant-gas↗

arXiv2018

Band Gap Closing in a Synthetic Hall Tube of Neutral Fermions

We report the experimental realization of a synthetic three-leg Hall tube with ultracold fermionic atoms in a one-dimensional optical lattice. The legs of the synthetic tube are composed of three hyperfine spin states of the atoms, and the cyclic inter-leg links are generated by two-photon Raman transitions between the spin states, resulting in a uniform gauge flux $ϕ$ penetrating each side plaquette of the tube. Using quench dynamics, we investigate the band structure of the Hall tube system for a commensurate flux $ϕ=2π/3$. Momentum-resolved analysis of the quench dynamics reveals that a critical point of band gap closing as one of the inter-leg coupling strengths is varied, which is consistent with a topological phase transition predicted for the Hall tube system.

cond-mat.quant-gas↗

arXiv2018

Realization of a cross-linked chiral ladder with neutral fermions in an optical lattice by orbital-momentum coupling

We report the experimental realization of a cross-linked chiral ladder with ultracold fermionic atoms in an optical lattice. In the ladder, the legs are formed by the orbital states of the optical lattice and the complex inter-leg links are generated by the orbital-changing Raman transitions that are driven by a moving lattice potential superimposed onto the optical lattice. The effective magnetic flux per ladder plaquette is tuned by the spatial periodicity of the moving lattice, and the chiral currents are observed from the asymmetric momentum distributions of the orbitals. The effect of the complex cross links is demonstrated in quench dynamics by measuring the momentum dependence of the inter-orbital coupling strength. We discuss the topological phase transition of the chiral ladder system for the variations of the complex cross links.

cond-mat.quant-gas↗

arXiv2018

Critical Vortex Shedding in a Strongly Interacting Fermionic Superfluid

We study the critical vortex shedding in a strongly interacting fermionic superfluid of $^{6}$Li across the BEC-BCS crossover. By moving an optical obstacle in the sample and directly imaging the vortices after time of flight, the critical velocity $v_{\rm c}$ for vortex shedding is measured as a function of the obstacle travel distance $L$. The observed $v_{\rm c}$ increases with decreasing $L$, where the rate of increase is the highest in the unitary regime. In the deep BEC regime, an empirical dissipation model well captures the dependence of $v_{\rm c}$ on $L$, characterized by a constant value of $η= -\frac{{\rm d}(1/v_{\rm c})}{{\rm d}(1/L)}$. However, as the system is tuned across the resonance, a step increase of $η$ develops about a characteristic distance $L_{\rm c}$ as $L$ is increased, where $L_{\rm c}$ is comparable to the obstacle size. This bimodal behavior is strengthened as the system is tuned towards the BCS regime. We attribute this evolution of $v_{\rm c}$ to the participation of pair breaking in the vortex shedding dynamics of a fermionic superfluid.

cond-mat.quant-gas↗

arXiv2017

Photoassociation spectroscopy of ultracold $^{173}$Yb atoms near the intercombination line

We report on photoassociation (PA) spectroscopy of a degenerate Fermi gas of $^{173}$Yb atoms near the dissociation limit of the spin-forbidden ${^1}S_0-{^3}P_1$ intercombination transition. An atom-loss spectrum is measured from a trapped sample for a spectral range down to $-1$~GHz with respect to the $f=5/2 \rightarrow f^\prime=7/2$ atomic resonance. The spectrum shows eighty PA resonances, revealing the high nuclear spin nature of the system. We investigate the Zeeman effect on the spectrum near a detuning of $-0.8$~GHz, where we examine the quantum numbers of the Zeeman levels using various two-component spin mixture samples. Finally, we measure the atom loss rate under PA light for several pronounced PA resonances.

physics.atom-ph↗

arXiv2017

Critical spin superflow in a spinor Bose-Einstein condensate

We investigate the critical dynamics of spin superflow in an easy-plane antiferromagnetic spinor Bose-Einstein condensate. Spin-dipole oscillations are induced in a trapped condensate by applying a linear magnetic field gradient and we observe that the damping rate increases rapidly as the field gradient increases above a certain critical value. The onset of dissipation is found to be associated with the generation of dark-bright solitons due to the modulation instability of the counterflow of two spin components. Spin turbulence emerges as the solitons decay because of their snake instability. We identify another critical point for spin superflow, in which transverse magnon excitations are dynamically generated via spin-exchanging collisions, which leads to the transient formation of axial polar spin domains.

cond-mat.quant-gas↗

arXiv2017

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 $\sim 0.5 T_c$, where $T_c$ 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.

cond-mat.quant-gas↗

arXiv2017

Emergence and scaling of spin turbulence in quenched antiferromagnetic spinor Bose-Einstein condensates

We investigate the phase transition dynamics of a quasi-2D antiferromagnetic spin-1 Bose-Einstein condensate from the easy-axis polar phase to the easy-plane polar phase, which is initiated by suddenly changing the sign of the quadratic Zeeman energy $q$. We observe the emergence and decay of spin turbulence and the formation of half-quantum vortices (HQVs) in the quenched condensate. The characteristic time and length scales of the turbulence generation dynamics are proportional to $|q|^{-1/2}$ as inherited from the dynamic instability of the initial state. In the evolution of the spin turbulence, spin wave excitations develop from large to small length scales, suggesting a direct energy cascade, and the spin population for the axial polar domains exhibit a nonexponential decay. The final equilibrated condensate contains HQVs, and the number is found to increase and saturate with increasing $|q|$. Our results demonstrate the time-space scaling properties of the phase transition dynamics near the critical point and the peculiarities of the spin turbulence state of the antiferromagnetic spinor condensate.

cond-mat.quant-gas↗

arXiv2017

Double resonance of Raman transitions in a degenerate Fermi gas

We measure momentum-resolved Raman spectra of a spin-polarized degenerate Fermi gas of $^{173}$Yb atoms for a wide range of magnetic fields, where the atoms are irradiated by a pair of counterpropagating Raman laser beams as in the conventional spin-orbit coupling scheme. Double resonance of first- and second-order Raman transitions occurs at a certain magnetic field and the spectrum exhibits a doublet splitting for high laser intensities. The measured spectral splitting is quantitatively accounted for by the Autler-Townes effect. We show that our measurement results are consistent with the spinful band structure of a Fermi gas in the spatially oscillating effective magnetic field generated by the Raman laser fields.

cond-mat.quant-gas↗

arXiv2016

Observation of von Kármán Vortex Street in an Atomic Superfluid Gas

We report on the experimental observation of vortex cluster shedding from a moving obstacle in an oblate atomic Bose-Einstein condensate. At low obstacle velocities $v$ above a critical value, vortex clusters consisting of two like-sign 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 regular-to-turbulent 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.

cond-mat.quant-gas↗

arXiv2016

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 $\bf 90$, 063627 (2014)] and, separately, the thermal friction coefficient $α$ for vortex motion was measured from the long-time evolution of a corotating vortex pair in a BEC [Phys. Rev. A $\bf 92$, 051601(R) (2015)]. We present a comparative analysis of the experimental results, and find that the vortex decay rate $Γ$ is almost linearly proportional to $α$. 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 $Γ$ on $α$ 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.

cond-mat.quant-gas↗