arXiv subjects

Kenichi Kasamatsu

Publications and source records attributed to Kenichi Kasamatsu.

At least 19 recordsLinked to original sources

arXiv2024

Critical velocity for wake vortex generation behind a plate in a superflow

We study theoretically the critical velocity $U_c$ for quantum vortex generation by a thin plate-shaped obstacle moving through a uniform Bose-Einstein condensate. Our results based on the Gross-Pitaevskii theory reveal that the critical velocity monotonically decreases with increasing plate size $L$. In the limit of large $L$, the critical velocity is asymptotic to $L^{-1/2}$ predicted by the potential flow theory for an incompressible ideal fluid with a phenomenological length correction. As $L$ decreases, however, the incompressible analysis breaks down quantitatively. By performing a perturbative analysis to incorporate compressibility into the potential flow theory, we have successfully reproduced the numerical results analytically over a wide parameter range. It is also shown that the critical velocity increases with finite plate thickness.

cond-mat.quant-gas↗

arXiv2023

Impact of density inhomogeneity on the critical velocity for vortex shedding in a harmonically trapped Bose-Einstein condensate

We report on a numerical study of the critical velocity for creation of quantized vortices by a moving Gaussian obstacle in a trapped Bose-Einstein condensate, modeled by the Gross-Pitaevskii equation. We pay attention to impact of density inhomogeneity associated with the global inverted-parabolic profile by a trapping potential as well as the local density suppression around the Gaussian obstacle. When the width of the Gaussian potential is large, the wake dynamics is significantly influenced by the nonuniformity around the obstacle potential. The critical velocity, estimated through the time interval between the first and second vortex emission, can be explained by the local sound velocity by taking into account the above two contributions. We also find that the ratio of the critical velocity to the sound velocity at the center of the system is insensitive to the nonlinear coefficient of the Gross-Pitaevskii equation, which supports the universal discussion even in a inhomogeneous trapped condensate under the local density approximation.

cond-mat.quant-gas↗

arXiv2022

Kelvin wave in miscible two-component Bose-Einstein condensates

We study the dispersion of Kelvin waves propagating along single- and half-quantum vortices in miscible two-component Bose-Einstein condensates based on the analysis of the Bogoloubov-de Genne equation. With the help of the interpolating formula connecting the dispersion relations at low- and high-wavenumber regime, we reveal the nontrivial dependence of the dispersion relation of the Kelvin waves on the intercomponent interaction through the change of the vortex core size of the vortical component. We also find the splitting of the Kelvin mode dispersion into gapless and gapfull branches when both components have overlapping single-quantized vortices.

cond-mat.quant-gas↗

arXiv2021

Dynamics of two quantized vortices belonging to different components of binary Bose-Einstein condensates in a circular box potential

This study aims to research on dynamics of two quantized vortices in miscible two-component Bose--Einstein condensates, trapped by a circular box potential by using the Gross--Pitaevskii equation. We consider a situation in which two vortices belong to different components and they are initially close. Their dynamics are significantly different from those of a vortex pair in a single-component condensate. When two vortices are initially co-located, they are split by dynamical instability or remain co-located depending on their intercomponent interaction. If the two split vortices have the same sign of circulation, they rotate around each other. Then, the angular velocity can be given as a function of the distance between two vortices, which is understood through the equations of motion and their interaction. If the circulations of two close vortices have different signs, however, they move in the same direction initially, then overlap during the dynamics. This subsequent overlap can be categorized into two types based on the initial distance between them. The mechanism of this overlapping can be understood from the interaction between the vortex and its image vortex, which makes the two vortices close to each other even if the intrinsic interaction between them is repulsive.

cond-mat.quant-gas↗

arXiv2021

Quantum droplet of a two-component Bose gas in an optical lattice near the Mott insulator transition

We theoretically study dynamical formation of a quantum droplet in a two-component Bose-Hubbard system with an external trap potential. Specifically, the superfluid in the central region surrounded by the Mott insulator with double filling forms a quantum droplet, which is self-bound thanks to the discontinuous nature of the quantum phase transition between the two phases. We show how to induce the characteristic behavior of the droplet through the control of the trap potential by using the time-dependent Gutzwiller simulations in a two-dimensional system. The static and dynamical properties of the droplet can be described qualitatively by the effective Ginzburg-Landau field theory with cubic-quintic nonlinearities, where the attractive cubic nonlinearlity emerges although all the bare interparticle interactions are repulsive.

cond-mat.quant-gas↗

arXiv2021

Pattern formation of quantum Kelvin-Helmholtz instability in binary superfluids

We study theoretically nonlinear dynamics induced by shear-flow instability in segregated two-component Bose-Einstein condensates in terms of the Weber number, defined by extending the past theory on the Kelvin-Helmholtz instability in classical fluids. Numerical simulations of the Gross-Pitaevskii equations demonstrate that dynamics of pattern formation is well characterized by the Weber number $We$, clarifying the microscopic aspects unique to the quantum fluid system. For $We \lesssim 1$, the Kelvin-Helmholtz instability induces flutter-finger patterns of the interface and quantized vortices are generated at the tip of the fingers. The associated nonlinear dynamics exhibits a universal behavior with respect to $We$. When $We \gtrsim 1$ in which the interface thickness is larger than the wavelength of the interface mode, the nonlinear dynamics is effectively initiated by the counter-superflow instability. In a strongly segregated regime and a large relative velocity, the instability causes transient zipper pattern formation instead of generating vortices due to the lack of enough circulation to form a quantized vortex per a finger. While, in a weakly segregating regime and a small relative velocity, the instability leads to sealskin pattern in the overlapping region, in which the frictional relaxation of the superflow cannot be explained only by the homogeneous counter-superflow instability. We discuss the details of the linear and nonlinear characteristics of this dynamical crossover from small to large Weber numbers, where microscopic properties of the interface become important for the large Weber number.

cond-mat.quant-gas↗

arXiv2021

Application of the inhomogeneous Kibble-Zurek mechanism to quench dynamics in the transition from a Mott-insulator to a superfluid in a finite system

We apply the theory of inhomogeneous Kibble-Zurek mechanism to understand quench dynamics from the Mott insulator to the superfluid in a cold Bose gases confined in both a two-dimensional optical lattice and a harmonic trap. The local quench time and the freeze-out region associated with the nonadiabatic transition take a nontrivial positional dependence due to the Mott-lobe structure of the ground state phase diagram of the Bose-Hubbard model. We demonstrate that the quench dynamics through the time-dependent Gutzwiller simulations, revealing inhomogeneous properties of the growth of the superfluid order parameter. The inhomogeneous Kibble-Zurek theory is applicable for the shallow harmonic trap.

cond-mat.quant-gas↗

arXiv2020

Decay of two-dimensional quantum turbulence in binary Bose-Einstein condensates

We study two-dimensional quantum turbulence in miscible binary Bose-Einstein condensates in either a harmonic trap or a steep-wall trap through the numerical simulations of the Gross-Pitaevskii equations. The turbulence is generated through a Gaussian stirring potential. When the condensates have unequal intra-component coupling strengths or asymmetric trap frequencies, the turbulent condensates undergo a dramatic decay dynamics to an interlaced array of vortex-antidark structures, a quasi-equilibrium state, of like-signed vortices with an extended size of the vortex core. The time of formation of this state is shortened when the parameter asymmetry of the intra-component couplings or the trap frequencies are enhanced. The corresponding spectrum of the incompressible kinetic energy exhibits two noteworthy features: (i) a $k^{-3}$ power-law around the range of the wave number determined by the spin healing length (the size of the extended vortex-core) and (ii) a flat region around the range of the wave number determined by the density healing length. The latter is associated with the small scale phase fluctuation relegated outside the Thomas-Fermi radius and is more prominent as the strength of intercomponent interaction approaches the strength of intra-component interaction. We also study the impact of the inter-component interaction to the cluster formation of like-signed vortices in an elliptical steep-wall trap, finding that the inter-component coupling gives rise to the decay of the clustered configuration.

cond-mat.quant-gas↗

arXiv2020

Semiclassical dynamics of a dark soliton in a one-dimensional bosonic superfluid in an optical lattice

We study quantum dynamics of a dark soliton in a one-dimensional Bose gas in an optical lattice within the truncated Wigner approximation. A previous work has revealed that in the absence of quantum fluctuations, dynamical stability of the dark soliton significantly depends on whether its phase kink is located at a lattice site or a link of two neighboring sites. It has also shown that the dark soliton is unstable in a regime of strong quantum fluctuations regardless of the phase-kink position. To bridge the gap between the classical and strongly quantum regimes, we investigate the dynamical stability of the dark soliton in a regime of weak quantum fluctuations. We find that the position dependence of the dynamical stability gradually diminishes and eventually vanishes as the strength of quantum fluctuations increases. This classical-to-quantum crossover of the soliton stability remains even in the presence of a parabolic trapping potential. We suggest that the crossover behavior can be used for experimentally diagnosing whether the instability of a dark soliton is due to quantum fluctuations or classical dynamical instability.

cond-mat.quant-gas↗

arXiv2020

Modulational instability, inter-component asymmetry and formation of quantum droplets in one-dimensional binary Bose gases

Quantum droplets are ultradilute liquid states which emerge from the competitive interplay of two Hamiltonian terms, the mean-field energy and beyond-mean-field correction, in a weakly interacting binary Bose gas. We relate the formation of droplets in symmetric and asymmetric two-component one-dimensional boson systems to the modulational instability of a spatially uniform state driven by the beyond-mean-field term. Asymmetry between the components may be caused by their unequal populations or unequal intra-component interaction strengths. Stability of both symmetric and asymmetric droplets is investigated. Robustness of the symmetric solutions against symmetry-breaking perturbations is confirmed.

cond-mat.quant-gas↗

arXiv2019

Transverse instability and disintegration of domain wall of relative phase in coherently coupled two-component Bose-Einstein condensates

We study transverse instability and disintegration dynamics of a domain wall of a relative phase in two-component Bose-Einstein condensates with a coherent Rabi coupling. We obtain analytically the stability phase diagram of the stationary solution of the domain wall for the one-dimensional coupled Gross-Pitaevskii equations in the plane of the Rabi frequency and the intercomponent coupling constant. Outside the stable region, the domain wall is dynamically unstable for the transverse modulation along the direction perpendicular to the phase kink. The nonlinear evolution associated with the instability is demonstrated through numerical simulations for both the domain wall without edges and that with edges formed by the quantized vortices.

cond-mat.quant-gas↗

arXiv2019

Effects of a magnetic field on vortex states in superfluid $^3$He-B

Superfluid $^3$He-B possesses three locally stable vortices known as a normal-core vortex ($o$-vortex), an A-phase-core vortex ($v$-vortex), and a double-core vortex ($d$-vortex). In this work, we study the effects of a magnetic field parallel or perpendicular to the vortex axis on these structures by solving the two-dimensional Ginzburg-Landau equation for two different sets of strong coupling correction. The energies of the $v$- and $d$-vortices have nontrivial dependence on the magnetic field. As a longitudinal magnetic field increases, the $v$-vortex is energetically unstable even for high pressures and the $d$-vortex becomes energetically most stable for all possible range of pressure. For a transverse magnetic field the energy of the $v$-vortex becomes lower than that of the $d$-vortex in the high pressure side. In addition, the orientation of the double cores in the $d$-vortex prefers to be parallel to the magnetic field at low pressures, while the $d$-vortex with the double cores perpendicular to the magnetic field is allowed to continuously deform into the $v$-vortex by increasing the pressure.

cond-mat.supr-con↗

arXiv2018

Modulation instability associated nonlinear dynamics of spin-orbit coupled Bose-Einstein condensates

We study pattern-forming nonlinear dynamics starting from a continuous wave state of quasi-one-dimensional two-component Bose-Einstein condensates with synthetic spin-orbit coupling induced by Raman lasers. Modulation instability can occur even when the miscibility condition due to the interatomic interactions is satisfied. We find that the initial stage of the nonlinear development is consistent with the prediction of modulation instability, where the two primary and secondary instability bands lead to the spontaneous growth of the modulation and the subsequent complicated dynamics of pattern formation. At later stages of the evolution, the wave functions undergo clear separation in the momentum space, reflected in the dispersion of the single particle Hamiltonian.

cond-mat.quant-gas↗

arXiv2018

Stripes and honeycomb lattice of quantized vortices in rotating two-component Bose-Einstein condensates

We study numerically the structure of a vortex lattice in two-component Bose-Einstein condensates with equal atomic masses and equal intra- and inter-component coupling strengths. The numerical simulations of the Gross-Pitaevskii equation show that the quantized vortices form uncertain lattice configurations accompanying the vortex stripes, honeycomb lattices, and their complexes. This is a result of the degeneracy of the system for the SU(2) symmetric operation, which makes a continuous transformation between the above structures. In terms of the pseudospin representation, the complex lattice structures are identified to a hexagonal lattice of doubly-winding half-skyrmions.

cond-mat.quant-gas↗

arXiv2018

Is a doubly quantized vortex dynamically unstable in uniform superfluids?

We revisit the fundamental problem of the splitting instability of a doubly quantized vortex in uniform single-component superfluids at zero temperature. We analyze the system-size dependence of the excitation frequency of a doubly quantized vortex through large-scale simulations of the Bogoliubov--de Gennes equation, and find that the system remains dynamically unstable even in the infinite-system-size limit. Perturbation and semi-classical theories reveal that the splitting instability radiates a damped oscillatory phonon as an opposite counterpart of a quasi-normal mode.

cond-mat.quant-gas↗

arXiv2017

Quantum simulation of $(1+1)$-dimensional U(1) gauge-Higgs model on a lattice by cold Bose gases

We present a theoretical study of quantum simulations of $(1+1)$-dimensional U(1) lattice gauge-Higgs models, which contain a compact U(1) gauge field and a Higgs matter field, by using ultra-cold bosonic gases on a one-dimensional optical lattice. Starting from the extended Bose-Hubbard model with on-site and nearest-neighbor interactions, we derive the U(1) lattice gauge-Higgs model as a low-energy effective theory. The derived gauge-Higgs model exhibits nontrivial phase transitions between confinement and Higgs phases, and we discuss the relation with the phase transition in the extended Bose-Hubbard model. Finally, we study real-time dynamics of an electric flux by the Gross-Pitaevskii equations and the truncated Wigner approximation. The dynamics is governed by a bosonic analog of the Schwinger mechanism, i.e., shielding of an electric flux by a condensation of Higgs fields, which occurs differently in the Higgs and the confinement phase. These results, together with the obtained phase diagrams, shall guide experimentalists in designing quantum simulations of the gauge-Higgs models by cold gases.

cond-mat.quant-gas↗

arXiv2016

Atomic quantum simulation of a three-dimensional U(1) gauge-Higgs model

In this paper, we study theoretically atomic quantum simulations of a U(1) gauge-Higgs model on a three-dimensional (3D) spatial lattice by using an extended Bose-Hubbard model with intersite repulsions on a 3D optical lattice. Here, the phase and density fluctuations of the boson variable on each site of the optical lattice describe the vector potential and the electric field on each link of the gauge-model lattice, respectively. The target gauge model is different from the standard Wilson-type U(1) gauge-Higgs model because it has plaquette and Higgs interactions with asymmetric couplings in the space-time directions. Nevertheless, the corresponding quantum simulation is still important as it provides us with a platform to study unexplored time-dependent phenomena characteristic of each phase in the general gauge-Higgs models. To determine the phase diagram of the gauge-Higgs model at zero temperature, we perform Monte-Carlo simulations of the corresponding 3+1-dimensional U(1) gauge-Higgs model, and obtain the confinement and Higgs phases. To investigate the dynamical properties of the gauge-Higgs model, we apply the Gross-Pitaevskii equations to the extended Bose-Hubbard model. We simulate the time-evolution of an electric flux that initially is put on a straight line connecting two external point charges. We also calculate the potential energy between this pair of charges and obtain the string tension in the confinement phase. Finally, we propose a feasible experimental setup for the atomic simulations of this quantum gauge-Higgs model on the 3D optical lattice. These results may serve as theoretical guides for future experiments.

cond-mat.quant-gas↗

arXiv2015

Dynamics of quantized vortices in Bose-Einstein condensates with laser-induced spin-orbit coupling

We study vortex dynamics in trapped two-component Bose-Einstein condensates with a laser- induced spin-orbit coupling using the numerical analysis of the Gross-Pitaevskii equation. The spin-orbit coupling leads to three distinct ground state phases, which depend on some experimentally controllable parameters. When a vortex is put in one or both of the two-component condensates, the vortex dynamics exhibits very different behaviors in each phase, which can be observed in experiments. These dynamical behaviors can be understood by clarifying the stable vortex structure realized in each phase.

cond-mat.quant-gas↗