2,050 research outputs found
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.Comment: 9 pages, 9 figure
Nambu-Goldstone modes in segregated Bose-Einstein condensates
Nambu-Goldstone modes in immiscible two-component Bose-Einstein condensates
are studied theoretically. In a uniform system, a flat domain wall is
stabilized and then the translational invariance normal to the wall is
spontaneously broken in addition to the breaking of two U(1) symmetries in the
presence of two complex order parameters. We clarify properties of the
low-energy excitations and identify that there exist two Nambu-Goldstone modes:
in-phase phonon with a linear dispersion and ripplon with a fractional
dispersion. The signature of the characteristic dispersion can be verified in
segregated condensates in a harmonic potential.Comment: 5 pages, 3 figure
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.Comment: 9 pages, 6 figure
Josephson Current Flowing in Cyclically Coupled Bose-Einstein Condensates
The Josephson effect in cyclically coupled Bose-Einstein condensates is
studied theoretically. We analyze the simultaneous Gross-Pitaevskii equations
with coupling terms between adjacent condensates. Depending on the initial
relative phases between condensates, Josephson current flows cyclically to make
a quantized vortex. Reducing the coupling between condensates changes the
motion from periodic to chaotic, thus suppressing the cyclic current. The
relation to the Kibble-Zurek mechanism is discussed.Comment: 4 pages, 3 figure, to be published in Journal of Physical Society of
Japa
Quantized vortices in atomic Bose-Einstein condensates
In this review, we give an overview of the experimental and theoretical
advances in the physics of quantized vortices in dilute atomic-gas
Bose--Einstein condensates in a trapping potential, especially focusing on
experimental research activities and their theoretical interpretations. Making
good use of the atom optical technique, the experiments have revealed many
novel structural and dynamic properties of quantized vortices by directly
visualizing vortex cores from an image of the density profiles. These results
lead to a deep understanding of superfluid hydrodynamics of such systems.
Typically, vortices are stabilized by a rotating potential created by a laser
beam, magnetic field, and thermal gas. Finite size effects and inhomogeneity of
the system, originating from the confinement by the trapping potential, yield
unique vortex dynamics coupled with the collective excitations of the
condensate. Measuring the frequencies of the collective modes is an accurate
tool for clarifying the character of the vortex state. The topics included in
this review are the mechanism of vortex formation, equilibrium properties, and
dynamics of a single vortex and those of a vortex lattice in a rapidly rotating
condensate.Comment: 51 pages, 12 figures, to be published in Progress in Low Temperature
Physics, vol. 1
First-principles investigation of polarization and ion conduction mechanisms in hydroxyapatite
We report first-principles simulation of polarization mechanisms in
hydroxyapatite to explain the underlying mechanism behind the reported ion
conductivities and polarization under electrical poling at elevated
temperatures. It is found that ion conduction occurs mainly in the column of
OH ions along the -axis through a combination of the flipping of OH
ions, exchange of proton vacancies between OH ions, and the hopping of the
OH vacancy. The calculated activation energies are consistent with those
found in conductivity measurements and thermally stimulated depolarization
current measurements
Direct coupling of first-principles calculations with replica exchange Monte Carlo sampling of ion disorder in solids
We demonstrate the feasibility of performing sufficient configurational
sampling of disordered oxides directly from first principles without resorting
to the use of fitted models such as cluster expansion. This is achieved by
harnessing the power of modern-day cluster supercomputers using the replica
exchange Monte Carlo method coupled directly with structural relaxation and
energy calculation performed by density functional codes. The idea is applied
successfully to the calculation of the temperature-dependence of the degree of
inversion in the cation sublattice of MgAlO spinel oxide. The
possibility of bypassing fitting models will lead to investigation of
disordered systems where cluster expansion is known to perform badly: for
example, systems with large lattice deformation due to defects, or systems
where long-range interactions dominate such as electrochemical interfaces.Comment: 6 pages, 4 figure
Quantized vortices and quantum turbulence
We review recent important topics in quantized vortices and quantum
turbulence in atomic Bose--Einstein condensates (BECs). They have previously
been studied for a long time in superfluid helium. Quantum turbulence is
currently one of the most important topics in low-temperature physics. Atomic
BECs have two distinct advantages over liquid helium for investigating such
topics: quantized vortices can be directly visualized and the interaction
parameters can be controlled by the Feshbach resonance. A general introduction
is followed by a description of the dynamics of quantized vortices,
hydrodynamic instability, and quantum turbulence in atomic BECs.Comment: 18 pages, 5 figures, short revie
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.Comment: 8 pages, 6 figure
Atomic Quantum Simulation of Lattice Gauge-Higgs Model: Higgs Couplings and Emergence of Exact Local Gauge Symmetry
Recently, the possibility of quantum simulation of dynamical gauge fields was
pointed out by using a system of cold atoms trapped on each link in an optical
lattice. However, to implement exact local gauge invariance, fine-tuning the
interaction parameters among atoms is necessary. In the present paper, we study
the effect of violation of the U(1) local gauge invariance by relaxing the
fine-tuning of the parameters and showing that a wide variety of cold atoms is
still to be a faithful quantum simulator for a U(1) gauge-Higgs model
containing a Higgs field sitting on sites. Clarification of the dynamics of
this gauge-Higgs model sheds some lights upon various unsolved problems
including the inflation process of the early universe. We study the phase
structure of this model by Monte Carlo simulation, and also discuss the atomic
characteristics of the Higgs phase in each simulator.Comment: 5 pages, 2 figures, Version to appear in Phys. Rev. Lett,
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