669 research outputs found
Modulation instability and solitary wave formation in two-component Bose-Einstein condensates
We investigate nonlinear dynamics induced by the modulation instability of a
two-component mixture in an atomic Bose-Einstein condensate. The nonlinear
dynamics is examined using numerical simulations of the time-dependent coupled
Gross-Pitaevskii equations. The unstable modulation grows from initially
miscible condensates into various types of vector solitary waves, depending on
the combinations of the sign of the coupling constants (intracomponent and
intercomponent). We discuss the detailed features of the modulation
instability, dynamics of solitary wave formation, and an analogy with the
collapsing dynamics in a single-component condensate with attractive
interactions.Comment: 15 pages, 10 figure
Nonlocal transport in the charge density waves of -TaS
We studied the nonlocal transport of a quasi-one dimensional conductor
-TaS. Electric transport phenomena in charge density waves include the
thermally-excited quasiparticles, and collective motion of charge density waves
(CDW). In spite of its long-range correlation, the collective motion of a CDW
does not extend far beyond the electrodes, where phase slippage breaks the
correlation. We found that nonlocal voltages appeared in the CDW of
-TaS, both below and above the threshold field for CDW sliding. The
temperature dependence of the nonlocal voltage suggests that the observed
nonlocal voltage originates from the CDW even below the threshold field.
Moreover, our observation of nonlocal voltages in both the pinned and sliding
states reveals the existence of a carrier with long-range correlation, in
addition to sliding CDWs and thermally-excited quasiparticles.Comment: 8 pages, 4 figure
Spin textures in rotating two-component Bose-Einstein condensates
We investigate two kinds of coreless vortices with axisymmetric and
nonaxisymmetric configurations in rotating two-component Bose-Einstein
condensates. Starting from the Gross-Pitaevskii energy functional in a rotating
frame, we derive a nonlinear sigma model generalized to the two-component
condensates. In terms of a pseudospin representation, an axisymmetric vortex
and a nonaxisymmetric one correspond to spin textures referred to as a
"skyrmion" and a "meron-pair", respectively. A variational method is used to
investigate the dependence of the sizes of the stable spin textures on system
parameters, and the optimized variational function is found to reproduce well
the numerical solution. In the SU(2) symmetric case, the optimal skyrmion and
meron-pair are degenerate and transform to each other by a rotation of the
pseudospin. An external rf-field that couples coherently the hyperfine states
of two components breaks the degeneracy in favor of the meron-pair texture due
to an effective transverse pseudomagnetic field. The difference between the
intracomponent and intercomponent interactions yields a longitudinal
pseudomagnetic field and a ferromagnetic or an antiferromagnetic pseudospin
interaction, leading to a meron-pair texture with an anisotropic distribution
of vorticity.Comment: 14 pages, 15 figure
Drag force on an oscillating object in quantum turbulence
This paper reports results of the computation of the drag force exerted on an
oscillating object in quantum turbulence in superfluid He. The drag force
is calculated on the basis of numerical simulations of quantum turbulent flow
about the object. The drag force is proportional to the square of the magnitude
of the oscillation velocity, which is similar to that in classical turbulence
at high Reynolds number. The drag coefficient is also calculated, and its value
is found to be of the same order as that observed in previous experiments. The
correspondence between quantum and classical turbulences is further clarified
by examining the turbulence created by oscillating objects.Comment: 7 pages, 5 figures, 1 tabl
Vortex molecules in coherently coupled two-component Bose-Einstein condensates
A vortex molecule is predicted in rotating two-component Bose-Einstein
condensates whose internal hyperfine states are coupled coherently by an
external field. A vortex in one component and that in the other are connected
by a domain wall of the relative phase, constituting a "vortex molecule", which
features a nonaxisymmetric (pseudo)spin texture with a pair of merons. The
binding mechanism of the vortex molecule is discussed based on a generalized
nonlinear sigma model and a variational ansatz. The anisotropy of vortex
molecules is caused by the difference in the scattering lengths, yielding a
distorted vortex-molecule lattice in fast rotating condensates.Comment: 4 pages, 4 figures, greatly revised versio
Spin echo in spinor dipolar Bose-Einstein condensates
We theoretically propose and numerically realize spin echo in a spinor
Bose--Einstein condensate (BEC). We investigate the influence on the spin echo
of phase separation of the condensate. The equation of motion of the spin
density exhibits two relaxation times. We use two methods to separate the
relaxation times and hence demonstrate a technique to reveal magnetic
dipole--dipole interactions in spinor BECs.Comment: 4 pages, 5 figure
Transition to superfluid turbulence governed by an intrinsic parameter
Hydrodynamic flow in both classical and quantum fluids can be either laminar
or turbulent. To describe the latter, vortices in turbulent flow are modelled
with stable vortex filaments. While this is an idealization in classical
fluids, vortices are real topologically stable quantized objects in
superfluids. Thus superfluid turbulence is thought to hold the key to new
understanding on turbulence in general. The fermion superfluid 3He offers
further possibilities owing to a large variation in its hydrodynamic
characteristics over the experimentally accessible temperatures. While studying
the hydrodynamics of the B phase of superfluid 3He, we discovered a sharp
transition at 0.60Tc between two regimes, with regular behaviour at
high-temperatures and turbulence at low-temperatures. Unlike in classical
fluids, this transition is insensitive to velocity and occurs at a temperature
where the dissipative vortex damping drops below a critical limit. This
discovery resolves the conflict between existing high- and low-temperature
measurements in 3He-B: At high temperatures in rotating flow a vortex loop
injected into superflow has been observed to expand monotonically to a single
rectilinear vortex line, while at very low temperatures a tangled network of
quantized vortex lines can be generated in a quiescent bath with a vibrating
wire. The solution of this conflict reveals a new intrinsic criterion for the
existence of superfluid turbulence.Comment: Revtex file; 5 pages, 2 figure
Quantum Kelvin-Helmholtz instability in phase-separated two-component Bose-Einstein condensates
We theoretically study the Kelvin-Helmholtz instability in phase-separated
two-component Bose-Einstein condensates using the Gross-Pitaevskii and
Bogoliubov-de Gennes models. A flat interface between the two condensates is
shown to deform into sawtooth or Stokes-like waves, leading to the formation of
singly quantized vortices on the peaks and troughs of the waves. This scenario
of interface instability in quantum fluids is quite different from that in
classical fluids.Comment: 5 pages, 4 figure
Vortex sheet in rotating two-component Bose-Einstein condensates
We investigate vortex states of immiscible two-component Bose-Einstein
condensates under rotation through numerical simulations of the coupled
Gross-Pitaevskii equations. For strong intercomponent repulsion, the two
components undergo phase separation to form several density domains of the same
component. In the presence of the rotation, the nucleated vortices are aligned
between the domains to make up winding chains of singly quantized vortices, a
vortex sheet, instead of periodic vortex lattices. The vortices of one
component are located at the region of the density domains of the other
component, which results in the serpentine domain structure. The sheet
configuration is stable as long as the imbalance of the intracomponent
parameter is small. We employ a planar sheet model to estimate the distance
between neighboring sheets, determined by the competition between the surface
tension of the domain wall and the kinetic energy of the superflow via
quantized vortices. By comparing the several length scales in this system, the
phase diagram of the vortex state is obtained.Comment: 8 pages, 7 figure
Crossover between Kelvin-Helmholtz and counter-superflow instabilities in two-component Bose-Einstein condensates
Dynamical instabilities at the interface between two Bose--Einstein
condensates that are moving relative to each other are investigated using
mean-field and Bogoliubov analyses. Kelvin--Helmholtz instability is dominant
when the interface thickness is much smaller than the wavelength of the
unstable interface mode, whereas the counter-superflow instability becomes
dominant in the opposite case. These instabilities emerge not only in an
immiscible system but also in a miscible system where an interface is produced
by external potential. Dynamics caused by these instabilities are numerically
demonstrated in rotating trapped condensates.Comment: 10 pages, 9 figure
- …