228 research outputs found
Semiclassical dynamics of quasi-one-dimensional, attractive Bose-Einstein condensates
The strongly interacting regime for attractive Bose-Einstein condensates
(BECs) tightly confined in an extended cylindrical trap is studied. For
appropriately prepared, non-collapsing BECs, the ensuing dynamics are found to
be governed by the one-dimensional focusing Nonlinear Schr\"odinger equation
(NLS) in the semiclassical (small dispersion) regime. In spite of the
modulational instability of this regime, some mathematically rigorous results
on the strong asymptotics of the semiclassical limiting solutions were obtained
recently. Using these results, "implosion-like" and "explosion-like" events are
predicted whereby an initial hump focuses into a sharp spike which then expands
into rapid oscillations. Seemingly related behavior has been observed in
three-dimensional experiments and models, where a BEC with a sufficient number
of atoms undergoes collapse. The dynamical regimes studied here, however, are
not predicted to undergo collapse. Instead, distinct, ordered structures,
appearing after the "implosion", yield interesting new observables that may be
experimentally accessible.Comment: 9 pages, 3 figure
Vortex-antivortex proliferation from an obstacle in thin film ferromagnets
Magnetization dynamics in thin film ferromagnets can be studied using a
dispersive hydrodynamic formulation. The equations describing the
magnetodynamics map to a compressible fluid with broken Galilean invariance
parametrized by the longitudinal spin density and a magnetic analog of the
fluid velocity that define spin-density waves. A direct consequence of these
equations is the determination of a magnetic Mach number. Micromagnetic
simulations reveal nucleation of nonlinear structures from an impenetrable
object realized by an applied magnetic field spot or a defect. In this work,
micromagnetic simulations demonstrate vortex-antivortex pair nucleation from an
obstacle. Their interaction establishes either ordered or irregular
vortex-antivortex complexes. Furthermore, when the magnetic Mach number exceeds
unity (supersonic flow), a Mach cone and periodic wavefronts are observed,
which can be well-described by solutions of the steady, linearized equations.
These results are reminiscent of theoretical and experimental observations in
Bose-Einstein condensates, and further supports the analogy between the
magnetodynamics of a thin film ferromagnet and compressible fluids. The
nucleation of nonlinear structures and vortex-antivortex complexes using this
approach enables the study of their interactions and effects on the stability
of spin-density waves.Comment: 23 pages, 7 figure
Symmetry-broken dissipative exchange flows in thin-film ferromagnets with in-plane anisotropy
Planar ferromagnetic channels have been shown to theoretically support a
long-range ordered and coherently precessing state where the balance between
local spin injection at one edge and damping along the channel establishes a
dissipative exchange flow, sometimes referred to as a spin superfluid. However,
realistic materials exhibit in-plane anisotropy, which breaks the axial
symmetry assumed in current theoretical models. Here, we study dissipative
exchange flows in a ferromagnet with in-plane anisotropy from a dispersive
hydrodynamic perspective. Through the analysis of a boundary value problem for
a damped sine-Gordon equation, dissipative exchange flows in a ferromagnetic
channel can be excited above a spin current threshold that depends on material
parameters and the length of the channel. Symmetry-broken dissipative exchange
flows display harmonic overtones that redshift the fundamental precessional
frequency and lead to a reduced spin pumping efficiency when compared to their
symmetric counterpart. Micromagnetic simulations are used to verify that the
analytical results are qualitatively accurate, even in the presence of nonlocal
dipole fields. Simulations also confirm that dissipative exchange flows can be
driven by spin transfer torque in a finite-sized region. These results
delineate the important material parameters that must be optimized for the
excitation of dissipative exchange flows in realistic systems.Comment: 20 pages, 5 figure
Interactions of large amplitude solitary waves in viscous fluid conduits
The free interface separating an exterior, viscous fluid from an intrusive
conduit of buoyant, less viscous fluid is known to support strongly nonlinear
solitary waves due to a balance between viscosity-induced dispersion and
buoyancy-induced nonlinearity. The overtaking, pairwise interaction of weakly
nonlinear solitary waves has been classified theoretically for the Korteweg-de
Vries equation and experimentally in the context of shallow water waves, but a
theoretical and experimental classification of strongly nonlinear solitary wave
interactions is lacking. The interactions of large amplitude solitary waves in
viscous fluid conduits, a model physical system for the study of
one-dimensional, truly dissipationless, dispersive nonlinear waves, are
classified. Using a combined numerical and experimental approach, three classes
of nonlinear interaction behavior are identified: purely bimodal, purely
unimodal, and a mixed type. The magnitude of the dispersive radiation due to
solitary wave interactions is quantified numerically and observed to be beyond
the sensitivity of our experiments, suggesting that conduit solitary waves
behave as "physical solitons." Experimental data are shown to be in excellent
agreement with numerical simulations of the reduced model. Experimental movies
are available with the online version of the paper.Comment: 13 pages, 4 figure
Transverse instabilities of stripe domains in magnetic thin films with perpendicular magnetic anisotropy
Stripe domains are narrow, elongated, reversed regions that exist in magnetic
materials with perpendicular magnetic anisotropy. Stripe domains appear as a
pair of domain walls that can exhibit topology with a nonzero chirality. Recent
experimental and numerical investigations identify an instability of stripe
domains in the long direction as a means of nucleating isolated magnetic
skyrmions. Here, the onset and nonlinear evolution of transverse instabilities
for a dynamic stripe domain known as the bion stripe are investigated. Both
non-topological and topological variants of the bion stripe are shown to
exhibit a long-wavelength transverse instability with different characteristic
features. In the former, small transverse variations in the stripe's width lead
to a neck instability that eventually pinches the non-topological stripe into a
chain of two-dimensional breathers composed of droplet soliton pairs. In the
latter case, small variations in the stripe's center results in a snake
instability whose topological structure leads to the nucleation of dynamic
magnetic skyrmions and antiskyrmions as well as perimeter-modulated droplets.
Quantitative, analytical predictions for both the early, linear evolution and
the long-time, nonlinear evolution are achieved using an averaged Lagrangian
approach that incorporates both exchange (dispersion) and anisotropy
(nonlinearity). The method of analysis is general and can be applied to other
filamentary structures.Comment: 8 figures, 13 page
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