11,100 research outputs found
FAST SYNTHESIS OF DYNAMIC COLOUR TEXTURES
The textural appearance of many real-world materials is not static, but changes over time. If such change is spatially and temporally homogeneous, these materials can be represented by means of dynamic textures. Dynamic textures modelling is a challenging problem that can improve the quality of computer graphics applications. During the recent years, a novel hybrid method for dynamic colour textures modelling was developed
The Intrinsic Magnetization of Antiferromagnetic Textures
Antiferromagnets (AFMs) exhibit intrinsic magnetization when the order
parameter spatially varies. This intrinsic spin is present even at equilibrium
and can be interpreted as a twisting of the homogeneous AFM into a state with a
finite spin. Because magnetic moments couple directly to external magnetic
fields, the intrinsic magnetization can alter the dynamics of antiferromagnetic
textures under such influence. Starting from the discrete Heisenberg model, we
derive the continuum limit of the free energy of AFMs in the exchange
approximation and explicitly rederive that the spatial variation of the
antiferromagnetic order parameter is associated with an intrinsic magnetization
density. We calculate the magnetization profile of a domain wall and discuss
how the intrinsic magnetization reacts to external forces. We show
conclusively, both analytically and numerically, that a spatially inhomogeneous
magnetic field can move and control the position of domain walls in AFMs. By
comparing our model to a commonly used alternative parametrization procedure
for the continuum fields, we show that the physical interpretations of these
fields depend critically on the choice of parametrization procedure for the
discrete-to-continuous transition. This can explain why a significant amount of
recent studies of the dynamics of AFMs, including effective models that
describe the motion of antiferromagnetic domain walls, have neglected the
intrinsic spin of the textured order parameter.Comment: 12 pages, 7 figure
Periodic spin textures in a degenerate F=1 Rb spinor Bose gas
We report on the spin textures produced by cooling unmagnetized Rb F=1
spinor gases into the regime of quantum degeneracy. At low temperatures,
magnetized textures form that break translational symmetry and display
short-range periodic magnetic order characterized by one- or two-dimensional
spatial modulations with wavelengths much smaller than the extent of the
quasi-two-dimensional degenerate gas. Spin textures produced upon cooling spin
mixtures with a non-zero initial magnetic quadrupole moment also show
ferromagnetic order that, at low temperature, coexists with the spatially
modulated structure.Comment: 6 pages, revised substantially following reviewer comments and
further analysi
Instability-induced formation and non-equilibrium dynamics of phase defects in polariton condensates
We study, theoretically and numerically, the onset and development of
modulational instability in an incoherently pumped spatially homogeneous
polariton condensate. Within the framework of mean-field theory, we identify
regimes of modulational instability in two cases: 1) Strong feedback between
the condensate and reservoir, which may occur in scalar condensates, and 2)
Parametric scattering in the presence of polarization splitting in spinor
condensates. In both cases we investigate the instability induced textures in
space and time including non-equilibrium dynamics of phase dislocations and
vortices. In particular we discuss the mechanism of vortex destabilization and
formation of spiraling waves. We also identify the presence of topological
defects, which take the form of half-vortex pairs in the spinor case, giving an
"eyelet" structure in intensity and dipole type structure in the spin
polarization. In the modulationally stable parameter domains, we observe
formation of the phase defects in the process of condensate formation from an
initially spatially incoherent low-density state. In analogy to the
Kibble-Zurek type scaling for nonequilibrium phase transitions, we find that
the defect density scales with the pumping rate.Comment: 13 pages, 9 figures, revised manuscript sent to Phys. Rev.
Spontaneously modulated spin textures in a dipolar spinor Bose-Einstein condensate
Helical spin textures in a Rb F=1 spinor Bose-Einstein condensate are
found to decay spontaneously toward a spatially modulated structure of spin
domains. This evolution is ascribed to magnetic dipolar interactions that
energetically favor the short-wavelength domains over the long-wavelength spin
helix. This is confirmed by eliminating the dipolar interactions by a sequence
of rf pulses and observing a suppression of the formation of the short-range
domains. This study confirms the significance of magnetic dipole interactions
in degenerate Rb F=1 spinor gases
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
Thin-Film Magnetization Dynamics on the Surface of a Topological Insulator
We theoretically study the magnetization dynamics of a thin ferromagnetic
film exchange-coupled with a surface of a strong three-dimensional topological
insulator. We focus on the role of electronic zero modes imprinted by domain
walls (DW's) or other topological textures in the magnetic film.
Thermodynamically reciprocal hydrodynamic equations of motion are derived for
the DW responding to electronic spin torques, on the one hand, and fictitious
electromotive forces in the electronic chiral mode fomented by the DW, on the
other. An experimental realization illustrating this physics is proposed based
on a ferromagnetic strip, which cuts the topological insulator surface into two
gapless regions. In the presence of a ferromagnetic DW, a chiral mode
transverse to the magnetic strip acts as a dissipative interconnect, which is
itself a dynamic object that controls (and, inversely, responds to) the
magnetization dynamics.Comment: 7 pages, 2 figure
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