3,321 research outputs found
Asymmetric vortex solitons in nonlinear periodic lattices
We reveal the existence of asymmetric vortex solitons in ideally symmetric
periodic lattices, and show how such nonlinear localized structures describing
elementary circular flows can be analyzed systematically using the
energy-balance relations. We present the examples of rhomboid, rectangular, and
triangular vortex solitons on a square lattice, and also describe novel
coherent states where the populations of clockwise and anti-clockwise vortex
modes change periodically due to a nonlinearity-induced momentum exchange
through the lattice. Asymmetric vortex solitons are expected to exist in
different nonlinear lattice systems including optically-induced photonic
lattices, nonlinear photonic crystals, and Bose-Einstein condensates in optical
lattices.Comment: 4 pages, 5 figure
Observation of Vortex Matching Phenomena in Antidot Array of NbN Thin Film
We report vortex matching phenomenon in rectangular antidot array fabricated
on epitaxial NbN thin film. The antidot array was fabricated using Focussed Ion
Beam milling technique. The magneto-transport measurements points to a period
doubling transition at higher magnetic field for rectangular lattices. The
results are discussed within the light of several models including the
multi-vortex model, the matched lattice model and the super-matched lattice
model.Comment: Added references, modified abstract and discussions and corrected
typo-graphic errors. Accepted for proceedings of M2S-IX 2009, Tokyo (Physica
C
Order in driven vortex lattices in superconducting Nb films with nanostructured pinning potentials
Driven vortex lattices have been studied in a material with strong pinning,
such as Nb films. Samples in which natural random pinning coexists with
artificial ordered arrays of defects (submicrometric Ni dots) have been
fabricated with different geometries (square, triangular and rectangular).
Three different dynamic regimes are found: for low vortex velocities, there is
a plastic regime in which random defects frustrate the effect of the ordered
array; then, for vortex velocities in the range 1-100 m/s, there is a sudden
increase in the interaction between the vortex lattice and the ordered dot
array, independent on the geometry. This effect is associated to the onset of
quasi long range order in the vortex lattice leading to an increase in the
overlap between the vortex lattice and the magnetic dots array. Finally, at
larger velocities the ordered array-vortex lattice interaction is suppresed
again, in agreement with the behavior found in numerical simulations.Comment: 8 text pages + 4 figure
Two-component Bose-Einstein Condensates with Large Number of Vortices
We consider the condensate wavefunction of a rapidly rotating two-component
Bose gas with an equal number of particles in each component. If the
interactions between like and unlike species are very similar (as occurs for
two hyperfine states of Rb or Na) we find that the two components
contain identical rectangular vortex lattices, where the unit cell has an
aspect ratio of , and one lattice is displaced to the center of the
unit cell of the other. Our results are based on an exact evaluation of the
vortex lattice energy in the large angular momentum (or quantum Hall) regime.Comment: 4 pages, 2 figures, RevTe
Simple Vortex States in Films of Type-I Ginzburg-Landau Superconductor
Sufficiently thin films of type-I superconductor in a perpendicular magnetic
field exhibit a triangular vortex lattice, while thick films develop an
intermediate state. To elucidate what happens between these two regimes,
precise numerical calculations have been made within Ginzburg-Landau theory at
and 0.25 for a variety of vortex lattice structures with one flux
quantum per unit cell. The phase diagram in the space of mean induction and
film thickness includes a narrow wedge in which a square lattice is stable,
surrounded by the domain of stability of the triangular lattice at thinner
films/lower fields and, on the other side, rectangular lattices with
continuously varying aspect ratio. The vortex lattice has an anomalously small
shear modulus within and close to the square lattice phase.Comment: 21 pages, 6 figure
Tkachenko modes and structural phase transitions of the vortex lattice of a two component Bose-Einstein condensate
We consider a rapidly rotating two-component Bose-Einstein condensate (BEC)
containing a vortex lattice. We calculate the dispersion relation for small
oscillations of vortex positions (Tkachenko modes) in the mean-field quantum
Hall regime, taking into account the coupling of these modes with density
excitations. Using an analytic form for the density of the vortex lattice, we
numerically calculate the elastic constants for different lattice geometries.
We also apply this method to calculate the elastic constant for the
single-component triangular lattice. For a two-component BEC, there are two
kinds of Tkachenko modes, which we call acoustic and optical in analogy with
phonons. For all lattice types, acoustic Tkachenko mode frequencies have
quadratic wave-number dependence at long-wavelengths, while the optical
Tkachenko modes have linear dependence. For triangular lattices the dispersion
of the Tkachenko modes are isotropic, while for other lattice types the
dispersion relations show directional dependence consistent with the symmetry
of the lattice. Depending on the intercomponent interaction there are five
distinct lattice types, and four structural phase transitions between them. Two
of these transitions are second-order and are accompanied by the softening of
an acoustic Tkachenko mode. The remaining two transitions are first-order and
while one of them is accompanied by the softening of an optical mode, the other
does not have any dramatic effect on the Tkachenko spectrum. We also find an
instability of the vortex lattice when the intercomponent repulsion becomes
stronger than the repulsion within components.Comment: 24 pages, 13 figures, typos corrected, references added, final
versio
Skyrmionic vortex lattices in coherently coupled three-component Bose-Einstein condensates
We show numerically that a harmonically trapped and coherently Rabi-coupled
three-component Bose-Einstein condensate can host unconventional vortex
lattices in its rotating ground state. The discovered lattices incorporate
square and zig-zag patterns, vortex dimers and chains, and doubly quantized
vortices, and they can be quantitatively classified in terms of a skyrmionic
topological index, which takes into account the multicomponent nature of the
system. The exotic ground-state lattices arise due to the intricate interplay
of the repulsive density-density interactions and the Rabi couplings as well as
the ubiquitous phase frustration between the components. In the frustrated
state, domain walls in the relative phases can persist between some components
even at strong Rabi coupling, while vanishing between others. Consequently, in
this limit the three-component condensate effectively approaches a
two-component condensate with only density-density interactions. At
intermediate Rabi coupling strengths, however, we face unique vortex physics
that occurs neither in the two-component counterpart nor in the purely
density-density-coupled three-component system.Comment: 13 pages, 16 color figures; v2 is identical in content to the
published articl
Dipolar ground state of planar spins on triangular lattices
An infinite triangular lattice of classical dipolar spins is usually
considered to have a ferromagnetic ground state. We examine the validity of
this statement for finite lattices and in the limit of large lattices. We find
that the ground state of rectangular arrays is strongly dependent on size and
aspect ratio. Three results emerge that are significant for understanding the
ground state properties: i) formation of domain walls is energetically favored
for aspect ratios below a critical valu e; ii) the vortex state is always
energetically favored in the thermodynamic limit of an infinite number of
spins, but nevertheless such a configuration may not be observed even in very
large lattices if the aspect ratio is large; iii) finite range approximations
to actual dipole sums may not provide the correct ground sta te configuration
because the ferromagnetic state is linearly unstable and the domain wall energy
is negative for any finite range cutoff.Comment: Several short parts have been rewritten. Accepted for publication as
a Rapid Communication in Phys. Rev.
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