1,941 research outputs found
Lattice symmetry breaking perturbations for spiral waves
Spiral waves in two-dimensional excitable media have been observed
experimentally and studied extensively. It is now well-known that the symmetry
properties of the medium of propagation drives many of the dynamics and
bifurcations which are experimentally observed for these waves. Also,
symmetry-breaking induced by boundaries, inhomogeneities and anisotropy have
all been shown to lead to different dynamical regimes as to that which is
predicted for mathematical models which assume infinite homogeneous and
isotropic planar geometry. Recent mathematical analyses incorporating the
concept of forced symmetry-breaking from the Euclidean group of all planar
translations and rotations have given model-independent descriptions of the
effects of media imperfections on spiral wave dynamics. In this paper, we
continue this program by considering rotating waves in dynamical systems which
are small perturbations of a Euclidean-equivariant dynamical system, but for
which the perturbation preserves only the symmetry of a regular square lattice
Modelling spin waves in noncollinear antiferromagnets: spin-flop states, spin spirals, skyrmions and antiskyrmions
Spin waves in antiferromagnetic materials have great potential for
next-generation magnonic technologies. However, their properties and their
dependence on the type of ground-state antiferromagnetic structure are still
open questions. Here, we investigate theoretically spin waves in one- and
two-dimensional model systems with a focus on noncollinear antiferromagnetic
textures such as spin spirals and skyrmions of opposite topological charges. We
address in particular the nonreciprocal spin excitations recently measured in
bulk antiferromagnet -- utilizing
inelastic neutron scattering experiments [Phys.\ Rev.\ Lett.\ \textbf{119},
047201 (2017)], where we help to characterize the nature of the detected
spin-wave modes. Furthermore, we discuss how the Dzyaloshinskii-Moriya
interaction can lift the degeneracy of the spin-wave modes in antiferromagnets,
resembling the electronic Rashba splitting. We consider the spin-wave
excitations in antiferromagnetic spin-spiral and skyrmion systems and discuss
the features of their inelastic scattering spectra. We demonstrate that
antiskyrmions can be obtained with an isotropic Dzyaloshinskii-Moriya
interaction in certain antiferromagnets.Comment: 26 pages, 9 figure
Towards a holographic realization of the quarkyonic phase
Large-N_c QCD matter at intermediate baryon density and low temperatures has
been conjectured to be in the so-called quarkyonic phase, i.e., to have a quark
Fermi surface and on top of it a confined spectrum of excitations. It has been
suggested that the presence of the quark Fermi surface leads to a homogeneous
phase with restored chiral symmetry, which is unstable towards creating
condensates breaking both the chiral and translational symmetry. Motivated by
these exotic features, we investigate properties of cold baryonic matter in the
single flavor Sakai-Sugimoto model searching for a holographic realization of
the quarkyonic phase. We use a simplified mean-field description and focus on
the regime of parametrically large baryon densities, of the order of the square
of the 't Hooft coupling, as they turn out to lead to new physical effects
similar to the ones occurring in the quarkyonic phase. One effect, the
appearance of a particular marginally stable mode breaking translational
invariance and linked with the presence of the Chern-Simons term in the flavor
brane Lagrangian, is known to occur in the deconfined phase of the
Sakai-Sugimoto model, but turns out to be absent here. The other, completely
new phenomenon that we, preliminarily, study using strong simplifying
assumptions are density-enhanced interactions of the flavor brane gauge field
with holographically represented baryons. These seem to significantly affect
the spectrum of vector and axial mesons and might lead to approximate chiral
symmetry restoration in the lowest part of the spectrum, where the mesons start
to qualitatively behave like collective excitations of the dense baryonic
medium. We discuss the relevance of these effects for holographic searches of
the quarkyonic phase and conclude with a discussion of various subtleties
involved in constructing a mean-field holographic description of a dense
baryonic medium.Comment: 31 pages, 16 figures; v2: inset plot in Fig. 10 removed, coloring in
Fig. 13 fixed, typos fixed, matches published versio
An introduction to the Ginzburg-Landau theory of phase transitions and nonequilibrium patterns
This paper presents an introduction to phase transitions and critical
phenomena on the one hand, and nonequilibrium patterns on the other, using the
Ginzburg-Landau theory as a unified language. In the first part, mean-field
theory is presented, for both statics and dynamics, and its validity tested
self-consistently. As is well known, the mean-field approximation breaks down
below four spatial dimensions, where it can be replaced by a scaling
phenomenology. The Ginzburg-Landau formalism can then be used to justify the
phenomenological theory using the renormalization group, which elucidates the
physical and mathematical mechanism for universality. In the second part of the
paper it is shown how near pattern forming linear instabilities of dynamical
systems, a formally similar Ginzburg-Landau theory can be derived for
nonequilibrium macroscopic phenomena. The real and complex Ginzburg-Landau
equations thus obtained yield nontrivial solutions of the original dynamical
system, valid near the linear instability. Examples of such solutions are plane
waves, defects such as dislocations or spirals, and states of temporal or
spatiotemporal (extensive) chaos
Competing Patterns of Signaling Activity in Dictyostelium discoideum
Quantitative experiments are described on spatio-temporal patterns of
coherent chemical signaling activity in populations of {\it Dictyostelium
discoideum} amoebae. We observe competition between spontaneously firing
centers and rotating spiral waves that depends strongly on the overall cell
density. At low densities, no complete spirals appear and chemotactic
aggregation is driven by periodic concentric waves, whereas at high densities
the firing centers seen at early times nucleate and are apparently entrained by
spiral waves whose cores ultimately serve as aggregation centers. Possible
mechanisms for these observations are discussed.Comment: 10 pages, RevTeX, 4 ps figures, accepted in PR
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