2,240 research outputs found
Spin texture of generic helical edge states
We study the spin texture of a generic helical liquid, the edge modes of a
two-dimensional topological insulator with broken axial spin-symmetry. By
considering honeycomb and square lattice realizations of topological
insulators, we show that in all cases the generic behavior of a
momentum-dependent rotation of the spin quantization axis is realized. Here we
establish this mechanism also for disk geometries with continuous rotational
symmetry. Finally, we demonstrate that the rotation of spin-quantization axis
remains intact for arbitrary geometries, i.e. in the absence of any continuous
symmetry. We also calculate the dependence of this rotation on the model and
material parameters. Finally we propose a spectroscopy measurement which should
directly reveal the rotation of the spin-quantization axis of the helical edge
states.Comment: 16 pages, 17 figure
Phase field approach to optimal packing problems and related Cheeger clusters
In a fixed domain of we study the asymptotic behaviour of optimal
clusters associated to -Cheeger constants and natural energies like the
sum or maximum: we prove that, as the parameter converges to the
"critical" value , optimal Cheeger clusters
converge to solutions of different packing problems for balls, depending on the
energy under consideration. As well, we propose an efficient phase field
approach based on a multiphase Gamma convergence result of Modica-Mortola type,
in order to compute -Cheeger constants, optimal clusters and, as a
consequence of the asymptotic result, optimal packings. Numerical experiments
are carried over in two and three space dimensions
Plasmonic surface lattice resonances on arrays of different lattice symmetry
Arrays of metallic particles may exhibit optical collective excitations known as surface lattice resonances (SLRs). These SLRs occur near the diffraction edge of the array and can be sharper than the plasmon resonance associated with the isolated single particle response. We have fabricated and modeled arrays of silver nanoparticles of different geometries. We show that square, hexagonal, and honeycomb arrays show similar SLRs; no one geometry shows a clear advantage over the others in terms of resonance linewidth. We investigate the nature of the coupling between the particles by looking at rectangular arrays. Our results combine experiment and modeling based on a simple coupled-dipole model.Royal SocietyLeverhulme Trus
Toward the Jamming Threshold of Sphere Packings: Tunneled Crystals
We have discovered a new family of three-dimensional crystal sphere packings
that are strictly jammed (i.e., mechanically stable) and yet possess an
anomalously low density. This family constitutes an uncountably infinite number
of crystal packings that are subpackings of the densest crystal packings and
are characterized by a high concentration of self-avoiding "tunnels" (chains of
vacancies) that permeate the structures. The fundamental geometric
characteristics of these tunneled crystals command interest in their own right
and are described here in some detail. These include the lattice vectors (that
specify the packing configurations), coordination structure, Voronoi cells, and
density fluctuations. The tunneled crystals are not only candidate structures
for achieving the jamming threshold (lowest-density rigid packing), but may
have substantially broader significance for condensed matter physics and
materials science.Comment: 19 pages, 5 figure
Direct observation of Dirac cones and a flatband in a honeycomb lattice for polaritons
Two-dimensional lattices of coupled micropillars etched in a planar
semiconductor microcavity offer a workbench to engineer the band structure of
polaritons. We report experimental studies of honeycomb lattices where the
polariton low-energy dispersion is analogous to that of electrons in graphene.
Using energy-resolved photoluminescence we directly observe Dirac cones, around
which the dynamics of polaritons is described by the Dirac equation for
massless particles. At higher energies, we observe p orbital bands, one of them
with the nondispersive character of a flatband. The realization of this
structure which holds massless, massive and infinitely massive particles opens
the route towards studies of the interplay of dispersion, interactions, and
frustration in a novel and controlled environment
Artificial graphene as a tunable Dirac material
Artificial honeycomb lattices offer a tunable platform to study massless
Dirac quasiparticles and their topological and correlated phases. Here we
review recent progress in the design and fabrication of such synthetic
structures focusing on nanopatterning of two-dimensional electron gases in
semiconductors, molecule-by-molecule assembly by scanning probe methods, and
optical trapping of ultracold atoms in crystals of light. We also discuss
photonic crystals with Dirac cone dispersion and topologically protected edge
states. We emphasize how the interplay between single-particle band structure
engineering and cooperative effects leads to spectacular manifestations in
tunneling and optical spectroscopies.Comment: Review article, 14 pages, 5 figures, 112 Reference
- âŠ