6,513 research outputs found
Spectral properties of microwave graphs with local absorption
The influence of absorption on the spectra of microwave graphs has been
studied experimentally. The microwave networks were made up of coaxial cables
and T junctions. First, absorption was introduced by attaching a 50 Ohm load to
an additional vertex for graphs with and without time-reversal symmetry. The
resulting level-spacing distributions were compared with a generalization of
the Wigner surmise in the presence of open channels proposed recently by Poli
et al. [Phys. Rev. Lett. 108, 174101 (2012)]. Good agreement was found using an
effective coupling parameter. Second, absorption was introduced along one
individual bond via a variable microwave attenuator, and the influence of
absorption on the length spectrum was studied. The peak heights in the length
spectra corresponding to orbits avoiding the absorber were found to be
independent of the attenuation, whereas, the heights of the peaks belonging to
orbits passing the absorber once or twice showed the expected decrease with
increasing attenuation.Comment: 7 pages, 7 figure
Experimental Observation of a Fundamental Length Scale of Waves in Random Media
Waves propagating through a weakly scattering random medium show a pronounced
branching of the flow accompanied by the formation of freak waves, i.e.,
extremely intense waves. Theory predicts that this strong fluctuation regime is
accompanied by its own fundamental length scale of transport in random media,
parametrically different from the mean free path or the localization length. We
show numerically how the scintillation index can be used to assess the scaling
behavior of the branching length. We report the experimental observation of
this scaling using microwave transport experiments in quasi-two-dimensional
resonators with randomly distributed weak scatterers. Remarkably, the scaling
range extends much further than expected from random caustics statistics.Comment: 5 pages, 5 figure
Experimental Observation of the Spectral Gap in Microwave n-Disk Systems
Symmetry reduced three-disk and five-disk systems are studied in a microwave
setup. Using harmonic inversion the distribution of the imaginary parts of the
resonances is determined. With increasing opening of the systems, a spectral
gap is observed for thick as well as for thin repellers and for the latter case
it is compared with the known topological pressure bounds. The maxima of the
distributions are found to coincide for a large range of the distance to radius
parameter with half of the classical escape rate. This confirms theoretical
predictions based on rigorous mathematical analysis for the spectral gap and on
numerical experiments for the maxima of the distributions.Comment: 5 pages, 4 figure
Disordered graphene and boron nitride in a microwave tight-binding analog
Experiments on hexagonal graphene-like structures using microwave measuring
techniques are presented. The lowest transverse-electric resonance of coupled
dielectric disks sandwiched between two metallic plates establishes a
tight-binding configuration. The nearest-neighbor coupling approximation is
investigated in systems with few disks. Taking advantage of the high
flexibility of the disks positions, consequences of the disorder introduced in
the graphene lattice on the Dirac points are investigated. Using two different
types of disks, a boron-nitride-like structure (a hexagonal lattice with a
two-atom basis) is implemented, showing the appearance of a band gap.Comment: 12 pages, 14 figure
First experimental realization of the Dirac oscillator
We present the first experimental microwave realization of the
one-dimensional Dirac oscillator, a paradigm in exactly solvable relativistic
systems. The experiment relies on a relation of the Dirac oscillator to a
corresponding tight-binding system. This tight-binding system is implemented as
a microwave system by a chain of coupled dielectric disks, where the coupling
is evanescent and can be adjusted appropriately. The resonances of the finite
microwave system yields the spectrum of the one-dimensional Dirac oscillator
with and without mass term. The flexibility of the experimental set-up allows
the implementation of other one-dimensional Dirac type equations.Comment: 6 figures, 5 page
On the theory of cavities with point-like perturbations. Part II: Rectangular cavities
We consider an application of a general theory for cavities with point-like
perturbations for a rectangular shape. Hereby we concentrate on experimental
wave patterns obtained for nearly degenerate states. The nodal lines in these
patterns may be broken, which is an effect coming only from the experimental
determination of the patterns. These findings are explained within a framework
of the developed theory.Comment: 14 pages, 3 figure
Dynamical tunneling in mushroom billiards
We study the fundamental question of dynamical tunneling in generic
two-dimensional Hamiltonian systems by considering regular-to-chaotic tunneling
rates. Experimentally, we use microwave spectra to investigate a mushroom
billiard with adjustable foot height. Numerically, we obtain tunneling rates
from high precision eigenvalues using the improved method of particular
solutions. Analytically, a prediction is given by extending an approach using a
fictitious integrable system to billiards. In contrast to previous approaches
for billiards, we find agreement with experimental and numerical data without
any free parameter.Comment: 4 pages, 4 figure
Nodal domains in open microwave systems
Nodal domains are studied both for real and imaginary part
of the wavefunctions of an open microwave cavity and found to show the same
behavior as wavefunctions in closed billiards. In addition we investigate the
variation of the number of nodal domains and the signed area correlation by
changing the global phase according to
. This variation can be
qualitatively, and the correlation quantitatively explained in terms of the
phase rigidity characterising the openness of the billiard.Comment: 7 pages, 10 figures, submitted to PR
Dirac Point and Edge States in a Microwave Realization of Tight-Binding Graphene-like Structures
We present a microwave realization of finite tight-binding graphene-like
structures. The structures are realized using discs with a high index of
refraction. The discs are placed on a metallic surface while a second surface
is adjusted atop the discs, such that the waves coupling the discs in the air
are evanescent, leading to the tight-binding behavior. In reflection
measurements the Dirac point and a linear increase close to the Dirac point is
observed, if the measurement is performed inside the sample. Resonances due to
edge states are found close to the Dirac point if the measurements are
performed at the zigzag-edge or at the corner in case of a broken benzene ring.Comment: 4 pages, 6 figure
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