1,329 research outputs found
Transmission Studies of Left-handed Materials
Left-handed materials are studied numerically using an improved version of
the transfer-matrix method. The transmission, reflection, the phase of the
reflection and the absorption are calculated and compared with experiments for
both single split-ring resonators (SRR) with negative permeability and
left-handed materials (LHMs) which have both the permittivity and permeability
negative. Our results suggest ways of positively identifying materials that
have both permittivity and permeability negative, from materials that have
either permeability or permittivity negative
Transmission Losses in Left-handed Materials
We numerically analyze the origin of the transmission losses in left-handed
structures. Our data confirms that left handed structures can have very good
transmission properties, in spite of the expectable dispersion of their
effective permeability and refraction index. The large permittivity of the
metallic components improves the transmission. High losses, observed in recent
experiments, could be explained by the absorption of the dielectric board
Absorption losses in periodic arrays of thin metallic wires
We analyze the transmission and reflection of the electromagnetic wave
calculated from transfer matrix simulations of periodic arrangements of thin
metallic wires. The effective permittivity and the absorption is determined.
Their dependence on the wire thickness and the conductance of the metallic
wires is studied. The cutoff frequency or effective plasma frequency is
obtained and is compared with analytical predictions. It is shown that the
periodic arrangement of wires exhibits a frequency region in which the real
part of the permittivity is negative while its imaginary part is very small.
This behavior is seen for wires with thickness as small as 17 m with a
lattice constant of 3.33 mm
Disorder-driven splitting of the conductance peak at the Dirac point in graphene
The electronic properties of a bricklayer model, which shares the same
topology as the hexagonal lattice of graphene, are investigated numerically. We
study the influence of random magnetic-field disorder in addition to a strong
perpendicular magnetic field. We found a disorder-driven splitting of the
longitudinal conductance peak within the narrow lowest Landau band near the
Dirac point. The energy splitting follows a relation which is proportional to
the square root of the magnetic field and linear in the disorder strength. We
calculate the scale invariant peaks of the two-terminal conductance and obtain
the critical exponents as well as the multifractal properties of the chiral and
quantum Hall states. We found approximate values for the
quantum Hall states, but for the divergence of the
correlation length of the chiral state at E=0 in the presence of a strong
magnetic field. Within the central Landau band, the multifractal
properties of both the chiral and the split quantum Hall states are the same,
showing a parabolic distribution with .
In the absence of the constant magnetic field, the chiral critical state is
determined by
Time-reversal in dynamically-tuned zero-gap periodic systems
We show that short pulses propagating in zero-gap periodic systems can be
reversed with 100% efficiency by using weak non-adiabatic tuning of the wave
velocity at time-scales that can be much slower than the period. Unlike
previous schemes, we demonstrate reversal of {\em broadband} (few cycle) pulses
with simple structures. Our scheme may thus open the way to time-reversal in a
variety of systems for which it was not accessible before.Comment: Accepted for publication in Phys. Rev. Letter
Intensity distribution of scalar waves propagating in random media
Transmission of the scalar field through the random medium, represented by
the system of randomly distributed dielectric cylinders is calculated
numerically. System is mapped to the problem of electronic transport in
disordered two-dimensional systems. Universality of the statistical
distribution of transmission parameters is analyzed in the metallic and in the
localized regimes.In the metallic regime the universality of the transmission
statistics in all transparent channels is observed. In the band gaps, we
distinguish the disorder induced (Anderson) localization from the tunneling
through the system due to the gap in the density of states. We show also that
absorption causes rapid decrease of the mean conductance, but, contrary to the
localized regime, the conductance is self-averaged with a
Gaussian distribution
Impact of the inherent periodic structure on the effective medium description of left-handed and related meta-materials
We study the frequency dependence of the effective electromagnetic parameters
of left-handed and related meta-materials of the split ring resonator and wire
type. We show that the reduced translational symmetry (periodic structure)
inherent to these meta-materials influences their effective electromagnetic
response. To anticipate this periodicity, we formulate a periodic effective
medium model which enables us to distinguish the resonant behavior of
electromagnetic parameters from effects of the periodicity of the structure. We
use this model for the analysis of numerical data for the transmission and
reflection of periodic arrays of split ring resonators, thin metallic wires,
cut wires as well as the left-handed structures. The present method enables us
to identify the origin of the previously observed resonance/anti-resonance
coupling as well as the occurrence of negative imaginary parts in the effective
permittivities and permeabilities of those materials. Our analysis shows that
the periodicity of the structure can be neglected only for the wavelength of
the electromagnetic wave larger than 30 space periods of the investigated
structure.Comment: 23 pages, 14 figure
Determination of Effective Permittivity and Permeability of Metamaterials from Reflection and Transmission Coefficients
We analyze the reflection and transmission coefficients calculated from
transfer matrix simulations on finite lenghts of electromagnetic metamaterials,
to determine the effective permittivity and permeability. We perform this
analysis on structures composed of periodic arrangements of wires, split ring
resonators (SRRs) and both wires and SRRs. We find the recovered
frequency-dependent permittivity and permeability are entirely consistent with
analytic expressions predicted by effective medium arguments. Of particular
relevance are that a wire medium exhibits a frequency region in which the real
part of permittivity is negative, and SRRs produce a frequency region in which
the real part of permeability is negative. In the combination structure, at
frequencies where both the recovered real part of permittivity and permeability
are simultaneously negative, the real part of the index-of-refraction is found
also to be unambigously negative.Comment: *.pdf file, 5 figure
Resonant and anti-resonant frequency dependence of the effective parameters of metamaterials
We present a numerical study of the electromagnetic response of the
metamaterial elements that are usedto construct materials with negative
refractive index. For an array of split ring resonators (SRR) we find that the
resonant behavior of the effective magnetic permeability is accompanied by an
anti-resonant behavior of the effective permittivity. In addition, the
imaginary parts of the effective permittivity and permeability are opposite in
sign. We also observe an identical resonant versus anti-resonant frequency
dependence of the effective materials parameters for a periodic array of thin
metallic wires with cuts placed periodically along the length of the wire, with
roles of the permittivity and permeability reversed from the SRR case. We show
in a simple manner that the finite unit cell size is responsible for the
anti-resonant behavior
Universal Conductance and Conductivity at Critical Points in Integer Quantum Hall Systems
The sample averaged longitudinal two-terminal conductance and the respective
Kubo-conductivity are calculated at quantum critical points in the integer
quantum Hall regime. In the limit of large system size, both transport
quantities are found to be the same within numerical uncertainty in the lowest
Landau band, and , respectively. In
the 2nd lowest Landau band, a critical conductance is
obtained which indeed supports the notion of universality. However, these
numbers are significantly at variance with the hitherto commonly believed value
. We argue that this difference is due to the multifractal structure
of critical wavefunctions, a property that should generically show up in the
conductance at quantum critical points.Comment: 4 pages, 3 figure
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