135,394 research outputs found
Transport in graphene antidot barriers and tunneling devices
Periodic arrays of antidots, i.e. nanoscale perforations, in graphene enable
tight confinement of carriers and efficient transport barriers. Such barriers
evade the Klein tunneling mechanism by being of the mass rather than
electrostatic type. While all graphene antidot lattices (GALs) may support
directional barriers, we show, however, that a full transport gap exists only
for certain orientations of the GAL. Moreover, we assess the applicability of
gapped graphene and the Dirac continuum approach as simplified models of
various antidot structures showing that, in particular, the former is an
excellent approximation for transport in GALs supporting a bulk band gap.
Finally, the transport properties of a GAL based resonant tunneling diode is
analyzed indicating that such advanced graphene based devices may, indeed, be
realized using GAL structures.Comment: 12 pages, 9 figures, accepted for publication on Journal of Applied
Physic
Tight-binding study of the magneto-optical properties of gapped graphene
We study the optical properties of gapped graphene in presence of a magnetic
field. We consider a model based on the Dirac equation, with a gap introduced
via a mass term, for which analytical expressions for the diagonal and Hall
optical conductivities can be derived. We discuss the effect of the mass term
on electron-hole symmetry and - symmetry and its implications for
the optical Hall conductivity. We compare these results with those obtained
using a tight-binding model, in which the mass is modeled via a staggered
potential and a magnetic field is included via a Peierls substitution.
Considering antidot lattices as the source of the mass term, we focus on the
limit where the mass term dominates the cyclotron energy. We find that a large
gap quenches the effect of the magnetic field. The role of overlap between
neighboring orbitals is investigated, and we find that the overlap has
pronounced consequences for the optical Hall conductivity that are missed in
the Dirac model.Comment: 10 pages, 9 figures, submitted for Physical Review
Development of a low-cost automated sample presentation and analysis system for counting and classifying nematode eggs : a thesis presented in partial fulfilment of the requirements for the degree of Master of Engineering in Mechatronics at Massey University, Manawatu, New Zealand
This thesis discusses the concept development and design of a low-cost, automated, sample presentation system for faecal egg counting, and classification. The system developed uses microfluidics to present nematode eggs for digital imaging to produce images suitable for image analysis and classification. The system costs are kept low by using simple manufacturing methods and commonly available equipment to produce microfluidic counting chambers, which can be interfaced with conventional microscopes. This thesis includes details of the design and implementation of the software developed to allow capture and processing of images from the presentation system. This thesis also includes details on the measures taken to correct for the optical aberrations introduced by the sample presentation system
Modelling of Path Arrival Rate for In-Room Radio Channels with Directive Antennas
We analyze the path arrival rate for an inroom radio channel with directive
antennas. The impulse response of this channel exhibits a transition from early
separate components followed by a diffuse reverberation tail. Under the
assumption that the transmitter's (or receiver's) position and orientation are
picked uniformly at random we derive an exact expression of the mean arrival
rate for a rectangular room predicted by the mirror source theory. The rate is
quadratic in delay, inversely proportional to the room volume, and proportional
to the product of beam coverage fractions of the transmitter and receiver
antennas. Making use of the exact formula, we characterize the onset of the
diffuse tail by defining a "mixing time" as the point in time where the arrival
rate exceeds one component per transmit pulse duration. We also give an
approximation for the power-delay spectrum. It turns out that the power-delay
spectrum is unaffected by the antenna directivity. However, Monte Carlo
simulations show that antenna directivity does indeed play an important role
for the distribution of instantaneous mean delay and rms delay spreadComment: Submitted to IEEE Trans. Antennas and Propagatio
Stochastic Multipath Model for the In-Room Radio Channel based on Room Electromagnetics
We propose a stochastic multipath model for the received signal for the case
where the transmitter and receiver, both with directive antennas, are situated
in the same rectangular room. This scenario is known to produce channel impulse
responses with a gradual specular-to-diffused transition in delay. Mirror
source theory predicts the arrival rate to be quadratic in delay, inversely
proportional to room volume and proportional to the product of the antenna beam
coverage fractions. We approximate the mirror source positions by a homogeneous
spatial Poisson point process and their gain as complex random variables with
the same second moment. The multipath delays in the resulting model form an
inhomogeneous Poisson point process which enables derivation of the
characteristic functional, power/kurtosis delay spectra, and the distribution
of order statistics of the arrival delays in closed form. We find that the
proposed model matches the mirror source model well in terms of power delay
spectrum, kurtosis delay spectrum, order statistics, and prediction of mean
delay and rms delay spread. The constant rate model, assumed in e.g. the
Saleh-Valenzuela model, is unable to reproduce the same effects.Comment: 14 pages, Manuscript Submitted to IEEE Transaction on Antennas and
Propagatio
- β¦