20,629 research outputs found
Analysis of DVB-H network coverage with the application of transmit diversity
This paper investigates the effects of the Cyclic Delay Diversity (CDD) transmit diversity scheme on DVB-H networks. Transmit diversity improves reception and Quality of Service (QoS) in areas of poor coverage such as sparsely populated or obscured locations. The technique not only povides robust reception in mobile environments thus improving QoS, but it also reduces network costs in terms of the transmit power, number of
infrastructure elements, antenna height and the frequency reuse factor over indoor and outdoor environments. In this paper, the benefit and effectiveness of CDD transmit diversity is tackled
through simulation results for comparison in several scenarios of coverage in DVB-H networks. The channel model used in the simulations is based on COST207 and a basic radio planning
technique is used to illustrate the main principles developed in this paper. The work reported in this paper was supported by
the European Commission IST project—PLUTO (Physical Layer DVB Transmission Optimization)
Electromagnetic duality and light-front coordinates
We review the light-front Hamiltonian approach for the Abelian gauge theory
in 3+1 dimensions, and then study electromagnetic duality in this framework.Comment: 18 pages, LaTeX, 2 references and a typo in an eqn. (19) corrected,
minor revisions in response to referee's repor
Quantum and classical criticality in a dimerized quantum antiferromagnet
A quantum critical point (QCP) is a singularity in the phase diagram arising
due to quantum mechanical fluctuations. The exotic properties of some of the
most enigmatic physical systems, including unconventional metals and
superconductors, quantum magnets, and ultracold atomic condensates, have been
related to the importance of the critical quantum and thermal fluctuations near
such a point. However, direct and continuous control of these fluctuations has
been difficult to realize, and complete thermodynamic and spectroscopic
information is required to disentangle the effects of quantum and classical
physics around a QCP. Here we achieve this control in a high-pressure,
high-resolution neutron scattering experiment on the quantum dimer material
TlCuCl3. By measuring the magnetic excitation spectrum across the entire
quantum critical phase diagram, we illustrate the similarities between quantum
and thermal melting of magnetic order. We prove the critical nature of the
unconventional longitudinal ("Higgs") mode of the ordered phase by damping it
thermally. We demonstrate the development of two types of criticality, quantum
and classical, and use their static and dynamic scaling properties to conclude
that quantum and thermal fluctuations can behave largely independently near a
QCP.Comment: 6 pages, 4 figures. Original version, published version available
from Nature Physics websit
Ripple modulated electronic structure of a 3D topological insulator
3D topological insulators, similar to the Dirac material graphene, host
linearly dispersing states with unique properties and a strong potential for
applications. A key, missing element in realizing some of the more exotic
states in topological insulators is the ability to manipulate local electronic
properties. Analogy with graphene suggests a possible avenue via a topographic
route by the formation of superlattice structures such as a moir\'e patterns or
ripples, which can induce controlled potential variations. However, while the
charge and lattice degrees of freedom are intimately coupled in graphene, it is
not clear a priori how a physical buckling or ripples might influence the
electronic structure of topological insulators. Here we use Fourier transform
scanning tunneling spectroscopy to determine the effects of a one-dimensional
periodic buckling on the electronic properties of Bi2Te3. By tracking the
spatial variations of the scattering vector of the interference patterns as
well as features associated with bulk density of states, we show that the
buckling creates a periodic potential modulation, which in turn modulates the
surface and the bulk states. The strong correlation between the topographic
ripples and electronic structure indicates that while doping alone is
insufficient to create predetermined potential landscapes, creating ripples
provides a path to controlling the potential seen by the Dirac electrons on a
local scale. Such rippled features may be engineered by strain in thin films
and may find use in future applications of topological insulators.Comment: Nature Communications (accepted
Quantification of vaporized targeted nanodroplets using high-frame-rate ultrasound and optics
Owing to their ability to efficiently deliver biological cargo and sense the intracellular milieu, vertical arrays of high aspect ratio nanostructures, known as nanoneedles,are being developed as minimally invasive tools for cell manipulation. However, little is known of the mechanisms of cargo transfer across the cell membrane-nanoneedle interface. Particularly,the contributions of membrane piercing, modulation of membrane permeability and endocytosis to cargo transfer remain largelyunexplored. Here, combining state-of-the-art electron and scanning ion conductance microscopy with molecular biology techniques, we show that porous silicon nanoneedle arrays concurrently stimulate independent endocytic pathways which contribute to enhanced biomolecule delivery into human mesenchymal stem cells. Electron microscopy of the cell membrane at nanoneedle sites shows an intact lipid bilayer, accompanied by an accumulation of clathrin-coated pits and caveolae. Nanoneedles enhance the internalisation of biomolecular markers of endocytosis, highlighting the concurrent activation of caveolae-and clathrin-mediated endocytosis, alongside macropinocytosis. These events contribute to the nanoneedle-mediated delivery (nanoinjection) of nucleic acids into human stem cells, which distribute across the cytosol and the endolysosomal system. This data extends the understanding of how nanoneedles modulate biological processes to mediate interaction with the intracellular space, providing indications for the rational design of improved cell-manipulation technologies
Bulk high-Tc superconductors with drilled holes: how to arrange the holes to maximize the trapped magnetic flux ?
Drilling holes in a bulk high-Tc superconductor enhances the oxygen annealing
and the heat exchange with the cooling liquid. However, drilling holes also
reduces the amount of magnetic flux that can be trapped in the sample. In this
paper, we use the Bean model to study the magnetization and the current line
distribution in drilled samples, as a function of the hole positions. A single
hole perturbs the critical current flow over an extended region that is bounded
by a discontinuity line, where the direction of the current density changes
abruptly. We demonstrate that the trapped magnetic flux is maximized if the
center of each hole is positioned on one of the discontinuity lines produced by
the neighbouring holes. For a cylindrical sample, we construct a polar
triangular hole pattern that exploits this principle; in such a lattice, the
trapped field is ~20% higher than in a squared lattice, for which the holes do
not lie on discontinuity lines. This result indicates that one can
simultaneously enhance the oxygen annealing, the heat transfer, and maximize
the trapped field
A Deep Dive into Adversarial Robustness in Zero-Shot Learning
Machine learning (ML) systems have introduced significant advances in various
fields, due to the introduction of highly complex models. Despite their
success, it has been shown multiple times that machine learning models are
prone to imperceptible perturbations that can severely degrade their accuracy.
So far, existing studies have primarily focused on models where supervision
across all classes were available. In constrast, Zero-shot Learning (ZSL) and
Generalized Zero-shot Learning (GZSL) tasks inherently lack supervision across
all classes. In this paper, we present a study aimed on evaluating the
adversarial robustness of ZSL and GZSL models. We leverage the well-established
label embedding model and subject it to a set of established adversarial
attacks and defenses across multiple datasets. In addition to creating possibly
the first benchmark on adversarial robustness of ZSL models, we also present
analyses on important points that require attention for better interpretation
of ZSL robustness results. We hope these points, along with the benchmark, will
help researchers establish a better understanding what challenges lie ahead and
help guide their work.Comment: To appear in ECCV 2020, Workshop on Adversarial Robustness in the
Real Worl
Gate-controlled Guiding of Electrons in Graphene
Ballistic semiconductor structures have allowed the realization of
optics-like phenomena in electronics, including magnetic focusing and lensing.
An extension that appears unique to graphene is to use both n and p carrier
types to create electronic analogs of optical devices having both positive and
negative indices of refraction. Here, we use gate-controlled density with both
p and n carrier types to demonstrate the analog of the fiber-optic guiding in
graphene. Two basic effects are investigated: (1) bipolar p-n junction guiding,
based on the principle of angle-selective transmission though the graphene p-n
interface, and (2) unipolar fiber-optic guiding, using total internal
reflection controlled by carrier density. Modulation of guiding efficiency
through gating is demonstrated and compared to numerical simulations, which
indicates that interface roughness limits guiding performance, with
few-nanometer effective roughness extracted. The development of p-n and
fiber-optic guiding in graphene may lead to electrically reconfigurable wiring
in high-mobility devices.Comment: supplementary materal at
http://marcuslab.harvard.edu/papers/OG_SI.pd
Absence of magnetic field effect on static magnetic order in electron-doped superconductor Nd_{1.86}Ce_{0.14}CuO_4
Neutron-scattering experiments were performed to study the magnetic field
effect on the electron-doped cuprate superconductor Nd_{1.86}Ce_{0.14}CuO_4,
which shows the coexistence of magnetic order and superconductivity. The (1/2
3/2 0) magnetic Bragg intensity, which mainly originates from the order of both
the Cu and Nd moments at low temperatures, shows no magnetic field dependence
when the field is applied perpendicular to the CuO_{2} plane up to 10 T above
the upper critical field. This result is significantly different from that
reported for the hole-doped cuprate superconductors, in which the quasi-static
magnetic order is noticeably enhanced under a magnetic field.Comment: 4 pages, 3 figure
The Quantized Hall Insulator: A New Insulator in Two-Dimensions
Quite generally, an insulator is theoretically defined by a vanishing
conductivity tensor at the absolute zero of temperature. In classical
insulators, such as band insulators, vanishing conductivities lead to diverging
resistivities. In other insulators, in particular when a high magnetic field
(B) is added, it is possible that while the magneto-resistance diverges, the
Hall resistance remains finite, which is known as a Hall insulator. In this
letter we demonstrate experimentally the existence of another, more exotic,
insulator. This insulator, which terminates the quantum Hall effect series in a
two-dimensional electron system, is characterized by a Hall resistance which is
approximately quantized in the quantum unit of resistance h/e^2. This insulator
is termed a quantized Hall insulator. In addition we show that for the same
sample, the insulating state preceding the QHE series, at low-B, is of the HI
kind.Comment: 4 page
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