4,140 research outputs found
Friedel oscillations due to Fermi arcs in Weyl semimetals
Weyl semimetals harbor unusual surface states known as Fermi arcs, which are
essentially disjoint segments of a two dimensional Fermi surface. We describe a
prescription for obtaining Fermi arcs of arbitrary shape and connectivity by
stacking alternate two dimensional electron and hole Fermi surfaces and adding
suitable interlayer coupling. Using this prescription, we compute the local
density of states -- a quantity directly relevant to scanning tunneling
microscopy -- on a Weyl semimetal surface in the presence of a point scatterer
and present results for a particular model that is expected to apply to
pyrochlore iridate Weyl semimetals. For thin samples, Fermi arcs on opposite
surfaces conspire to allow nested backscattering, resulting in strong Friedel
oscillations on the surface. These oscillations die out as the sample thickness
is increased and Fermi arcs from the bottom surface retreat and weak
oscillations, due to scattering between the top surface Fermi arcs alone,
survive. The surface spectral function -- accessible to photoemission
experiments -- is also computed. In the thermodynamic limit, this calculation
can be done analytically and separate contributions from the Fermi arcs and the
bulk states can be seen.Comment: 5 pages, 2 figures; minor changes in figures and text, typos
correcte
Noise radar technology as an interference prevention method
In some applications, such as automotive and marine/navigation, hundreds of radars may operate in a small environment (e.g., a road complex or a strait) and in an allocated frequency band with limited width. Therefore, a compatibility problem between different radars arises that is not easily solved by time, frequency, space, or polarization diversity. The advent of fast digital signal processing and signal generation techniques makes it possible to use waveform diversity to solve this problem that will be exacerbated in the next future. Ideal waveforms for the diversity are supplied by Noise Radar Technology (NRT), whose application is promising in some military applications as well as in the civilian applications considered in this paper. In addition to being orthogonal as much as possible, the random signals to be transmitted have to satisfy requirements concerning side lobe level and crest factor, calling for novel, original design and generation processes
The strange quark condensate in the nucleon in 2+1 flavor QCD
We calculate the "strange quark content of the nucleon", ,
which is important for interpreting the results of some dark matter detection
experiments. The method is to evaluate quark-line disconnected correlations on
the MILC lattice ensembles, which include the effects of dynamical strange
quarks. After continuum and chiral extrapolations, the result is <N |s s_bar
|N> = 0.69 +- 0.07(statistical) +- 0.09(systematic), in the modified minimal
subtraction scheme (2 GeV), or for the renormalization scheme invariant form,
m_s partial{M_N}/partial{m_s} = 59(6)(8) MeV.Comment: Added figures and references, especially for fit range choice. Other
changes for clarity. Version to appear in publicatio
Stochastic simulation techniques as related to innovation in communications-navigation-surveillance and air traffic management (CNS/ATM)
The design and operational tuning of the instruments and procedures employed in communications-navigation-surveillance (CNS) and air traffic management (ATM) often relies on stochastic simulation techniques. In this paper the application areas of simulation in the CNS/ATM context are reviewed together with the simulation methods that can help solve the main problems encountered, i.e. quick simulation techniques for the simulation of rare events, and the bootstrap technique for the evaluation of the accuracy of the results
Fundamentals of Heterogeneous Cellular Networks with Energy Harvesting
We develop a new tractable model for K-tier heterogeneous cellular networks
(HetNets), where each base station (BS) is powered solely by a self-contained
energy harvesting module. The BSs across tiers differ in terms of the energy
harvesting rate, energy storage capacity, transmit power and deployment
density. Since a BS may not always have enough energy, it may need to be kept
OFF and allowed to recharge while nearby users are served by neighboring BSs
that are ON. We show that the fraction of time a k^{th} tier BS can be kept ON,
termed availability \rho_k, is a fundamental metric of interest. Using tools
from random walk theory, fixed point analysis and stochastic geometry, we
characterize the set of K-tuples (\rho_1, \rho_2, ... \rho_K), termed the
availability region, that is achievable by general uncoordinated operational
strategies, where the decision to toggle the current ON/OFF state of a BS is
taken independently of the other BSs. If the availability vector corresponding
to the optimal system performance, e.g., in terms of rate, lies in this
availability region, there is no performance loss due to the presence of
unreliable energy sources. As a part of our analysis, we model the temporal
dynamics of the energy level at each BS as a birth-death process, derive the
energy utilization rate, and use hitting/stopping time analysis to prove that
there exists a fundamental limit on \rho_k that cannot be surpassed by any
uncoordinated strategy.Comment: submitted to IEEE Transactions on Wireless Communications, July 201
When Plasmonic Colloids Meet Optical Vortices -- A Brief Review
Structured light has emerged as an important tool to interrogate and
manipulate matter at micron and sub-micron scale. One form of structured light
is an optical vortex beam. The helical wavefront of these vortices carry
orbital angular momentum which can be transferred to a Brownian colloid. When
the colloid is made of metallic nanostructures, such as silver and gold,
resonant optical effects play a vital role, and the interaction leads to
complex dynamics and assembly. This brief review aims to discuss some recent
work on trapping plasmonic colloids with optical vortices and their lattices.
The role of optical scattering and absorption has important implications on the
underlying forces and torques, which is specifically enunciated. The effect of
spin and orbital angular momentum in an optical vortex can lead to spin-orbit
coupling dynamics, and these effects are highlighted with examples from the
literature. In addition to assembly and dynamics, enhanced Brownian motion of
plasmonic colloids under the influence of a vortex-lattice is discussed. The
pedagogical aspects to understand the interaction between optical vortex and
plasmonic colloids is emphasized.Comment: Comments welcome. Invited review article (150 references). Submitted
to Indian Journal of Physics and Applied Physics (IJPAP) - special issue on
nanophotonic
On the determination of constitutive parametersin a hyperelastic model for a soft tissue
The aim of this paper is to study a model of hyperelastic materials and itsapplications into soft tissue mechanics. In particular, we first determine an unbounded domain of the constitutive parameters of the model making our smoothstrain energy function to be polyconvex and hence satisfying the Legendre–Hadamard condition. Thus, physically reasonable material behaviour are described by our model with these parameters and a plently of tissues can betreated. Furthermore, we localize bounded subsets of constitutive parameters in fixed physical and very general bounds and then introduce a family of descrete stress–strain curves. Whence, various classes of tissues are characterized. Ourgeneral approach is based on a detailed analytical study of the first Piola–Kirchhoff stress tensor through its dependence on the invariants and on the constitutive parameters. The uniqueness of parameters for one tissue is discussed by introducing the notion of manifold of constitutive parameters, whichis locally represented by possibly different physical quantities. The advantage of our study is that we show a possible way to improve of the usual approachesshown in the literature which are mainly based on the minimization of a costfunction as the difference between experimental and model results
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