617 research outputs found
First-principles Simulations of a Graphene Based Field-Effect Transistor
We improvise a novel approach to carry out first-principles simulations of
graphene-based vertical field effect tunneling transistors that consist of a
graphene{\it h}-BNgraphene multilayer structure. Within the density
functional theory framework, we exploit the effective screening medium (ESM)
method to properly treat boundary conditions for electrostatic potentials and
investigate the effect of gate voltage. The distribution of free carriers and
the band structure of both top and bottom graphene layers are calculated
self-consistently. The dielectric properties of {\it h}-BN thin films
sandwiched between graphene layers are computed layer-by-layer following the
theory of microscopic permittivity. We find that the permittivities of BN
layers are very close to that of crystalline {\it h}-BN. The effect of
interface with graphene on the dielectric properties of {\it h}-BN is weak,
according to an analysis on the interface charge redistribution.Comment: 6 pages, 6 figure
Does Silicene on Ag(111) Have a Dirac Cone?
We investigate the currently debated issue of the existence of the Dirac cone
in silicene on an Ag(111) surface, using first-principles calculations based on
density functional theory to obtain the band structure. By unfolding the band
structure in the Brillouin zone of a supercell to that of a primitive cell,
followed by projecting onto Ag and silicene subsystems, we demonstrate that the
Dirac cone in silicene on Ag(111) is destroyed. Our results clearly indicate
that the linear dispersions observed in both angular-resolved photoemission
spectroscopy (ARPES) [P. Vogt et al, Phys. Rev. Lett. 108, 155501 (2012)] and
scanning tunneling spectroscopy (STS) [L. Chen et al, Phys. Rev. Lett. 109,
056804 (2012)] come from the Ag substrate and not from silicene.Comment: 5 pages, 3 figure
Electron Transport Through Ag-Silicene-Ag Junctions
For several years the electronic structure properties of the novel
two-dimensional system silicene have been studied extensively. Electron
transport across metal-silicence junctions, however, remains relatively
unexplored. To address this issue, we developed and implemented a theoretical
framework that utilizes the tight-binding Fisher-Lee relation to span
non-equilibrium Green's function (NEGF) techniques, the scattering method, and
semiclassical Boltzmann transport theory. Within this hybrid quantum-classical,
two-scale framework, we calculated transmission and reflection coefficients of
monolayer and bilayer Ag-silicene-Ag junctions using the NEGF method in
conjunction with density functional theory; derived and calculated the group
velocities; and computed resistance using the semi-classical Boltzmann
equation. We found that resistances of these junctions are {} 0.08 \fom
for monolayer silicene junctions and {} 0.3 \fom for bilayer ones,
factors of 8 and 2, respectively, smaller than Sharvin resistances
estimated via the Landauer formalism.Comment: 5 pages, 4 figure
Prospects for CP & P violation in decay at STCF
CP violation is an excellent tool for probing flavor dynamics as we learnt
first with and later also with the weak decays of beauty
mesons. LHCb 2019 data have shown CP violation for the first time in vs. . Searching for CP asymmetries is of great
interest in charm quark sector in the Standard Model (SM) or even more beyond
it. In charm hadron decays, lots of work had focused on two-body final states,
and the measurements of CP asymmetries in three- or four-body final states are
rare. Dalitz plots have shown an excellent record for three-body final states,
and more results are desired for four-body ones. In this work we study CP
asymmetries in the decays //, where the SM gives zero values for the first
two channels, while for the last one due to oscillation. We performed a fast Monte Carlo simulation study by using
electron-positron annihilation data of 1~ at center-of-mass
energy GeV. The data is expected to be available by the
next generation Super Tau Charm Facility proposed by China and Russia with one
year (or even less) data taking operation. The results indicate that a
sensitivity at the level of 0.20.5% is accessible for these processes,
which would be enough to measure nonzero CP-violating asymmetries as large as
1%.Comment: final version, to appear in PR
Direct Symbol Decoding using GA-SVM in Chaotic Baseband Wireless Communication System
To retrieve the information from the serious distorted received signal is the
key challenge of communication signal processing. The chaotic baseband
communication promises theoretically to eliminate the inter-symbol interference
(ISI), however, it needs complicated calculation, if it is not impossible. In
this paper, a genetic algorithm support vector machine (GA-SVM) based symbol
detection method is proposed for chaotic baseband wireless communication system
(CBWCS), by this way, treating the problem from a different viewpoint, the
symbol decoding process is converted to be a binary classification through
GA-SVM model. A trained GA-SVM model is used to decode the symbols directly at
the receiver, so as to improve the bit error rate (BER) performance of the
CBWCS and simplify the symbol detection process by removing the channel
identification and the threshold calculation process as compared to that using
the calculated threshold to decode symbol in the traditional methods. The
simulation results show that the proposed method has better BER performance in
both the static and time-varying wireless channels. The experimental results,
based on the wireless open-access research platform, indicate that the BER of
the proposed GA-SVM based symbol detection approach is superior to the other
counterparts under a practical wireless multipath channel.Comment: 31 pages, 16 figure
Electronic resistances of multilayered two-dimensional crystal junctions
We carry out a layer-by-layer investigation to understand electron transport
across metal-insulator-metal junctions. Interfacial structures of junctions
were studied and characterized using first-principles density functional theory
within the generalized gradient approximation. We found that as a function of
the number of crystal layers the calculated transmission coefficients of
multilayer silicene junctions decay much slower than for BN-based junctions We
revisited the semiclassical Boltzmann theory of electronic transport and
applied to multilayer silicene and BN-based junctions. The calculated
resistance in the high-transmission regime is smaller than that provided by the
Landauer formula. As the thickness of the barrier increases, results from the
Boltzmann and the Landauer formulae converge. We provide a upper limit in the
transmission coefficient below which, the Landauer method becomes valid.
Quantitatively, when the transmission coefficient is lower than
per channel, the error introduced by the Landauer formula for calculating the
resistance is negligible. In addition, we found that the resistance of a
junction is not entirely determined by the averaged transmission, but also by
the distribution of the transmission over the first Brillouin zone.Comment: 11 pages, 7 figure
Tunneling Field-Effect Junctions with WS barrier
Transition metal dichalcogenides (TMDCs), with their two-dimensional
structures and sizable bandgaps, are good candidates for barrier materials in
tunneling field-effect transistor (TFET) formed from atomic precision vertical
stacks of graphene and insulating crystals of a few atomic layers in thickness.
We report first-principles study of the electronic properties of the
Graphene/WS/Graphene sandwich structure revealing strong interface effects
on dielectric properties and predicting a high ON/OFF ratio with an appropriate
WS thickness and a suitable range of the gate voltage. Both the band
spin-orbit coupling splitting and the dielectric constant of the WS layer
depend on its thickness when in contact with the graphene electrodes,
indicating strong influence from graphene across the interfaces. The dielectric
constant is significantly reduced from the bulk WS value. The effective
barrier height varies with WS thickness and can be tuned by a gate voltage.
These results are critical for future nanoelectronic device designs.Comment: 18 pages, 5 figure
Autocorrelation Invariance Property of Chaos for Wireless Communication
A new feature of the chaotic signal generated by chaotic shape-forming filter
(CSF) is uncovered in this work. We find that, the autocorrelation function
(ACF) of the transmitting signal generated by CSF keeps the same as that of the
base function of CSF, no matter what information is encoded. We derive the
analytical equation to describe the relation between the ACF of the received
signal and the wireless channel parameters using the ACF of the transmitted
signal as prior knowledge revealed by the finding in this work. This new
property can be utilized together with different wireless communication systems
to improve the system performance. Specially, to demonstrate the improvement,
channel state information (CSI) is identified using the chaotic baseband
wireless communication as a paradigm. Two significant benefits by using the new
property are 1) the CSI can be identified without the probe information known
to the receiver as done in the conventional wireless communication systems,
which improves the bandwidth efficiency, especially in the time-varying
channel; 2) the correlation operation is insensitive to the channel noise,
which improves the identification accuracy as compared to the commonly used
methods.Comment: 5 pages,4 figure
Echo State Network based Symbol Detection in Chaotic Baseband Wireless Communication
In some Internet of Things (IoT) applications, multi-path propagation is a
main constraint of communication channel. Recently, the chaotic baseband
wireless communication system (CBWCS) is promising to eliminate the
inter-symbol interference (ISI) caused by multipath propagation. However, the
current technique is only capable of removing the partial effect of ISI, due to
only past decoded bits are available for the suboptimal decoding threshold
calculation. However, the future transmitting bits also contribute to the
threshold. The unavailable future information bits needed by the optimal
decoding threshold are an obstacle to further improve the bit error rate (BER)
performance. Different from the previous method using echo state network (ESN)
to predict one future information bit, the proposed method in this paper
predicts the optimal threshold directly using ESN. The proposed ESN-based
threshold prediction method simplifies the symbol decoding operation by
removing the threshold calculation from the transmitting symbols and channel
information, which achieves better BER performance as compared to the previous
method. The reason for this superior result lies in two folds, first, the
proposed ESN is capable of using more future symbols information conveyed by
the ESN input to get more accurate threshold; second, the proposed method here
does not need to estimate the channel information using Least Square method,
which avoids the extra error caused by inaccurate channel information
estimation. By this way, the calculation complexity is decreased as compared to
the previous method. Simulation results and experiment based on a wireless
open-access research platform under a practical wireless channel, show the
effectiveness and superiority of the proposed method.Comment: 12 pages, 15 figure
DFT+DMFT calculations of the complex band and tunneling behavior for the transition metal monoxides MnO, FeO, CoO and NiO
We report complex band structure (CBS) calculations for the four late
transition metal monoxides, MnO, FeO, CoO and NiO, in their paramagnetic phase.
The CBS is obtained from density functional theory plus dynamical mean field
theory (DMFT) calculations to take into account correlation effects. The
so-called parameters, governing the exponential decay of the
transmission probability in the non-resonant tunneling regime of these oxides,
are extracted from the CBS. Different model constructions are examined in the
DMFT part of the calculation. The calculated parameters provide
theoretical estimation for the decay length in the evanescent channel, which
would be useful for tunnel junction applications of these materials.Comment: 16 pages, 13 figure
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