40,090 research outputs found
Klein tunneling through an oblique barrier in graphene ribbons
We study a transmission coefficient of graphene nanoribbons with a top gate
which acts as an oblique barrier. Using a Green function method based on the
Dirac-like equation, scattering among transverse modes due to the oblique
barrier is taken into account numerically. In contrast to the 2-dimensional
graphene sheet, we find that the pattern of transmission in graphene ribbons
depends strongly on the electronic structure in the region of the barrier.
Consequently, irregular structures in the transmission coefficient are
predicted while perfect transmission is still calculated in the case of
metallic graphene independently of angle and length of the oblique barrier
Systems analysis of the space shuttle
Developments in communications systems, computer systems, and power distribution systems for the space shuttle are described. The use of high speed delta modulation for bit rate compression in the transmission of television signals is discussed. Simultaneous Multiprocessor Organization, an approach to computer organization, is presented. Methods of computer simulation and automatic malfunction detection for the shuttle power distribution system are also described
Higgs bosons of a supersymmetric model at the ILC
We study the scalar Higgs sector of the next-to-minimal supersymmetric
standard model with an extra U(1), which has two Higgs doublets and a Higgs
singlet, in the light leptophobic scenario where the extra neutral gauge
boson does not couple to charged leptons. In this model, we find that the
sum of the squared coupling coefficients of the three neutral scalar Higgs
bosons to , normalized by the corresponding SM coupling coefficient is
noticeably smaller than unity, due to the effect of the extra U(1), for a
reasonable parameter space of the model, whereas it is unity in the
next-to-minimal supersymmetric standard model. Thus, these two models may be
distinguished if the coupling coefficients of neutral scalar Higgs bosons to
are measured at the future International Linear Collider by producing them
via the Higgs-strahlung, fusion, and fusion processes.Comment: 12 pages, 2 figures, 1 table, PR
Ultrafast spectroscopy of propagating coherent acoustic phonons in GaN/InGaN heterostructures
We show that large amplitude, coherent acoustic phonon wavepackets can be
generated and detected in InGaN/GaN epilayers and heterostructures
in femtosecond pump-probe differential reflectivity experiments. The amplitude
of the coherent phonon increases with increasing Indium fraction and unlike
other coherent phonon oscillations, both \textit{amplitude} and \textit{period}
are strong functions of the laser probe energy. The amplitude of the
oscillation is substantially and almost instantaneously reduced when the
wavepacket reaches a GaN-sapphire interface below the surface indicating that
the phonon wavepackets are useful for imaging below the surface. A theoretical
model is proposed which fits the experiments well and helps to deduce the
strength of the phonon wavepackets. Our model shows that localized coherent
phonon wavepackets are generated by the femtosecond pump laser in the epilayer
near the surface. The wavepackets then propagate through a GaN layer changing
the local index of refraction, primarily through the Franz-Keldysh effect, and
as a result, modulate the reflectivity of the probe beam. Our model correctly
predicts the experimental dependence on probe-wavelength as well as epilayer
thickness.Comment: 11 pages, 14 figure
Testing tripartite Mermin inequalities by spectral joint-measurements of qubits
It is well known that Bell inequality supporting the local realism can be
violated in quantum mechanics. Numerous tests of such a violation have been
demonstrated with bipartite entanglements. Using spectral jointmeasurements of
the qubits, we here propose a scheme to test the tripartite Mermin inequality
(a three-qubit Bell-type inequality) with three qubits dispersively-coupled to
a driven cavity. First, we show how to generate a three-qubit
Greenberger-Horne-Zeilinger (GHZ) state by only one-step quantum operation.
Then, spectral joint-measurements are introduced to directly confirm such a
tripartite entanglement. Assisted by a series of single-qubit operations, these
measurements are further utilized to test the Mermin inequality. The
feasibility of the proposal is robustly demonstrated by the present numerical
experiments.Comment: 7pages,3figure
HeII Recombination Lines From the First Luminous Objects
The hardness of the ionizing continuum from the first sources of UV radiation
plays a crucial role in the reionization of the intergalactic medium (IGM).
While usual stellar populations have soft spectra, mini-quasars or metal-free
stars with high effective temperatures may emit hard photons, capable of doubly
ionizing helium and increasing the IGM temperature. Absorption within the
source and in the intervening IGM will render the ionizing continuum of
high-redshift sources inaccessible to direct observation. Here we show that
HeII recombination lines from the first luminous objects are potentially
detectable by the Next Generation Space Telescope. Together with measurements
of the hydrogen Balmer alpha emission line, this detection can be used to infer
the ratio of HeII to HI ionizing photons. A measurement of this ratio would
shed light on the nature and emission mechanism of the first luminous sources,
with important astrophysical consequences for the reheating and reionization of
the IGM.Comment: ApJ published version. Due to an error in one of the references, the
strength of the 1640 A line was underestimated in a previous version; this
line is now brighter by a factor of 1
Real Forms of the Oscillator Quantum Algebra and its Representations
We consider the conditions under which the -oscillator algebra becomes a
Hopf -algebra. In particular, we show that there are at least two real forms
associated with the algebra. Furthermore, through the representations, it is
shown that they are related to with different
conjugations.Comment: 10 pages, Ams-Tex, To be published in Letters in Mathematical physic
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