12,072 research outputs found
First-principles Analysis of Photo-current in Graphene PN Junctions
We report a first principles investigation of photocurrent generation by
graphene PN junctions. The junctions are formed by either chemically doping
with nitrogen and boron atoms, or by controlling gate voltages. Non-equilibrium
Green's function (NEGF) formalism combined with density functional theory (DFT)
is applied to calculate the photo-response function. The graphene PN junctions
show a broad band photo-response including the terahertz range. The dependence
of the response on the angle between the light polarization vector and the PN
interface is determined. Its variation against photon energy is
calculated in the visible range. The essential properties of chemically doped
and gate-controlled PN junctions are similar, but the former shows fingerprints
of dopant distribution.Comment: 7 pages, 6 figure
Non-Arrhenius modes in the relaxation of model proteins
We have investigated the relaxational dynamics for a protein model at various
temperatures. Theoretical analysis of this model in conjunction with numerical
simulations suggests several relaxation regimes, including a single
exponential, a power law and a logarithmic time dependence. Even though a
stretched exponential form gives a good fit to the simulation results in the
crossover regime between a single exponential and a power law decay, we have
not been able to directly deduce this form from the theoretical analysis.Comment: 5 figures, 12 page
From the Complete Yang Model to Snyder's Model, de Sitter Special Relativity and Their Duality
By means of Dirac procedure, we re-examine Yang's quantized space-time model,
its relation to Snyder's model, the de Sitter special relativity and their
UV-IR duality. Starting from a dimensionless dS_5-space in a 5+1-d Mink-space a
complete Yang model at both classical and quantum level can be presented and
there really exist Snyder's model, the dS special relativity and the duality.Comment: 7 papge
I-V characteristics and differential conductance fluctuations of Au nanowires
Electronic transport properties of Au nano-structure are investigated using
both experimental and theoretical analysis. Experimentally, stable Au nanowires
were created using mechanically controllable break junction in air, and
simultaneous current-voltage (I-V) and differential conductance data were measured. The atomic device scale structures are
mechanically very stable up to bias voltage and have a life time
of a few . Facilitated by a shape function data analysis technique
which finger-prints electronic properties of the atomic device, our data show
clearly differential conductance fluctuations with an amplitude at room
temperature, and a nonlinear I-V characteristics. To understand the transport
features of these atomic scale conductors, we carried out {\it ab initio}
calculations on various Au atomic wires. The theoretical results demonstrate
that transport properties of these systems crucially depend on the electronic
properties of the scattering region, the leads, and most importantly the
interaction of the scattering region with the leads. For ideal, clean Au
contacts, the theoretical results indicate a linear I-V behavior for bias
voltage . When sulfur impurities exist at the contact junction,
nonlinear I-V curves emerge due to a tunnelling barrier established in the
presence of the S atom. The most striking observation is that even a single S
atom can cause a qualitative change of the I-V curve from linear to nonlinear.
A quantitatively favorable comparison between experimental data and theoretical
results is obtained. We also report other results concerning quantum transport
through Au atomic contacts.Comment: 11 pages and 9 figures, submitted to PR
The Conditional Colour-Magnitude Distribution: I. A Comprehensive Model of the Colour-Magnitude-Halo Mass Distribution of Present-Day Galaxies
We formulate a model of the conditional colour-magnitude distribution (CCMD)
to describe the distribution of galaxy luminosity and colour as a function of
halo mass. It consists of two populations of different colour distributions,
dubbed pseudo-blue and pseudo-red, respectively, with each further separated
into central and satellite galaxies. We define a global parameterization of
these four colour-magnitude distributions and their dependence on halo mass,
and we infer parameter values by simultaneously fitting the space densities and
auto-correlation functions of 79 galaxy samples from the Sloan Digital Sky
Survey defined by fine bins in the colour-magnitude diagram (CMD). The model
deprojects the overall galaxy CMD, revealing its tomograph along the halo mass
direction. The bimodality of the colour distribution is driven by central
galaxies at most luminosities, though at low luminosities it is driven by the
difference between blue centrals and red satellites. For central galaxies, the
two pseudo-colour components are distinct and orthogonal to each other in the
CCMD: at fixed halo mass, pseudo-blue galaxies have a narrow luminosity range
and broad colour range, while pseudo-red galaxies have a narrow colour range
and broad luminosity range. For pseudo-blue centrals, luminosity correlates
tightly with halo mass, while for pseudo-red galaxies colour correlates more
tightly (redder galaxies in more massive haloes). The satellite fraction is
higher for redder and for fainter galaxies, with colour a stronger indicator
than luminosity. We discuss the implications of the results and further
applications of the CCMD model.Comment: 32 pages, 26 figures, accepted for publication in MNRA
Efficient quantum key distribution over a collective noise channel
We present two efficient quantum key distribution schemes over two different
collective-noise channels. The accepted hypothesis of collective noise is that
photons travel inside a time window small compared to the variation of noise.
Noiseless subspaces are made up of two Bell states and the spatial degree of
freedom is introduced to form two nonorthogonal bases. Although these protocols
resort to entangled states for encoding the key bit, the receiver is only
required to perform single-particle product measurements and there is no basis
mismatch. Moreover, the detection is passive as the receiver does not switch
his measurements between two conjugate measurement bases to get the key.Comment: 6 pages, 1 figure; the revised version of the paper published in
Phys. Rev. A 78, 022321 (2008). Some negligible errors on the error rates of
eavesdropping check are correcte
Improving the security of secure direct communication based on secret transmitting order of particles
We analyzed the security of the secure direct communication protocol based on
secret transmitting order of particles recently proposed by Zhu, Xia, Fan, and
Zhang [Phys. Rev. A 73, 022338 (2006)], and found that this scheme is insecure
if an eavesdropper, say Eve, wants to steal the secret message with Trojan
horse attack strategies. The vital loophole in this scheme is that the two
authorized users check the security of their quantum channel only once. Eve can
insert another spy photon, an invisible photon or a delay one in each photon
which the sender Alice sends to the receiver Bob, and capture the spy photon
when it returns from Bob to Alice. After the authorized users check the
security, Eve can obtain the secret message according to the information about
the transmitting order published by Bob. Finally, we present a possible
improvement of this protocol.Comment: 4 pages, no figur
Conductance spectra of metallic nanotube bundles
We report a first principles analysis of electronic transport characteristics
for (n,n) carbon nanotube bundles. When n is not a multiple of 3, inter-tube
coupling causes universal conductance suppression near Fermi level regardless
of the rotational arrangement of individual tubes. However, when n is a
multiple of 3, the bundles exhibit a diversified conductance dependence on the
orientation details of the constituent tubes. The total energy of the bundle is
also sensitive to the orientation arrangement only when n is a multiple of 3.
All the transport properties and band structures can be well understood from
the symmetry consideration of whether the rotational symmetry of the individual
tubes is commensurate with that of the bundle
Enzymes and genes involved in the betalain biosynthesis in higher plants
Betalains, a class of water-soluble nitrogen-containing pigments, replace anthocyanins and serve the analogous functions in 13 families of the order, caryophyllales. They modulate the attractive appearance of plants and protect them against destructive oxidative damage. Their antioxidant roles, radicalscavenging properties in human health and their potential uses in food and pharmaceutical industries have made significant progress achieved in the detection, purification, quantification, structure elucidation of betalains, and in particular in the understanding of biosynthetic pathways of the pigments,the enzymes and their genes involved in the pathways. In this paper, major progress in betalain biosynthesis and the enzymes and genes involved in the biosynthetic pathways in higher plant are reviewed, and the perspectives discussed
Information filtering based on transferring similarity
In this Brief Report, we propose a new index of user similarity, namely the
transferring similarity, which involves all high-order similarities between
users. Accordingly, we design a modified collaborative filtering algorithm,
which provides remarkably higher accurate predictions than the standard
collaborative filtering. More interestingly, we find that the algorithmic
performance will approach its optimal value when the parameter, contained in
the definition of transferring similarity, gets close to its critical value,
before which the series expansion of transferring similarity is convergent and
after which it is divergent. Our study is complementary to the one reported in
[E. A. Leicht, P. Holme, and M. E. J. Newman, Phys. Rev. E {\bf 73} 026120
(2006)], and is relevant to the missing link prediction problem.Comment: 4 pages, 4 figure
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