13,723 research outputs found
Theoretical Raman fingerprints of -, -, and -graphyne
The novel graphene allotropes -, -, and -graphyne
derive from graphene by insertion of acetylenic groups. The three graphynes are
the only members of the graphyne family with the same hexagonal symmetry as
graphene itself, which has as a consequence similarity in their electronic and
vibrational properties. Here, we study the electronic band structure, phonon
dispersion, and Raman spectra of these graphynes within an
\textit{ab-initio}-based non-orthogonal tight-binding model. In particular, the
predicted Raman spectra exhibit a few intense resonant Raman lines, which can
be used for identification of the three graphynes by their Raman spectra for
future applications in nanoelectronics
Comparative study of the two-phonon Raman bands of silicene and graphene
We present a computational study of the two-phonon Raman spectra of silicene
and graphene within a density-functional non-orthogonal tight-binding model.
Due to the presence of linear bands close to the Fermi energy in the electronic
structure of both structures, the Raman scattering by phonons is resonant. We
find that the Raman spectra exhibit a crossover behavior for laser excitation
close to the \pi-plasmon energy. This phenomenon is explained by the
disappearance of certain paths for resonant Raman scattering and the appearance
of other paths beyond this energy. Besides that, the electronic joint density
of states is divergent at this energy, which is reflected on the behavior of
the Raman bands of the two structures in a qualitatively different way.
Additionally, a number of Raman bands, originating from divergent phonon
density of states at the M point and at points, inside the Brillouin zone, is
also predicted. The calculated spectra for graphene are in excellent agreement
with available experimental data. The obtained Raman bands can be used for
structural characterization of silicene and graphene samples by Raman
spectroscopy
On the Gap and Time Interval between the First Two Maxima of Long Continuous Time Random Walks
We consider a one-dimensional continuous time random walk (CTRW) on a fixed
time interval where at each time step the walker waits a random time
, before performing a jump drawn from a symmetric continuous probability
distribution function (PDF) , of L\'evy index . Our
study includes the case where the waiting time PDF has a power law
tail, , with , such that
the average time between two consecutive jumps is infinite. The random motion
is sub-diffusive if ).
We investigate the joint PDF of the gap between the first two highest
positions of the CTRW and the time separating these two maxima. We show
that this PDF reaches a stationary limiting joint distribution in the
limit of long CTRW, . Our exact analytical results show a very
rich behavior of this joint PDF in the plane, which we study in
great detail. Our main results are verified by numerical simulations. This work
provides a non trivial extension to CTRWs of the recent study in the discrete
time setting by Majumdar et al. (J. Stat. Mech. P09013, 2014).Comment: 36 pages, 10 figures. arXiv admin note: text overlap with
arXiv:1405.122
Survival Probability of Random Walks and L\'evy Flights on a Semi-Infinite Line
We consider a one-dimensional random walk (RW) with a continuous and
symmetric jump distribution, , characterized by a L\'evy index , which includes standard random walks () and L\'evy flights
(). We study the survival probability, , representing the
probability that the RW stays non-negative up to step , starting initially
at . Our main focus is on the -dependence of for
large . We show that displays two distinct regimes as
varies: (i) for ("quantum regime"), the discreteness of the jump
process significantly alters the standard scaling behavior of and
(ii) for ("classical regime") the discrete-time nature of
the process is irrelevant and one recovers the standard scaling behavior (for
this corresponds to the standard Brownian scaling limit). The purpose
of this paper is to study how precisely the crossover in occurs
between the quantum and the classical regime as one increases .Comment: 20 pages, 3 figures, revised and accepted versio
On the Gap and Time Interval between the First Two Maxima of Long Random Walks
In the context of order statistics of discrete time random walks (RW), we
investigate the statistics of the gap, , and the number of time steps,
, between the two highest positions of a Markovian one-dimensional random
walker, starting from , after time steps (taking the -axis
vertical). The jumps are independent and identically
distributed random variables drawn from a symmetric probability distribution
function (PDF), , the Fourier transform of which has the small
behavior , with . For ,
the variance of the jump distribution is finite and the RW (properly scaled)
converges to a Brownian motion. For , the RW is a L\'evy flight of
index . We show that the joint PDF of and converges to a well
defined stationary bi-variate distribution as the RW duration goes
to infinity. We present a thorough analytical study of the limiting joint
distribution , as well as of its associated marginals
and , revealing a rich variety of behaviors depending on the
tail of (from slow decreasing algebraic tail to fast decreasing
super-exponential tail). We also address the problem for a random bridge where
the RW starts and ends at the origin after time steps. We show that in the
large limit, the PDF of and converges to the {\it same}
stationary distribution as in the case of the free-end RW. Finally, we
present a numerical check of our analytical predictions. Some of these results
were announced in a recent letter [S. N. Majumdar, Ph. Mounaix, G. Schehr,
Phys. Rev. Lett. {\bf 111}, 070601 (2013)].Comment: 52 pages, 8 figures. Published version (typos corrected). Accepted
for publication in J. Stat. Mec
Principles and applications of CVD powder technology
Chemical vapor deposition (CVD) is an important technique for surface modification of powders through either grafting or deposition of films and coatings. The efficiency of this complex process primarily depends on appropriate contact between the reactive gas phase and the solid particles to be treated. Based on this requirement, the first part of this review focuses on the ways to ensure such contact and particularly on the formation of fluidized beds. Combination of constraints due to both fluidization and chemical vapor deposition leads to the definition of different types of reactors as an alternative to classical fluidized beds, such as spouted beds, circulating beds operating in turbulent and fast-transport regimes or vibro-fluidized beds. They operate under thermal but also plasma activation of the reactive gas and their design mainly depends on the type of powders to be treated. Modeling of both reactors and operating conditions is a valuable tool for understanding and optimizing these complex processes and materials. In the second part of the review, the state of the art on materials produced by fluidized bed chemical vapor deposition is presented. Beyond pioneering applications in the nuclear power industry, application domains, such as heterogeneous catalysis, microelectronics, photovoltaics and protection against wear, oxidation and heat are potentially concerned by processes involving chemical vapor deposition on powders. Moreover, simple and reduced cost FBCVD processes where the material to coat is immersed in the FB, allow the production of coatings for metals with different wear, oxidation and corrosion resistance. Finally, large-scale production of advanced nanomaterials is a promising area for the future extension and development of this technique
Gravitational sensing with weak value based optical sensors
Using weak values amplification angular resolution limits, we theoretically
investigate the gravitational sensing of objects. By inserting a force-sensing
pendulum into a weak values interferometer, the optical response can sense
accelerations to a few 10's of
, with optical powers of
. We convert this precision into range and mass sensitivity,
focusing in detail on simple and torsion pendula. Various noise sources present
are discussed, as well as the necessary cooling that should be applied to reach
the desired levels of precision.Comment: 9 pages, 4 figures, Quantum Stud.: Math. Found. (2018
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