280 research outputs found
Phonon Dispersion Effects and the Thermal Conductivity Reduction in GaAs/AlAs Superlattices
The experimentally observed order-of-magnitude reduction in the thermal
conductivity along the growth axis of (GaAs)_n/(AlAs)_n (or n x n)
superlattices is investigated theoretically for (2x2), (3x3) and (6x6)
structures using an accurate model of the lattice dynamics. The modification of
the phonon dispersion relation due to the superlattice geometry leads to
flattening of the phonon branches and hence to lower phonon velocities. This
effect is shown to account for a factor-of-three reduction in the thermal
conductivity with respect to bulk GaAs along the growth direction; the
remainder is attributable to a reduction in the phonon lifetime. The
dispersion-related reduction is relatively insensitive to temperature (100 < T
< 300K) and n. The phonon lifetime reduction is largest for the (2x2)
structures and consistent with greater interface scattering. The thermal
conductivity reduction is shown to be appreciably more sensitive to GaAs/AlAs
force constant differences than to those associated with molecular masses.Comment: 5 figure
Market opportunity recognition in the Chilean wine industry: traditional versus relational marketing approaches
The thermal conductivity reduction in HgTe/CdTe superlattices
The techniques used previously to calculate the three-fold thermal
conductivity reduction due to phonon dispersion in GaAs/AlAs superlattices
(SLs) are applied to HgTe/CdTe SLs. The reduction factor is approximately the
same, indicating that this SL may be applicable both as a photodetector and a
thermoelectric cooler.Comment: 5 pages, 2 figures; to be published in Journal of Applied Physic
On equations over sets of integers
Systems of equations with sets of integers as unknowns are considered. It is
shown that the class of sets representable by unique solutions of equations
using the operations of union and addition S+T=\makeset{m+n}{m \in S, \: n \in
T} and with ultimately periodic constants is exactly the class of
hyper-arithmetical sets. Equations using addition only can represent every
hyper-arithmetical set under a simple encoding. All hyper-arithmetical sets can
also be represented by equations over sets of natural numbers equipped with
union, addition and subtraction S \dotminus T=\makeset{m-n}{m \in S, \: n \in
T, \: m \geqslant n}. Testing whether a given system has a solution is
-complete for each model. These results, in particular, settle the
expressive power of the most general types of language equations, as well as
equations over subsets of free groups.Comment: 12 apges, 0 figure
A wide band gap metal-semiconductor-metal nanostructure made entirely from graphene
A blueprint for producing scalable digital graphene electronics has remained
elusive. Current methods to produce semiconducting-metallic graphene networks
all suffer from either stringent lithographic demands that prevent
reproducibility, process-induced disorder in the graphene, or scalability
issues. Using angle resolved photoemission, we have discovered a unique one
dimensional metallic-semiconducting-metallic junction made entirely from
graphene, and produced without chemical functionalization or finite size
patterning. The junction is produced by taking advantage of the inherent,
atomically ordered, substrate-graphene interaction when it is grown on SiC, in
this case when graphene is forced to grow over patterned SiC steps. This
scalable bottomup approach allows us to produce a semiconducting graphene strip
whose width is precisely defined within a few graphene lattice constants, a
level of precision entirely outside modern lithographic limits. The
architecture demonstrated in this work is so robust that variations in the
average electronic band structure of thousands of these patterned ribbons have
little variation over length scales tens of microns long. The semiconducting
graphene has a topologically defined few nanometer wide region with an energy
gap greater than 0.5 eV in an otherwise continuous metallic graphene sheet.
This work demonstrates how the graphene-substrate interaction can be used as a
powerful tool to scalably modify graphene's electronic structure and opens a
new direction in graphene electronics research.Comment: 11 pages, 7 figure
On Varieties of Ordered Automata
The Eilenberg correspondence relates varieties of regular languages to
pseudovarieties of finite monoids. Various modifications of this correspondence
have been found with more general classes of regular languages on one hand and
classes of more complex algebraic structures on the other hand. It is also
possible to consider classes of automata instead of algebraic structures as a
natural counterpart of classes of languages. Here we deal with the
correspondence relating positive -varieties of languages to
positive -varieties of ordered automata and we present various
specific instances of this correspondence. These bring certain well-known
results from a new perspective and also some new observations. Moreover,
complexity aspects of the membership problem are discussed both in the
particular examples and in a general setting
Ion Collisions in Very Strong Electric Fields
A Classical Trajectory Monte Carlo (CTMC) simulation has been made of
processes of charge exchange and ionization between an hydrogen atom and fully
stripped ions embedded in very strong static electric fields (
V/m), which are thought to exist in cosmic and laser--produced plasmas.
Calculations show that the presence of the field affects absolute values of the
cross sections, enhancing ionization and reducing charge exchange. Moreover,
the overall effect depends upon the relative orientation between the field and
the nuclear motion. Other features of a null-field situation, such as scaling
laws, are revisited.Comment: Latex, 13 pages, 11 figures (available upon request), to be published
in Journal of Physics
Lattice Dynamics of II-VI materials using adiabatic bond charge model
We extend the adiabatic bond charge model, originally developed for group IV
semiconductors and III-V compounds, to study phonons in more ionic II-VI
compounds with a zincblende structure. Phonon spectra, density of states and
specific heats are calculated for six II-VI compounds and compared with both
experimental data and the results of other models. We show that the 6-parameter
bond charge model gives a good description of the lattice dynamics of these
materials. We also discuss trends in the parameters with respect to the
ionicity and metallicity of these compounds.Comment: 16 pages of RevTex with 3 figures submitted as a uuencode compressed
tar fil
Disorder induced collapse of the electron phonon coupling in MgB observed by Raman Spectroscopy
The Raman spectrum of the superconductor MgB has been measured as a
function of the Tc of the film. A striking correlation is observed between the
onset and the frequency of the mode. Analysis of the data with
the McMillan formula provides clear experimental evidence for the collapse of
the electron phonon coupling at the temperature predicted for the convergence
of two superconducting gaps into one observable gap. This gives indirect
evidence of the convergence of the two gaps and direct evidence of a transition
to an isotropic state at 19 K. The value of the electron phonon coupling
constant is found to be 1.22 for films with T 39K and 0.80 for films with
T19K.Comment: 5 pages, 4 figure
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