1,978 research outputs found
Crosstalk between nanotube devices: contact and channel effects
At reduced dimensionality, Coulomb interactions play a crucial role in
determining device properties. While such interactions within the same carbon
nanotube have been shown to have unexpected properties, device integration and
multi-nanotube devices require the consideration of inter-nanotube
interactions. We present calculations of the characteristics of planar carbon
nanotube transistors including interactions between semiconducting nanotubes
and between semiconducting and metallic nanotubes. The results indicate that
inter-tube interactions affect both the channel behavior and the contacts. For
long channel devices, a separation of the order of the gate oxide thickness is
necessary to eliminate inter-nanotube effects. Because of an exponential
dependence of this length scale on dielectric constant, very high device
densities are possible by using high-k dielectrics and embedded contacts
Activated aging dynamics and negative fluctuation-dissipation ratios
In glassy materials aging proceeds at large times via thermal activation. We
show that this can lead to negative dynamical response functions and novel and
well-defined violations of the fluctuation-dissipation theorem, in particular,
negative fluctuation-dissipation ratios. Our analysis is based on detailed
theoretical and numerical results for the activated aging regime of simple
kinetically constrained models. The results are relevant to a variety of
physical situations such as aging in glass-formers, thermally activated domain
growth and granular compaction.Comment: 4 pages, 4 figs; v2 final version (minor modifs) published in Phys.
Rev. Let
Multiple Functionality in Nanotube Transistors
Calculations of quantum transport in a carbon nanotube transistor show that
such a device offers unique functionality. It can operate as a ballistic
field-effect transistor, with excellent characteristics even when scaled to 10
nm dimensions. At larger gate voltages, channel inversion leads to resonant
tunneling through an electrostatically defined nanoscale quantum dot. Thus the
transistor becomes a gated resonant tunelling device, with negative
differential resistance at a tunable threshold. For the dimensions considered
here, the device operates in the Coulomb blockade regime, even at room
temperature.Comment: To appear in Phys. Rev. Let
Properties of short channel ballistic carbon nanotube transistors with ohmic contacts
We present self-consistent, non-equilibrium Green's function calculations of
the characteristics of short channel carbon nanotube transistors, focusing on
the regime of ballistic transport with ohmic contacts. We first establish that
the band lineup at the contacts is renormalized by charge transfer, leading to
Schottky contacts for small diameter nanotubes and ohmic contacts for large
diameter nanotubes, in agreement with recent experiments. For short channel
ohmic contact devices, source-drain tunneling and drain-induced barrier
lowering significantly impact the current-voltage characteristics. Furthermore,
the ON state conductance shows a temperature dependence, even in the absence of
phonon scattering or Schottky barriers. This last result also agrees with
recently reported experimental measurements.Comment: Nanotechnology, in pres
Phase ordering and roughening on growing films
We study the interplay between surface roughening and phase separation during
the growth of binary films. Already in 1+1 dimension, we find a variety of
different scaling behaviors depending on how the two phenomena are coupled. In
the most interesting case, related to the advection of a passive scalar in a
velocity field, nontrivial scaling exponents are obtained in simulations.Comment: 4 pages latex, 6 figure
Améliorer les apprentissages en sciences en milieu francophone minoritaire : résultats de l’expérimentation d’un modèle pédagogique
Selon les postulats thĂ©oriques, les faibles rĂ©sultats scolaires chez les populations linguistiques minoritaires s’expliqueraient par des facteurs socioculturels et Ă©conomiques, on recommande donc une pĂ©dagogie qui composerait avec ces Ă©lĂ©ments. Dans le cadre de cette recherche, les auteurs ont crĂ©Ă© et expĂ©rimentĂ© un modèle pĂ©dagogique pour enseigner les sciences en milieu minoritaire tenant compte de ces facteurs. Lors d’une intervention de deux mois, des Ă©lèves de cinquième annĂ©e ont Ă©tudiĂ© les marais salĂ©s Ă partir de ce modèle pĂ©dagogique. L’expression Ă©crite et orale de leurs observations demeurait une prioritĂ©. Les rĂ©sultats ont rĂ©vĂ©lĂ© une attitude positive envers l’approche, un apprentissage scientifique important et une progression lexicale aux niveaux oral et Ă©crit.Socio-cultural and economic factors may in part explain poor academic results for linguistic minorities. In such a case, pedagogical considerations in light of such factors should be put in place. This research created and experimented a pedagogical model for teaching science to minority language students, while considering socio-cultural factors. During a two-month intervention, grade five students studied salt-water marshes in a unit conceived within this model. Oral and written expression of their observations constituted an important part of the experiment. Results indicate a favourable attitude towards the pedagogical approach and progression in science and lexical learning in both written and oral language.Los postulados teĂłricos indican que los bajos resultados escolares en las poblaciones de lenguas minoritarias se justifican por factores socioculturales y econĂłmicos. Por lo tanto, se requiere de una pedagogĂa que tome en cuenta estos elementos. En el marco de esta investigaciĂłn, los autores han creado y experimentado un modelo pedagĂłgico para enseñar las ciencias en un ámbito minoritario tomando en cuenta estos factores. En una intervenciĂłn de una duraciĂłn de dos meses, alumnos de quinto grado estudiaron las marismas saladas a partir de un modelo pedagĂłgico. QuedĂł prioritaria la expresiĂłn escrita y oral de sus observaciones. Los resultados revelaron una actitud positiva hacia el enfoque, un aprendizaje cientĂfico importante asĂ como una progresiĂłn lexical a nivel oral y escrito
Following microscopic motion in a two dimensional glass-forming binary fluid
The dynamics of a binary mixture of large and small discs are studied at
temperatures approaching the glass transition using an analysis based on the
topology of the Voronoi polygon surrounding each atom. At higher temperatures
we find that dynamics is dominated by fluid-like motion that involves particles
entering and exiting the nearest-neighbour shells of nearby particles. As the
temperature is lowered, the rate of topological moves decreases and motion
becomes localised to regions of mixed pentagons and heptagons. In addition we
find that in the low temperature state particles may translate significant
distances without undergoing changes in their nearest neig hbour shell. These
results have implications for dynamical heterogeneities in glass forming
liquids.Comment: 12 pages, 7 figure
Epitaxial growth in dislocation-free strained alloy films: Morphological and compositional instabilities
The mechanisms of stability or instability in the strained alloy film growth
are of intense current interest to both theorists and experimentalists. We
consider dislocation-free, coherent, growing alloy films which could exhibit a
morphological instability without nucleation. We investigate such strained
films by developing a nonequilibrium, continuum model and by performing a
linear stability analysis. The couplings of film-substrate misfit strain,
compositional stress, deposition rate, and growth temperature determine the
stability of film morphology as well as the surface spinodal decomposition. We
consider some realistic factors of epitaxial growth, in particular the
composition dependence of elastic moduli and the coupling between top surface
and underlying bulk of the film. The interplay of these factors leads to new
stability results. In addition to the stability diagrams both above and below
the coherent spinodal temperature, we also calculate the kinetic critical
thickness for the onset of instability as well as its scaling behavior with
respect to misfit strain and deposition rate. We apply our results to some real
growth systems and discuss the implications related to some recent experimental
observations.Comment: 26 pages, 13 eps figure
RLZAP: Relative Lempel-Ziv with Adaptive Pointers
Relative Lempel-Ziv (RLZ) is a popular algorithm for compressing databases of
genomes from individuals of the same species when fast random access is
desired. With Kuruppu et al.'s (SPIRE 2010) original implementation, a
reference genome is selected and then the other genomes are greedily parsed
into phrases exactly matching substrings of the reference. Deorowicz and
Grabowski (Bioinformatics, 2011) pointed out that letting each phrase end with
a mismatch character usually gives better compression because many of the
differences between individuals' genomes are single-nucleotide substitutions.
Ferrada et al. (SPIRE 2014) then pointed out that also using relative pointers
and run-length compressing them usually gives even better compression. In this
paper we generalize Ferrada et al.'s idea to handle well also short insertions,
deletions and multi-character substitutions. We show experimentally that our
generalization achieves better compression than Ferrada et al.'s implementation
with comparable random-access times
Rotationally induced Penning ionization of ultracold photoassociated helium dimers
We have studied photoassociation of metastable \tripS helium atoms near the
\tripS-\tripP asymptote by both ion detection in a magneto-optical trap and
trap-loss measurements in a magnetic trap. A detailed comparison between the
results of the two experiments gives insight into the mechanism of the Penning
ionization process. We have identified four series of resonances corresponding
to vibrational molecular levels belonging to different rotational states in two
potentials. The corresponding spin states become quasi-purely quintet at small
interatomic distance, and Penning ionization is inhibited by spin conservation
rules. Only a weak rotational coupling is responsible for the contamination by
singlet spin states leading to a detectable ion signal. However, for one of
these series Bose statistics does not enable the rotational coupling and the
series detected through trap-loss does not give rise to sufficient ionization
for detection.Comment: 7 pages, 4 figures, submitted to EuroPhysics Letter
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