2,190 research outputs found
Tunable bandgaps and excitons in doped semiconducting carbon nanotubes made possible by acoustic plasmons
Doping of semiconductors is essential in modern electronic and photonic
devices. While doping is well understood in bulk semiconductors, the advent of
carbon nanotubes and nanowires for nanoelectronic and nanophotonic applications
raises some key questions about the role and impact of doping at low
dimensionality. Here we show that for semiconducting carbon nanotubes, bandgaps
and exciton binding energies can be dramatically reduced upon experimentally
relevant doping, and can be tuned gradually over a broad range of energies in
contrast to higher dimensional systems. The later feature is made possible by a
novel mechanism involving strong dynamical screening effects mediated by
acoustic plasmons.Comment: 5 pages, 4 figures, published in Phys. Rev. Lett
Atomistic study of an ideal metal/thermoelectric contact: the full-Heusler/half-Heusler interface
Half-Heusler alloys such as the (Zr,Hf)NiSn intermetallic compounds are
important thermoelectric materials for converting waste heat into electricity.
Reduced electrical resistivity at the hot interface between the half-Heusler
material and a metal contact is critical for device performance, however this
has yet to be achieved in practice. Recent experimental work suggests that a
coherent interface between half-Heusler and full-Heusler compounds can form due
to diffusion of transition metal atoms into the vacant sublattice of the
half-Heusler lattice. We study theoretically the structural and electronic
properties of such an interface using a first-principles based approach that
combines {\it ab initio} calculations with macroscopic modeling. We find that
the prototypical interface HfNiSn/HfNiSn provides very low contact
resistivity and almost ohmic behavior over a wide range of temperatures and
doping levels. Given the potential of these interfaces to remain stable over a
wide range of temperatures, our study suggests that full-Heuslers might provide
nearly ideal electrical contacts to half-Heuslers that can be harnessed for
efficient thermoelectric generator devices.Comment: 8 pages, 8 figure
Lifetime of dynamic heterogeneity in strong and fragile kinetically constrained spin models
Kinetically constrained spin models are schematic coarse-grained models for
the glass transition which represent an efficient theoretical tool to study
detailed spatio-temporal aspects of dynamic heterogeneity in supercooled
liquids. Here, we study how spatially correlated dynamic domains evolve with
time and compare our results to various experimental and numerical
investigations. We find that strong and fragile models yield different results.
In particular, the lifetime of dynamic heterogeneity remains constant and
roughly equal to the alpha relaxation time in strong models, while it increases
more rapidly in fragile models when the glass transition is approached.Comment: Submitted to the proceedings of the 6th EPS Liquid Matter Conference,
Utrecht 2-6 July 200
Ab initio calculations of low-energy quasiparticle lifetimes in bilayer graphene
Motivated by recent experimental results we calculate from first-principles
the lifetime of low-energy quasiparticles in bilayer graphene (BLG). We take
into account the scattering rate arising from electron-electron interactions
within the approximation for the electron self-energy and consider several
p-type doping levels ranging from to
holes/cm. In the undoped case we find that the average inverse lifetime
scales linearly with energy away from the charge neutrality point, with values
in good agreement with experiments. The decay rate is approximately three times
larger than in monolayer graphene, a consequence of the enhanced screening in
BLG. In the doped case, the dependence of the inverse lifetime on quasiparticle
energy acquires a non-linear component due to the opening of an additional
decay channel mediated by acoustic plasmons.Comment: 15 pages, 3 figures, accepeted for publication in Applied Physics
Letter
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
Theory of enhancement of thermoelectric properties of materials with nanoinclusions
Based on the concept of band bending at metal/semiconductor interfaces as an
energy filter for electrons, we present a theory for the enhancement of the
thermoelectric properties of semiconductor materials with metallic
nanoinclusions. We show that the Seebeck coefficient can be significantly
increased due to a strongly energy-dependent electronic scattering time. By
including phonon scattering, we find that the enhancement of ZT due to electron
scattering is important for high doping, while at low doping it is primarily
due to a decrease in the phonon thermal conductivity
A JKO splitting scheme for Kantorovich-Fisher-Rao gradient flows
In this article we set up a splitting variant of the JKO scheme in order to
handle gradient flows with respect to the Kantorovich-Fisher-Rao metric,
recently introduced and defined on the space of positive Radon measure with
varying masses. We perform successively a time step for the quadratic
Wasserstein/Monge-Kantorovich distance, and then for the Hellinger/Fisher-Rao
distance. Exploiting some inf-convolution structure of the metric we show
convergence of the whole process for the standard class of energy functionals
under suitable compactness assumptions, and investigate in details the case of
internal energies. The interest is double: On the one hand we prove existence
of weak solutions for a certain class of reaction-advection-diffusion
equations, and on the other hand this process is constructive and well adapted
to available numerical solvers.Comment: Final version, to appear in SIAM SIM
Theme Analysis of Flora Nwapaâs Never Again: A Systemic Functional Approach
This paper deals with theme analysis of Flora Nwapaâs Never Again. The theory that underpins this study is systemic functional linguistics, which is one of the approaches proposed by scholars such as M.A.K Halliday, S. Eggins, J. R. Martin, R. Fowler, J. D. Benson to mention but a very few, for the study of language and its function(s). In this system, the study of language involves three functional labels: experiential, interpersonal and textual meanings. This study aims at describing and analysing linguistic features which connote textual meaning in Never Again so as to have a deep understanding of the novel and provide a new interpretation of it. It also aims at uncovering the deep messages conveyed through the novel. Both quantitative and qualitative methods have been used in this study. Thus, two extracts have been selected from Never Again and systemic functional linguistic theories have been applied to each of them for the purpose of exploring how language is used to organise itself. The study of the internal organisation and communicative nature of the selected Texts has enabled the researchers to come up with valuable findings. Indeed, through Mood adjuncts that conflate interpersonal themes, participants in Text 1 have expressed particular attitudes and judgments about the limited power of women and the roles of women and children during war whereas those in Text 2 have expressed a high degree of certainty and inclination. This study enables the researchers to provide a deeper understanding of Never Again by contending that the deep messages conveyed through the novel are the Nigerian civil war and the roles of women and children in our societies especially during war time. Keywords: Metafunctions, register variables, systemic functional linguistics, theme patterns
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