1,941 research outputs found
Nonradiative Recombination of Excitons in Carbon Nanotubes Mediated by Free Charge Carriers
Free electrons or holes can mediate the nonradiative recombination of
excitons in carbon nanotubes. Kinematic constraints arising from the quasi
one-dimensional nature of excitons and charge carriers lead to a thermal
activation barrier for the process. However, a model calculation suggests that
the rate for recombination mediated by a free electron is the same order of
magnitude as that of two-exciton recombination. Small amounts of doping may
contribute to the short exciton lifetimes and low quantum yields observed in
carbon nanotubes.Comment: 18 pages, 4 figures. Submitted to Physical Review
Formation of Subgap States in Carbon Nanotubes Due to a Local Transverse Electric Field
We introduce two simple models to study the effect of a spatially localized
transverse electric field on the low-energy electronic structure of
semiconducting carbon nanotubes. Starting from the Dirac Hamiltonian for the
low energy states of a carbon nanotube, we use scattering theory to show that
an arbitrarily weak field leads to the formation of localized electronic states
inside the free nanotube band gap. We study the binding energy of these subgap
states as a function of the range and strength of the electrostatic potential.
When the range of the potential is held constant and the strength is varied,
the binding energy shows crossover behavior: the states lie close to the free
nanotube band edge until the potential exceeds a threshold value, after which
the binding energy increases rapidly. When the potential strength is held
constant and the range is varied, we find resonant behavior: the binding energy
passes through a maximum as the range of the potential is increased. Large
electric fields confined to a small region of the nanotube are required to
create localized states far from the band edge.Comment: 15 pages + 5 figures, 1 table in RevTe
Language and Identities: The Exceptional Normality of Italy
Language issues loom large in current debates on Italian identity/identities, indigenous minorities in Italy and, of course, immigration. While the context of language debates in early 21st century Italy presents new realities and challenges, the fundamental issues are the same as those originally defined by the first European language planner, Dante, and reworked by successive theorists. The debates turn on exclusions and inclusions, on levels of multiple identities, on understandings of otherness. It is no accident that language is at once as a provocation for debates on identity and a metaphor of those debates, for the tensions that run through the debates lie at the heart of language itself. All cultures have a narrative that explains diversity among languages and cultures, either as the result of a mistake or as divine punishment. The Biblical accounts of Creation, Babel and Pentecost provide the framework for European understandings of language diversity. These accounts capture the paradoxical nature of human language, which characterizes us a species and is a tool for building unity between persons and groups, but is, by its nature, always and inevitably an expression of diversity, in time and space. These contradictions are being played out in current language debates as emigration, return migration, internal migration and immigration elicit new constructions of ‘Italianness’, the literary canon and the social weight of the different varieties of language present on Italian soil and in Italian communities abroad
Approximating strongly correlated spin and fermion wavefunctions with correlator product states
We explore correlator product states for the approximation of correlated
wavefunctions in arbitrary dimensions. We show that they encompass many
interesting states including Laughlin's quantum Hall wavefunction, Huse and
Elser's frustrated spin states, and Kitaev's toric code. We further establish
their relation to common families of variational wavefunctions, such as matrix
and tensor product states and resonating valence bond states. Calculations on
the Heisenberg and spinless Hubbard models show that correlator product states
capture both two-dimensional correlations (independent of system width) as well
as non-trivial fermionic correlations (without sign problems). In
one-dimensional simulations, correlator product states appear competitive with
matrix product states with a comparable number of variational parameters,
suggesting they may eventually provide a route to practically generalise the
density matrix renormalisation group to higher dimensions.Comment: Table 1 expanded, Table 2 updated, optimization method discussed,
discussions expanded in some sections, earlier work on similar wavefunctions
included in text and references, see also (arXiv:0905.3898). 5 pages, 1
figure, 2 tables, submitted to Phys. Rev.
Perfect Reflection of Chiral Fermions in Gated Graphene Nanoribbons
We describe the results of a theoretical study of transport through gated
metallic graphene nanoribbons using a non-equilibrium Green function method.
Although analogies with quantum field theory predict perfect transmission of
chiral fermions through gated regions in one dimension, we find \emph{perfect
reflection} of chiral fermions in armchair ribbons for specific configurations
of the gate. This effect should be measurable in narrow graphene constrictions
gated by a charged carbon nanotube.Comment: 9 pages, 3 figures. Submitted to Nano Letter
Promoting multimedia projects among university language students
Este proyecto multimedia es un experimento 'por primera vez' por los dos autores, y se integraba en un curso lingüístico de nivel avanzado, cuyos objetivos eran exponer a los estudiantes una gama de registros del francés contemporáneo, de lo formal a lo grosero. El formato informático multimedia convenía bien a esa materia, en cuanto permitía presentar una larga gama de material (audio-visual y textual), ofreciendo a los estudiantes la libertad de explorarlo interactivamente según sus exigencias y nivel de competencia. En el curso del proyecto, los profesores y los estudiantes han debido a aprender los fundamentos de la creación multimedia. Este artículo describe el proyecto, sus resultados y los procedimientos de validación
Striped spin liquid crystal ground state instability of kagome antiferromagnets
The Dirac spin liquid ground state of the spin 1/2 Heisenberg kagome
antiferromagnet has potential instabilities[1-4]. This has been suggested as
the reason why it is not strongly supported in large-scale numerical
calculations[5]. However, previous attempts to observe these instabilities have
failed. We report on the discovery of a projected BCS state with lower energy
than the projected Dirac spin liquid state which provides new insight into the
stability of the ground state of the kagome antiferromagnet. The new state has
three remarkable features. First, it breaks both spatial symmetry in an unusual
way that may leave spinons deconfined along one direction. Second, it breaks
the U(1) gauge symmetry down to . Third, it has the spatial symmetry of a
previously proposed "monopole" suggesting that it is an instability of the
Dirac spin liquid. The state described herein also shares a remarkable
similarity to the distortion of the kagome lattice observed at low Zn
concentrations in Zn-Paratacamite suggesting it may already be realized in
these materials.Comment: 4+ pages, 3 figure
To Like Or Not to Like, That Is the Question
Perhaps the most ubiquitous and basic affective decision of daily life is
deciding whether we like or dislike something/somebody, or, in terms of
psychological emotion theories, whether the object/subject has positive or
negative valence. Indeed, people constantly make such liking decisions within
a glimpse and, importantly, often without expecting any obvious benefit or
knowing the exact reasons for their judgment. In this paper, we review
research on such elementary affective decisions (EADs) that entail no direct
overt reward with a special focus on Neurocognitive Poetics and discuss
methods and models for investigating the neuronal and cognitive-affective
bases of EADs to verbal materials with differing degrees of complexity. In
line with evolutionary and appraisal theories of (aesthetic) emotions and data
from recent neurocognitive studies, the results of a decision tree modeling
approach simulating EADs to single words suggest that a main driving force
behind EADs is the extent to which such high-dimensional stimuli are
associated with the “basic” emotions joy/happiness and disgust
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