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 $Z_2$. 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

“If I were the mother…”: fostering perspective taking in German teacher education

Higher perspective taking skills are associated with better social functioning and improved social relationships. Generally, teachers are willing to take the perspective of their students, but it is unclear whether the same is true for the perspective of parents. As communication and conflicts with parents are pervasive, the motivation and willingness to adopt the perspective of parents in counseling situations should be promoted during university teacher training. Therefore, we investigated the promotion of perspective taking among teachers in training and focused mainly on perspective taking toward parents. We developed a case-based learning task in which teachers in training from Freie Universität Berlin ( N  = 515) prepared for a fictitious upcoming consultation with a mother about her son. Because it is unclear if direct instruction for perspective taking is necessary in order to promote it, we also used indirect instruction to investigate whether preparing for the consultation under these instructions fostered the willingness to adopt the perspective of students and parents. In the direct instruction participants were directly told to take the perspective of the fictitious mother when evaluating and developing formulations for the consultation. The indirect instruction did not mention the concept of perspective taking but asked participants to focus on the comprehensibility of the formulations. We obtained three measures: the willingness to take a perspective, the attitude toward another person, and the emotional and empathic language used in written texts. With our main result we demonstrated that the willingness to adopt the perspective of both students and parents could be significantly promoted by both instructions. We further demonstrated that a higher willingness to take another’s perspective is associated with a more positive attitude toward the mother, as well as increased positive emotions and empathic concern. Additionally, we replicated results of a previous study showing a generally higher willingness to take the student perspective prior to the intervention. Results are discussed regarding the benefits of promoting perspective taking, especially toward parents, in teacher education

“If I were the mother…”: fostering perspective taking in German teacher education

Higher perspective taking skills are associated with better social functioning and improved social relationships. Generally, teachers are willing to take the perspective of their students, but it is unclear whether the same is true for the perspective of parents. As communication and conflicts with parents are pervasive, the motivation and willingness to adopt the perspective of parents in counseling situations should be promoted during university teacher training. Therefore, we investigated the promotion of perspective taking among teachers in training and focused mainly on perspective taking toward parents. We developed a case-based learning task in which teachers in training from Freie Universität Berlin (N = 515) prepared for a fictitious upcoming consultation with a mother about her son. Because it is unclear if direct instruction for perspective taking is necessary in order to promote it, we also used indirect instruction to investigate whether preparing for the consultation under these instructions fostered the willingness to adopt the perspective of students and parents. In the direct instruction participants were directly told to take the perspective of the fictitious mother when evaluating and developing formulations for the consultation. The indirect instruction did not mention the concept of perspective taking but asked participants to focus on the comprehensibility of the formulations. We obtained three measures: the willingness to take a perspective, the attitude toward another person, and the emotional and empathic language used in written texts. With our main result we demonstrated that the willingness to adopt the perspective of both students and parents could be significantly promoted by both instructions. We further demonstrated that a higher willingness to take another’s perspective is associated with a more positive attitude toward the mother, as well as increased positive emotions and empathic concern. Additionally, we replicated results of a previous study showing a generally higher willingness to take the student perspective prior to the intervention. Results are discussed regarding the benefits of promoting perspective taking, especially toward parents, in teacher education