678 research outputs found
Optical excitations in hexagonal nanonetwork materials
Optical excitations in hexagonal nanonetwork materials, for example,
Boron-Nitride (BN) sheets and nanotubes, are investigated theoretically. The
bonding of BN systems is positively polarized at the B site, and is negatively
polarized at the N site. There is a permanent electric dipole moment along the
BN bond, whose direction is from the B site to the N site. When the exciton
hopping integral is restricted to the nearest neighbors, the flat band of the
exciton appears at the lowest energy. The higher optical excitations have
excitation bands similar to the electronic bands of graphene planes and carbon
nanotubes. The symmetry of the flat exciton band is optically forbidden,
indicating that the excitons related to this band will show quite long lifetime
which will cause strong luminescence properties.Comment: 4 pages; 3 figures; proceedings of "XVIth International Winterschool
on Electronic Properties of Novel Materials (IWEPNM2002)
Rendimiento de grano de cultivares de trigo en una serie histórica en la Región Semiárida Pampeana. Efecto de la fertilización nitrogenada bajo riego con agua de alto contenido de sales
In the Semiarid Central Pampa Region, with ow nitrogen content of the soils, about 26 % of the area cultivated with wheat is fertilized with nitrogen. However, water availability limits wheat production due to tow rainfall and a very marked water deficit during winter and summer. Therefore, complementary irrigation might contribute to increase the production or this crop, but the implementation of this technology has important restrictions, due to the abundance of highly saline ground water. In order to explain the yield increases due to a higher utilization or nitrogen in wheat, in modem cultivars respect to old cultivars we studied: a) a historical sequence of wheat cultivars in the Semiarid Pampa Region with respect to vegetative growth parameters and yield and b) the yield, with response of the genotypes to nitrogen fertilization with high - salt water irrigation. The experiment was carried out in, five genotypes released between 1954 and 1989. two levels of nitrogen fertilization and two levels of water availability. Our results showed that modem cultivars had a better yield index than the old ones. The yield response to nitrogen fertilization was different and the cultivars also differed in growth and yield in response to the irrigation with saline water. The grain yield response to nitrogen fertilization, in some genotypes, may not occur when irrigation with high salt content water is applied.La región semiárida central pampeana, dado las características de su suelo, de bajo contenido de nitrógeno, se fertiliza el 26% de la superficie cultivada con trigo. Sin embargo, la disponibilidad hídrica es el factor ambiental más Importante que podría limitar la producción de trigo en esta región, debido a las escasas precipitaciones y a un marcado déficit hídrico en invierno y verano por lo que, la aplicación de riego complementario podría contribuir a la mejora de la producción de este cultivo; sin embargo, la utilización de este, tiene importantes restricciones debido a la abundancia de acuíferos con alto contenido de sales En funci6n de una explicación del aumento del rendimiento en base a una mejor utilizaci6n del nitrógeno en trigo, en los cultivares modernos respecto de los cultivares antiguos, se estudió: a) en una serie histórica de cultivares de trigo, en la Región Semiárida Pampeana el comportamiento de parámetros de crecimiento vegetativo y rendimiento en grano y b) la respuesta de los genotipos a la fertilización nitrogenada bajo riego con agua de alto contenido de sales. El experimento se realizó a campo, y se trabajó con 5 genotipos liberados entre 1954 y 1989, dos niveles de nitrógeno y dos niveles de disponibilidad hídrica. Nuestros resultados mostraron una mejora en el índice de cosecha de los cultivares modernos respecto de los antiguos. La respuesta de los cultivares a la fertilización nitrogenada fue diferente en los parámetros de crecimiento y rendimiento en función del riego con agua salina. La respuesta del rendimiento en grano a la fertilización nitrogenada, en algunos genotipos, puede no manifestarse en condiciones de riego con agua de alto contenido de sales
Chemically active substitutional nitrogen impurity in carbon nanotubes
We investigate the nitrogen substitutional impurity in semiconducting zigzag
and metallic armchair single-wall carbon nanotubes using ab initio density
functional theory. At low concentrations (less than 1 atomic %), the defect
state in a semiconducting tube becomes spatially localized and develops a flat
energy level in the band gap. Such a localized state makes the impurity site
chemically and electronically active. We find that if two neighboring tubes
have their impurities facing one another, an intertube covalent bond forms.
This finding opens an intriguing possibility for tunnel junctions, as well as
the functionalization of suitably doped carbon nanotubes by selectively forming
chemical bonds with ligands at the impurity site. If the intertube bond density
is high enough, a highly packed bundle of interlinked single-wall nanotubes can
form.Comment: 4 pages, 4 figures; major changes to the tex
Mandelbrot set in coupled logistic maps and in an electronic experiment
We suggest an approach to constructing physical systems with dynamical
characteristics of the complex analytic iterative maps. The idea follows from a
simple notion that the complex quadratic map by a variable change may be
transformed into a set of two identical real one-dimensional quadratic maps
with a particular coupling. Hence, dynamical behavior of similar nature may
occur in coupled dissipative nonlinear systems, which relate to the Feigenbaum
universality class. To substantiate the feasibility of this concept, we
consider an electronic system, which exhibits dynamical phenomena intrinsic to
complex analytic maps. Experimental results are presented, providing the
Mandelbrot set in the parameter plane of this physical system.Comment: 9 pages, 3 figure
BN domains included into carbon nanotubes: role of interface
We present a density functional theory study on the shape and arrangement of
small BN domains embedded into single-walled carbon nanotubes. We show a strong
tendency for the BN hexagons formation at the simultaneous inclusion of B and N
atoms within the walls of carbon nanotubes. The work emphasizes the importance
of a correct description of the BN-C frontier. We suggest that BN-C interface
will be formed preferentially with the participation of N-C bonds. Thus, we
propose a new way of stabilizing the small BN inclusions through the formation
of nitrogen terminated borders. The comparison between the obtained results and
the available experimental data on formation of BN plackets within the single
walled carbon nanotubes is presented. The mirror situation of inclusion of
carbon plackets within single walled BN nanotubes is considered within the
proposed formalism. Finally, we show that the inclusion of small BN plackets
inside the CNTs strongly affects the electronic character of the initial
systems, opening a band gap. The nitrogen excess in the BN plackets introduces
donor states in the band gap and it might thus result in a promising way for
n-doping single walled carbon nanotubes
Supersymmetric solutions of PT-/non-PT-symmetric and non-Hermitian Screened Coulomb potential via Hamiltonian hierarchy inspired variational method
The supersymmetric solutions of PT-symmetric and Hermitian/non-Hermitian
forms of quantum systems are obtained by solving the Schrodinger equation for
the Exponential-Cosine Screened Coulomb potential. The Hamiltonian hierarchy
inspired variational method is used to obtain the approximate energy
eigenvalues and corresponding wave functions.Comment: 13 page
Subsequent Hematopoietic Stem Cell Transplantation (HSCT) Associated with Longer Survival in Patients with Relapsed/Refractory (R/R) Acute Myelogenous Leukemia (AML) After Clo+Ara-C or Ara-C Alone: A Landmark Analysis from the Classic I Trial
A Theoretical Analysis of the Feasibility of a Singularity-Induced Micro-Electroporation System
Electroporation, the permeabilization of the cell membrane lipid bilayer due to a pulsed electric field, has important implications in the biotechnology, medicine, and food industries. Traditional macro and micro-electroporation devices have facing electrodes, and require significant potential differences to induce electroporation. The goal of this theoretical study is to investigate the feasibility of singularity-induced micro-electroporation; an electroporation configuration aimed at minimizing the potential differences required to induce electroporation by separating adjacent electrodes with a nanometer-scale insulator. In particular, this study aims to understand the effect of (1) insulator thickness and (2) electrode kinetics on electric field distributions in the singularity-induced micro-electroporation configuration. A non-dimensional primary current distribution model of the micro-electroporation channel shows that while increasing insulator thickness results in smaller electric field magnitudes, electroporation can still be performed with insulators thick enough to be made with microfabrication techniques. Furthermore, a secondary current distribution model of the singularity-induced micro-electroporation configuration with inert platinum electrodes and water electrolyte indicates that electrode kinetics do not inhibit charge transfer to the extent that prohibitively large potential differences are required to perform electroporation. These results indicate that singularity-induced micro-electroporation could be used to develop an electroporation system that consumes minimal power, making it suitable for remote applications such as the sterilization of water and other liquids
Colonization, disability, and the intranet: the ethnic cleansing of space?
The article analyzes teacher’s emplacement of the image of disability within school’s intranet sites in England. The image unearthed within such sites was problematic as it did not display a positive or realistic image of disability or disabled people. Within the article historical archaeology and colonialism are employed as theoretic framework to interpret this artifact of disability. The article also provides an ethnographic subscript to the creation of a space of possibilities and how this became striated by missionary teachers who colonized this brave new intranet world. Deciphering of the organization and representation of the disabled indigene, through this theoretical framework, unearthed a cartography inscribed by the scalpel of old world geometry
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