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)

A time-dependent model for improved biogalvanic tissue characterisation

Measurement of the passive electrical resistance of biological tissues through biogalvanic characterisation has been proposed as a simple means of distinguishing healthy from diseased tissue. This method has the potential to provide valuable real-time information when integrated into surgical tools. Characterised tissue resistance values have been shown to be particularly sensitive to external load switching direction and rate, bringing into question the stability and efficacy of the technique. These errors are due to transient variations observed in measurement data that are not accounted for in current electrical models. The presented research proposes the addition of a time-dependent element to the characterisation model to account for losses associated with this transient behaviour. Influence of switching rate has been examined, with the inclusion of transient elements improving the repeatability of the characterised tissue resistance. Application of this model to repeat biogalvanic measurements on a single ex vivo human colon tissue sample with healthy and cancerous (adenocarcinoma) regions showed a statistically significant difference (p 0.05) between tissue types was found when measurements were subjected to the current model, suggesting that the proposed model may allow for improved biogalvanic tissue characterisation

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

Descripción botánica del hibrido interespecífico entre Zea mays x Zea diploperennis

En la Facultad de Agronomía de la Universidad Nacional de La Pampa se obtuvieron híbridos entre Zea mays L. y Zea diploperennis Litis usando a Zea mays L. como madre. Para lograr la inducción fotoperiódica de Zea diploperennis se le suministraron aproximadamente 12 hs de oscuridad (para ello se cubrían las plantas a las 20 hs, y se descubrían a las 8 hs, del día siguiente); después de 15 días de inducción se constató en el ápice vegetativo su pasaje a reproductivo. A mediados de febrero de 1987 se dispuso de polen de Zea diploperennis L para fertilizar las plantas de maíz. Las semillas, producto de cruzamiento, se sembraron a campo en noviembre de 1988 y se lograron siete plantas híbridas

Dental status of persons engaged in sports swimming

To date, information on the problem is contradictory and ambiguous. It is difficult to trace cause – effect relationships between the condition of hard dental tissues, oral fluid, the dental status of swimmers, and the characteristics of the water environment of training pools. The article presents an analysis and systematization of literature on the dental condition in swimming athletes, as well as formulates modern ideas on the subject and highlights topics for further research

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

Condition of hard tissues of teeth and oral fluid in athletes-swimmers

Objective: to determine the state of dental hard tissues and saliva parameters changing in competitive swimmers, before and after a 2­hour training session in the chlorinated water swimming pool.Materials and methods: saliva parameters of competitive swimmers trained in the sport club “Grifon”, city of St. Petersburg, Russia (n = 21), age of participants 18.1 ± 3.5 years, qualifications (1 category­Master of Sports) and track and field athletes of the sports schools “Nevsky district” and “Orlyonok”, city of St. Petersburg, Russia (n = 18), age of participants 18.80 ± 4.54 years, qualifications (1 category — Master of Sports), have been analyzed by various tests include a study of calcium (Ca), phosphorus (P), fluorine (F) levels, and pH before and after training sessions. All athletes in the experimental and control groups have passed an examination by dentist, including: examination of the oral cavity with the determination of the KPU index (the sum of carious, filled and removed permanent teeth in the subject), examination of the most common places of occurrence of dental enamel erosion (medical binoculars) and filling out specially designed questionnaires.Results: there was a tendency to decrease of the average saliva pH in competitive swimmers’ group after a training session, varying from 6.9 ± 0.1 (before training) to 6.5 ± 0.1 (after training) (p > 0.05). In track and field athletes, saliva pH did not change significantly (7.1 ± 0.2 before training and 7.0 ± 0.1 after training) (p < 0.05). The competitive swimmers showed a statistically significant increase in Calcium (Ca) levels (1.25 ± 0.15 mmol/L before exercise and 1.56 ± 0.11 mmol/L after exercise) and Fluorine (F) (0.0010 ± 0.0003 mmol / L before exercise and 0.0090 ± 0.0004 mmol / L after training session) in saliva. The Phosphorus (P) level in saliva was significantly reduced after the training session (from 6.09 ± 0.39 to 3.89 ± 0.46 mmol / L) (p > 0.05). In track and field athletes, there were no significant changes in Ca, F and P levels before and after a training session. As a result of the dentist’s examination, competitive swimmers were found to have 3 athletes (14.3 %) with lesions of the teeth enamel, corresponding to dental erosion and caused by localized demineralization. In the control group of athletes, no such pathologies were revealed.Conclusion: discovered saliva parameters changing in competitive swimmers (decrease in free salivation, pH level, and increase in the level of Ca and F in saliva) can contribute to the demineralization of tooth enamel. Timely and controlled use of fluorides (as part of mouth rinses, application gels, fluoride varnishes), regular preventive dental examinations, in order to prevent the loss of mineral composition of the teeth when swimming in chlorinated pools, will minimize the risk of staining and dental enamel erosions

Agent-Centered Decision Making in Normal and Abnormal Cognition

Much of human cognition is “agent-centered,” subjective, and in that sense relative, directed at deciding, “What is best for me”. This is very different  from  “veridical”  cognition,  directed  at  finding  an  objectively  correct solution inherent in the task and independent of the agent. The frontal lobes in particular are central to agent-centered decision making. Yet very little is available  in  the  arsenal  of  cognitive  paradigms  used  in  the  cognitive neuroscience  research  and  in  clinical  neuropsychology  test  design  to examine  “agent-centered”  decision  making.  Current  paradigms  and  tests used  to  measure  decision  making  clinically  and  experimentally  are veridical  in  nature  and  as  such  miss  the  essence  of  “agent-centered” cognition.  The  dearth  of  “agent-centered”  cognitive  paradigms  severely limits  our  ability  to  understand  fully  the  function  and  dysfunction  of  the frontal  lobes.  The  Cognitive  Bias  Task  (CBT)  is  an  agent-centered paradigm  designed  to  fill  this  gap.  CBT  has  been  used  as  a  cognitive activation task in fMRI, SPECT, and EEG, as well as in studies of normal development,  addiction,  dementia,  focal  lesions,  and  schizophrenia.  This resulted  in  a  range  of  findings  that  eluded  more  traditional  “veridical” paradigms and are reviewed here.  Key Words:  Decision Making;  Cognitive Bias Task; Cognition    La Toma de Decisiones Centrada en el Agente en la Cognición Normal y  Anormal:  Gran  parte  de  la  cognición  humana  está  "centrada  en  el agente", es subjetiva, y en ese sentido, está dirigida a decidir: "¿Qué es  lo mejor para mí?". Esta es una perspectiva muy diferente a la de la cognición "verídica",  que  está  más  dirigida  a  encontrar  una  solución  objetivamente correcta,  inherente  a  la  tarea,  e  independiente  del  agente.  Los  lóbulos frontales,  en  particular,  son  fundamentales  para  la  toma  de  decisiones "centrada en el agente". Sin embargo, existen pocos paradigmas cognitivos en  el  marco  de  la  neurociencia  cognitiva  y  de  la  neuropsicología  clínica, que  se  hayan  diseñado  para  evaluar  la  toma  de  decisiones  "centrada  en  el agente".  Los  paradigmas  actuales  y  las  pruebas  utilizadas  para  medir  la toma  de  decisiones  clínica  y  experimentalmente  son  "verídicos"  en  su naturaleza  y,  por  lo  mismo,  no  son  adecuados  para  la  evaluación  de  la cognición "centrada en el agente". La escasez de paradigmas "centrados en el agente" limita severamente nuestra capacidad de entender plenamente la función  y  la  disfunción  de  los  lóbulos  frontales.  Al  respecto,  la  Tarea  de Sesgo Cognitivo (o Cognitive Bias Task, CBT) es un paradigma "centrado en el agente" diseñado para llenar este vacío. La CBT se ha utilizado como una tarea de activación cognitiva en estudios de IRMf, SPECT y EEG, así como  en  estudios  del  desarrollo  normal,  adicciones,  demencia, lesiones focales, y esquizofrenia. Consecuentemente, se han obtenido hallazgos que evitan  algunas  de  las  limitaciones  de  los  paradigmas  más  tradiciones, "verídicos", y que se revisan en este artículo. Palabras claves:  Toma de Decisiones; Tarea Con Preferencia Cognitiva; Cognición

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

Comparative high pressure Raman study of boron nitride nanotubes and hexagonal boron nitride

High pressure Raman experiments on boron nitride multi-walled nanotubes show that the intensity of the vibrational mode at ~1367 cm−1 vanishes at ~12 GPa and it does not recover under decompression. In comparison, the high pressure Raman experiments on hexagonal boron nitride show a clear signature of a phase transition from hexagonal to wurtzite at ~13 GPa which is reversible on decompression. These results are contrasted with the pressure behavior of carbon nanotubes and graphite