viernes, 21 de enero de 2011

Utilidad múltiple del grafeno


Utilidad múltiple del grafeno
El grafeno es un nuevo material de sólo dos dimensiones, con propiedades asombrosas para la física teórica. El investigador García Naumis del Departamento de Física Cuántica de la UNAM está llevando a cabo una investigación teórica que pueda predecir cómo controlar los electrones del grafeno. Esta investigación ha permitido elaborar una primera ecuación que permite obtener la respuesta de esos electrones con campos electromagnéticos. La solución de esta ecuación tiene utilidad práctica: con ella es posible diseñar transistores de ese material, aparatos electrónicos innovadores, transistores más eficientes, procesadores más veloces, nuevos paneles de luz, además de otros productos fuertes, elásticos y translúcidos así como estudiar fenómenos a nivel cuántico relativista.
FUENTE | Universidad de México (UNAM)10/01/2011

México
De la punta de un lápiz se puede obtener algo nunca antes visto para la física teórica, un nuevo material de sólo dos dimensiones, con propiedades asombrosas: el grafeno.

En 2004, los físicos Andre Geim y Konstantin Novoselov, de la Universidad de Manchester, Inglaterra, obtuvieron en laboratorio capas de grafeno a partir de un experimento que consistió en despegar repetidas veces una cinta adhesiva doblada e impregnada de hojuelas de grafito.

Seis años después, los científicos rusos ganaron el premio Nobel en Física por sus aportaciones a la ciencia básica relacionadas con las propiedades de ese cristal de carbono bidimensional (algo que no podía existir) y con sus posibles aplicaciones tecnológicas. "Los experimentos realizados con el grafeno suponen un punto de inflexión en los fenómenos de la física cuántica", dijo el comité de los premios Nobel.

"El grafeno es el material más duro y delgado jamás hallado (es un millón de veces menos grueso que una hoja de papel); es también el mejor conductor de electricidad y de calor; además, tiene propiedades ópticas interesantes", aseguró Gerardo García Naumis, quien encabeza un equipo de investigadores dedicado a su estudio teórico en el Instituto de Física (IF) de la UNAM.

Con el grafeno se podrán fabricar, en un futuro próximo, aparatos electrónicos innovadores, transistores más eficientes que los actuales de silicio, procesadores más veloces, nuevos paneles de luz, celdas solares, además de otros productos y componentes fuertes y, al mismo tiempo, delgados, elásticos y translúcidos. Además, mezclado con plásticos será un conductor de electricidad resistente al calor.

TRANSISTORES DE GRAFENO
El grafeno es una lámina formada por átomos de carbono dispuestos en los vértices de una red hexagonal que se parece a un panal de abejas; es como una autopista, donde la movilidad electrónica es 10 veces mayor que en los mejores materiales conductores, lo que permite que los electrones conduzcan la electricidad con mucha más rapidez.

Con esa cualidad, se podrán sustituir los transistores de silicio, que ya no se pueden hacer más pequeños sin correr el riesgo de degradarse rápidamente y generar calor en extremo; con los de grafeno se podrán elaborar procesadores de computadoras más veloces y ahorradores de energía.

El primer trabajo de García Naumis sobre el grafeno, publicado en 2006, es una predicción teórica para resolver el problema que implica construir transistores con ese material.

"En el grafeno es difícil 'detener' los electrones, y en un transistor, éstos tienen que ser controlados con una especie de llave, que a veces hay que cerrar y abrir, de manera que se puedan hacer unos y ceros, como si fueran pulsos de corriente", indicó.

Entonces, propuso dopar el grafeno con una cierta concentración de átomos ligeros (de hidrógeno y litio, por ejemplo) para generar un material semiconductor, en el que sí se pueda controlar el flujo de electrones.

En 2009, un grupo de investigación de la Universidad de California, en Estados Unidos, demostró que los cálculos de esa propuesta son correctos y que sí se pueden hacer transistores de grafeno.

Este trabajo sobre el dopaje del grafeno fue incluido en el Virtual Journal of Nanotecnology, que cada mes publica los mejores artículos en la materia a nivel mundial.

PREDICCIÓN TEÓRICA
Otra alternativa para controlar los electrones de carbono es la irradiación del grafeno con ondas electromagnéticas, sean de radio o de luz. Mediante este proceso, los electrones adquieren una "masa efectiva" y, por lo tanto, se genera una fuerte respuesta no lineal.

"Pronosticamos que si el sistema es perturbado con una frecuencia dada, genera lo que se llaman armónicos, es decir, responde con el doble o el triple de frecuencia. Por este efecto no lineal, el grafeno podría trabajar a frecuencias mucho más altas de las esperadas, operar a velocidades más rápidas de reloj", explicó.

Esta solución para controlar los electrones, postulada con base en un enfoque cuántico relativista por García Naumis y su alumno de doctorado Francisco López Rodríguez fue comprobada por investigadores de la Universidad de Massachusetts, EU.

Una vez publicada por la editorial inglesa Francis & Taylor, la solución fue incluida en el Philosophical Magazine y seleccionada como uno de los siete artículos científicos más importantes relacionados con el premio Nobel de Física 2010.

"Como somos teóricos, hicimos una predicción para controlar los electrones del grafeno por medio de la introducción de impurezas y establecimos una primera ecuación que nos permite obtener la respuesta de esos electrones con campos electromagnéticos. La solución de esta ecuación tiene utilidad práctica: con ella es posible diseñar transistores de ese material, así como estudiar fenómenos a nivel cuántico relativista", añadió el investigador universitario.

ANALOGÍA
"Visto al microscopio, el grafeno es como una sábana arrugada: plano, con ligeras ondulaciones, como suponía la física teórica que debería ser un cristal bidimensional".

En la actualidad, García Naumis estudia qué sucede a los electrones cuando "sienten" esas ondulaciones, lo que es equivalente a considerar partículas en un espacio curvo.

Para lograr una descripción del movimiento de los electrones en el grafeno, en el marco de la gravedad cuántica relativista, el investigador trabaja en una analogía en colaboración con el cosmólogo Richard Kerner, de la Universidad de París, Francia. Un resultado preliminar del proyecto fueron algunas ecuaciones que describen los electrones en ese espacio curvo, parecidas a las ecuaciones relativistas. "Aún hay que explorar más en esa analogía; quizás podría dar pistas que ayuden a relacionar la mecánica cuántica relativista con la gravedad, algo que no alcanzó a hacer Albert Einstein?. Esto, en principio, a partir de un material que puede desprenderse de la punta de un lápiz, concluyó.

Brasil, atractivo para jóvenes científicos


Brasil, atractivo para jóvenes científicos
La revista británica The Economist afirma que Brasil se está convirtiendo en un destino cada vez más atractivo para la realización de investigaciones científicas y académicas.
FUENTE | Inovação tecnológica10/01/2011

Brasil
La revista presenta datos que muestran el crecimiento de esta área en el país, y cita las iniciativas del gobierno que impulsan la investigación, y que es un líder mundial en investigación, especialmente en las áreas de la medicina tropical, la bioenergía y la biología botánica.

Sin embargo, afirma que el activo más importante de Brasil son las posibilidades que ofrecen las universidades brasileñas para que los investigadores puedan seguir adelante. "Usted puede tener su propio laboratorio de aquí y puede incluso iniciar una nueva línea de investigación. En este caso, usted es un pionero", dice una genetista botánica de la Unicamp, en la publicación de The Economist.

Además, la revista destaca el hecho de que las becas para investigadores principiantes tienen un valor comparable a los estándares mundiales, pero añade que esto mismo no ocurre con la becas para académicos con más experiencia. "Sin embargo, Fapesp está tratando de cambiar este escenario. La institución ha publicado un anuncio en la revista Nature referido a un programa de dos años para estudiar en universidades de Sao Paulo", dice la publicación. "Y mientras que la mayoría de las respuestas proceden de los científicos al inicio de su carrera, son los más experimentados con los que se contacta para charlar. Y FAPESP espera que durante esos dos años, aprendan portugués y -algunos de ellos - decidan quedarse en el país".

PROBLEMAS TROPICALES
Según la revista, en Brasil se graduaron 500.000 estudiantes en la educación superior y 10.000 doctores en un año - diez veces más que hace 20 años. El país también ha aumentado su cuota en el volumen total de los estudios científicos publicados en el mundo: del 1,7% en 2002 al 2,7% en 2008.The Economist afirma que formar parte de la iniciativa científica mundial también está relacionado con el orgullo nacional: "Al invertir en ciencia en su propio territorio, los países tropicales garantizan que los problemas que se resuelven no sean únicamente los de las naciones ricas." 

To Really Learn, Quit Studying and Take a Test


To Really Learn, Quit Studying and Take a Test

Taking a test is not just a passive mechanism for assessing how much people know, according to new research. It actually helps people learn, and it works better than a number of other studying techniques.

The research, published online Thursday in the journal Science
, found that students who read a passage, then took a test asking them to recall what they had read, retained about 50 percent more of the information a week later than students who used two other methods.
One of those methods — repeatedly studying the material — is familiar to legions of students who cram before exams. The other — having students draw detailed diagrams documenting what they are learning — is prized by many teachers because it forces students to make connections among facts.
These other methods not only are popular, the researchers reported; they also seem to give students the illusion that they know material better than they do.
In the experiments, the students were asked to predict how much they would remember a week after using one of the methods to learn the material. Those who took the test after reading the passage predicted they would remember less than the other students predicted — but the results were just the opposite.
“I think that learning is all about retrieving, all about reconstructing our knowledge,” said the lead author, Jeffrey Karpicke, an assistant professor of psychology at Purdue University. “I think that we’re tapping into something fundamental about how the mind works when we talk about retrieval.”
Several cognitive scientists and education experts said the results were striking.
The students who took the recall tests may “recognize some gaps in their knowledge,” said Marcia Linn, an education professor at the University of California, Berkeley, “and they might revisit the ideas in the back of their mind or the front of their mind.”
When they are later asked what they have learned, she went on, they can more easily “retrieve it and organize the knowledge that they have in a way that makes sense to them.”
The researchers engaged 200 college students in two experiments, assigning them to read several paragraphs about a scientific subject — how the digestive system works, for example, or the different types of vertebrate muscle tissue.
In the first experiment, the students were divided into four groups. One did nothing more than read the text for five minutes. Another studied the passage in four consecutive five-minute sessions.
A third group engaged in “concept mapping,” in which, with the passage in front of them, they arranged information from the passage into a kind of diagram, writing details and ideas in hand-drawn bubbles and linking the bubbles in an organized way.
The final group took a “retrieval practice” test. Without the passage in front of them, they wrote what they remembered in a free-form essay for 10 minutes. Then they reread the passage and took another retrieval practice test.
A week later all four groups were given a short-answer test that assessed their ability to recall facts and draw logical conclusions based on the facts.
The second experiment focused only on concept mapping and retrieval practice testing, with each student doing an exercise using each method. In this initial phase, researchers reported, students who made diagrams while consulting the passage included more detail than students asked to recall what they had just read in an essay.
But when they were evaluated a week later, the students in the testing group did much better than the concept mappers. They even did better when they were evaluated not with a short-answer test but with a test requiring them to draw a concept map from memory.
Why retrieval testing helps is still unknown. Perhaps it is because by remembering information we are organizing it and creating cues and connections that our brains later recognize.
“When you’re retrieving something out of a computer’s memory, you don’t change anything — it’s simple playback,” said Robert Bjork, a psychologist at the University of California, Los Angeles, who was not involved with the study.
But “when we use our memories by retrieving things, we change our access” to that information, Dr. Bjork said. “What we recall becomes more recallable in the future. In a sense you are practicing what you are going to need to do later.”
It may also be that the struggle involved in recalling something helps reinforce it in our brains.
Maybe that is also why students who took retrieval practice tests were less confident about how they would perform a week later.
“The struggle helps you learn, but it makes you feel like you’re not learning,” said Nate Kornell, a psychologist at Williams College. “You feel like: ‘I don’t know it that well. This is hard and I’m having trouble coming up with this information.’ ”
By contrast, he said, when rereading texts and possibly even drawing diagrams, “you say: ‘Oh, this is easier. I read this already.’ ”
The Purdue study supports findings of a recent spate of research showing learning benefits from testing, including benefits when students get questions wrong. But by comparing testing with other methods, the study goes further.

“It really bumps it up a level of importance by contrasting it with concept mapping, which many educators think of as sort of the gold standard,” said Daniel Willingham, a psychology professor at the University of Virginia. Although “it’s not totally obvious that this is shovel-ready — put it in the classroom and it’s good to go — for educators this ought to be a big deal.”
Howard Gardner, an education professor at Harvard who advocates constructivism — the idea that children should discover their own approach to learning, emphasizing reasoning over memorization — said in an e-mail that the results “throw down the gauntlet to those progressive educators, myself included.”
“Educators who embrace seemingly more active approaches, like concept mapping,” he continued, “are challenged to devise outcome measures that can demonstrate the superiority of such constructivist approaches.”
Testing, of course, is a highly charged issue in education, drawing criticism that too much promotes rote learning, swallows valuable time for learning new things and causes excessive student anxiety.
“More testing isn’t necessarily better,” said Dr. Linn, who said her work with California school districts had found that asking students to explain what they did in a science experiment rather than having them simply conduct the hands-on experiment — a version of retrieval practice testing — was beneficial. “Some tests are just not learning opportunities. We need a different kind of testing than we currently have.”
Dr. Kornell said that “even though in the short term it may seem like a waste of time,” retrieval practice appears to “make things stick in a way that may not be used in the classroom.
“It’s going to last for the rest of their schooling, and potentially for the rest of their lives.”

The Greatest




The Greatest

HERE goes. This article completes my two-week project to select the top 10 classical music composers in history, not including those still with us. The argument, laid out in a series of articles, online videos and blog posts, was enlivened by the more than 1,500 informed, challenging, passionate and inspiring comments from readers of The New York Times. As often as I could, I answered direct questions online and jumped into the discussion.
Thomas Fuchs
The Greatest

The Top 10 Composers

Anthony Tommasini explores the qualities that make a classical composer great, maybe even the best of all time.

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Left, 1. Johann Sebastian Bach (1685-1750). From top left, 2. Ludwig van Beethoven (1770-1827), 3. Wolfgang Amadeus Mozart (1756 — 91). 4. Franz Peter Schubert (1797-1828). From middle left, 5. Claude Achille Debussy (1862 — 1918), 6. Igor Stravinsky (1882 — 1971), 7. Johannes Brahms (1833 — 97). From bottom left, 8. Giuseppe Verdi (1813 — 1901), 9. Richard Wagner (1813 — 83), 10. Bela Bartok (1881 — 1945).

Readers' Comments

I am about to reveal my list, though as those who have been with me on this quest already know, I’ve dropped hints along the way. And the winner, the all-time great, is ... Bach!
To step back for a moment, I began this project with bravado, partly as an intellectual game but also as a real attempt to clarify — for myself, as much as for anyone else — what exactly about the master composers makes them so astonishing. However preposterous the exercise may seem, when I found myself debating whether to push Brahms or Haydn off the list to make a place for Bartok or Monteverdi, it made me think hard about their achievements and greatness.
Ah, greatness. Early on I received a friendly challenge from a reader (“Scott”) who questioned the whole notion of greatness in music. He cited the title essay in “Listen to This,” a collection of astute, lively writings by Alex Ross, the music critic for The New Yorker and my good friend, which was published last year (Farrar, Straus & Giroux). In this essay he argues that the very term “classical music” makes this vibrant art form seem dead. Indeed, as he writes, “greatness” and “seriousness” are not classical music’s defining characteristics; it can also “be stupid, vulgar and insane.”
All true. Yet what came through in the comments from readers and, I hope, my articles and videos is that for most of us these composers are not monumental idols but living, compelling presences. Just as we organize our lives by keeping those we love in a network of support, we do something similar with the composers we rely on.
I was moved by how many readers could not wait to share their lists of favorite composers, whom, naturally, they also considered the greats. Even many of those who dismissed the exercise jumped right in: “This is absurd, of course. But here’s my list. And don’t you dare leave out Mahler.” OrBerg. Or Ligeti. Or, from one Baroque music enthusiast, Albinoni!
As a longtime champion of contemporary music, I was gratified to receive so many objections to my decision to eliminate living composers from consideration. Still, for me there was no other way. We are too close to living composers to have perspective. Besides, assessing greatness is the last thing on your mind when you are listening to an involving, exciting or baffling new piece.
So humbled by the discerning music lovers who wrote in, I now offer my own list. And remember: my editors gave the go-ahead for this project on condition that I go all the way and rank my 10 in order.
My top spot goes to Bach, for his matchless combination of masterly musical engineering (as one reader put it) and profound expressivity. Since writing about Bach in the first article of this series I have been thinking more about the perception that he was considered old-fashioned in his day. Haydn was 18 when Bach died, in 1750, and Classicism was stirring. Bach was surely aware of the new trends. Yet he reacted by digging deeper into his way of doing things. In his austerely beautiful “Art of Fugue,” left incomplete at his death, Bach reduced complex counterpoint to its bare essentials, not even indicating the instrument (or instruments) for which these works were composed.
On his own terms he could be plenty modern. Though Bach never wrote an opera, he demonstrated visceral flair for drama in his sacred choral works, as in the crowd scenes in the Passions where people cry out with chilling vehemence for Jesus to be crucified. In keyboard works like the Chromatic Fantasy and Fugue, Bach anticipated the rhapsodic Romantic fervor of Liszt, even Rachmaninoff. And as I tried to show in the first video for this project, through his chorales alone Bach explored the far reaches of tonal harmony.
The obvious candidates for the second and third slots are Mozart and Beethoven. If you were to compare just Mozart’s orchestral and instrumental music to Beethoven’s, that would be a pretty even match. But Mozart had a whole second career as a path-breaking opera composer. Such incredible range should give him the edge.
Still, I’m going with Beethoven for the second slot. Beethoven’s technique was not as facile as Mozart’s. He struggled to compose, and you can sometimes hear that struggle in the music. But however hard wrought, Beethoven’s works are so audacious and indestructible that they survive even poor performances.
I had an epiphany about Beethoven during the early 1980s when I heard the composer Leon Kirchner conduct the Harvard Chamber Orchestra. He began with a Piston symphony, a fresh, inventive Neo-Classical piece from the 1950s. “La Mer” by Debussy came next, and Kirchner, who had studied with Schoenberg and had a Germanic orientation, brought weighty, Wagnerian intensity to this landmark score, completed in 1905. The Debussy came across as more modern than the Piston.
After intermission Peter Serkin joined Kirchner for a performance of Beethoven’s Fourth Piano Concerto that brought out the mysticism, poetic reverie and wildness of the music. The Beethoven sounded like the most radical work in the program by far: unfathomable and amazing. I’m giving Beethoven the second slot, and Mozart No. 3.
Four? Schubert. You have to love the guy, who died at 31, ill, impoverished and neglected except by a circle of friends who were in awe of his genius. For his hundreds of songs alone — including the haunting cycle “Winterreise,” which will never release its tenacious hold on singers and audiences — Schubert is central to our concert life. The baritone Sanford Sylvan once told me that hearing the superb pianist Stephen Drury give searching accounts of the three late Schubert sonatas on a single program was one of the most transcendent musical experiences of his life. Schubert’s first few symphonies may be works in progress. But the “Unfinished” and especially the Ninth Symphony are astonishing. The Ninth paves the way for Bruckner and prefigures Mahler.




Debussy, who after hundreds of years of pulsating Germanic music proved that there could be tension in timelessness, is my No. 5. With his pioneering harmonic language, the sensual beauty of his sound and his uncanny, Freudian instincts for tapping the unconscious, Debussy was the bridge over which music passed into the tumultuous 20th century.

Readers' Comments

One who later walked that bridge was Stravinsky, my No. 6. During the years when “The Firebird” and “The Rite of Spring” were shaking up Paris, Stravinsky was swapping ideas with his friend Debussy, who was 20 years older. Yet Stravinsky was still around in the 1960s, writing serial works that set the field of contemporary music abuzz. One morning in 1971 I arrived at the door of the music building at Yale, on which someone had posted an index card with this simple news: “Igor Stravinsky died today.” It felt as if the floor had dropped out from under the musical world I inhabited. Stravinsky had been like a Beethoven among us.
I’m running out of slots. In some ways, as I wrote to one reader, either a list of 5 or a list of 20 would have been much easier. By keeping it to 10, you are forced to look for reasons to push out, say, Handel or Shostakovich to make a place for someone else.
Some musicians I respect have no trouble finding shortcomings in Brahms. He did sometimes become entangled in an attempt to extend the Classical heritage while simultaneously taking progressive strides into new territory. But at his best (the symphonies, the piano concertos, the violin concerto, the chamber works with piano, the solo piano pieces, especially the late intermezzos and capriccios that point the way to Schoenberg) Brahms has the thrilling grandeur and strangeness of Beethoven. Brahms is my No. 7.
In an earlier installment of this series I tried to weasel out of picking Romantic composers other than Brahms by arguing that the era fostered originality and personal expression above all. To a genius like Chopin, having a distinctive voice and giving vent to his inspirations were more important than achieving some level of quantifiable greatness.
But the dynamic duo of 19th-century opera, Verdi and Wagner, aimed high. As I already let slip, they both make my list. That a new production of a Verdi opera, like Willy Decker’s spare, boldlyreimagined staging of “La Traviata” at the Metropolitan Opera, can provoke such heated passions among audiences is testimony to the enduring richness of Verdi’s works. A production of Wagner’s “Ring” cycle has become the entry card for any opera company that wants to be considered big time. The last 20 minutes of “Die Walküre” may be the most sadly beautiful music ever written.
But who ranks higher? They may be tied as composers but not as people. Though Verdi had an ornery side, he was a decent man, an Italian patriot and the founder of a retirement home for musicians still in operation in Milan. Wagner was an anti-Semitic, egomaniacal jerk who transcended himself in his art. So Verdi is No. 8 and Wagner No. 9.
One slot left. May Haydn forgive me, but one of the Vienna Four just had to go, and Haydn’s great legacy was carried out by his friend Mozart, his student Beethoven and the entire Classical movement. My apologies to Mahler devotees, so impressively committed to this visionary composer. Would that I could include my beloved Puccini.
I was heartened by the hundreds of readers who championed 20th-century composers like Ligeti, Messiaen, Shostakovich, Ives, Schoenberg, Prokofiev and Copland, all of whom are central to my musical life. Then there is Berg, who wrote arguably the two greatest operas of the 20th century. His Violin Concerto, as I explained in my first video, would make my list of top 10 pieces. I was disappointed that an insignificant number of readers made a case for Britten. I have some advocacy work to do.
I received the most forceful challenges from readers who thought that pre-Bach composers simply had to be included, especially Monteverdi. Though Monteverdi did not invent opera, he took one look at what was going in Florence around 1600 and figured out how this opera thing should really be done. In 1607 he wrote “Orfeo,” the first great opera. His books of madrigals brought the art of combining words and music to new heights. The Monteverdi contingent is probably right.
But forced to pick only one more composer, I’m going with Bartok. In an earlier piece I made my case for Bartok, as an ethnomusicologist whose work has empowered generations of subsequent composers to incorporate folk music and classical traditions from whatever culture into their works, and as a formidable modernist who in the face of Schoenberg’s breathtaking formulations showed another way, forging a language that was an amalgam of tonality, unorthodox scales and atonal wanderings.
So that’s my list.
And now, in an act of contrition, I am beginning a personal project to listen nonstop to recordings of Britten, Haydn, Chopin, Monteverdi, Ligeti and those composers whom I could not squeeze in but whose music carries me through my days.