Thursday, January 13, 2011


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European and U.S. physicists are offering up the strongest evidence yet that magnetism is the driving force behind unconventional superconductivity. The findings by researchers from Rice University, the Max Planck Institute for Chemical Physics of Solids (MPI-CPfS) in Dresden, Germany, and other institutions were published online in Nature Physics.

The findings follow more than three decades of research by the team that discovered unconventional superconductivity in 1979. That breakthrough, which was led by MPI-CPfS Director Frank Steglich, preceded by seven years the more widely publicized discovery of unconventional superconductivity at high temperatures. In the latest study, the team revisited the same heavy-fermion material -- a mix of cerium, copper and silicon -- that was used in 1979, applying new experimental techniques and theoretical knowledge unavailable 30 years ago.

"In 1979, there was not much understanding of quantum criticality or of the collective way that electrons behave at the border of magnetism," said Rice physicist Qimiao Si, the lead theorist and co-author of the new paper. "Today, we know a great deal about such collective behavior in the regime where materials transition to a superconducting state. The question we examined in this study is, How does all of that new knowledge translate into an understanding of the superconducting state itself?"

Magnetism -- the phenomenon that drives compass needles and keeps notes stuck to refrigerators the world over -- arises when the electrons in a material are oriented in a particular way. Every electron is imbued with a property called spin, and electron spins are oriented either up or down. In most materials, the arrangement of electron spins is haphazard, but in everyday refrigerator magnets -- which scientists call ferromagnets -- electron spins are oriented collectively, in the same direction.

Classical superconductors, which were discovered almost a century ago, were the first materials known to conduct electrons without losing energy due to resistance. Electrons typically bump and ricochet from atom to atom as they travel down a wire, and this jostling leads to a loss of energy in the form of electrical resistance. Resistance costs the energy industry billions of dollars per year in lost power, so scientists have been keen to put superconducting wires to widespread use, but it hasn't been easy.

It took physicists almost 50 years to explain classical superconductivity: At extremely low temperatures, electrons pair up and move in unison, thus avoiding the jostling they experience by themselves. These electron twosomes are called Cooper pairs, and physicists began trying to explain how they form in unconventional superconductors as soon as Steglich's findings were published in 1979. Si said theorists studying the question have increasingly been drawn to the collective behavior of electrons, particularly at the border of magnetism -- the critical point where a material changes from one magnetic state to another.

In the new experiments, Steglich, the lead experimentalist co-author, and his group collaborated with physicists at the Jülich Centre for Neutron Science at the Institut Laue-Langevin in Grenoble, France, to bombard heavy fermion samples with neutrons. Because neutrons also have spin, those experiments allowed the team to probe the spin states of the electrons in the heavy fermions.

"Our neutron-scattering data provide convincing evidence that the cerium-based heavy fermion compound is located near a quantum critical point," said Oliver Stockert, a study co-author and a neutron-scattering specialist from MPI-CPfS. "Moreover, the data revealed how the magnetic spectrum changes as the material turns into a superconductor."

From the data, Si and co-author Stefan Kirchner, a theorist from the Max Planck Institute for the Physics of Complex Systems and a former postdoctoral fellow at Rice, determined the amount of magnetic energy that was saved when the system entered the superconducting state.

"We have calculated that the saved magnetic energy is more than 10 times what is needed for the formation of the Cooper pairs," Kirchner said.

"Why the magnetic exchange in the superconductor yields such a large energy saving is a new and intriguing question," said Si, Rice's Harry C. and Olga K. Wiess Professor of Physics and Astronomy. He said one possible origin is the electronic phenomenon known as the "Kondo effect," which is involved in a class of unconventional quantum critical points advanced by Si and colleagues in a theoretical paper published in Nature in 2001. Regardless of the final answer, Si said the present study already constitutes a definitive proof that "collective fluctuations of the electrons at the border of magnetism are capable of driving superconductivity."

Si and Steglich found it remarkable that the notion of quantum criticality is providing fresh insights into the workings of the very first unconventional superconductor ever discovered. At the same time, both said more studies are needed to determine the precise way that quantum-critical fluctuations give rise to heavy-fermion superconductivity. And thanks to key differences between the heavy-fermion materials and high-temperature superconductors, additional work must be done to determine whether the same findings apply to both.

"We are certain that we are on the right track with our investigations, however," Steglich said.

Rice University


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You can touch a functioning light bulb and know right away that it's hot. Ouch! But you can't touch a single molecule and get the same feedback.

Rice University researchers say they have the next best thing -- a way to determine the temperature of a molecule or flowing electrons by using Raman spectroscopy combined with an optical antenna.

A new paper from the lab of Douglas Natelson, a Rice professor of physics and astronomy, details a technique that measures the temperature of molecules set between two gold nanowires and heated either by current applied to the wires or laser light. The paper was published this week in the online edition of Nature Nanotechnology.

Natelson, postdoctoral research associate Dan Ward and their colleagues found that while measuring heat at the nanoscale can be much more complicated than taking the temperature of macro objects, it can be done with a level of accuracy that will be of interest to the molecular electronics community or anyone who wants to know how heating and dissipation work at very small scales.

"When you get down to making small electronic devices or tiny junctions, you have to worry about how energy ends up in the form of heat," Natelson said. "In the case of macroscopic objects, like the filament in a light bulb, you can attach a thermocouple -- a thermometer -- and measure it." When light bulbs get hot, they also glow. "If you look at the spectrum of the light coming out, you can figure out how hot it is," he said.

That's an over-simplified version of what Natelson and Ward are doing. One can't see the glow of a molecule. However, the researchers can send in light as a probe and detect the wavelength of the light that molecule is returning when heated. "In Raman scattering, you send in light that interacts with your target. When it comes back, it will either have more energy than you put in, or the same, or less. And we can see that and figure out the effective temperature of whatever is scattering the light."

The new work follows a paper published in September about the lab's creation of nano antennas that concentrate and magnify light up to 1,000 times. That paper focused on the intensity of laser light shot into a gap between the tips of two gold nanowires.

This time, Natelson and Ward spread molecules -- either oligophenylene vinylene or 1-dodecanethiol -- on the surface of a gold nanowire and then broke the wire, leaving a nanoscale gap. When they were fortunate enough to find molecules in the gap -- "the sweet spot" being where the metal wires are closest, Natelson said -- they'd power up and read the resulting spectra.

The experiments were carried out in a vacuum with materials cooled to 80 kelvins (-315 degrees Fahrenheit). The researchers found they could easily detect temperature fluctuations of up to 20 degrees in the molecules.

On the macro level, Natelson said, "You're usually looking at something that's essentially cold. You send in light, it dumps some of the energy into the thing you're looking at and the light comes out with less energy than when you started. With Raman scattering, you can actually see particular molecular vibrational modes."

But the opposite can happen if the atoms are already vibrating with stored energy. "The light can grab some of that and come out with more energy than when it started," he explained.

The effect is most dramatic when current is supplied through the nanowires. "As we crank up the current through this junction, we can watch these different vibrations shaking more and more. We can watch this thing heat up."

Natelson, named by Discover magazine in 2008 as one of the nation's top 20 scientists under age 40, said the experiments show not only how molecules wedged into the nanogap heat up, but also their interaction with the metal wires. "The vibrations show up as sharp peaks in the spectra," he said. "They have very definite energies. Underneath all that, there's this sort of diffuse smear where the light instead is interacting with the electrons in the metal, the actual metal wires."

Natelson said it's extremely hard to get direct information about how heating and dissipation work on nano scales. "In general, you can't do it. There's a lot of modeling, but in terms of experimental things you can actually measure that tell you what's happening, everything is very indirect. This is an exception. This is special. You can see what's happening.

"In our fantasy experiment, we'd say, 'Boy, I wish I could go in with a thermometer,' or, 'I wish I could see each molecule and see how much it's shaking.' And this is effectively a way of doing that. We can really watch these things heat up."

Rice University


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A new mathematical model developed by researchers at the University of Pennsylvania has offered even more evidence of the correctness of evolutionary theory.

Herbert Wilf, Penn’s Thomas A. Scott Emeritus Professor of Mathematics, and Warren Ewens, emeritus professor of biology, say their model directly challenges the long-standing contention among some doubters that evolution couldn't have happened because the small changes in species outlined by the theory simply would have taken too much time to be completed.

Their works shows that, under a very reasonable model of mutations and natural selection, the time required to evolve a very complex organism is vastly smaller than might be presumed. As a result, the idea that evolution would require "too much time" to be true is proved false.

Wilf and Ewens’ model is described in the paper "There's Plenty of Time for Evolution," which will appear in an upcoming issue of Proceedings of the National Academy of Sciences USA.

According to Wilf, the understanding of evolution reached in the paper can best be illustrated by thinking about the two different ways a hacker might try to break into a computer.

Suppose for a moment that a computer's password is 12 letters long. Simple math dictates that because there are 12 characters in the password and 26 letters in the alphabet, there are approximately 10,000,000,000,000,000 (26 to the 12th power) possible iterations of the password.

One way to hack this password would be to guess a random string of 12 letters and keep doing so until the right combination was found. That process, however, would take an extremely long time.

A better strategy, Wilf said, would be to use a "spy." After each guess, the spy could tell the hacker which, if any, of the 12 letters were correct. If, for instance, the spy told the hacker that two of the 12 letters were correct, it would leave only 10 letters to be discovered. Extrapolate that spying-and-guessing process over the entire hack attempt, and it's clear that the amount of time required would be greatly reduced.

"When you have this spy inside, it means that each letter is essentially operating independently in the [password] you're trying to guess," Wilf said. "Instead of trying to worry about the whole word, you just have to worry about each letter individually. When you get it right, it stays there; it doesn't change."

But what does hacking have to do with the evolution of species?

Simple, Wilf said. In the case of evolution, the hacker is evolution itself. The password is the string of codons that describes, for example, a butterfly. And the spy is natural selection.

"If, when we guess the full string of letters [for a new species], one of the letters is correct — for instance, one that describes correctly the eyes of a butterfly — then that letter has survival value," he said.

"It will not be discarded as future mutations take place because the intermediate creatures are seeing very well, and they will live and reproduce. So although it seems at first glance that the process of random mutations will take a very long time to produce a higher organism, thanks to the spying of natural selection, the process can go very rapidly.

“In the paper, these ideas are precisely quantified, according to this model, and the extent of the speedup is found. It is enormous, and shows that there is indeed plenty of time for evolution."

University of Pennsylvania


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Researchers have discovered evidence of a distinct group of "archaic" humans existing outside of Africa more than 30,000 years ago at a time when Neanderthals are thought to have dominated Europe and Asia. But genetic testing shows that members of this new group were not Neanderthals, and they interbred with the ancestors of some modern humans who are alive today.

The journal Nature reported the finding this week. The National Science Foundation's Behavioral and Cognitive Sciences Division partially funded the research.

An international team of scientists led by Svante Pääbo at the Max Planck Institute of Evolutionary Anthropology in Leipzig, Germany, used a combination of genetic data and dental analysis to identify a previously unknown population of early humans, whom the researchers call "Denisovans." The name was taken from Denisova Cave in southern Siberia where archaeologists from the Russian Academy of Sciences recovered a bone in 2008.

Genetic sequencing of DNA extracted from a finger bone of a 5-10-year-old girl from the cave revealed that she was neither Neanderthal nor a modern human, but shared an ancient origin with Neanderthals. The genetic analysis also showed she had a very different history since splitting from Neanderthals, the researchers concluded.

A tooth, also found in the Denisova Cave, complemented the genetic evidence. "The tooth is just amazing," said Bence Viola, a paleoanthropologist at the Max Planck Institute. "It allows us to connect the morphological and genetic information."

Analysis of the tooth revealed a shape that falls outside normal tooth variation typically seen in Neanderthals and modern humans, providing further evidence the Denisova hominins are an evolutionarily distinct group.

Another type of analysis reported by the study's authors showed Denisovans contributed 4-6 percent of their genetic material to the genomes of present-day New Guineans. "They are ancestors of people in Papua New Guinea but not of the great majority of people in Eurasia," said David Reich, a geneticist at Harvard Medical School in Boston, who led the research's population genetics analysis.

By comparing the genetic material of the Denisovans to diverse modern humans, the authors disclose a previously uncharacterized episode of gene flow between "archaic" and modern humans.

Until last year, the mainstream view in genetics was that modern humans inherited essentially their entire DNA makeup from Neanderthal-related individuals when they migrated from Africa 40,000-55,000 years ago. It was surmised they completely replaced the humans who migrated before them, including the Neanderthals whose ancestors likely made the pilgrimage hundreds of thousands of years earlier.

But sequencing and analysis of the Neanderthal genome earlier this year showed this was not the case. Neanderthals were not completely replaced, but instead contributed 1-4 percent of their genetic material to all modern non-Africans before dying out. The finding, based on Neanderthals discovered at Vindija Cave in Croatia, showed that modern humans outside of Africa are not all descended from a single out-of-Africa migration.

"We have now found evidence for a second gene flow event as well from a different source population and into a narrower set of modern human groups," said Reich."The first gene flow event appears to have been from a population closely related to the Neanderthals, while the second gene flow event was from a population much more closely related to Denisovans."

The new research suggests rather than being an irregular occurrence, intermixing between diverged human populations may have been common. "In combination with the Neanderthal genome sequence, the Denisovan genome suggests a complex picture of genetic interactions between our ancestors and different ancient hominin groups," said Pääbo, a founder of the field of ancient DNA.

Denisovans are likely to have been widespread across a broad swath of Eurasia, since Denisovans must have existed not just in Siberia, but also thousands of miles to the south along the path of modern humans migrating out of Africa on the way to New Guinea. However, far less is known about this population archaeologically or morphologically than about the Neandertals and modern humans who were their contemporaries in western Eurasian and in Africa.

"We hope that these results will spur archaeologists and paleontologists to study sites occupied by Denisovans," said Reich. "All we have now is a finger bone, a tooth, and a genome. However, we now know that this population existed, and new archaeological discoveries should reveal much more about their morphology and material culture."

"Technically, the discovery and definition of this new population based on its DNA patterns--rather than morphology--is also fascinating," said Reich. Traditionally, hominin populations are defined based on studies of their physical form and structure. Defining them based on DNA is something made possible only by recent advances in DNA technology, and may be a harbinger for the future.

This study also involved contributions from researchers at the Broad Institute of Harvard and Massachusetts Institute of Technology; the University of California at Santa Cruz and Berkeley; the University of Tübingen, Germany; Emory University, Georgia; the University of Montana; the University of Washington; the Institute of Evolutionary Biology, Barcelona, Spain; the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, Beijing, China; the University of British Columbia, Vancouver, Canada; and the Institute of Archaeology & Ethnography, Russian Academy of Sciences, Siberian Branch, Novosibirsk, Russia.

(Photo: David Reich, Harvard Medical School)

National Science Foundation


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In a new study that would have made Rudyard Kipling proud, a UTSC graduate student has provided the most definitive answer yet to the curious question, “How did flying fish get their wings?”

Eric Lewallen, a PhD student in professor Nathan Lovejoy’s ecology and evolutionary biology lab at UTSC, is the lead author on the first molecular study of genetic relatedness among species of flying fish. Appearing in the Biological Journal of the Linnean Society, Lewallen’s paper confirms what scientists have long hypothesized—that the wide variety of “flying” strategies found in fish around the world are all the result of a single evolutionary chain of events.

“Our results show that flying fish are monophyletic, which means they all share a common ancestor,” said Lewallen. “This suggests that true gliding behavior in fish evolved just once, and all the modifications we see today can be traced back to that one event.”

There are approximately 50 species of flying fish to be found in the tropical and subtropical regions of the Pacific, Atlantic and Indian oceans. Their “wings” are really just enlarged fins, accompanied by specialized muscles, which together allow them to burst out of the water and glide above the ocean surface for short periods of time.

Some species have two pectoral wings, while others have two pectoral and two pelvic wings. The two-winged species can exit the water quickly and usually glide in a straight line. Four-winged species can glide for hundreds of metres at a time and can even manouevre in mid-air to change direction.

Scientists believe fish evolved various gliding abilities in order to evade specific predators such as tunas, dolphins and seabirds.

Over the last few years, Lewallen has had his fair share of adventures while collecting his fish specimens. Due to the high cost of conducting research on the high seas, he has often worked aboard boats operated by the National Oceanic and Atmospheric Administration (NOAA) in return for the opportunity to collect specimens. By day, he would work as an independent marine mammal observer for NOAA studies. By night, he would catch his flying fishes using spotlights and dipnets.

“The shocking part,” said Lewallen, “is that flying fishes are so abundant—they’re found in every major tropical ocean—yet many basic questions regarding their ecology and evolution remain unanswered.” Lewallen’s paper, which will serve as the first chapter of his PhD dissertation at UTSC, provides an exciting foundation for future studies involving open ocean organisms.

“There are many complex questions I would like to address regarding these creatures and their habitats,” he said. “But we’ve got to lay some of the groundwork first.”

Lewallen’s work provides further evidence of UTSC’s emerging leadership in the field of conservation biology.

Noted Professor Lovejoy, “The great success of Eric’s study is that it highlights UTSC’s growing strength in field biology, graduate research and internationally collaborative science.”

University of Toronto


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Is red seaweed a viable future biofuel? Now that a University of Illinois metabolic engineer has developed a strain of yeast that can make short work of fermenting galactose, the answer is an unequivocal yes.

"When Americans think about biofuel crops, they think of corn, miscanthus, and switchgrass. In small island or peninsular nations, though, the natural, obvious choice is marine biomass," said Yong-Su Jin, a U of I assistant professor of microbial genomics and a faculty member in its Institute for Genomic Biology.

Producers of biofuels made from terrestrial biomass crops have had difficulty breaking down recalcitrant fibers and extracting fermentable sugars. The harsh pretreatment processes used to release the sugars also resulted in toxic byproducts, inhibiting subsequent microbial fermentation, he said.

But marine biomass can be easily degraded to fermentable sugars, and production rates and range of distribution are higher than terrestrial biomass, he said.

"However, making biofuels from red seaweed has been problematic because the process yields both glucose and galactose, and until now galactose fermentation has been very inefficient," he said.

But Jin and his colleagues have recently identified three genes in Saccharomyces cerevisiae, the microbe most often used to ferment the sugars, whose overexpression increased galactose fermentation by 250 percent when compared to a control strain.

"This discovery greatly improves the economic viability of marine biofuels," he said.

Overexpression of one gene in particular, a truncated form of the TUP1 gene, sent galactose fermentation numbers soaring. The new strain consumed both sugars (glucose and galactose) almost three times faster than the control strain—8 versus 24 hours, he said.

"When we targeted this protein, the metabolic enzymes in galactose became very active. We can see that this gene is part of a regulating or controlling system," he said.

According to Jin, galactose is one of the most abundant sugars in marine biomass so its enhanced fermentation will be industrially useful for seaweed biofuel producers.

Marine biomass is an attractive renewable source for the production of biofuels for three reasons:

* production yields of marine plant biomass per unit area are much higher than those of terrestrial biomass

* marine biomass can be depolymerized relatively easily compared to other biomass crops because it does not contain recalcitrant lignin and cellulose crystalline structures

* the rate of carbon dioxide fixation by marine biomass is much higher than by terrestrial biomass, making it an appealing option for sequestration and recycling of carbon dioxide, he said.

University of Illinois




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