Proyectos Frío y Calor Tenerife

miércoles, 21 de diciembre de 2016

Majorana fermions predicted in a superconducting material

A low-temperature material made from the elements praseodymium, osmium, and antimony should be able to host subatomic particles known as Majorana fermions, MIT researchers have shown in a theoretical analysis.

Majorana fermions, first predicted by physicists in 1937, can be thought of as electrons split into two parts, each of which behaves as independent particles. These fermions do not exist as elementary particles in nature but can emerge in certain superconducting materials near absolute zero temperature. In superconducting materials, electrons flow without resistance generating little or no heat.

The new analysis by graduate student Vladyslav Kozii, postdoc Jörn Venderbos, and Lawrence C. (1944) and Sarah W. Biedenharn Career Development Assistant Professor Liang Fu predicts this special state should occur in a praseodymium, osmium and antimony compound, PrOs₄Sb_12, and similar materials made of heavy metals.

Physicists describe electrons by their energy, momentum, and spin. An electron can occupy a possible energy level, and an unoccupied level is called a hole. In the new analysis, Majorana fermions emerge as a quantum superposition of an electron and a hole that move freely, with each having the same direction, or spin. This Majorana fermion spin can interact with the spin of atomic nuclei in the material, so it ought to be seen using nuclear magnetic resonance techniques, they predict.

“We address a certain class of superconductors, show that they have Majorana fermions as freely propagating quasiparticles in the bulk, and then look at how they can be detected and what other properties these materials have that one could use in the future for interesting functionality,” says Venderbos. “I think it very nicely bridges the gap between experiment and theory and it can be used by experimentalists right now.” Their paper was published this month in the journal Science Advances.

A key physics concept in this work is that of time-reversal symmetry. Such symmetry means that equations of motions governing an object or particle stay the same if one could reverse the direction of time — with time flowing backward rather forward. If the equation of motion of electrons in a material is different when time flows backwards — as is true in magnets, for instance — then time-reversal symmetry is said to be broken. This gives physicists an important way to distinguish different materials. In the proposed antimony-compound based superconductor, analysis shows that the Majorana fermions can only exist when time reversal symmetry is broken. Upon reversing the motion in time, the spin of the Majorana fermions is reversed — for example, from clockwise to counterclockwise — and this implies a different equation of motion for Majorana fermions going backward in time. “Regarding the material that we proposed, actually there is one recent experiment that confirms that time-reversal symmetry is broken in the superconducting state of this material. This reinforces our conclusion that it is indeed a very promising candidate for our theory to apply,” Kozii explains.

Majorana fermions were first proposed by Italian physicist Ettore Majorana as a special mathematical solution for quantum behavior of electrons. Princeton University researchers reported detection of a zero-dimensional realization of these particles at the end of an atom chain in October 2014. The MIT theorists now show that the three-dimensional propagating Majorana fermions they predict are governed by Majorana’s original equation. “The extensive study we have performed shows that this peculiar particle may now find its realization in solid state physics in a real material,” Venderbos says.

Electrons in materials such as metals and semiconductors can fill only certain energy levels, or bands, with excluded, or forbidden, energy levels referred to as a bandgap. In a superconductor, this is also called the superconducting gap. Ordinarily, it takes outside energy in order to lift a lower energy electron to a higher energy level, especially when it has to cross a bandgap. The Fu groups’ analysis of praseodymium, osmium, and antimony reveals that there are some special points in its electronic excitation spectrum where the bandgap vanishes in its superconducting state, which means that low energy excitations are possible. “However low energy you take, there will be always excitation at this energy. These excitations are exactly these Majorana fermions we were talking about,” Kozii explains. Venderbos adds, “There are some excitations for which you don’t have to put in any energy or just an infinitesimally tiny amount and you can still create the excitation.”

Noting that Fu has made “some fantastic predictions in the past,” Princeton University professor of chemistry Robert J. Cava, who was not involved in this research, suggests: “Experimentalists should listen to what he has to say. ... I am very happy to see that he and his coworkers have presented an analysis of real materials in which their ideas might be embodied.”

Kozii, Venderbos, and Fu analyzed these unconventional superconductors for a year. For Kozii, the work will become part of his doctoral thesis.

The researchers hope their work will inspire experimentalists to look again at some previously studied materials to identify ones that host superconducting states with Majorana fermions. “I think the first step would be just to find a material in which everyone can agree that it has these Majorana fermions. That would be really exciting and constitute the discovery of a new type of superconductor in experiment,” Venderbos says. “The next step would be to think about functionalization of these materials, what could be the specific applications.” Trying to make quantum devices out of these materials is one possible direction. “We hope this research ultimately brings closer efforts from the quantum material and quantum device community in finding out the many facets of Majorana fermions,” Fu adds.

The Department of Energy Office of Basic Energy Sciences, Division of Materials Sciences and Engineering supported Fu’s and Kozii’s work. The Netherlands Organization for Scientific Research supported Venderbos through a Rubicon grant.

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MIT continues to show support for community members impacted by Cambridge fire

In the weeks since a devastating 10-alarm fire in East Cambridge, the MIT community has banded together to offer aid and relief to the students, staff, and community residents who lost their belongings or homes during the Dec. 3 emergency on Berkshire and York Streets. From opening the doors to on-campus housing to making an Institute donation to the Mayor’s Fire Relief Fund, and inviting members of the community to do the same, MIT is standing with the impacted individuals and families. And now, thanks to the Graduate Student Council (GSC), there is a new way to provide sustained support for MIT graduate students impacted by the tragedy: a fire relief gift registry.

Spearheaded by the GSC’s Housing and Community Affairs Committee, the Amazon gift registry was designed to meet the direct needs of the eight graduate students who lost their belongings or homes. The GSC asked them to register for the household supplies and goods they need now and in the coming months and then invited the MIT community to purchase items off the registry by following these easy instructions:

Visit amazon.com.
Under “Accounts and Lists,” click on “Find a List or Registry.”
Search for the email address gsc-hca@mit.edu to find the “Cambridge Fire Relief” registry.
Purchase items registered by displaced community members.

"This registry provides an opportunity for us to show compassion for members of our own community who have suffered,” states Ryan Gillis, GSC Housing and Community Affairs Committee co-chair. “I hope that we can reach out together and help the victims of this fire.”

The GSC’s generosity is in keeping with MIT’s larger response to the fire. In the immediate aftermath and in the weeks since the blaze, administrators, faculty, students, and staff have offered support in a variety of ways, including:

The Division of Student Life contacted impacted students on the day of the fire to offer them open rooms in Tang Hall. Altogether, six students and their partners moved into Tang, and MIT Dining helped them get meals. Another family was moved into an open Westgate apartment.
Human Resources highlighted its MyLife Services so that MIT employees (and family members) can access free assistance in the following areas: emotional support and counseling, as well as personalized legal, financial, and relocation help and resources. This benefit is free and is available 24/7 by calling: 844-405-5433. All calls are confidential.
Human Resources informed employees about backup child care and backup adult care services. These services, provided by the Work-Life Center vendor partner, Care.com, can be accessed by visiting mit.care.com and completing the enrollment form. Then, call Care.com at 1-855-781-1303, extension 2, during regular business hours, Monday through Friday 9 a.m. – 6 p.m., to schedule care.

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Distinctive brain pattern may underlie dyslexia

A distinctive neural signature found in the brains of people with dyslexia may explain why these individuals have difficulty learning to read, according to a new study from MIT neuroscientists.

The researchers discovered that in people with dyslexia, the brain has a diminished ability to acclimate to a repeated input — a trait known as neural adaptation. For example, when dyslexic students see the same word repeatedly, brain regions involved in reading do not show the same adaptation seen in typical readers.

This suggests that the brain’s plasticity, which underpins its ability to learn new things, is reduced, says John Gabrieli, the Grover M. Hermann Professor in Health Sciences and Technology, a professor of brain and cognitive sciences, and a member of MIT’s McGovern Institute for Brain Research.

“It’s a difference in the brain that’s not about reading per se, but it’s a difference in perceptual learning that’s pretty broad,” says Gabrieli, who is the study’s senior author. “This is a path by which a brain difference could influence learning to read, which involves so many demands on plasticity.”

Former MIT graduate student Tyler Perrachione, who is now an assistant professor at Boston University, is the lead author of the study, which appears in the Dec. 21 issue of Neuron.

Reduced plasticity

The MIT team used magnetic resonance imaging (MRI) to scan the brains of young adults with and without reading difficulties as they performed a variety of tasks. In the first experiment, the subjects listened to a series of words read by either four different speakers or a single speaker.

The MRI scans revealed distinctive patterns of activity in each group of subjects. In nondyslexic people, areas of the brain that are involved in language showed neural adaption after hearing words said by the same speaker, but not when different speakers said the words. However, the dyslexic subjects showed much less adaptation to hearing words said by a single speaker.

Neurons that respond to a particular sensory input usually react strongly at first, but their response becomes muted as the input continues. This neural adaptation reflects chemical changes in neurons that make it easier for them to respond to a familiar stimulus, Gabrieli says. This phenomenon, known as plasticity, is key to learning new skills.

“You learn something upon the initial presentation that makes you better able to do it the second time, and the ease is marked by reduced neural activity,” Gabrieli says. “Because you’ve done something before, it’s easier to do it again.”

The researchers then ran a series of experiments to test how broad this effect might be. They asked subjects to look at series of the same word or different words; pictures of the same object or different objects; and pictures of the same face or different faces. In each case, they found that in people with dyslexia, brain regions devoted to interpreting words, objects, and faces, respectively, did not show neural adaptation when the same stimuli were repeated multiple times.

“The brain location changed depending on the nature of the content that was being perceived, but the reduced adaptation was consistent across very different domains,” Gabrieli says.

He was surprised to see that this effect was so widespread, appearing even during tasks that have nothing to do with reading; people with dyslexia have no documented difficulties in recognizing objects or faces.

He hypothesizes that the impairment shows up primarily in reading because deciphering letters and mapping them to sounds is such a demanding cognitive task. “There are probably few tasks people undertake that require as much plasticity as reading,” Gabrieli says.

Early appearance

In their final experiment, the researchers tested first and second graders with and without reading difficulties, and they found the same disparity in neural adaptation.

“We got almost the identical reduction in plasticity, which suggests that this is occurring quite early in learning to read,” Gabrieli says. “It’s not a consequence of a different learning experience over the years in struggling to read.”

Guinevere Eden, a professor of pediatrics and director of the Center for the Study of Learning at Georgetown University Medical Center, described the study as “groundbreaking.”

“For children with dyslexia, we know that the brain looks different in terms of anatomy and function, but we have not been able to establish why,” says Eden, who was not involved in the research. “This study makes an important step in that direction: It gets to the true characteristics of the properties of the neurons in these brain regions, not just their outward appearance.”

Gabrieli’s lab now plans to study younger children to see if these differences might be apparent even before children begin to learn to read. They also hope to use other types of brain measurements such as magnetoencephalography (MEG) to follow the time course of the neural adaptation more closely.

The research was funded by the Ellison Medical Foundation, the National Institutes of Health, and a National Science Foundation Graduate Research Fellowship.

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Majorana fermions predicted in a semiconducting material

Kozii, Venderbos, and Fu analyzed these unconventional superconductors for a year. For Kozii, the work will become part of his doctoral thesis.

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Letter regarding MIT's updated policy against discrimination and harassment

The following email was sent today to the MIT community by President L. Rafael Reif.

To the members of the MIT community:

I write for two reasons – one official, one personal. Officially, I share MIT's updated policy against discrimination and harassment, including sexual harassment. Thanks to thoughtful insights from many people, this revision now offers more concrete examples and is clearer about boundaries and consequences.

None of us, however, loves the MIT community because its policies aspire to be excellent. What we love is that the energy, creativity, standards, principles and values that we each contribute make a magnificent whole – a precious shared home.

So I offer a few personal observations on our MIT community, and why it matters, especially now.

We live in a moment when, in our public conversation, familiar standards of decency and open, respectful discourse can no longer be taken for granted. These unwritten rules of a healthy society are especially significant in a community like ours; at MIT, our differences of opinion, perspective, training, identity and background, and our ability to navigate and learn from those differences, are essential to our creative excellence. These shared values have even greater importance in this charged time, when incidents of discrimination across the country have left many members of our community uneasy about their own safety.

In that context, I would like to try to articulate a few of our unwritten rules, so we can reflect on them together.

At MIT, when our community is at its best, bigotry and discrimination are out of bounds, period. Diminishing or excluding others on the basis of their identity – whether race, religion, gender, sexual orientation, disability, social class, nationality or any other aspect – would be unthinkable. It's also out of the question to bully others, period. Such behavior is simply beneath us – because we value each other as members of our community and respect each other as fellow human beings.

The mission we pursue together is too important and too difficult for any of us to spend time erecting artificial barriers to anyone else's achievement; we need to welcome and embrace all the great talent we are fortunate enough to attract. Intellectually, we are a community where prejudice – pre-judging – is anathema. In the MIT community I love, our personal interactions benefit when we behave as we do in our intellectual work: Assume less and ask more, to learn more. Refrain from jumping to conclusions on superficial evidence. And listen as closely and as much as we can.

At the same time, free expression of ideas is a fundamental MIT value. In my experience, valuing free expression at MIT means accepting each other's right to express deep disagreement and candid criticism, sometimes in very strong terms, whether the subject is science or philosophy or politics. As long as such arguments are governed by mutual respect, they are part of how we make each other smarter. The capacity to listen to each other through passionate disagreement is an indispensable tool for learning; we shouldn't trade or compromise that for anything.

I believe our magnificently diverse community can and must be part of making a better world. Through that work, we speak clearly and proudly for our community's deeply American ideals. And we do that best if we succeed in living out those values here at home: if we remember always that all of us are here on our merits and by choice, and that we need each other and benefit and learn from each other. In striving to accomplish this, we might even offer a model of human possibility.

For the privilege of belonging to this community, each of us has a responsibility to live up to its values, to care for it – and to make it better. I have said that I would like MIT to be famous not only for what we know, and what we know how to do, but for being a community where we treat one another with sympathy, humility, decency, respect and kindness. We may not be there yet. But thanks to the sustained loving effort of many individuals, I believe we are on our way.

In these unsettled times, let's commit ourselves – mind, hand and heart – to achieving this aspiration.

With gratitude for what we have accomplished together and with high hopes for the future,

L. Rafael Reif

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martes, 20 de diciembre de 2016

Driverless platoons

As driverless cars merge into our transportation system in the coming years, some researchers believe autonomous vehicles may save fuel by trailing each other in large platoons. Like birds and fighter jets flying in formation, or bikers and race car drivers drafting in packs, vehicles experience less aerodynamic drag when they drive close together.

But assembling a vehicle platoon to deliver packages between distribution centers, or to transport passengers between stations, requires time. The first vehicle to arrive at a station must wait for others to show up before they can all leave as a platoon, creating inevitable delays.

Now MIT engineers have studied a simple vehicle-platooning scenario and determined the best ways to deploy vehicles in order to save fuel and minimize delays. Their analysis, presented this week at the International Workshop on the Algorithmic Foundations of Robotics, shows that relatively simple, straightforward schedules may be the optimal approach for saving fuel and minimizing delays for autonomous vehicle fleets. The findings may also apply to conventional long-distance trucking and even ride-sharing services.

“Ride-sharing and truck platooning, and even flocking birds and formation flight, are similar problems from a systems point of view,” says Sertac Karaman, the Class of 1948 Career Development Associate Professor of Aeronautics and Astronautics

at MIT. “People who study these systems only look at efficiency metrics like delay and throughput. We look at those same metrics, versus sustainability such as cost, energy, and environmental impact. This line of research might really turn transportation on its head.”

Karaman is a co-author of the paper, along with Aviv Adler, a graduate student in the Department of Electrical Engineering and Computer Science, and David Miculescu, a graduate student in the Department of Aeronautics and Astronautics.

Pushing through drag

Karaman says that for truck-driving — particularly over long distances — most of a truck’s fuel is spent on trying to overcome aerodynamic drag, that is, to push the truck through the surrounding air. Scientists have previously calculated that if several trucks were to drive just a few meters apart, one behind the other, those in the middle should experience less drag, saving fuel by as much as 20 percent, while the last truck should save 15 percent — slightly less, due to air currents that drag behind.

If more vehicles are added to a platoon, more energy can collectively be saved. But there is a cost in terms of the time it takes to assemble a platoon.

Karaman and his colleagues developed a mathematical model to study the effects of different scheduling policies on fuel consumption and delays. They modeled a simple scenario in which multiple trucks travel between two stations, arriving at each station at random times. The model includes two main components: a formula to represent vehicle arrival times, and another to predict the energy consumption of a vehicle platoon.

The group looked at how arrival times and energy consumption changed under two general scheduling policies: a time-table policy, in which vehicles assemble and leave as a platoon at set times; and a feedback policy, in which vehicles assemble and leave as a platoon only when a certain number of vehicles are present — a policy that Karaman first experienced in Turkey.

“I grew up in Turkey, where there are two types of public transportation buses: normal buses that go out at certain time units, and another set where the driver will sit there until the bus is full, and then will go,” Karaman says.

When to stay, when to go

In their modeling of vehicle platooning, the researchers analyzed many different scenarios under the two main scheduling policies. For example, to evaluate the effects of time-table scheduling, they modeled scenarios in which platoons were sent out at regular intervals — for instance, every five minutes — versus over more staggered intervals, such as every three and seven minutes. Under the feedback policy, they compared scenarios in which platoons were deployed once a certain number of trucks reached a station, versus sending three trucks out one time, then five trucks out the next time.

Ultimately, the team found the simplest policies incurred the least delays while saving the most fuel. That is, time tables set to deploy platoons at regular intervals were more sustainable and efficient than those that deployed at more staggered times. Similarly, feedback scenarios that waited for the same number of trucks before deploying every time were more optimal than those that varied the number of trucks in a platoon.

Overall, feedback policies were just slightly more sustainable than time-table policies, saving only 5 percent more fuel.

“You’d think a more complicated scheme would save more energy and time,” Karaman says. “But we show in a formal proof that in the long run, it’s the simpler policies that help you.”

Ahead of the game

Karaman is currently working with trucking companies in Brazil that are interested in using the group’s model to determine how to deploy truck platoons to save fuel. He hopes to use data from these companies on when trucks enter highways to compute delay and energy tradeoffs with his mathematical model.

Eventually, he says, the model may suggest that trucks follow each other at very close range, within 3 to 4 meters, which is difficult for a driver to maintain. Ultimately, Karaman says, truck platoons may require autonomous driving systems to kick in during long stretches of driving, to keep the platoon close enough together to save the most fuel.

“There are already experimental trials testing autonomous trucks [in Europe],” Karaman says. “I imagine truck platooning is something we might see early in the [autonomous transportation] game.”

The researchers are also applying their simulations to autonomous ride-sharing services. Karaman envisions a system of driverless shuttles that transport passengers between stations, at rates and times that depend on the overall system’s energy capacity and schedule requirements. The team’s simulations could determine, for instance, the optimal number of passengers per shuttle in order to save fuel or prevent gridlock.

“We believe that ultimately this thinking will allow us to build new transportation systems in which the cost of transportation will be reduced substantially,” Karaman says.

This research was funded, in part, by the National Science Foundation.

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35 exceptional MIT students named Burchard Scholars for 2017

The School of Humanities, Arts, and Social Sciences (SHASS) has named 35 exceptional MIT undergraduates as Burchard Scholars for 2017. The award honors sophomores and juniors who demonstrate academic excellence in the humanities, arts, and social sciences, as well as in science and engineering.

Burchard Scholars can come from any school or department of the Institute, and this year’s honorees major and minor in a range of disciplines, including physics, history, biology, economics, Chinese, French, engineering, mathematics, international studies, management, and philosophy. "What binds the group together," says Margery Resnick, professor of literature and director of the Burchard Program, "is a powerful curiosity about ideas."

“The Burchard scholars are some of MIT’s liveliest undergraduates,” she says. “Selection is extremely competitive, and the students chosen are unafraid to wrestle with new ideas.”

Ideas and conversation at dinner seminars

Named in honor of the school's first dean, John Ely Burchard, the Burchard Scholars Program brings undergraduates together with distinguished members of the faculty for a series of eight dinner seminars that reflect the range of MIT's research in the humanities, arts, and social science fields. Past gatherings have featured talks on: the politics of aid to Haitian trauma survivors; what philosophy tells us about how to make big decisions; U.S. grand strategy in foreign policy; the art of discovery; and a political history of gravity.

“The Burchard dinners are, for faculty and students alike, an oasis in our busy lives,” Resnick notes. “I look forward to every dinner as the powerful ideas generated by our faculty are reflected upon, challenged, and enjoyed by this wonderful group of young scholars and citizens."

Expanding horizons, in community

The Burchard gatherings are famous not only for presenting leading edge research, but for building a warm, supportive community, and giving students experience in the art of give-and-take in intellectual conversations — a skill students value for success in every field.

“Engaging and engaged are defining characteristics of MIT's Burchard Scholars,” Resnick says, noting that many past winners have gone on to receive other distinguished honors, including Rhodes, Marshall, and Truman scholarships and fellowships.

Melissa Nobles, the Kenan Sahin Dean of SHASS, will congratulate the new class of Burchard Scholars at a reception to be held in their honor in February 2017.

The 2017 Burchard Scholars are:

Meia Alsup, sophomore in materials science and engineering

Natasha Batten, sophomore in chemical engineering

Jaclyn Baughman, sophomore in physics

Margaret Bertoni, sophomore in mechanical engineering

Phoebe Cai, junior in mathematics and economics

Samantha Cawthon, sophomore in brain and cognitive sciences

Ann Chen, junior in mechanical engineering with a minor in history

Herng Yi Cheng, junior in mathematics

Matthew Coupin, junior in materials science and engineering

Caralyn Cutlip, junior in mechanical engineering with a minor in German

Anshula Gandhi, sophomore in electrical engineering and computer science

Ying Gao, junior in mathematics and economics

Benjamin Harpt, junior in physics

Christopher Hillenbrand, sophomore in chemistry

Nicholas Hoffman, junior in Earth, atmospheric, and planetary sciences

Yuliya Klochan, junior in philosophy and political science

Tinna-Solveig Kosoko-Thoroddsen, sophomore in chemical engineering with a minor in Chinese

Jennifer Lee, sophomore in electrical engineering and computer science

Feng Ping "Angela" Leong, junior in mechanical engineering

Jessy Lin, sophomore in electrical engineering and computer science

Stacie Lin, sophomore in biology

Ian MacFarlane, junior in physics

Oghenefejiro Oruerio, junior in mechanical engineering

Audrey Pillsbury, sophomore in chemical engineering

Mary Jane Porzenheim, sophomore in biology

Ankita Reddy, sophomore in biology

Uma Roy, sophomore in mathematics

Megan Schussman, sophomore in brian and cognitive sciences and philosophy

Jacqueline Shen, junior in biology with a minor in applied international studies

Nafisa Syed, sophomore in biology

Emily Tang, sophomore in electrical engineering and computer science

Amanda Wu, junior in brian and cognitive sciences

Sefa Yakpo, junior in management with a minor in French

Rachel Yang, junior in electrical engineering and computer science

Jingyi Zhao, junior in mathematics

Story prepared by MIT SHASS Communications

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