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Monday, February 22nd, 2016
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| 11:00a |
How to make electrons behave like a liquid Electrical resistance is a simple concept: Rather like friction slowing down an object rolling on a surface, resistance slows the flow of electrons through a conductive material. But two physicists have now found that electrons can sometimes cooperate to turn resistance on its head, producing vortices and backward flow of electric current.
The prediction of “negative resistance” is just one of a set of counterintuitive and bizarre fluid-like effects encountered under certain exotic circumstances, involving systems of strongly interacting particles in a sheet of graphene, a two-dimensional form of carbon. The findings are described in a paper appearing today in the journal Nature Physics, by MIT professor of physics Leonid Levitov and Gregory Falkovich, a professor at Israel’s Weizmann Institute of Science.
Electrons in graphene move in a neatly coordinated way, in many ways resembling the movement of viscous fluids through a tube where they are strongly affected by turbulence and vortices. This is due to interactions producing a long-range current-field response, quite different from the simple “individualist” behavior expected under ordinary circumstances, when electrons move in straight lines like pinballs bouncing among the ions, as described by Ohm's law, the researchers say.
The notion of electron viscosity had been suggested before in theory, but it had proven difficult to test because nobody had come up with a way to directly observe such phenomena. Now, Levitov and Falkovich say they have figured out a set of signs that can serve as an indicator of such collective effects in electron flows.
This work is “a remarkable application of theoretical insight to the prediction of a new experimentally observable effect,” says Subir Sachdev, a professor of physics at Harvard University who was not involved in this work. He says this insight is “very significant and opens a new chapter in the study of electron flow in metals.”
A benchmark system
“There was always a kind of dichotomy between what’s easy to do in theory and what’s easy to do in experiments,” Levitov says. “There was a search for an ideal system that would be easy for experimentalists to work with and also be a benchmark system with strong interactions that would show strong interactive phenomena.” Now, he says, graphene is providing many of the sought-after qualities of such a system.
On a graphene surface, Levitov says, “you have electrons behaving as relativistic particles coupled by interactions that are long-range and pretty strong.” With a possible exception of exotic fluids such as quark-gluon plasmas, he says, graphene may be closer to the notion of a perfect “strongly interacting fluid,” an important theoretical concept in quantum physics, than any other system we currently know.
The collective behavior of the charge carriers in such strongly interacting systems is quite peculiar. “In fact, it’s not so different from fluid mechanics,” Levitov says. The way fluids move can be calculated “with very little knowledge of how individual atoms of the liquid interact. We don’t care that much” about the individual motions; it’s the collective behavior that matters in such situations, he says.
In the graphene environment, quantum effects, which are ordinarily insignificant at scales larger than that of individual particles, play a dominant role, he says. In this setting, “we show that [the way charge carriers move] has collective behavior similar to other strongly interacting fluids, like water.”
How to detect it?
But while that’s true in theory, he says, “the question is, even if we have it” — that is, this fluid-like behavior — “how do we detect it? Unlike ordinary fluids, where you can directly track the flow by putting some beads in it, for example, in this system we don’t have a way to view the flow directly.” But because of the two-dimensional structure of graphene, while electrons are moving through the material “we can get information from electrical measurements” done from the outside, where it is possible to place probes at any point on the sheet.
The new approach relies on the fact that “if you have a viscous flow, you expect the different parts of the liquid to drag on each other and produce whirlpools. They will create a flow that will drag on neighboring particles and will drive a vortex,” Levitov says. Specifically, a direct flow in the middle of a graphene ribbon will be accompanied by whirlpools developing along the sides. In those whirlpools, electrons can actually flow in the direction opposite that of the applied electric field — resulting in what the physicists refer to as negative resistance.
While the whirlpools themselves can’t be observed directly, the backward movement of the electron flow in certain parts of the material can be measured and compared with the theoretical predictions.
While Levitov and Falkovich have not personally carried out such experiments, Levitov says that some recent enigmatic findings do seem to fit the predicted pattern. In an experiment that has just been reported, he says “researchers saw something similar, where the voltage on the side turns negative. It’s very tempting to say” that what they saw is a manifestation of the phenomena predicted by this work.
Not just analogy
The comparison of electron behavior in graphene to fluid dynamics “is not just an analogy, but a direct correspondence,” Levitov says. But there are important differences, including the fact that this fluid bears electrical charge, so it behaves not exactly like water flowing in a pipe but rather in a way similar to some plasmas, which are essentially clouds of charged particles.
Because this is early-stage work, Levitov says, it’s too early to tell whether it might ever have any practical applications. But one surprising implication of this work is that heat transport can couple strongly to charge transport. That is, heat can ride atop of charge flow and propagate in a wave-like fashion much faster than under ordinary conditions — perhaps as much as 10 to 100 times faster. This behavior, if achieved, might be harnessed at some point, perhaps in sensing devices with very fast response times, he speculates.
Andre Geim, a professor of condensed matter physics at the University of Manchester in the U.K. who was not involved in this work, says, “It is a brilliant piece of theory, which agrees very well with our recent experimental findings.” Those experiments, he says, “detected the vortices predicted by Levitov's group and showed that the electron liquid in graphene was 100 times more viscous than honey, contrary to the universal belief that electrons behave like a gas.”
Geim adds that graphene is becoming increasingly used in a variety of applications, and says, “Electronic engineers cannot really utilize the material without an understanding of its electronic properties. Whether your electrons move like bullets or swim in treacle creating whirlpools obviously makes a huge difference.”
The work was supported by the National Science Foundation, the Israeli Science Foundation, the Russian Science Foundation, MISTI MIT-Israel Seed Fund, the U.S. Army Research Laboratory and the U.S. Army Research Office through the Institute for Soldier Nanotechnologies at MIT. | | 3:00p |
Title for ‘Earth’s first animal’ likely goes to simple sea creature The first animal to appear on Earth was very likely the simple sea sponge.
New genetic analyses led by MIT researchers confirm that sea sponges are the source of a curious molecule found in rocks that are 640 million years old. These rocks significantly predate the Cambrian explosion — the period in which most animal groups took over the planet, 540 million years ago — suggesting that sea sponges may have been the first animals to inhabit the Earth.
“We brought together paleontological and genetic evidence to make a pretty strong case that this really is a molecular fossil of sponges,” says David Gold, a postdoc in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS). “This is some of the oldest evidence for animal life.”
The results are published today in the Proceedings of the National Academy of Sciences. Gold is the lead author on the paper, along with senior author and EAPS Professor Roger Summons.
Ancient molecular clues
Paleontologists have unearthed an extraordinary number of fossils from the period starting around 540 million years ago. Based on the fossil record, some scientists have argued that contemporary animal groups essentially “exploded” onto Earth, very quickly morphing from single-celled organisms to complex multicellular animals in a relatively short geological time span. However, the fossils that are known from before the Cambrian explosion are peculiar in many respects, making it extremely difficult to determine which type of animal was the first to the evolutionary line.
Summons’ lab has been looking for the answer in molecular fossils — trace amounts of molecules that have survived in ancient rocks long after the rest of an animal has decayed away.
“There’s a feeling that animals should be much older than the Cambrian, because a lot of animals are showing up at the same time, but fossil evidence for animals before that has been contentious,” Gold says. “So people are interested in the idea that some of these biomarkers and chemicals, molecules left behind, might help resolve these debates.”
In particular, he and his colleagues have focused on 24-isopropylcholestane, or 24-ipc for short — a lipid molecule, or sterol, that is a modified version of cholesterol. In 1994, Summons was part of a team, led by Mark McCaffrey PhD ’90, that first found 24-ipc, in unusually high amounts, in Cambrian and slightly older rocks. They speculated that sponges or their ancestors might be the source.
In 2009, a team led by University of California at Riverside Professor Gordon Love, then a postdoc in Summons’ lab, did the first detailed study of rocks in Oman. The researchers confirmed the presence of 24-ipc in 640-million-year-old rock samples, potentially representing the oldest evidence for animal life. That work utilized high precision uranium-lead dating techniques developed by EAPS Professor Samuel Bowring.
“This research topic has a 20-plus-year history intimately connected to MIT scientists,” Summons notes. “Now, in 2016 David Gold has been able to apply his skills and the new tools of the genomic era, to add a further layer of evidence supporting the ‘sponge biomarker hypothesis.’”
Growing an evolutionary tree
It’s known that some modern sea sponges and certain types of algae produce 24-ipc today, but which organism was around to make the molecule 640 million years ago? To answer this question, Summons and Gold sought to first identify the gene responsible for making 24-ipc, then find the organisms that carry this gene, and finally trace when the gene evolved in those organisms.
The team looked through the genomes of about 30 different organisms, including plants, fungi, algae, and sea sponges, to see what kinds of sterols each organism produces and to identify the genes associated with those sterols.
“What we found was this really interesting pattern across most of eukaryotic life,” Gold says.
By comparing genomes, they identified a single gene, sterol methyltransferase, or SMT, responsible for producing certain kinds of sterols depending on the number of copies of the gene an organism carries. The researchers found that sea sponge and algae species that produce 24-ipc have an extra copy of SMT when compared with their close relatives.
The researchers compared the copies to determine how they were all related and when each copy of the gene first appeared. They then mapped the relationships onto an evolutionary tree and used evidence from the fossil record to determine when each SMT gene duplication occurred.
No matter how they manipulated the timing of the evolutionary tree, the researchers found that sea sponges evolved the extra copy of SMT much earlier than algae, and they did so around 640 million years ago — the same time period in which 24-ipc was found in rocks.
Their results provide strong evidence that sea sponges appeared on Earth 640 million years ago, much earlier than any other life form.
“This brings up all these new questions: What did these organisms look like? What was the environment like? And why is there this big gap in the fossil record?” Gold says. “This goes to show how much we still don’t know about early animal life, how many discoveries there are left, and how useful, when done properly, these molecular fossils can be to help fill in those gaps.”
This research is supported, in part, by the Agouron Institute and the NASA Astrobiology Institute. | | 11:59p |
Celebrations planned for centennial of MIT’s river crossing It was a grand gala event in 1916 when MIT, just over a half-century old, moved from a series of scattered buildings in Boston to a newly designed campus on a large parcel of freshly-created filled land in Cambridge. The celebratory events included a water parade across the river to symbolize the move, led by an elaborate replica of a Venetian grand barge that was built for the occasion.
To celebrate the centenary of that grand crossing, MIT is planning a series of public events this spring, culminating in a grand parade across the river by water and bridge, and a rare Open House at which tens of thousands of visitors are expected to explore the campus. The anniversary events also include a symposium, a day of service for the entire MIT community, a concert, a play, and a major special exhibit at the MIT Museum.
The land on which the new campus was built, like the Back Bay neighborhood across the Charles River, was reclaimed from a large area of tidal mudflats, and originally was also envisioned as a residential neighborhood. Only one major building was put in place as part of that original vision, however: the Riverbank Hotel, at the corner of Massachusetts Avenue and Memorial Drive, which has since been converted into Maseeh Hall, now MIT’s largest dormitory.
Planners spent many years debating the design of the new Cambridge campus, says John Ochsendorf, who is chair of the MIT 2016 steering committee, the Class of 1942 Professor of Architecture, and a professor of civil and environmental engineering. Several different architects worked on the plans and suggested different elements, but nearly all the designs, starting early in the process, included a large domed structure as a centerpiece to the campus, he says.
The relocation was an epochal event in MIT’s history. “What a leap in scale!” says Ochsendorf, when the Institute moved from “a hodgepodge of small buildings and lab space” in Boston to a parcel that originally occupied some 50 acres, which from the start included classrooms, labs, dormitories, and athletic fields. It was a “grand, audacious leap, to this huge interconnected campus in Cambridge.”
That interconnectedness was a key part of the innovative design of the campus, Ochsendorf explains. Most college campuses until then had been built as collections of separate buildings, each housing a particular department or school. But MIT wanted to encourage cross-disciplinary research, and having all the buildings connected together was — and still is — seen as an important facilitator for such collaborative work. The new connected campus design owed more to the layout of major industrial buildings than to any traditional academic structures, Ochsendorf says. That connectedness “really allowed MIT to become a preeminent problem-solving institution, because the barriers were broken down” he says. “That really shaped the spirit of MIT today.”
Richard Maclaurin, MIT's president at the time, was able to proceed with plans for the new campus after securing a donation from George Eastman, the founder of Eastman Kodak. President L. Rafael Reif says: "President Maclaurin had the highest aspirations for what MIT could become, and in George Eastman, he found the perfect patron and co-conspirator. In Eastman's words, they believed that 'The future of Technology should be big!' And with the audacious leap to this grand, interconnected campus in Cambridge, they made it so."
This centennial of the Cambridge campus, Ochsendorf says, represents “an opportunity to reflect on how and why” the campus was created the way it was, on a “blank slate” of new land.
Kicking off the series of centennial festivities, the MIT Museum will hold an opening reception on Monday, Feb. 29 of a new exhibit celebrating the design and construction of the new campus, titled “Imagining New Technology: Building MIT in Cambridge.”
The exhibits “include many original materials that have never been seen before,” says Deborah Douglas, the museum’s director of collections. These include early architectural sketches and plans, and even some original blueprints that have never been seen in public. There are also a variety of photographs and artifacts that show the dramatic impact the design of the Institute has had on the city. The museum will also host a series of programs and workshops in coming months related to the exhibit, and will house a crowd-sourced, detailed, 3-D replica of the entire campus and surrounding area. Anyone who has access to a 3-D printer and wants to participate can download the plans from the museum’s website and add their printed buildings to the 3-D map, Douglas says.
She says the rare drawings and photographs reveal some little-known aspects of this century-old campus. For example, they show how the “Main Group” buildings, though built in a classical style with their colonnades, domes, and limestone facings, have interiors that are “actually the antithesis of that,” she says: They are actually structurally very advanced and modern for their time, built on steel and reinforced concrete frames that give much more flexibility in modifying the interiors than traditional construction would have allowed.
There will be two symposia featuring discussions of MIT and its campus. First, a two-day event on March 30-31, called “The Campus — Then, Now, Next,” will feature discussions about the past, present, and future of MIT and its role as an innovative campus.
Then, an afternoon symposium will be held in Kresge Auditorium on April 12, called “Beyond 2016 — MIT's Frontiers of the Future.” This will feature a cross-section of MIT faculty members giving short talks about the frontiers of their own work as well as on visions for the future of MIT. This selection, intended to showcase the wide variety of activities taking place at MIT, will help to “bring the campus to life,” Ochsendorf says.
Emphasizing the deep, ongoing connections between MIT and the city it calls home, the centennial celebrations will also include an MIT Day of Service, on April 19. “We will be matching local organizations in need with volunteers from campus,” Ochsendorf says. Volunteers may include any graduate or undergraduate students, faculty, staff and alumni, as individuals or in teams, who want to pitch in and help out at service organizations in the area. And an important emphasis of this service will be to establish links for ongoing volunteer work in the city by members of the MIT community.
On April 23, the celebration’s biggest event will take place: a campus-wide “Open House” to which everyone is invited, with labs and demonstrations open for view all across the Institute. “We expect tens of thousands of visitors,” Ochsendorf says.
The culmination of the commemorative activities will take place on May 7 with the “Moving Day: Crossing the Charles” parade and competitions, featuring a variety of methods of crossing the river designed by teams of MIT community members. The crossing will feature an “innovative procession over land and water” to celebrate the Institute’s historic relocation.
Entries in the competition “may move autonomously or not; they may be representations of transport through artistic expression; and they may demonstrate types of transport other than physical, such as that of thought or emotion,” according to the official rules for the event. Teams are encouraged to use creativity, humor, elegant engineering, beauty, and “a sense of ‘only at MIT.’” Onlookers are encouraged to come and cheer on the various teams and take part in the parade.
In a fitting touch, the Grand Master of the parade will be none other than Oliver Smoot ’62, the MIT fraternity member whose body was used by his fraternity brothers as a living yardstick to measure the length of the Harvard Bridge that leads to the campus from Boston. The story of that long-ago prank is so well-known that even Google Earth allows “smoots” as an optional unit of measurement. And as the painted markings on the bridge, refreshed every year with new paint, attest, that bridge measures exactly 364.4 smoots and one ear.
The day of celebration will conclude with a pageant in Killian Court, featuring a multimedia spectacle, after which participants will spread out to a series of dance parties all across the campus. |
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