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  • richardmitnick 8:37 am on May 7, 2016 Permalink | Reply
    Tags: "Achieving zero resistance in energy flow", , MIT Physics   

    From MIT: “Achieving zero resistance in energy flow” 

    MIT News
    MIT News
    MIT Widget

    May 6, 2016
    Denis Paiste

    MIT postdoc Cui-Zu Chang makes a spintronic breakthrough in the Moodera group.

    MIT postdoc Cui-Zu Chang works with equipment that can monitor topological insulator thin-film quality as it grows. The bright green vertical bar on computer screen is an indicator of very high-quality film growth. Chang led work in the Moodera group showing the first zero-resistance edge state in a circuit. Photo: Denis Paiste/Materials Processing Center

    Laptop computer users operating their devices on their laps will be familiar with the heat they generate, which comes from electrical resistance converting waste energy to heat. Scientists dream of creating electronic devices with little or no resistance to the flow of electricity, in order to reduce heat output, save energy, and extend device capabilities. In the last several years theorists and experimentalists have been trying to achieve this goal using extremely thin materials with special physical properties, called topological insulators (TIs). Recently there has been a breakthrough* towards this goal: Dissipationless flow of current has been achieved in TIs when it enters a quantum state without any external magnetic fields — although, as of now, only at extremely low temperatures, its potential can be significant if the operating temperature could be raised.

    Topological insulators allow the free flow of electrons only on their surface while blocking the flow of electrons through their bulk. MIT postdoc Cui-Zu Chang, then a doctoral student at Tsinghua University in China, and colleagues at Chinese Academy of Sciences-Institute of Physics, Tsinghua, and Stanford University, reported the experimental demonstration of electrons flowing only along the edge of a topological insulator film circuit, driven by an internal magnetic field, which physicists call the quantum anomalous Hall effect. To provide internal magnetism for their circuit, they added chromium to their material, which was composed of bismuth, antimony, and tellurium. However, the Tsinghua system still showed remnants of electrical resistance to the edge current, frustratingly close to zero resistance.

    Dissipationless transport

    Improving upon his earlier work, Chang and colleagues in the group of Jagadeesh Moodera, along with collaborators from Penn State, Stanford and Northeastern University, achieved robust quantum anomalous Hall state and near dissipationless electron transport in topological insulators. Chang and colleagues at MIT replaced chromium with vanadium to obtain atomically thin layers of their magnetic topological insulators. They stacked sample films of this material on a base of strontium titanate. They reported** early results of this work in Nature Materials in May 2015, achieving very slight resistance to current flowing lengthwise along their sample.

    Via local and nonlocal measurements, Chang and colleagues at MIT and Penn State University with further optimization achieved zero resistance to current flowing lengthwise along the edge of their sample circuit at the extremely low temperature of 25 millikelvins (0.025 kelvins), a state physicists call “dissipationless chiral edge transport.” This lack of resistance is independent of length, they say in a Physical Review Letters paper*** published in July 2015. Moodera’s group is part of the Francis Bitter Magnet Laboratory and MIT Department of Physics.

    “In this system, there is a very special edge channel,” Chang explains. “The bulk is insulating but the chiral edge channel is metallic and spin polarized, so it’s very useful for the next generation electronics and spintronics with low power consumption.”

    “A signal entering this system can propagate a long distance without losing any of its energy. While presently it can only be realized at very low temperatures, there are indications that this can be raised,” Chang says. Observing this kind of quantum anomalous Hall state below 1 kelvin requires a special piece of equipment called a cryostat, so work continues to produce this effect at a higher temperature.

    Vanadium advantages

    Adding an extra element such as chromium or vanadium to introduce a special property (such as magnetism) to a material is known as doping. The vanadium-doped system showed three distinct advantages over the chromium-doped system:

    • twofold increase in the temperature above which the material loses magnetism (its Curie temperature), allowing the vanadium system to operate at zero resistance at a slightly higher but still very cold temperature;
    • 10 times increase in the stability of its intrinsic magnetism (its coercive field); and
    • one-half reduction in its carrier density.

    The vanadium system spontaneously shows magnetism at below about 23 kelvins. Results show this quantum anomalous Hall state can survive in a vanadium-doped system up to 5 kelvins (-450 degrees Fahrenheit). However, above 5 kelvins, the effect disappears and the normal resistance of the bulk material appears.

    While their sample film is still extremely thin — about 4 nanometers — the device studied is about 1 mm long by 0.4 mm wide, which is relatively large compared with other studies of quantum spintronic phenomena. “We make this kind of sample so big to preserve the delicate properties of the film. These films are very sensitive to water and air, which degrades the film properties,” Chang explains.

    Chang worked for five years in his doctoral studies at Tsinghua University searching for the quantum anomalous Hall effect, which was predicted in 1988 by F. Duncan M. Haldane at Princeton, he notes. “In a recent theoretical paper, no quantum anomalous Hall effect was predicted in a vanadium-doped topological insulator, whereas we experimentally showed the opposite is true, that this system is better for observing quantum anomalous Hall effect!” Chang says.

    Three conditions needed

    The 2006 discovery of topological insulators made the realization of quantum anomalous Hall effect practical. Chang cites three conditions to realize this effect: atomically flat thin TI film; introducing magnetism into the TI film; and tuning the chemical potential (Fermi level) into the gap induced by magnetism. After an intense search, Chang first observed the quantum anomalous Hall state in Oct. 9, 2012, in a sample of chromium-doped bismuth antimony, simultaneously showing a noticeable decrease in longitudinal resistance, according to a report on the evolution of their work published Feb. 26 in the Journal of Physics: Condensed Matter. Separately, a group at Tokyo University which included Joseph Checkelsky, now assistant professor of physics at MIT, confirmed the Tsinghua work and also observed the quantum anomalous Hall effect in the same system, Chang says.

    “If you can realize this effect at room temperature, it will significantly change our life. You can use this kind of effect to develop quantum electronics including the quantum computer,” Chang says. “In this kind of computer, there is minimal heating effect; the current flow is completely dissipationless; and you can also communicate over very long distance.”

    Although a superconductor can also reach zero resistance at low temperature, it is not spin-polarized, so it can transfer only electrical information but not spin information, Chang explains. The advantage of the quantum anomalous Hall effect, or topological edge state, is that the edge current is spin-polarized and robust, so it can be used to transfer information.

    Chang, 30, is originally from the Chinese kite-making city of Weifang, in Shandong province. He received his bachelor’s in optical engineering at Shandong University in China and doctorate in physics at Tsinghua University. His wife, Jia Song, has a PhD in mathematics from Tsinghua University. Chang’s work is supported by the Center for Integrated Quantum Materials under NSF grant DMR-1231319. A third-year postdoc in the Moodera group, he is looking for a faculty position in the fall.

    *Science paper:
    Experimental Observation of the Quantum Anomalous Hall Effect in a Magnetic Topological Insulator

    **Science paper:
    High-precision realization of robust quantum anomalous Hall state in a hard ferromagnetic topological insulator

    ***Science paper:
    Zero-Field Dissipationless Chiral Edge Transport and the Nature of Dissipation in the Quantum Anomalous Hall State

    See the full article here .

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  • richardmitnick 3:02 pm on September 11, 2014 Permalink | Reply
    Tags: , , MIT Physics   

    From M.I.T.: “Physicists find a new way to push electrons around” 

    MIT News

    September 11, 2014
    David L. Chandler | MIT News Office

    When moving through a conductive material in an electric field, electrons tend to follow the path of least resistance — which runs in the direction of that field.


    But now physicists at MIT and the University of Manchester have found an unexpectedly different behavior under very specialized conditions — one that might lead to new types of transistors and electronic circuits that could prove highly energy-efficient.

    They’ve found that when a sheet of graphene — a two-dimensional array of pure carbon — is placed atop another two-dimensional material, electrons instead move sideways, perpendicular to the electric field. This happens even without the influence of a magnetic field — the only other known way of inducing such a sideways flow.

    What’s more, two separate streams of electrons would flow in opposite directions, both crosswise to the field, canceling out each other’s electrical charge to produce a “neutral, chargeless current,” explains Leonid Levitov, an MIT professor of physics and a senior author of a paper describing these findings this week in the journal Science.

    The exact angle of this current relative to the electric field can be precisely controlled, Levitov says. He compares it to a sailboat sailing perpendicular to the wind, its angle of motion controlled by adjusting the position of the sail.

    Levitov and co-author Andre Geim at Manchester say this flow could be altered by applying a minute voltage on the gate, allowing the material to function as a transistor. Currents in these materials, being neutral, might not waste much of their energy as heat, as occurs in conventional semiconductors — potentially making the new materials a more efficient basis for computer chips.

    “It is widely believed that new, unconventional approaches to information processing are key for the future of hardware,” Levitov says. “This belief has been the driving force behind a number of important recent developments, in particular spintronics” — in which the spin of electrons, not their electric charge, carries information.

    The MIT and Manchester researchers have demonstrated a simple transistor based on the new material, Levitov says.

    “It is quite a fascinating effect, and it hits a very soft spot in our understanding of complex, so-called topological materials,” Geim says. “It is very rare to come across a phenomenon that bridges materials science, particle physics, relativity, and topology.”

    In their experiments, Levitov, Geim, and their colleagues overlaid the graphene on a layer of boron nitride — a two-dimensional material that forms a hexagonal lattice structure, as graphene does. Together, the two materials form a superlattice that behaves as a semiconductor.

    This superlattice causes electrons to acquire an unexpected twist — which Levitov describes as “a built-in vorticity” — that changes their direction of motion, much as the spin of a ball can curve its trajectory.

    Electrons in graphene behave like massless relativistic particles. The observed effect, however, has no known analog in particle physics, and extends our understanding of how the universe works, the researchers say.

    Whether or not this effect can be harnessed to reduce the energy used by computer chips remains an open question, Levitov concedes. This is an early finding, and while there is clearly an opportunity to reduce energy loss to heat locally, other parts of such a system may counterbalance those gains. “This is a fascinating question that remains to be resolved,” Levitov says.

    Francisco Guinea, a research professor at Spain’s Instituto de Ciencia de Materiales de Madrid, who was not connected with this research, calls the approach taken by this team “novel and imaginative. … The characterization of these currents in graphene is a very important advance in the understanding of two-dimensional materials.”

    The work has great potential, Guinea adds, because “two-dimensional materials with special topological properties are the basis of new technologies for the manipulation of quantum information.”

    In addition to Levitov and Geim, the research team included Roman Gorbachev, a research fellow at Manchester; Justin Song, a graduate student at MIT who is now at Caltech; Geliang Yu, a graduate student at Manchester; Freddie Withers, Yang Cao, and Artem Mishchenko, who are postdocs at Manchester; and Manchester professors Irina Grigorieva and Konstantin Novoselov. The work was supported by the European Research Council, the Royal Society, the National Science Foundation, the Office of Naval Research, and the Air Force Office of Scientific Research.

    See the full article here.

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  • richardmitnick 7:52 am on October 4, 2013 Permalink | Reply
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    From M.I.T.: “New kind of microscope uses neutrons” 

    October 4, 2013
    David L. Chandler, MIT News Office

    Researchers at MIT, working with partners at NASA, have developed a new concept for a microscope that would use neutrons — subatomic particles with no electrical charge — instead of beams of light or electrons to create high-resolution images.

    No image credit

    Among other features, neutron-based instruments have the ability to probe inside metal objects — such as fuel cells, batteries, and engines, even when in use — to learn details of their internal structure. Neutron instruments are also uniquely sensitive to magnetic properties and to lighter elements that are important in biological materials.

    The new concept has been outlined in a series of research papers this year, including one published this week in Nature Communications by MIT postdoc Dazhi Liu, research scientist Boris Khaykovich, professor David Moncton, and four others.

    See the full article here.

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  • richardmitnick 2:54 pm on July 25, 2013 Permalink | Reply
    Tags: , , , MIT Physics, , , , Superfluids   

    From M.I.T.: “Superfluid turbulence through the lens of black holes” 

    Study finds behavior of the turbulent flow of superfluids is opposite that of ordinary fluids.

    July 25, 2013
    Jennifer Chu, MIT News Office

    “A superfluid moves like a completely frictionless liquid, seemingly able to propel itself without any hindrance from gravity or surface tension. The physics underlying these materials — which appear to defy the conventional laws of physics — has fascinated scientists for decades.

    Black hole physics shows that superfluids in turbulence behave much like cigarette smoke. Image: Christine Daniloff

    Think of the assassin T-1000 in the movie “Terminator 2: Judgment Day” — a robotic shape-shifter made of liquid metal. Or better yet, consider a real-world example: liquid helium. When cooled to extremely low temperatures, helium exhibits behavior that is otherwise impossible in ordinary fluids. For instance, the superfluid can squeeze through pores as small as a molecule, and climb up and over the walls of a glass. It can even remain in motion years after a centrifuge containing it has stopped spinning.

    Now physicists at MIT have come up with a method to mathematically describe the behavior of superfluids — in particular, the turbulent flows within superfluids. They publish their results this week in the journal Science.

    ‘Turbulence provides a fascinating window into the dynamics of a superfluid,’ says Allan Adams, an associate professor of physics at MIT. ‘Imagine pouring milk into a cup of tea. As soon as the milk hits the tea, it flares out into whirls and eddies, which stretch and split into filigree. Understanding this complicated, roiling turbulent state is one of the great challenges of fluid dynamics. When it comes to superfluids, whose detailed dynamics depend on quantum mechanics, the problem of turbulence is an even tougher nut to crack.’

    To describe the underlying physics of a superfluid’s turbulence, Adams and his colleagues drew comparisons with the physics governing black holes. At first glance, black holes — extremely dense, gravitationally intense objects that pull in surrounding matter and light — may not appear to behave like a fluid. But the MIT researchers translated the physics of black holes to that of superfluid turbulence, using a technique called holographic duality.

    Consider, for example, a holographic image on a magazine cover. The data, or pixels, in the image exist on a flat surface, but can appear three-dimensional when viewed from certain angles. An engineer could conceivably build an actual 3-D replica based on the information, or dimensions, found in the 2-D hologram.

    ‘If you take that analogy one step further, in a certain sense you can regard various quantum theories as being a holographic image of a world with one extra dimension,’ says Paul Chesler, a postdoc in MIT’s Department of Physics.

    Taking this cosmic line of reasoning, Adams, Chesler and colleagues used holographic duality as a ‘dictionary’ to translate the very well-characterized physics of black holes to the physics of superfluid turbulence.

    To the researchers’ surprise, their calculations showed that turbulent flows of a class of superfluids on a flat surface behave not like those of ordinary fluids in 2-D, but more like 3-D fluids, which morph from relatively uniform, large structures to smaller and smaller structures. The result is much like cigarette smoke: From a burning tip, smoke unfurls in a single stream that quickly disperses into smaller and smaller eddies. Physicists refer to this phenomenon as an “energy cascade.”

    ‘For superfluids, whether such energy cascades exist is an open question,’ says Hong Liu, an associate professor of physics at MIT. ‘People have been making all kinds of claims, but there hasn’t been any smoking-gun type of evidence that such a cascade exists. In a class of superfluids, we produced very convincing evidence for the direction of this kind of flow, which would otherwise be very hard to obtain.’”

    See the full article here.

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  • richardmitnick 12:09 pm on January 11, 2013 Permalink | Reply
    Tags: , , MIT Physics,   

    From M.I.T. : “How to treat heat like light” 

    January 11, 2013
    David L. Chandler

    An MIT researcher has developed a technique that provides a new way of manipulating heat, allowing it to be controlled much as light waves can be manipulated by lenses and mirrors.
    The approach relies on engineered materials consisting of nanostructured semiconductor alloy crystals. Heat is a vibration of matter — technically, a vibration of the atomic lattice of a material — just as sound is. Such vibrations can also be thought of as a stream of phonons — a kind of “virtual particle” that is analogous to the photons that carry light. The new approach is similar to recently developed photonic crystals that can control the passage of light, and phononic crystals that can do the same for sound.

    Thermal lattices, shown here, are one possible application of the newly developed thermocrystals. In these structures, where precisely spaced air gaps (dark circles) control the flow of heat, thermal energy can be “pinned” in place by defects introduced into the structure (colored areas).
    Image courtesy of the researchers

    The spacing of tiny gaps in these materials is tuned to match the wavelength of the heat phonons, explains Martin Maldovan, a research scientist in MIT’s Department of Materials Science and Engineering and author of a paper on the new findings published Jan. 11 in the journal Physical Review Letters.”

    See the full article here.

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  • richardmitnick 11:42 am on August 31, 2012 Permalink | Reply
    Tags: , , MIT Physics, , ,   

    From MIT News: “A one-way street for spinning atoms” 

    Work correlating ultracold atoms’ spin with their direction of motion may help physicists model new circuit devices and unusual phases of matter.

    August 30, 2012
    News Office

    Elementary particles have a property called spin that can be thought of as rotation around their axes. In work reported this week in the journal Physical Review Letters, MIT physicists have imposed a stringent set of traffic rules on atomic particles in a gas: Those spinning clockwise can move in only one direction, while those spinning counterclockwise can move only in the other direction.

    Elementary particles have a fundamental property called ‘spin’ that determines how they align in a magnetic field. MIT researchers have created a new physical system in which atoms with clockwise spin move in only one direction, while atoms with counterclockwise spin move in the opposite direction.
    Graphic: Christine Daniloff

    Physical materials with this distinctive property could be used in “spintronic” circuit devices that rely on spin rather than electrical current for transferring information. The correlation between spin and direction of motion is crucial to creating a so-called topological superfluid, a key ingredient of some quantum-computing proposals.

    The MIT team, led by Martin Zwierlein, an associate professor of physics and a principal investigator in the Research Laboratory of Electronics (RLE), produced this spin-velocity correlation in an ultracold, dilute gas of atoms.

    The MIT research was funded in part by the National Science Foundation, the Air Force Office of Scientific Research, the Office of Naval Research, the Army Research Office with funding from the DARPA Optical Lattice Emulator program, and the David and Lucile Packard Foundation.

    See the full and important article here.

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  • richardmitnick 3:41 pm on July 31, 2012 Permalink | Reply
    Tags: , MIT Physics   

    From MIT News: “Alan Guth wins $3 million Fundamental Physics Prize” 

    Alan Guth ’69, SM ’69, PhD ’72, the Victor F. Weisskopf Professor of Physics at MIT, is among nine physicists worldwide selected as inaugural winners of the Fundamental Physics Prize, the Milner Foundation announced today.


    Congratulations to this graduate of Highland Park High School, Highland Park, NJ, USA, which also educated Eric, Jodi and Josh. What can I say, I could not let this go by.

    This year’s recipients — each of whom will receive $3 million in recognition of past research achievements in physics — will form a selection committee for future winners of the Fundamental Physics Prize. After this year, it is expected that the prize will be awarded to one physicist annually for what the Milner Foundation described in a statement as ‘transformative advances in the field.’”

  • richardmitnick 2:54 pm on July 25, 2012 Permalink | Reply
    Tags: , , , MIT Physics, , ,   

    From MIT News: “Single-photon transmitter could enable new quantum devices” 

    July 25, 2012
    David L. Chandler

    Long-sought goal for quantum devices — the ability to transmit single photons while blocking multiple photons — is finally achieved.

    In theory, quantum computers should be able to perform certain kinds of complex calculations much faster than conventional computers, and quantum-based communication could be invulnerable to eavesdropping. But producing quantum components for real-world devices has proved to be fraught with daunting challenges.

    An artist’s conception shows how any number of incoming photons (top) can be absorbed by a cloud of ultra-cold atoms (center), tuned so that only one single photon can pass through at a time. Being able to produce a controlled beam of single photons has been a goal of research toward creating quantum devices. Graphic: Christine Daniloff

    Now, a team of researchers at MIT and Harvard University has achieved a crucial long-term goal of such efforts: the ability to convert a laser beam into a stream of single photons, or particles of light, in a controlled way. The successful demonstration of this achievement is detailed in a paper published this week in the journal Nature by MIT doctoral student Thibault Peyronel and colleagues.

    See the full article here.

  • richardmitnick 9:32 am on June 5, 2012 Permalink | Reply
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    From M.I.T.: “NSE fusion program moves beyond plasma, towards practical power-plant issues” 

    “Nuclear fusion is a seemingly ideal energy source: carbon-free, fuel derived largely from seawater, no risk of runaway reactors and minimal waste issues. And the MIT Department of Nuclear Science and Engineering’s (NSE) long-standing fusion program is extending its leadership role in advancing the technology toward practical use.

    NSE’s Plasma Science and Fusion Center (PFSC), home of one of just three U.S. tokamak fusion reactors, has been a focal point of fusion research since its founding in 1976, developing substantial basic knowledge about creating and maintaining fusion reactions. And today, explains Professor Dennis Whyte, NSE’s fusion team is beginning a strategic pivot into the next stage of development, with a focus on interdisciplinary knowledge needed for the creation of functioning

    A tokamak

    ‘We’re basically making energy by creating a star,’ explains Whyte. ‘For power generation, the star has to turn on, and stay on for a year at a time, and we need a way to extract the energy it creates.’”

    See the full article here.

  • richardmitnick 3:06 pm on July 6, 2011 Permalink | Reply
    Tags: MIT Physics,   

    From MIT News: “A new way to build nanostructures” 

    Combining top-down and bottom-up approaches, new low-cost method could be a boon to research with a variety of applications.

    David L. Chandler, MIT News Office
    July 6, 2011

    “The making of three-dimensional nanostructured materials — ones that have distinctive shapes and structures at scales of a few billionths of a meter — has become a fertile area of research, producing materials that are useful for electronics, photonics, phononics and biomedical devices. But the methods of making such materials have been limited in the 3-D complexity they can produce. Now, an MIT team has found a way to produce more complicated structures by using a blend of current “top-down” and “bottom-up” approaches.

    The work is described in a paper published in June in the journal Nano Letters, co-authored by postdoc Chih-Hao Chang; George Barbastathis, the Singapore Research Professor of Optics and Professor of Mechanical Engineering; and six MIT graduate students.

    The new 3D nanofabrication method makes it possible to manufacture complex multi-layered solids all in one step. In this example, seen in these Scanning Electron Microscope images, a view from above (at top) shows alternating layers containing round holes and long bars. As seen from the side (lower image), the alternating shapes repeat through several layers. Image: Chih-Hao Chang

    See the full article here.

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