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Monday, March 21st, 2016
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| 12:00a | New way to control particle motions on 2-D materials Researchers at MIT and other institutions have found a new phenomenon in the behavior of a kind of quasiparticles called plasmons as they move along tiny ribbons of two-dimensional materials such as graphene and TMDs (transition metal dichalcogenides), which have a hexagonal structure resembling chicken wire. The team found that these plasmons can be separated into two different streams moving in opposite directions at the edges of the ribbons, like traffic on a two-lane highway, without the need for strong magnetic fields or other exotic conditions. The new research was carried out by MIT associate professor of mechanical engineering Nicholas X. Fang, recent PhD graduate from that department Anshuman Kumar, and four other researchers from the University of Wisconsin at Milwaukee, Hong Kong Polytechnic University, and the University of Minnesota. The work was reported in a paper in the journal Physical Review B. Other groups had previously observed such separated flows, Fang says, but that previous work required the use of powerful magnetic fields. Instead, the new process relies largely on optical effects, he says, using beams of circularly polarized light. The findings are based on exotic states of matter that can occur in two-dimensional materials that, unlike graphene, have a characteristic known as a bandgap, necessary for devices such as transistors or solar cells (and also in graphene that is modified to have a bandgap). These states of matter are based on quantum physics phenomena such as Berry curvature, which occur in configurations known as massive Dirac systems. Although such systems are a hot area of research these days, the researchers say this particular class of phenomena, involving surface electromagnetic properties known as surface plasmons, has been relatively unexplored until now. Clustering in “valleys” In the new work, the team showed that shining beams of circularly polarized light onto the graphene ribbons causes electrons in the material to cluster into two different “valleys” in the electronic band structure. The peculiar symmetry properties of this system gives rise to a phenomenon called Berry curvature, which can be thought of as an artificial magnetic field. Under these conditions, these valleys correspond to motions of the plasmons — which are a kind of oscillation of electron density in the material — in opposite directions on the two edges of the material. The graphene ribbons are just 50 nanometers (billionths of a meter) in width. This effective magnetic field can be measured by sending in a second polarized beam, whose transmission can then be detected so that the changes in its polarization give a direct measurement of the effects taking place in the surface plasmons. “This is exciting,” Fang explains, because it opens up a whole new approach to both manipulating the electromagnetic behavior of such systems and measuring the results of these manipulations. This could suggest possibilities for new kinds of electro-optical devices, he says. For example, some experimental photonic systems require devices called optical isolators, which prevent beams of light in precision optical systems from being reflected back to their source and causing interference. But these isolators, which require strong magnetic fields, are inherently bulky, he says, limiting the usefulness of such systems. “With this concept,” he says, “it’s possible to replace these bulky optical isolators with one monolayer of two-dimensional material.” Chip-scale isolation With such a system, Kumar says, it should be possible “to do chip-scale optical isolation without the need for a magnetic field.” To achieve the same degree of optical isolation that this system would provide with a beam of light, Kumar says, with a conventional system would require a magnetic field with a strength of 7 tesla — a very strong field that would require a special research facility. (By comparison, the Earth’s magnetic field measures just 32 millionths of a tesla). Theoretically, this could lead to applications such as new types of memory devices where information could be both written and read by using beams of polarized light, making them relatively immune to electromagnetic or other kinds of interference, the researchers say. “The concept presented in this paper is very interesting and exciting,” says Fengnian Xia, an assistant professor of engineering and science at Yale University, who was not involved in this work. He adds, “In the long run, it may be possible to construct an electrically tunable on-chip isolator based on this concept, which can be a very critical component in integrated optics.” In addition to Fang and Kumar, the team included Andrei Nemilentsau and George Hanson at the University of Wisconsin at Madison, Kin Hung Fung at Hong Kong Polytechnic University, and Tony Low at the University of Minnesota. The work was supported by the National Science Foundation and the Air Force Office of Scientific Research. |
| 12:00a | Physicists prove energy input predicts molecular behavior The world within a cell is a chaotic space, where the quantity and movement of molecules and proteins are in constant flux. Trying to predict how widely a protein or process may fluctuate is essential to knowing how well a cell is performing. But such predictions are hard to pin down in a cell’s open system, where everything can look hopelessly random. Now physicists at MIT have proved that at least one factor can set a limit, or bound, on a given protein or process’ fluctuations: energy. Given the amount of energy that a cell is spending, or dissipating, the fluctuations in a particular protein’s quantity, for example, must be within a specific range; fluctuations outside this range would be deemed impossible, according to the laws of thermodynamics. This idea also works in the opposite direction: Given a range of fluctuations in, say, the rate of a motor protein’s rotation, the researchers can determine the minimum amount of energy that the cell must be expending to drive that rotation. “This ends up being a very powerful, general statement about what is physically possible, or what is not physically possible, in a microscopic system,” says Jeremy England, the Thomas D. and Virginia W. Cabot Assistant Professor of Physics at MIT. “It’s also a generally applicable design constraint for the architecture of anything you want to make at the nanoscale.” For instance, knowing how energy and microscopic fluctuations relate will help scientists design more reliable nanomachines, for applications ranging from drug delivery to fuel cell technology. These tiny synthetic machines are designed to mimic a molecule’s motor-like behavior, but getting them to perform reliably at the nanoscale has proven extremely difficult. “This is a general proof that shows that how much energy you feed the system is related in a quantitative way to how reliable you’ve made it,” England says. “Having this constraint immediately gives you intuition, and a sort of road-ready yardstick to hold up to whatever it is you’re trying to design, to see if it’s feasible, and to direct it toward things that are feasible.” England and his colleagues, including Physics of Living Systems Fellow Todd Gingrich, postdoc Jordan Horowitz, and graduate student Nikolay Perunov, have published their results this week in Physical Review Letters. Making sense of microscopic motions The researchers’ paper was inspired by another study published last summer by scientists in Germany, who speculated that a cell’s energy dissipation might shape the fluctuations in certain microscopic processes. That paper addressed only typical fluctuations. England and his colleagues wondered whether the same results could be extended to include rare, “freak” instances, such as a sudden, temporary spike in a cell’s protein quantity. The team started with a general master equation, a model that describes motion of small systems, be it in the number or directional rotation for a given protein. The researchers then employed large deviation theory, which is a mathematical technique that is used to determine the probability distributions of processes that occur over a long period time, to evaluate how a microscopic system such as a rotating protein would behave. They then calculated, essentially, how the system fluctuated over a long period of time — for instance, how often a protein rotated clockwise versus counterclockwise — and then developed a probability distribution for those fluctuations. That distribution turns out to have a general form, which the team found could be bounded, or limited, by a simple mathematical expression. When they translated this expression into thermodynamic terms, to apply to the fluctuations in cells and other microscopic systems, they found that the bound boiled down to energy dissipation. In other words, how a microscopic system fluctuates is constrained by the energy put into the system. “We have in mind trying to make some sense of molecular systems,” Gingrich says. “What this proof tells us is, even without observing every single feature, by measuring the amount of energy lost from the system to the environment, it teaches us and limits the set of possibilities of what could be going on with the microscopic motions.” Pushing out of equilibrium The team found that the minimum amount of energy required to produce a given distribution of fluctuations is related to a state that is “near-equilibrium.” Systems that are at equilibrium are essentially at rest, with no energy coming in or out of the system. Any movement within the system is entirely due to the effect of the surrounding temperature, and therefore, fluctuations in whether a protein turns clockwise or counterclockwise, for example, are completely random, with an equal chance of rotating in either direction. Near-equilibrium systems are close to this state of rest; directional motion is generated by a small input of energy, but many features of the motion still appear as they do in equilibrium. Most living systems, however, operate far from equilibrium, with so much energy constantly flowing into and out of a cell that the fluctuations of molecular proteins and processes do not resemble anything in equilibrium. Lacking a similarity to equilibrium, it has been hard for scientists to uncover many general features of nonequilibrium fluctuations. England and his colleagues have shown that a comparison can nevertheless be made: Fluctuations occurring far from equilibrium must be at least as large as those that occur near equilibrium. The team says scientists can use the relationships established in its proof to understand the energy requirements in certain cellular systems, as well as to design reliable synthetic molecular machines. “One of the things that’s confusing about life is, it happens on a microscopic scale where there are a lot of processes that look pretty random,” Gingrich says. “We view this proof as a signpost: Here is one thing that at least must be true, even in those extreme, far-from-equilibrium situations where life is operating.” This research was supported in part by the Gordon and Betty Moore Foundation. |
| 12:00p | 3 Questions: The origin of the cosmos’ heaviest elements Reticulum II is an ancient and faint dwarf galaxy discovered in images taken as part of the Dark Energy Survey. It orbits the Milky Way galaxy about 100,000 light years away from us. Though the galaxy looks unassuming at first, the chemical content of its stars may hold the key to unlocking a 60-year-old mystery about the cosmic origin of the heaviest elements in the periodic table. Today in the journal Nature, a team of astronomers at MIT’s Kavli Institute for Astrophysics and Space Research and the Observatories of the Carnegie Institution of Washington report on observations of this unique galaxy using the Magellan telescopes at the Las Campanas Observatory in Chile’s Atacama Desert. Lead author and MIT physics graduate student Alex Ji explains more. Q: How are the heaviest elements in the periodic table created in the cosmos? A: Carl Sagan popularized the notion that we are all made of star stuff. He could say so with confidence because we actually know where nearly every element in the periodic table is made in the universe. But there's a hole in our understanding. The heaviest elements are made in what is called the "rapid neutron-capture process," or "r-process" for short, in which heavy elements are quickly built up from lighter seed nuclei. Gold, platinum, and uranium are r-process elements, as are more exotic elements like europium, neodymium, and gadolinium. The nuclear physics of the r-process was mostly worked out by 1957, but for almost 60 years astronomers have debated about the astrophysical site that could provide the extreme conditions for the r-process to occur. Synthesizing the r-process elements requires environments with a very large number of neutrons. The two best candidate sites are supernovae and merging neutron stars. Supernovae are the explosions that mark the end of a massive star's life. They often leave behind a remnant called a neutron star. During the formation of a neutron star, a large amount of neutrons is released. If two of these neutron stars happen to be orbiting each other, they will eventually merge to form one giant neutron star. During that explosion neutrons are released and r-process elements can form. Q: How does this dwarf galaxy help us understand the site of the r-process? A: Reticulum II is not the first ancient dwarf galaxy to have its chemical content examined; it's actually the 10th. But its chemical composition differs completely from those other galaxies. The stars in those first nine galaxies have unusually low amounts of r-process elements. Reticulum II, on the other hand, is chock-full of r-process elements. Its stars display some of the highest r-process enhancements we have ever seen. It's almost literally a gold mine. What this means is that a single rare event produced a rather large amount of this r-process material. All those elements were then incorporated into the surrounding gas and from there into the next generations of stars. It is those stars that we can still observe today. The single, prolific r-process event in this galaxy implies that a neutron star merger could have produced these elements in the early universe. A normal supernova would have produced less, and the observed enhancement could not have been as high, though it's also hypothesized that rare, magnetically-driven supernovae might be able to produce much more r-process material. Interestingly, there is indirect evidence that neutron star mergers do also synthesize r-process elements in the universe today. So it looks like neutron star mergers could be the primary r-process sites throughout cosmic time. It's amazing to think that Reticulum II preserved a signature of that extraordinary event for more than 12 billion years, just waiting for us to dig it up. Q: What was it like to be at the telescope and realize what you had found? A: Based on studying the other ultra-faint galaxies, I had expected to find stars with essentially none of these r-process elements and to further establish that these types of dwarf galaxies are devoid in these elements. So we had a plan to get some really good, low upper limits on the r-process content to push this issue. When we realized the stars in this galaxy were the complete opposite, and instead full of r-process elements, I was certain I had screwed something up. From the telescope in Chile I called my advisor Anna Frebel in Cambridge [Mass.] in the middle of the night to urgently talk about what was going on. Telescope time is precious and expensive after all and shouldn't be wasted. During the hour-long discussions that followed, I kept observing more stars while carrying out preliminary analyses of the data at hand to ensure that this was a real signal. At the observatory, astronomers work all night and sleep during the day. But after seeing the r-process elements in the first few stars, I couldn't sleep anymore; all I could do was stare out the window and hope the incoming clouds would go away again and the wind would die down. We were very lucky that it ended up being clear most of the four nights we had available. My last night there, the weather forecast, as translated from Spanish by Google, read "rain and wind." So I prepared myself to get no data that night. But it turned out Google had translated the word "despejado" incorrectly and in fact it was supposed to be "clear and wind." An important translation to get right, especially for astronomers! |
| 1:04p | Nanocrystal self-assembly sheds its secrets The secret to a long-hidden magic trick behind the self-assembly of nanocrystal structures is starting to be revealed. The transformation of simple colloidal particles — bits of matter suspended in solution — into tightly packed, beautiful lace-like meshes, or superlattices, has puzzled researchers for decades. Pretty pictures in themselves, these tiny superlattices, also called quantum dots, are being used to create more vivid display screens as well as arrays of optical sensory devices. The ultimate potential of quantum dots to make any surface into a smart screen or energy source hinges, in part, on understanding how they form. Through a combination of techniques including controlled solvent evaporation and synchrotron X-ray scattering, the real time self-assembly of nanocrystal structures has now become observable in-situ. The findings were reported in the journal Nature Materials in a paper by Assistant Professor William A. Tisdale and grad student Mark C. Weidman, both at MIT’s Department of Chemical Engineering, and Detlef-M. Smilgies at the Cornell High Energy Synchrotron Source (CHESS). The researchers anticipate their new findings will have implications for the direct manipulation of resulting superlattices, with the possibility of on-demand fabrication and the potential to generate principles for the formation of related soft materials such as proteins and polymers. Quantum dot disco Tisdale and his colleagues are among the many groups who study hard semiconductor nanocrystals with surfaces coated with organic molecules. These solution-processable electronic materials are on store shelves now under a variety of names, incorporated into everything from lighting displays to TVs. They also are being eyed for making efficient solar cells and other energy conversion devices due to their ease of fabrication and low-cost manufacturing processes. The broader adoption of these nanocrystals into other energy conversion technologies has been limited, in part, by the lack of knowledge about how they self-assemble, going from colloidal particles (like tiny Styrofoam balls suspended in a liquid) to superlattices (picture those same balls now dry, packed, and aligned). Techniques including electron microscopy and dynamic light scattering have uncovered some aspects of the starting colloidal state and the final superlattice structure, but they have not illuminated the transition between these two states. In fact, such foundational work dates back to the mid-1990s with Moungi Bawendi’s group at MIT. “In the past 10 to 15 years, a lot of progress has been made in making very beautiful nanocrystal structures,” Tisdale says. “However, there’s still a lot of debate about why they assemble into each configuration. Is it ligand entropy or the faceting of the nanocrystals? The depth of information provided by watching the entire self-organization process unfold in real time can help answer these questions.” Chamber of secrets
To make the nanoscale movie above, Tisdale’s graduate student and co-author Mark Weidman took advantage of a Cornell-developed experimental chamber and a recently developed dual detector setup with two fast area detectors, while environmental conditions were changed during the formation of superlattices. Using lead sulfide nanocrystals, Weidman was able to conduct simultaneous small-angle X-ray scattering (capturing the structure of the superlattice) and wide-angle X-ray scattering (capturing atomic scale orientation and alignment of single particles) observations during the evaporation of a solvent. “We believe this was the first experiment that has allowed us to watch in real time and in a native environment how self-assembly occurs,” Tisdale says. “These experiments would not have been possible without the experimental capabilities developed by Detlef and the CHESS team.” The use of nanocrystals with a heavy element (lead) and the brightness of the synchrotron X-ray source enabled sufficiently fast data collection that self-assembly could be observed in real time, resulting in compelling images and movies of the process. A fine mesh The discovery may lead to refined models for self-assembly of a wide range of organic soft materials. Moreover, the ability to watch the structure as it is evolving in real time also holds promise for intervening or directing the system into desired configurations, presaging a future how-to guide for creating superlattices. Tisdale says that much more work needs to be done to gain insights about why nanocrystals self-assemble they way they do. He and his team plan to use their new technique to manipulate parameters such as solvent conditions as well as the size and shape of nanocrystals, and to more closely study the ligands on the surface as they seem to be the key driver for self-assembly. “We hope that this study and technique will help to increase our understanding of colloidal self-assembly and, in the long term, enable us to direct nanoscale self-assembly toward a desired structure,” Weidman adds. The work was supported as part of the Center for Excitonics, an Energy Frontier Research Center funded by the U.S. Department of Energy Office of Basic Energy Sciences. The Cornell High Energy Synchrotron Source (CHESS) is supported by the National Science Foundation and the National Institutes of Health/National Institute of General Medical Sciences. |
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