Researchers investigate mechanics of lithium sulfides, which show promise as solid electrolytes.
|Using specialized equipment, the MIT team did tests in which they used a pyramidal-tipped probe to indent the surface of a piece of the sulfide-based material. Surrounding the resulting indentation (seen at center), cracks were seen forming in the material (indicated by arrows), revealing details of its mechanical properties. Courtesy of the researchers|
Most batteries are composed of two solid, electrochemically active layers called electrodes, separated by a polymer membrane infused with a liquid or gel electrolyte. But recent research has explored the possibility of all-solid-state batteries, in which the liquid (and potentially flammable) electrolyte would be replaced by a solid electrolyte, which could enhance the batteries’ energy density and safety.
Now, for the first time, a team at MIT has probed the mechanical properties of a sulfide-based solid electrolyte material, to determine its mechanical performance when incorporated into batteries.
The new findings were published this week in the journal Advanced Energy Materials, in a paper by Frank McGrogan and Tushar Swamy, both MIT graduate students; Krystyn Van Vliet, the Michael (1949) and Sonja Koerner Professor of Materials Science and Engineering; Yet-Ming Chiang, the Kyocera Professor of Materials Science and Engineering; and four others including an undergraduate participant in the National Science Foundation Research Experience for Undergraduate (REU) program administered by MIT’s Center for Materials Science and Engineering and its Materials Processing Center.
“Batteries with components that are all solid are attractive options for performance and safety, but several challenges remain,” Van Vliet says.
Read more at the MIT News Office.
David L. Chandler | MIT News Office
February 2, 2017
|Graduate student Frank McGrogan, left, supervised the work of 2016 Summer Scholar Erica Eggleton on lithium manganese oxide [LMO] electrodes for lithium ion batteries in the Van Vliet Lab. Photo, Denis Paiste, Materials Processing Center.|
Once fabricated, objects can be altered by adding new polymers.
|MIT chemists have now developed a 3-D printing technique that allows them to print objects and then go back and add new polymers that alter the materials’ chemical composition and mechanical properties. These new polymers can be reactivated by light. Image, Demin Liu and Jeremiah Johnson|
Three-dimensional printing technology makes it possible to rapidly manufacture objects by depositing layer upon layer of polymers in a precisely determined pattern. Once these objects are completed, the polymers that form the material are “dead” — that is, they cannot be extended to form new polymer chains.
MIT chemists have now developed a technique that allows them to print objects and then go back and add new polymers that alter the materials’ chemical composition and mechanical properties. The researchers can also fuse two or more printed objects together to form more complex structures.
“The idea is that you could print a material and subsequently take that material and, using light, morph the material into something else, or grow the material further,” says Jeremiah Johnson, the Firmenich Career Development Associate Professor of Chemistry at MIT.
This technique could greatly expand the complexity of objects that can be created with 3-D printing, says Johnson, the senior author of a paper describing the approach in the Jan. 13 issue of ACS Central Science. The paper’s lead authors are former MIT postdoc Mao Chen and graduate student Yuwei Gu.
Read more at the MIT News Office.
Anne Trafton | MIT News Office
January 13, 2017
Porous, 3-D forms of graphene developed at MIT can be 10 times as strong as steel but much lighter.
A team of researchers at MIT has designed one of the strongest lightweight materials known, by compressing and fusing flakes of graphene, a two-dimensional form of carbon. The new material, a sponge-like configuration with a density of just 5 percent, can have a strength 10 times that of steel.
In its two-dimensional form, graphene is thought to be the strongest of all known materials. But researchers until now have had a hard time translating that two-dimensional strength into useful three-dimensional materials.
The new findings show that the crucial aspect of the new 3-D forms has more to do with their unusual geometrical configuration than with the material itself, which suggests that similar strong, lightweight materials could be made from a variety of materials by creating similar geometric features.
Read more at the MIT News Office.
David L. Chandler | MIT News Office
January 6, 2017
MIT engineer’s research on surfaces is improving everything from power plants to ketchup bottles.
|“If you really understand the phenomena, you can reduce it to a few nondimensional parameters,” MIT mechanical engineer Kripa Varanasi says. That collapses the complexity into manageable formulas and phase diagrams, “and then we can design new processes, new products, and zero-tradeoff solutions.” That approach, he says, has been “at the heart of the companies we’ve started.”
Photo, Bryce Vickmark.
Ketchup’s sluggish pace as it oozes out of its bottle is a longstanding nuisance — but one that is about to be upended by a new product coming to market. The brainchild of MIT mechanical engineer Kripa Varanasi and his students, a new coating called LiquiGlide is set to make the transition from the laboratory to consumer and industrial markets.
LiquiGlide renders a surface highly slippery and allows every last drop of ketchup — or almost any other viscous product, from paint, to glue, to cosmetics — to flow from its container without sticking, saving billions of gallons of product from going waste.
“Viscous products sticking to the inside of containers leads to huge losses across industries,” Varanasi says. “For example, in paint manufacturing alone, paint sticking to the inside of mixing and holding tanks costs the industry more than 100 million gallons of lost product and billions of dollars per year in associated waste costs. Using the LiquiGlide platform, we are on a mission to eliminate waste generated across manufacturing applications, in areas ranging from food and agrochemical production to health care and energy, to usher in a new era of sustainable manufacturing.”
Read more at the MIT News Office.
David L. Chandler | MIT News Office
January 19, 2017
United Technologies Corp.’s subsidiary UTC Aerospace Systems is commercializing carbon nanotube-based de-icing technology co-developed by MIT Professor of Aeronautics and Astronautics Brian L. Wardle and Metis Design Corp., according to a press release.
“This technology strengthens UTC Aerospace Systems capability to deliver the most innovative solutions for aircraft ice protection systems," says Dr. Mauro Atalla, Vice President, Engineering and Technology, Sensors & Integrated Systems, UTC Aerospace Systems.
The de-icing system uses nanostitch technologies to weld carbon nanotubes to a wing. The electrically conductive nanotubes are heated to melt away ice.
Under a licensing agreement with Metis, UTC Aerospace Systems will integrate the new CNT technology at its facility in Uniontown, Ohio, with support from Metis Design in Boston and the United Technologies Research Center in East Hartford, Conn.
|Faculty Highlight: Brian Wardle|
Itamar Kimchi studies the physics underlying unusual behavior of electrons in compounds such as transition metal oxides.
|MIT Pappalardo Fellow in Physics Itamar Kimchi studies unusual behavior of electrons in transition metal oxides. Frustration of electrons in some materials at extremely low temperatures can force them to abandon their preference for classical magnetic states and instead enter unusual states of collective quantum superposition. Photo, Denis Paiste, Materials Processing Center.|
There are many kinds of frustration. There’s the kind of frustration of electrons in some materials at extremely low temperatures that forces them to abandon their preference for classical states such as spin up or spin down and instead enter unusual states of quantum superposition, which is what MIT Pappalardo Fellow in Physics Itamar Kimchi studies. Then there is the kind of scientific frustration when a research path down which Kimchi travels trying to explain these phenomena turns into a dead end.
“Especially in the kind of work I do, you don’t know what the research problem is until you’ve solved it, because you are exploring with a flashlight in the dark things that you only understand once you’ve finished exploring them, that you didn’t even know were there until you’ve understood them,” says Kimchi, who works closely as a postdoctoral associate with Professor of Physics Senthil Todadri.
“One of the things people don’t talk about that much, but I think is helpful to recognize for young people who may go into science, is this emotional roller coaster of scientific research, especially of this basic science, where you’re in the dark even when you try to define the research problem, you don’t know what’s going on, you run into a dead end, but then when you do discover something new that nobody discovered before, it is this high when things make sense, and it’s really something new. That’s beautiful. It’s an emotionally intensive exploration,” Kimchi, 30, says.
Kimchi [SB, ’08] double majored in mathematics and physics at MIT before obtaining his doctorate at the University of California-Berkeley. He returned to MIT as a Pappaladro Fellow in 2015.
Early interest in physics
Interest in math and physics came early to Kimchi, who was born in Jerusalem, Israel, but attended high school in the United States. “I knew I liked math and logic and understanding things. Popular science books that I was exposed to a little bit, like about relativity or the universe, were kind of fun. It felt like there was something neat about there being mysteries that you could uncover and then maybe part of that seems to combine nicely with thinking logically and mathematical reasoning,” he recalls.
Coming to MIT as an undergraduate, Kimchi was hooked from his first physics class. “I immediately could tell that what teachers in physics here wanted me to learn really fit what I wanted to learn as well, much better than any other class or department,” Kimchi says. “I felt like it was a good fit for the kind of problem sets I wanted to work on. The way that the class was taught, I think, really brought me into physics. That’s what cemented it.” He graduated Phi Beta Kappa.
It wasn’t until graduate school that Kimchi specialized in condensed matter physics. “It’s neat when you can use sophisticated math to try to understand real experimental results. That’s the draw of hard condensed matter physics theory, the correlated electron systems, for me. You have these really complicated quantum systems, and you see strange experimental results. And it’s always fun to try to understand them, and sometimes also it turns out that you need some powerful mathematics to make sense of the behavior, and that’s a kind of neat synergy or combination,” Kimchi explains.
In his graduate work, Kimchi discovered he liked the richness in the field of condensed matter physics and that he could accomplish projects on a short time scale. He notes that it overlaps with materials science, chemistry, theoretical mathematics and computer science. “My main project was on a series of magnetic insulator compounds where the magnetic atom is iridium, which is very heavy, far down in the periodic table. The nucleus is big, with a big charge,” Kimchi explains. “So the electrons move around it very fast, and that means that special relativity has some effect on the electronic properties of the material, in particular spin-orbit coupling.” His work on possible quantum spin liquid phases in lithium iridium oxide was published in a Physical Review B paper in 2014. Iridium is a transition metal, making this compound a transition metal oxide. “It turns out that it’s only when the electrons move very fast that their orbital motion can interact with this internal relativistic spin. The effects on the material properties, you don’t need to know special relativity for that, you just see the effects when you measure the magnetic properties of the iridium oxide.” These interactions can also lead to strange quantum entangled states of these electrons, that cannot be described by electronic band theory.
Looking back on the work, Kimchi says when he began making theoretical predictions about this particular interplay of spin-orbit coupling and lattice crystal structure in iridium oxides, it wasn’t clear that the crystal structures that he envisioned could be made. But when experimentalist James Analytis moved to Berkeley as a professor, he was able to grow crystals that turned out to form these crystal structures very closely related to the ones Kimchi was studying theoretically. “Then we could understand their magnetic properties, which are very unusual. People are still working on understanding them. The framework for understanding these properties that I and other people are using still comes from the initial theoretical work I did when I didn’t even know if these crystal theoretical structures could be made,” Kimchi says.
“We developed a model for this family of materials that involved frustration and spin-orbit coupling and crystal structure, all three tied up together. The model has some unusual predictions, one about the possibility of a spin liquid type of entangled state,” Kimchi explains. He is continuing work on other predictions from the model proposed in the paper about unusual spiral states, in which spins point in a very unusual spiral configuration. More recently, Kimchi co-authored work on these spiral states with Radu Coldea at Oxford University. “In this recent paper, I discuss what makes them unusual. It’s a little bit technical but turns out to be a very striking qualitative difference from usual spirals, this one is the first of its kind actually, and it comes from these ingredients of frustration and spin-orbit coupling and crystal structure that are in this model,” Kimchi says.
Dance of the electrons
Kimchi likens these mysterious behaviors of electrons to a quantum mechanical dance. “It can mean a lot for how the material behaves because the electrons are doing something completely different,” he says. He tries to develop theories that point to new experimental directions. “The role I see for myself is as a theorist who works with experimentalists. That’s been my role in my PhD as well,” he says.
“All the kinds of work that I do are understanding things that nobody has thought of before. It’s really thinking about the possibilities for how electrons can collectively act with quantum mechanics. So what are possibilities that people haven’t even considered?” Kimchi says.
“Before you can talk about technological applications, you need to understand it quantitatively, but before you can understand things quantitatively, you need to understand them qualitatively, when there is something completely new that you can’t even understand qualitatively. For example, this qualitatively new type of spiral, where you’re just understanding, what is this thing seen in the material? What’s different about it from everything that came before, but how can we use what we know about what came before to understand it?” he explains.
Frustration at triangles
In a very light atom, two properties of the electron, its momentum and its spin, are to a good approximation independent of each other. But in the heavy-element-based materials that Kimchi sometimes studies, relativistic effects couple the spin to the electron motion. At low temperatures, there may sometimes be just a few or one electron per atom [the outermost electron, i.e. a valence electron] involved in the low-temperature behavior, and its various states have low energy. Repulsive interactions between the electrons might lead a compound that should be metallic to instead become an insulator, with the electrons “stuck,” as in a traffic jam, on each atom. In such a so-called Mott insulator, an essential question is what happens to the electron spin. In many cases, the electrons on successive atoms form an alternating line of spin up and spin down electrons, preferring to alternate the spin on every site. In the context of his work, Kimchi says, magnetic frustration refers to cases where the electron doesn’t know whether it wants to be spin up or spin down. For example, this happens in crystals with structures where there are triangles in the lattice of the magnetic atom. In materials with this triangular type of arrangement, the crystal structure doesn’t allow a checkerboard pattern of alternating up/down spins. “This kind of frustration from the triangles is a way to amplify the effects of quantum mechanics,” Kimchi says. Electrons may be spin up at the first point of the triangle and spin down at the second, but the frustration at the third point can force the electron into a quantum superposition of both spin up and spin down. [This type of quantum superposition from coupling of spin up and spin has been shown in nitrogen-vacancy center diamond.] Spin-orbit coupling can have frustration effects similar to the triangles, again amplifying the collective quantum effects in the material.
He is currently working on quantum magnetic insulators that have both magnetic frustration and some disorder, which means there are irregularities in the arrangement of atoms. “There is some really interesting interplay between this quantum frustration and disorder, such as from impurities or other material randomness, and I am exploring how they combine. They can actually enhance each other’s effects, it turns out,” Kimchi says.
At Berkeley, Kimchi was advised by Prof. Ashvin Vishwanath, who also was a Pappalardo Fellow in Physics at MIT from 2001 to 2004. Like Kimchi, Vishwanath [who is now at Harvard] also worked closely with Physics Professor Senthil Todadri. During his PhD work, among other projects, Kimchi analyzed the role of spin-orbit coupling in quantum spin liquids, in which all of the spin moments of the electron are thought to be in a quantum superposition across the material. “Because they are quantum mechanical, one of the fundamental issues is there is no general direct way to see this quantum superposition,” he says. “One of my goals is to try to understand what are other experiments that one could do on other materials to try to understand this elusive behavior better.”
“There is a set of experiments that collectively form sufficient evidence that some kinds of spin liquids do occur in these certain materials, but what they are exactly, there is still some key understanding that’s lacking,” he suggests. “It’s a very hard problem, but it’s very interesting because what you have is a chunk of material where the electrons are in a quantum superposition state of the magnetic moments of their spins involving a subtle dance across the entire chunk of material.”
Although he was born in Israel, Kimchi considers himself to be more American, but he cherishes the multicultural research environment at MIT. “One of the fun things about science is getting to know people from across the world, how international it is,” he says.
Kimchi is married to Mollie Kimchi-Schwartz, who also is a physicist and a staff scientist at Lincoln Labs, where she works on quantum information and quantum computing. He enjoys traveling, hiking and being outdoors.
|PAPER: Three-dimensional quantum spin liquids in models of harmonic-honeycomb iridates and phase diagram in an infinite-D approximation|
|PAPER: Spin dynamics of counterrotating Kitaev spirals via duality|
A new chapter in magnetism
Quest to understand superconductivity leads MIT theoretical physicist Senthil Todadri to discoveries about new magnetic materials called quantum spin liquids.
|MIT Professor of Physics Senthil Todadri seeks to understand unusual metals that defy the textbook description. “We’ve been making slow but steady progress, not just I, myself, but the community as a whole, but I would really like that progress to accelerate,” he says. Photo, Denis Paiste, Materials Processing Center|
Mother nature is like a restless child who fidgets even when at rest, because electrons are never completely at rest, even at the coldest temperatures, MIT Physics Professor Senthil Todadri says. Imagine pushing a pendulum hanging from a clock. It will swing back and forth, but eventually it will come to a complete stop so that it has a velocity of zero, but we also can see that it has a definite position in space. In the quantum world of electrons, knowing both of these properties, velocity and position, with ultimate precision is forbidden by the Heisenberg uncertainty principle, Senthil explains. “Quantum mechanically, there is still some motion even in the default state, what we call the ground state,” he says. “It’s unavoidable motion that’s there, even in the default state, the lowest energy state that a quantum system can find itself in. There is still some motion.” And this basic fact of nature underlies a variety of unusual behaviors of electrons in materials. Explaining these hard-to-observe physical conditions such as new forms of magnetism is Senthil’s life’s work. Although his legal name is Senthil Todadri, he publishes under the name T. Senthil.
Everyday permanent magnetism in materials such as iron, where opposite poles attract and like poles repel, has been known for some 2,600 years, Senthil notes, and these are called ferromagnets, derived from the Latin word for iron. “A ferromagnet is something in which the electrons inside the material, they have tiny magnetic moments, and those tiny magnetic moments all line up together to form a giant magnet, and that’s what we see in the ordinary world as a ferromagnet,” he says.
Less than 100 years ago, physicists identified a new kind of emergent magnetism in materials that are called antiferromagnets. In an antiferromagnet, these tiny magnetic moments of individual electrons are frozen in space, but the pattern, or direction, of these tiny moments, oscillates in space on an atomic length scale. “From a macroscopic point of view, if you take an antiferromagnet, it’s very hard to know it has any kind of magnetism at all,” he says. That’s because this magnetism varies on a scale of one angstrom to a few angstroms, which is the size of just a few atoms, and technology to detect it didn’t become available until the 20th century.
“But now we know that antiferromagnetism is by far the most common form of magnetism. If you look around in magnetic materials, there are many, many more antiferromagnets than there are ferromagnets. It’s just that they are much harder to detect,” Senthil explains.
While it can be studied by itself, magnetism is intimately related to electricity, since every electron, including a magnetic one, carries an electric charge. A different class of materials, called superconductors, lose their resistance to electricity at very cold temperatures and there textbook analysis breaks down, says Senthil, 46, who is a theoretical physicist. As they are being chilled to a temperature of about 100 kelvins (minus -279.67 degrees Fahrenheit), these materials first show unusual metallic behavior, then become superconducting. Among materials that have been discovered to superconduct, many don’t do so until they reach a range from about 4 kelvins down to a fraction of 1 kelvin, or nearly absolute zero, the coldest possible temperature. So in the world of physics, superconducting at 100 kelvins is spoken of as high-temperature.
“From a fundamental science point of view what makes them remarkable is they seem to violate almost all of our textbook understanding of how electrons behave inside a solid,” Senthil explains. “Before they become superconducting, when the material is so hot that it’s not a superconductor yet, it’s a metal, but it’s a really unusual metal, and it’s out of this unusual metal that the superconductor is born.”
“Many of us think that understanding this unusual metal will give us a clue as to why the system is superconducting,” he says. “One of the hopes is that once we understand that clue, we can then have sensible ideas on what kinds of other materials might superconduct at relatively high temperatures and eventually maybe the field will discover room temperature superconductivity.”
These magnetic and superconducting behaviors appear to be intimately connected. One such material, lanthanum copper oxide, becomes an antiferromagnet at about 27 degrees Celsius (80 degrees Fahrenheit). “Certainly above room temperature in Boston right now,” Senthil says on a chilly winter day. “This is a very famous material. It’s famous because if you take this material and you remove electrons from this material, using chemical methods, it’s precisely this material that becomes one of those superconductors that I mentioned before. If you remove electrons, it losses its magnetism, but it becomes something else, it becomes a superconductor. Of course, it’s superconducting only at temperatures much colder than room temperature.”
“The thing I would really like to understand – that’s been a goal for more than 10 years now – is to understand these metals that defy the textbook description of a metal. In some sense, to me that’s the most outstanding challenge in this entire field, and over the years, we’ve been making slow but steady progress, not just I, myself, but the community as a whole, but I would really like that progress to accelerate. That’s my home base; that’s the problem I am always thinking about. Everything else almost feels like I am doing on the side, but that’s hard. There has been progress, enough to keep me encouraged, but not enough for me to jump up and down. So that’s my pet project. That’s what I see as the main thing I feel like I want to have done by the time I’m ready to retire,” Senthil reveals.
Yet another form of magnetism in solid materials is the quantum spin liquid, a name for magnetism drawn by analogy from fluid materials. In a spin liquid, Senthil says, individual electrons still have magnetic moments but they are randomly tumbling around all the time. “If you took a snapshot of what the magnetic moments are doing, different electrons with spins will be pointing in different directions, and if you took a different snapshot, the pattern would be completely different. So as a function of time, it’s moving all over the place, so there is no net magnetic moment. It’s neither a ferromagnet nor an antiferromagnet. We describe that by saying there is no magnetic ordering. ... That’s what’s called a spin liquid,” Senthil says.
|At extremely low temperatures in certain materials something unusual happens when an electron splits into fractional pieces. In the top image (which describes a metal), the orange balls represent positively charged metal ions and the smaller green objects are negatively charged electrons swimming freely. The bottom image represents a change from metallic behavior to an electrically insulating quantum spin liquid, in which the electrons split into fractional pieces. Here, the negative charge of the electron has attached itself to the positively charged ion. The remaining part of the electron continues to swim freely. Illustration, David Mross. Reproduced with permission, Physical Review B.|
Although there is no magnetic ordering, electrons in these spin liquids possess a property that no other form of magnetism has, that is, their magnetic moments are entangled quantum mechanically with the magnetic moments of other electrons far away from them. Quantum entanglement is one of the most counterintuitive concepts in physics. Its essence is that quantum mechanical systems that are separated from each other spatially still have some sort of contact with each other, what Einstein called “spooky action at a distance.” “The presence of quantum entanglement between distant parts of my sample, that is unusual, and it’s unprecedented in magnetism. So this is a new chapter in the study of magnetism. It leads to all kinds of unusual, bizarre phenomena that can potentially happen inside a solid,” Senthil explains.
An example of something unusual that can happen in these materials is that the electron may split into fractional pieces. How can that be when an electron is supposed to one of the fundamental particles of the universe? “An electron is supposed to be a fundamental particle in the vacuum of the universe, but inside a solid, the presence of long-distance quantum entanglement, enables the system to behave as some soup in which there are particle-like objects that move, but these objects are fractions of the electron,” he says. “What makes all these phenomena possible inside a system that looks very simple – it’s a collection of magnetic moments? What makes it possible is the long distance quantum mechanical entanglement that’s present between the localized magnetic moments. So I’ve been studying these kinds of magnetic matter. Not only is it fascinating by itself as a phenomenon, it turns out that studying these kinds of matter leads to all kinds of insights into other problems in condensed matter physics.”
In a February 2016 Physical Review B paper with Chong Wang [PhD ’15], who now is a postdoctoral associate at the Society of Fellows at Harvard, Senthil explored three-dimensional quantum spin liquids. Thinking about these unusual states of magnetism helped them map the connections between topological insulators, superconductors and quantum spin liquids. “There are some deep connections between many different phenomena in the field in many different kinds of systems that we realized only recently just in 2015, 2016, which ended up in some cases solving, in some cases showing the way forward, on questions that have been open for more than 20 years in the field. So there is enormous progress that has come about in theoretical physics as a whole because of thinking about these kinds of novel states of magnetism,” he says.
In particular, Senthil and Wang clarified how a particular metallic system chilled to very low temperatures in the presence of a large magnetic field can display the seemingly contradictory behavior of having both an overall current running at a right angles to an applied voltage while at the same time carrying charged particles that were shown in experiment to move in a straight line with the voltage. A long-standing theory from the early ’90s by co-authors Patrick A. Lee at MIT, Bertrand I. Halperin at Harvard and Nicholas Read at Yale proposed that this phenomenon could be explained by thinking of each charge carrier as having an electron that attaches to itself a bit of magnetic flux, so that even though each such composite particle moves in a straight line, it carries both an electrical charge and a magnetic flux. This movement of magnetic flux produces an electric field in the perpendicular [or transverse] direction. But the Lee-Halperin-Read theory couldn’t explain a kind of physical symmetry in this system known as particle-hole symmetry, Senthil says. A hole, perhaps most familiar in the context of semiconducting materials, is the absence of an electron in an atom where one is expected. Symmetry means that whether you view the system as a collection of electrons or view it as a collection of holes, the system is the same. “These are just two different viewpoints on what’s really the same system,” Senthil says. In experiments, replacing electrons with holes in this system didn’t make a physical difference, so it is considered to be in electron-hole symmetry, Senthil explains. Physicists model electrons mathematically through a complicated formula known as a wave function, which incorporates the electrons’ properties such as spin and momentum. Senthil says he and Wang were motivated by an idea proposed by Dam T. Son at the University of Chicago that this could be explained by thinking of the composite particle as carrying a “spin” that is locked into some definite angle to its momentum. A peculiar occurrence in quantum physics is that when spin of this composite particle, just like the electron spin itself, is rotated in a full circle, the particle’s quantum mechanical wave function turns from a positive number to a negative number. By coupling spin to momentum in this system, Senthil explains, “What we now understand in this story is that these objects that move in straight lines inside this medium, inside this two-dimensional collection of electrons in a strong field, that they have this feature that if you rotate their momentum on a full circle, the wave function changes sign, so that’s a different theory from the older theory.”
The technical term for these spin-momentum coupled particles is Dirac particles, from the Dirac equation that quantifies their quantum state. “Going from electrons to holes it turns out, flips the direction of the momentum of these Dirac particles, and it also has the effect of flipping, therefore, the direction of the spin, because the spin of this particle is tied to the momentum. So the momentum changes sign, the spin changes sign, so that’s all that happens,” he says. But importantly it explains how the electron-hole symmetry acts in this system. Wang and Senthil derived this solution from their understanding of other phenomena in condensed matter physics, such as topological insulators and quantum spin liquids. “In the process, we ended up learning a lot about all of these different systems,” Senthil says.
Senthil notes that these ideas also were pursued independently by Max Metlitski, who recently joined the MIT Physics faculty as Assistant Professor in the Condensed Matter Theory group, and Harvard Professor of Physics Ashvin Vishwanath. “These are friends of mine who it turns out were working on the same thing at the same time independently,” Senthil says. Vishwanath previously was a Pappalardo Fellow at MIT, serving as a postdoctoral associate in physics with Senthil.
Senthil supervises a small group of graduate students, Michael Pretko and Liujun Zou, who are working on quantum spin liquids, and Yahui Zhang, who is working on unconventional metallic states of matter through numerical calculations. “In experimental groups, the PI [principal investigator] tells the group what to work on. In theory groups, there is a lot more freedom. If a student comes to me and says, look, I have this idea, I encourage them to work on it, even if I am not working on it myself. ... It’s a great thing for students to come up with their own ideas,” Senthil says. Senthil also works with Pappalardo Fellows Inti Sodemann and Itamar Kimchi and Moore Fellow Sam Lederer and advises them on their work on magnetism, unconventional metals, and superconductivity.
This spring, Senthil will teach an advanced graduate course on Many Body Quantum Mechanics. Over the past four years, he taught quantum mechanics for first-year graduate students.
– Denis Paiste, Materials Processing Center
January 26, 2017
|Condensed Matter Theory Group|
|Theorists predict new forms of exotic insulating materials|
|PAPER: Symmetry-Protected Topological Phases of Quantum Matter|
New observations confirm an 80-year-old quantum theory.
|MIT researchers believe they have finally captured the process of quantum melting — a phase transition in quantum mechanics, in which electrons that have formed a crystalline structure purely through their quantum interactions melt into a more disordered fluid, in response to quantum fluctuations to their density.
Illustration, Jose-Luis Olivares, MIT (Wigner crystal image courtesy of Arunas.rv, CC BY-SA 3.0)
For the first time, MIT physicists have observed a highly ordered crystal of electrons in a semiconducting material and documented its melting, much like ice thawing into water. The observations confirm a fundamental phase transition in quantum mechanics that was theoretically proposed more than 80 years ago but not experimentally documented until now.
The team, led by MIT professor of physics Raymond Ashoori and his postdoc Joonho Jang, used a spectroscopy technique developed in Ashoori’s group. The method relies on electron “tunneling,” a quantum mechanical process that allows researchers to inject electrons at precise energies into a system of interest — in this case, a system of electrons trapped in two dimensions. The method uses hundreds of thousands of short electrical pulses to probe a sheet of electrons in a semiconducting material cooled to extremely low temperatures, just above absolute zero.
With their tunneling technique, the researchers shot electrons into the supercooled material to measure the energy states of electrons within the semiconducting sheet. Against a background blur, they detected a sharp spike in the data. After much analysis, they determined that the spike was the precise signal that would be given off from a highly ordered crystal of electrons vibrating in unison.
Read more at the MIT News Office.
Jennifer Chu | MIT News Office
December 20, 2016
“Invisible infrastructure” of nation’s scientific enterprise is eroding, MIT president warns in Wall Street Journal op-ed.
MIT President L. Rafael Reif. Photo, Dominick Reuter.
Writing in The Wall Street Journal, MIT President L. Rafael Reif has called for renewed federal support in basic science research, “for the nation’s long-term security, prosperity, competitiveness and health, and for generations of lasting new jobs.”
Noting that President-elect Trump has announced his intention to reinvest in public infrastructure, Reif noted in an op-ed published in Tuesday’s newspaper, “we must also rebuild another kind of infrastructure now eroding — by renewing our national commitment to fundamental science.”
For decades, federal research and development (R&D) funding provided the basis for much of the nation’s innovation and economic growth, Reif noted. The development of GPS technology, MRI scanners, and E-commerce, to name just a few examples, all rest on knowledge gleaned through fundamental research.
At its height in the 1970s, government funding for basic research represented more than 2 percent of the U.S. gross domestic product (GDP), according to Reif. Known as R&D intensity, this ratio measures society’s commitment to science, and by 2014, it had dropped to just 0.78 percent of the GDP.
Read more at the MIT News Office.
MIT News Office
December 8, 2016