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Behaviors in complex systems are often more than the sum of their parts, whether in the brain where neurons enable expression of personality, or in fast data storage devices enabled by magnetic interactions. Physicists call these distinctive collective behaviors “emergent” properties. MIT assistant professor of physics Joseph G. Checkelsky is working at the intersection of materials synthesis and quantum physics to discover new materials that host these emergent phenomena, which in turn may foster new technologies.
“In the context of solid state physics, emergent behavior often takes the form of collective behavior of electrons,” Checkelsky explains. “For example, looking at an individual electron, one would not imagine that it could form a coherent superconducting condensate by partnering with other electrons in a solid. Nor would one expect that different crystalline solids could give rise to the dynamic family of superconductors we know from experiment. Our goal is to look at materials where the quantum mechanical nature of the underlying electrons is brought out in new types of such collective properties.”
Checkelsky recently won a National Science Foundation (NSF) CAREER award to pursue research into particular kinds of quantum materials that combine so-called frustrated atomic lattices and mobile electrons.
An ordinary magnet gets its magnetism from the quantum mechanical property that electrons are indistinguishable and may lower their energy by aligning their magnetic moments. In some magnetic materials, known as antiferromagnets, magnetic moments of ions alternate in a repeating up/down pattern to lower their energy. The notion of a frustrated lattice, Checkelsky explains, is to create a repeating structure in the arrangement of atoms that confuses this ordering with competing interactions so that the system is unable to find a suitable way to orient itself. It is expected that this will lead to electrons with stronger quantum mechanical interactions that promote collective behavior. “By using these atomic arrangements one can force the electrons to have to deal with each other,” Checkelsky says.
In Checkelsky’s lab, graduate student Linda Ye and postdoc Takehito Suzuki are studying an example of this with various arrangements of iron and tin atoms on the Kagome lattice structure, a series of overlapping triangles and hexagons. This work was inspired by theoretical work from 2011 by Cecil and Ida Green Professor of Physics Xiao-Gang Wen, Checkelsky notes. Many frustrated systems are electrically insulating. Whereas the study of those types of frustrated materials is established (including seminal contributions from MIT), in these new experiments the researchers are adding free electrons to these systems and studying how they behave.
Ye explains that theorists proposed that ferromagnetic electrons on this Kagome lattice with strong spin-orbit coupling might give rise to topological bands with fractional excitations. “Generally this is a combination of strong spin-orbit interaction, magnetism, and the lattice degree of freedom,” she says. “It turns out to be a very interesting electronic system.” It differs from much of the low-temperature work in the lab because it displays interesting properties at room temperature. “It has strong ferromagnetism even at room temperature,” she says.
The crystal lattice, in a ratio of three iron to two tin atoms, is made with the chemical vapor transport method. The obtained single crystals are hexagonally shaped plates with dimensions of about 1 millimeter (about four one-hundreths of an inch), Ye says. “There is one trick about growing this crystal. It’s not a stable phase at room temperature,” she says. Her approach is to cool down the crystals rapidly from high temperature in cold water so the lattice doesn’t change its phase. It can take up to a full month in the furnace to grow these single-crystal compounds.
Ye is studying the mechanism underlying a phenomenon known as the anomalous Hall effect by measuring voltage made in the direction transverse (perpendicular) to a current flowing through a crystal of this material. Ye presented a report on the work today an American Physical Society meeting in Baltimore. Ye, 26, grew up in the city of Chengdu in Sichuan, China, and earned her bachelor’s degree at Tsinghua University in Beijing. She has a master’s in engineering (applied physics) from the University of Tokyo.
Checkelsky was named a Moore Foundation Fellow in Materials Synthesis in August 2013. The award helped him to establish a lab that combines the growth of new materials with analyzing their properties and creating new devices that exploit their unique behavior. “The Moore Foundation has allowed our laboratory the capabilities to both synthesize and study materials. This ranges from thin film growth by molecular beam epitaxy to cryogenic measurements. Because of their support at the inception of our lab, we have been able to make fast progress towards realizing exotic quantum materials and studying their properties,” Checkelsky explains.
The lab features a synthesis side with furnaces for growing materials and an analysis side with equipment such as cryogenic refrigerators and superconducting magnets for testing them. “It’s one room, but it has two distinct halves,” he says. “Every day the students are walking across the line between the two sides — we are trying to make this border disappear.”
The group includes postdoc Takehito Suzuki, graduate students Linda Ye and Aravind Devarakonda and undergraduate Christina Wicker. A second postdoc is expected to join the group this summer, and Checkelsky plans to add more graduate and undergraduate students. “It’s very exciting to have seen all the big equipment come one by one, and put [it] together,” Ye says. “Many of the things in the lab we design and make by ourselves.” She is making a high vacuum probe for the thermal measurements inside the cryogenic refrigerator.
Accurate measurements require using modern tools of nanoscience to probe the electronic properties of materials, Checkelsky says. “One type of life cycle for a project begins with an idea for physics of interest that we have in the office or learn through discussions with our theory colleagues. We then consider what materials systems are most likely to support such behavior and try to synthesize them. With the characterized material in hand we then design ways to incisively probe the physics of interest, which can involve high magnetic field, scattering, or making and measuring nanodevices from the material.” Checkelsky adds, “Accidental discoveries more interesting than our original targets also happen in the lab — which we are more than happy to embrace.”
Besides equipment in his own lab, Checkelsky and his research team use facilities on the MIT campus at the Microsystems Technology Lab (MTL), as well as tools at the Harvard Center for Nanoscale Systems, the NIST Center for Neutron Research, and the National High Magnetic Field Laboratory in Tallahassee. The Florida lab features the world’s largest static magnet. “When we finally have our best materials, we bring them down to the magnet lab. They have been very supportive of our projects,” he adds.
Checkelsky says he is looking forward to completion of MIT.nano, where he hopes to make use of the state-of-the-art facilities. “Then we will be able to go from thinking of projects to combining the powders to making the crystals to walking them next door to MIT.nano for the next step. It will be a really key facility for our projects,” he says.
Checkelsky also is part of the Center for Integrated Quantum Materials. He works closely with MIT experimentalists Nuh Gedik, Ray Ashoori, and Pablo Jarillo-Herrero and theorists Patrick Lee, Liang Fu, Senthil Todadri, Leonid Levitov, and Xiao-Gang Wen.
Heavy metal magnets
Graduate student Aravind Devarakonda is studying magnetic behavior of heavy metal compounds where static magnetism provides a source of correlation for the electrons. “The basic idea is that certain spinel compounds have a fixed level of magnetic order which can persist even with changing composition of other lattice constituents. By going from lighter to heavier elements surrounding these magnetic atoms, we can introduce topological electronic features into the material,” Devarakonda says.
“If we want to combine electronic correlation with the physics of topological insulators and related materials we have to make progress on several fronts,” Checkelsky explains. “In addition to following theoretical predictions, an experimental voyage into material systems is a key component of this. And indeed thus far the work has been experimentally driven. In this sense the theme has been to look at systems where the presence of non-trivial electronic topology is not well-resolved from theory due to the complications of electronic correlation but which we can grow and examine experimentally.”
Devarakonda, who earned his bachelor’s in applied science (physics) at Rutgers University, has become expert in the techniques of crystal growth, cryogenics, electrical measurements, nanofabrication, metal work, and first principles calculations — “the full tool belt,” Checkelsky says.
Checkelsky received his BS in physics at Harvey Mudd College and PhD in physics at Princeton University in 2010. Before joining the faculty at MIT in January 2014, Checkelsky was a postdoc at Japan’s Institute for Physical and Chemical Research (RIKEN) and a lecturer at the University of Tokyo. His work and continued collaboration there includes studies of the surface properties of topological insulators and more recently unusual electronic properties of a compound material with layers of three elements — bismuth, tellurium, and iodine. “The atoms line up in the same stacking sequence through the whole material,” Checkelsky explains. “So, the system breaks a spatial symmetry and has a direction associated with it.”
“This type of compound is called polar because of this directionality. That structural difference has profound implications for the electronic behavior,” he says. In particular this polarity changes the spin degree of freedom of the electron so that the way electrons move in the solid very strictly depends on the spin direction. Experiments published in a 2015 paper demonstrated a spin up channel and a spin down channel with different energy levels in measurement of their electrical resistance under a magnetic field. This work could have implications for future spintronic devices.
Checkelsky is in his third term of teaching 8.02 (Electromagnetism) in the Department of Physics. With a typical class size of 100 students, 8.02 is taught in a Technology Enabled Active Learning (TEAL) classroom with students around the teacher. “It gives you not only face-to-face communication with the students, but a technological link as well. They have a clicker with which they can respond to questions, which can offer a view into what they’re thinking,” Checkelsky explains. “I had the good fortune of starting my teaching in 8.02 with Peter Dourmashkin, who is a constant source of insight into how to approach the class. I really enjoy working with the students and trying to convey not only my enthusiasm for physics but also practical, effective ways for them to tackle the course material.”
Checkelsky’s lab has a partnership with the Wilson Creek Elementary School located in Johns Creek, Georgia, near his hometown north of Atlanta. With the help of their teachers, students there submit questions about science, and Checkelsky’s team comes up with the best answers they can. Analysis of student submissions showed that the most frequently occurring word in the students’ questions is “possible.” “They’re really at a very young age interested in what is possible for human beings and technology,” Checkelsky says. “We approach their questions as seriously as any kind of scientific question that comes up in our research. We have to read papers and discuss with colleagues; we really try to come up with a complete answer for students.” In addition to the support of the teachers and Principal Andrea Cushing, it is no doubt a great benefit that Checkelsky’s mom, Eileen Checkelsky, is the current principal’s secretary at Wilson Creek Elementary School.
Learning the recipe
Checkelsky’s office windows are lined with towering plants inherited from Tom Greytak, retired professor of physics, who previously had the office. “At first I had to figure out what the names of the plants were. But taking care of them has become a serious hobby of mine.” This dovetails with his interest in cooking. “I think it’s curious that what I do at MIT is synthesizing and studying inorganic materials and my hobbies are basically synthesizing and processing organic ones. It all revolves around finding the right recipe,” he says.