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Breaking the Secrets of a Emerging Branch of Physics MIT News



Than Nguyen has a habit of breaking down barriers. Take languages, for example: Nguyen, a third-year PhD candidate in nuclear science and engineering (NSE), wanted to “connect with other people and cultures” about his work and social life, he says, so he learned Vietnamese, French, German and Russian language and now attends a MIT course in Mandarin. But this quest to overcome obstacles really comes to the fore in his research, where Nguyen tries to penetrate the secrets of a new and thriving branch of physics.

“My dissertation focuses on neutron scattering on topological semimetals that were only discovered experimentally in 2015,” he says. “They have many special properties, but because they are so new, there are many things that are unknown, and neutrons offer a unique perspective to explore their properties at a new level of clarity.”

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Topological materials do not fit well into conventional categories of substances that occur in everyday life. They first materialized in the 1980s, but became practical only in the mid-2000s with a thorough understanding of topology, which refers to geometric objects whose properties remain the same even when the objects undergo extreme deformation. Researchers have experimentally discovered topological materials even earlier, using the tools of quantum physics.

In this domain, topological semimetals that share the properties of both metals and semiconductors are of particular interest to Nguyen. “They offer high levels of thermal and electrical conductivity and inherent stability, making them very promising for applications in microelectronics, energy conversion and quantum computing,” he says.

Intrigued by the possibilities that may arise from such “unconventional physics,” Nguyen pursues two related but separate fields of study: “On the one hand, I try to identify and then synthesize new, robust topological semimetals, and from another, I want to discover a fundamental new neutron physics and further design new devices. “

On a fast research path

Achieving these goals over the next few years may seem like a serious task. But at MIT, Nguyen used every opportunity to master the specialized techniques needed to conduct large-scale experiments with topological materials and obtain results. Led by his advisor Mingda Lee, Norman Assistant S. Rasmussen and director of the Quantum Matter group at NSE, Nguyen was able to immerse himself in significant research, even before stepping on campus.

“The summer before I joined the group, Mingda sent me on a trip to the National Laboratory in Argon for a very fun experiment that used synchrotron X-ray scattering to characterize topological materials,” Nguyen recalled. “Learning the techniques fascinated me in the field and I began to see my future.”

During his first two years of graduate school, he participated in four studies, serving as a lead author in three journal articles. In a remarkable project described earlier this year in Physical examination letters, Nguyen and fellow researchers at the Quantum Matter Group demonstrated, through experiments conducted in three national laboratories, unexpected phenomena involving the way electrons move through topological semimetallic, tantalum phosphide (TaP).

“These materials are essentially resistant to heat and disturbances and can conduct electricity with a level of strength,” says Nguyen. “With robust properties like this, some materials can conduct electricity better than the best metals, and in some circumstances superconductors – an improvement over the current generation of materials.”

This discovery opens the door to topological quantum computing. Modern quantum computing systems, where the elementary computing units are qubits that perform ultrafast calculations, require superconducting materials that function only in extremely cold conditions. Heat fluctuations can eject one of these systems.

“The properties inherent in materials like TaP could form the basis of future qubits,” says Nguyen. It envisages synthesizing TaP and other topological semimetals – a process involving the delicate cultivation of these crystal structures – and then characterizing their structural and excitatory properties using neutron and X-ray technology, which studies these materials at the atomic level. This would allow him to identify and implement the appropriate materials for specific applications.

“My goal is to create programmable artificial structured topological materials that can be applied directly as a quantum computer,” says Nguyen. “With infinitely better heat management, these quantum computing systems and devices can be incredibly energy efficient.”

Environmental physics

Energy efficiency and its benefits have long affected Nguyen. Born in Montreal, Quebec, with a penchant for math and physics and a concern for climate change, he devoted his final year to high school in environmental studies. “I worked on an initiative in Montreal to reduce the city’s heat islands by creating more city parks,” he said. “Climate change was important to me and I wanted to make an impact.”

He graduated in physics from McGill University. “I was fascinated by the problems in the area, but I also felt that I could eventually apply what I had learned to meet my environmental goals,” he said.

In both class and research, Nguyen immersed himself in various fields of physics. He worked in a high-energy physics lab for two years to make neutrino detectors, part of a much larger collaboration to test the standard model. In the fall of his final year at McGill, Nguyen’s interest turned to condensed matter research. “I really enjoyed the interaction between physics and chemistry in this area, and I especially enjoyed researching superconductivity issues that seemed to have very important applications,” he says. This spring, trying to add useful skills to his research repertoire, he worked in the laboratories of Chalk River in Ontario, where he learned to characterize materials using neutron spectroscopes and other instruments.

These academic and practical experiences served as the impetus for Nguyen’s current postgraduate course. “Mingda Li offered an interesting research plan, and although I didn’t know much about topological materials, I knew they had recently been discovered and I was excited to enter the field,” he said.

A man with a plan

Nguyen has outlined the remaining years of his doctoral program and they will prove to be demanding. “It’s hard to work with topological semimetals,” he says. “We still don’t know the optimal conditions for their synthesis, and we need to make these micrometer crystals large enough to allow testing.”

With the right materials in hand, he hopes to develop “a qubit structure that is less vulnerable to interference, advancing rapidly in quantum computing so that computations that now take years can only take minutes or seconds,” he says. “Significantly higher computational speeds can have a huge impact on issues such as climate, health or finances that have important implications for society.” If his research on topological materials “benefits the planet or improves people’s lifestyles,” says Nguyen, “I will be completely happy.”


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