Building Materials, Atom by Atom
Joon Sue Lee knows what he’s looking for before he lays down the first atom. A builder in reduced dimensions, he considers the physics before he chooses the elements. He crafts nanostructures, develops devices, and makes careful measurements to show the sophisticated quantum phenomena he envisioned when he started. This is the welcome expertise he brought to the physics department when he joined the faculty as an assistant professor at the outset of 2020.
A native of South Korea, Lee was born in Gwangju, meaning city of light, and grew up in the capital city, Seoul. Physics, it turns out, came quite naturally to him from the early days of childhood.
"I was a boy with curiosity," Lee said, who liked to disassemble small electronics like clocks and radios, some of which were never reassembled, unfortunately.
"It was fun to learn how things are connected and working," he continued. "I naturally liked physics, which is the study that tries to explain nature from small things like subatomic particles to the big things like the universe."
He attended Seoul National University for his undergraduate studies, with a dual major in physics education and electrical engineering. He explained that he chose the former because he valued his own education, which encouraged and cultivated his interest in physics. The latter was rooted in a desire to explore what applications would build on the physics fundamentals he had learned. That’s where he discovered the area that would become the basis for his research program.
"Applications based on quantum phenomena caught my attention," he said. "(That) brought me to experimental condensed matter physics. I learned that applications in quantum matter are largely determined by the properties of materials. I finally became interested in quantum materials and their applications in quantum information science and spintronics."
Lee went on to earn a master’s degree in physics at Seoul National University and then moved to the United States for doctoral work, earning a PhD at Penn State in 2014. He stayed in State College for a postdoctoral appointment, then moved to the University of California, Santa Barbara, for a second postdoctoral position. On January 1 he joined the UT faculty, where he’s part of a growing program in quantum science, both in the physics department and across the university. His specialty is in creating and characterizing materials on the quantum scale with an eye toward potential devices.
Quantum materials are a new frontier, as novel and interesting (called emergent) phenomena can show up when electrons are confined in reduced dimensions. Spintronics, for example, uses an electron’s spin to generate and maintain current much more efficiently than conventional electronics. Topological superconductors could be used for stable qubits needed to make scalable quantum computing a reality. With that promise, however, come a fair number of challenges.
"Quantum phenomena are delicate and require clean materials systems and careful measurement schemes," Lee explained. "For my research, synthesis of high-quality materials is the first challenge, device fabrication with minimal disorder is the second, and low-temperature transport measurements with minimal noise is the third."
Lee carefully builds quantum structures using molecular beam epitaxy (MBE), depositing atomic layers one on top of the other, like LEGOs. Often, however, the elements that comprise a material can impose limits on the system he wants to create.
"Some are toxic like arsenic, some are corrosive, and some are pyrophoric," he said.
He added that scientists may avoid hazardous elements, but if they choose to use them, they need a careful plan to mitigate exposure. Aside from potential hazards, there’s a host of other considerations in choosing elements: how well they can be controlled as they’re deposited on a base layer (substrate), how they respond to temperature and pressure, and whether there’s a substrate commercially available that will be a good match for the surface symmetry and lattice parameter of the desired materials—i.e., how well the LEGOs will “lock.”
The elements, however, aren’t really the starting point. When Lee is considering what kind of materials he wants to develop, he begins by thinking where he’d like to end up. First, there’s the physics he wants to pursue (for example, topological superconductivity). Then he searches for the most promising materials candidates that might realize this physics (say, a superconductor-semiconductor hybrid system). As he narrows down potential materials that fit with his MBE expertise, he keeps in mind the current challenges of the field, what the next experimental milestones will be, the design and fabrication of devices to address those goals, and ultimately, measurements that demonstrate the quantum phenomena he’s looking for are actually there.
Lee’s research is based at the UT-Oak Ridge National Laboratory Joint Institute for Advanced Materials, which provides a collaborative environment to combine his strengths with those of other UT physicists.
"I would like to interface dissimilar materials based on my expertise as well as other researchers’ expertise to synergistically create new functionalities," he said.
For example, he could grow an indium arsenide semiconductor to interface with a high-temperature superconductor grown in Hanno Weitering’s group. This would allow them to study topological superconductivity that can be induced at much higher temperatures compared to currently studied materials systems. Another example he proposed would be that his group could prepare superconductor-semiconductor systems to grow on magnetic materials created by either Jian Liu’s or Haidong Zhou’s group. Such a material could exploit internal magnetic coupling to induce topological superconductivity without applying an external magnetic field. Grown materials can be carefully characterized in Norman Mannella’s group, and materials characteristics and physics behind the superconducting/magnetic features can be further understood with the help of our condensed matter theory groups.
Lee will also participate in the Appalachian Quantum Initiative, an endeavor that leverages the resources of the university and its partners to strengthen regional research and education.
While he started at UT in January, Lee’s MBE equipment didn’t arrive until late October.
"I just left the starting line of my research," he said.
(Though for someone who just left the starting line, he’s doing quite well, having been recognized with the 2020 Outstanding Young Researcher Award by the Association of Korean Physicists in America.)
Once all equipment is installed, his group, including graduate students Pradip Adhikari and Anjali Rathore as well as undergraduate Lena Schwebs, will start growing and characterizing materials—semiconductors, superconductors, and hybrids of the two. As they progress to developing and testing devices, he’ll be looking for more help.
"My group will need more hands and brains," he said. "Highly motivated undergraduate and graduate students are always welcome."
Lee’s other students were those enrolled in his thermal physics course this past spring, an interesting semester that began in the classroom and ended online.
"My teaching in spring 2020 was full of challenges and fun," he said. "Everything was new, from developing face-to-face teaching schemes to transitioning to online teaching."
When the term began, Lee said he was using a whiteboard to write down equations and a projector to show slides with figures. When the class moved online after spring break, there was some trial and error to find the best approach, which turned out to be a OneNote page with blank spaces between figures for taking notes during class.
"I thank all the students who actively participated in both the offline and the online classes and provided feedback to improve the class," he said.
When he isn’t studying quantum materials or developing physics courses, Lee loves playing with his kids, ages three and one. He does let science carry over to his leisure time just a bit, though not with the precision his nanostructures demand. When it comes to books and movies, he’s a science fiction fan.