Two More NSF CAREER Awards for UT Physics

April 2, 2019

Assistant Professors Steve Johnston and Jian Liu have won prestigious early-career awards from the National Science Foundation for their research to understand and control materials on a quantum scale. The grants came through the NSF’s Faculty Early Career Development (CAREER) Program, an initiative supporting faculty at the outset of their careers who have shown potential as role models in research and education. These two awards bring the department’s total to seven NSF CAREER honors since 2012.

Both Johnston and Liu will focus on the properties of quantum materials, which are at the forefront of condensed matter physics research. The rules for macroscopic objects don’t necessarily hold in the realm of subatomic—a pitched baseball’s path and velocity, for example, aren’t mimicked by an electron working through a metal. Johnston will develop new theoretical tools that will help scientists interpret findings from powerful experimental techniques, while Liu will investigate the hidden symmetry of quantum systems to develop new two-dimensional magnetic materials. Both have incorporated elements in their awards to broaden the impact of their work through education and outreach.

Defining Drivers and Passengers

Johnston’s research will focus on providing new theory frameworks to support current experiments using Resonant Inelastic X-Ray Scattering (RIXS) as well as developing time-resolved RIXS techniques. This method scatters an x-ray beam off a material’s electrons and in doing so can probe multiple properties in a single experiment, resulting in rich, yet complicated, data. As Johnston explained, the remarkable properties of quantum materials often derive from a subtle interplay between their electronic, magnetic, and structural properties, and a central challenge for the community is to separate the drivers from the passengers. Tools like RIXS provide a way to study all of these interactions in a single experiment. But the tradeoff is that interpreting their findings can be a challenge without sophisticated theoretical models. Johnston will work to develop theoretical approaches and computational codes that can help experimentalists determine what their data is truly measuring and what can be determined from those findings. The goal is that these theoretical tools will work across a broad range of quantum materials, including high-temperature superconductors and quantum spin liquids.

Johnston described how, for example, scientists still aren’t certain if high-temperature superconductivity is driven purely by magnetic interactions, or if a material’s atomic lattice is lending a hand. He hopes to develop causal links for scenarios like this, in part by looking at time scales to see if effects are governed by slow or fast processes.

"This new information can give us clues as to which interaction is the driver and which is the passenger," he said.

Johnston will be working with scientists from the Paul Scherrer Institute in Switzerland and hopes to expand his collaborative efforts to RIXS experimental groups in the U.S. His proposal also includes broadening the impact of this research by establishing an APS Bridge Program in the UT Physics Department.

The American Physical Society Bridge Program is designed to strengthen the wider physics community by increasing the number of graduate students from under-represented minorities via transition programs, mentoring, and networking. Johnston will work with members within the department to develop the Bridge program at UT, as well as partner with existing efforts such as Tennessee Louis Stokes Alliance for Minority Participation (TLSAMP). He will also involve current students in his CAREER research. The grant, titled Advancing theory of Resonant Inelastic X-ray Scattering for Materials In- and Out-of-Equilibrium, begins in August and lasts for five years.

Magnets, One Atom Thick

Like Johnston, Jian Liu is also intrigued by the properties of quantum materials, specifically magnetism. As he explained in his NSF proposal, new magnetic materials are necessary for developing a next generation of processors, memories, and sensors with better security, faster speed, and smaller size. His goal is to build, atomic layer by atomic layer, quantum antiferromagnets that can be controlled externally.

Quantum antiferromagnets are model systems that have great impacts for condensed matter physics. For instance, high-temperature superconductivity is known to occur when metallizing quantum antiferromagnets. Their quantum mechanical nature can cause entanglement that is absent in ferromagnets. One chief challenge, however, is that antiferromagnets, unlike their ferromagnetic cousins, by nature resist being controlled by a magnetic field.

"By the name of antiferromagnet, essentially the material likes to be the opposite of a ferromagnet," Liu said. "And what that means is that it doesn’t like to be in a magnetic field. So typically you have two choices. You either apply a magnetic field and get no response, (or) you apply a very large magnetic field and simply just destroy the antiferromagnetic order. That’s usually the only two possible outcomes."

By using laser deposition to synthesize precise two-dimensional oxide materials and implement a hidden symmetry in their structure, Liu sees the promise of overcoming this challenge.

"If you can make things two-dimensional rather than three-dimensional, you save a lot of room—a lot of space,” he said. “You basically scale things down to as thick as one atom."

He will take advantage of the hidden rotational symmetry in the materials’ spin—where a magnetic field will couple to alternating quantum spins and in doing so cause the material to respond like a ferromagnet. This will require Liu to precisely engineer these tiny structures, designing every minute detail about their thickness, composition, and structural distortions and then study the magnetism in samples a million times thinner than a human hair. He’ll do this work in his lab at the Joint Institute for Advanced Materials, a UT-Oak Ridge National Laboratory institute located at the university’s research park at Cherokee Farm, and with collaborators at synchrotron facilities.

Liu will share his research publicly through several venues, including the department’s Saturday Morning Physics program and Faceback live chats. To engage budding scientists, he will also work with the university’s Governor’s School for the Sciences and Engineering and plans to reach out to local high schools to see if any students would be interested in working on some summer projects related to the research. As with Johnston’s CAREER work, Liu is encouraging students from under-represented groups to take part.

"We are going to invite students from minority-serving institutions to do a project in the summer,” he said. “We will try to build a long-term collaboration with those institutions."

Liu also has an eye on incorporating his research into the department’s undergraduate physics courses.

"After three years of teaching those classes, I feel like we’re using a lot of examples and demonstrations that are seminal but based on old techniques or old devices. But today all the students are dealing with smart phones; smart devices—very user-friendly, user-oriented devices—and they don’t get to see the physics behind because those devices are all packaged," he said. "There’s a lot of technology and physics that you can demonstrate by using those devices."

With the next revolution in technology coming with 5G and the Internet of Things, Liu would like to make a clear connection between developments in materials physics and the world students live in.

"That’s what they’re going to be dealing with in their future careers," he said. "It would be beneficial for them to see the bridge between the physics they learn in the textbook and the technology they embrace in life."

Liu’s grant—Engineering Artificial Oxide Layers with Hidden Spin Symmetry for Drivable 2D Quantum Magnetism—begins July 1 and like Johnston’s runs for five years. Since 2012, the UT Physics Department has won seven NSF CAREER Awards. Of the 26 current CAREER awards listed under the University of Tennessee on the NSF website, six are from the physics department.

Further Reading

• More about the NSF CAREER Program
• More on Steve Johnston and Jian Liu's research involving RIXS.
• More on Jian Liu's work with quantum systems and hidden symmetry.