Like different sides of a ledger, how quantum materials work and what everyday applications require are in opposite columns. With support from a prestigious Humboldt Research Fellowship, Associate Professor Jian Liu wants to balance the books with materials and devices that use quantum science to meet practical needs.

Good Friends Make Good Science

Currently Liu is spending the first of three summers at the Leibniz Institute for Solid State and Materials Research Dresden (IFW Dresden) in Germany. He was introduced to scientists there by UT Physics Assistant Professor Yang Zhang.

“He has good connections in Germany,” Liu said of Zhang. “He recommended (to) me that would be a nice place to visit and he connected me to some great colleagues there. We started talking and there was a lot of common interest.”

IFW Dresden focuses on investigating matter’s properties to develop new applications. That’s a great match for Liu, who studies quantum materials for innovative nanotechnologies. While quantum science is a rapidly-growing field for research and industry, it can be tricky ground to cover. Physics in the macroscopic world (the path of a baseball pitch, for example) is very different from the microscopic, or quantum, world (like the spin of an electron). The rules in quantum mechanics differ wildly from the predictable laws of classical mechanics. One key difference is temperature.

“For quantum phenomena to emerge, we need to go to very low temperature,” Liu explained. “If you read articles about quantum computers, you’ll see they have to go extremely low temperatures. That’s when the thermal effects are gone and then the quantum effects really show up. (It’s) the same for materials. If we want to measure the quantum properties of materials we have to go to very low temperatures.”

Very low in quantum-speak means near absolute zero. Liu explained that when things get too warm, quantum properties disappear.

“Thermal effects can de-cohere quantum properties,” he said. “The famous example would be superconductivity, where you need two electrons in a pair. The reason they could pair together is because their wave functions are coherent with each other. They know what the other is doing, so that they can act accordingly. But a thermal effect is going to come in and de-cohere (them). And eventually when you reach high enough temperatures superconductivity disappears.”

As scientists learn more about subatomic systems, they can find advanced uses for them like cyptography for secure communications or sophisticated sensors for precise navigation.

The Dresden group hosting Liu has instruments with the cooling power needed to make devices and measure their quantum properties.

Bridging Basic and Applied Science

Liu said he wants to start by making devices as simple as a Hall bar, which lets scientists measure both longitudinal and transverse voltage in a semiconductor.

“The problem is that in practical materials to measure those two things at the same time is not easy,” he explained. “You want to make a device where your electrodes are extremely symmetric on both sides of a narrow channel. That requires you to do nanofabrication.”

The tools at IFW make that possible and will help Liu build an even stronger research program at UT.

“We’re very strong in materials synthesis and we’re getting very comprehensive in terms of characterizations,” he said. “We can make all these new, amazing materials, but eventually if you want to turn them in to any kind of application, the first step will be to build a device. The device is the bridge between fundamental physics, basic science, to applied science. The problem we have is that we don’t have much device fabrication capability for quantum materials.”

Liu is the scientific director of the Electromagnetic Properties Lab (EMP) at UT’s Institute for Advanced Materials and Manufacturing (IAMM). As a core facility, EMP serves materials researchers from multiple departments and colleges. Liu said UT has invested in quantum science with facility upgrades and new hires and his time in Dresden will help him make the most of those resources and plan for the future.

“We don’t have as much on-campus experience of device fabrication as those folks in Germany,” he said. “One of the things I want to do is learn from them. If I learn device fabrication and see how things work (and) get the know-how, then I could help enhance that capability on our campus for the local community of materials research.”

Physics Professor and Department Head Adrian Del Maestro is among Liu’s colleagues who’ll benefit from this newly-gained expertise. He also studies quantum materials and holds leadership positions at IAMM through the National Science Foundation-supported Center for Advanced Materials and Manufacturing (CAMM).

“Professor Liu is operating at the cutting edge of quantum materials research, and this fellowship will enhance the EMP facility’s quantum device capabilities, moving UT up the technological readiness level scale,” he said. “Humboldt Fellowships are prestigious life-long opportunities that demonstrate the impact UT Physics and Astronomy is having on the international scientific enterprise.”

In true “Everywhere You Look, UT” style, Liu said while he’s abroad he’ll also promote Tennessee’s strengths in quantum science.

“(I’ll) let them know that we’re good—and growing,” he said.

Quantum materials have the potential to transform technology just as transistors did, but before that can happen scientists have to understand how their components interact—and how those interactions are manifested. UT’s physicists and their colleagues were asked for their expertise on how one experimental method can play a defining role in those discoveries. 

UT Physics Bains Professor Steven Johnston and Adjunct Professor Mark Dean (a physicist with the distinction of tenure at Brookhaven National Laboratory), along with their colleagues Matteo Mitrano (Harvard University) and Young-June Kim (University of Toronto), have published an authoritative perspective piece in Physical Review X on applications of resonant inelastic x-ray scattering (RIXS) to quantum materials.

PRX Perspectives judiciously survey and synthesize existing fields with a forward-facing outlook on how the technique can address significant questions for the field and are commissioned by the journal’s editors. The article “Exploring quantum materials with resonant inelastic x-ray scattering” marks the third in the series since its launch in 2022.

Understanding quantum materials—solids in which interactions among constituent electrons yield many novel emergent quantum phenomena — is a forefront challenge in modern condensed matter physics. This Perspective article highlights the potential for RIXS, which has experienced rapid growth as a probe of quantum materials, to explore these novel materials. Progress in instrumentation means that we are now at a watershed period of being able to apply RIXS with time and energy resolutions that match the fundamental energy scales of many quantum materials and solve key problems in this major area of condensed matter physics.

The article is available through open access and can be downloaded at  https://journals.aps.org/prx/abstract/10.1103/PhysRevX.14.040501.

–Courtesy of Bains Professor Steven Johnston

Above: The Kramers-Heisenberg process for resonant inelastic x-ray scattering (RIXS) and the different excitations that it can probe. The RIXS process, shown in the center, involves the resonant absorption of an x-ray photon, creating an intermediate state with a core hole and a valence excitation, before the hole is filled via the emission of another x-ray photon. By measuring the energy and momentum change of the x rays, one can infer the properties of the excitations created in the material. Around the outside, we illustrate the many different types of excitation that RIXS can probe, arranged clockwise in order of increasing energy scale, as denoted by the red-to-blue circular arrow.

Assistant Professor Yang Zhang hadn’t planned to go to Greece this summer, much less prepare an invited lecture. But when he learned he’d been chosen for the 2024 International Union of Pure and Applied Physics (IUPAP) Early Career Scientist Prize in Computational Physics, he was happy to put together some last-minute travel plans.

Watching Galaxies Form

The IUPAP comprises 20 international Commissions representing different subfields of physics. Each Commission recognizes outstanding physicists in the first stages of their careers with the Early Career Scientist Prize. C20, the Commission on Computational Physics, selected Zhang for this year’s award. He was cited “for his significant and innovative achievements in computational study of topological bands and quantum anomalous Hall states in two-dimensional semiconductors.”

As the C20 website explains, computational physics is where a computer becomes the basic tool for exploring natural laws. When experiments are impossible or impractical, computation provides simulated studies with closely-controlled conditions. Where data are overwhelming or unwieldy in terms of volume or intricacy, computational codes and models can work through them more easily.

Zhang has been intrigued by the field’s possibilities since his early studies.

“I first got interested in computational physics during my undergraduate research internship with Dr. Sverre Aarseth” of the University of Cambridge Institute of Astronomy, he said. “Seeing a galaxy form on the computer screen was mesmerizing. I learned to tweak parameters and to optimize the program even at hardware level, gaining a deeper understanding of the physics and computational techniques involved. The blend of physical intuition, mathematical rigor, and computational creativity ignited my passion for the field and set me on the path to further studies and research in computational physics.”

Zhang has taken that passion and applied it quantum materials, helping build UT’s research and teaching expertise in this growing and critical field.

Physics Professor and Department Head Adrian Del Maestro explained that Zhang “is driving innovation in quantum materials research by translating the latest advances in artificial intelligence and applying them to extraordinarily challenging problems in strongly interacting quantum systems.”

He added that with his strong collaboration network, Yang’s research has a truly global impact, while at the same he has a unique gift for developing new algorithmic methods and communicating these discoveries to UT’s undergraduate and graduate students.

Del Maestro works with Zhang through their leadership roles in UT’s Center for Advanced Materials and Manufacturing, a National Science Foundation-supported Materials Research Science and Engineering Center (MRSEC). Both hold joint appointments in the Department of Physics and Astronomy and the Min H. Kao Department of Electrical Engineering and Computer Science.

An Unexpected Honor

Zhang joined UT in 2023 after a postdoctoral appointment at the Massachusetts Institute of Technology following completion of a PhD at the Max Planck Institute Dresden. He has won several awards, including two Overall Winner Awards in the World Supercomputing Contest, the SFB Best Doctoral Thesis Award, the Tschirnhaus Medal from the Leibniz Association, and the Otto-Hahn Medal of the Max Planck Society. Now he adds the prestigious IUPAP Early Career Scientist Prize to that list.

“Receiving this award means a great deal to me,” Zhang said. “It recognizes my hard work and dedication in computational quantum matter and highlights the importance of my method development, as well as my contributions in semiconductor superlattice. This recognition motivates me to continue pushing the boundaries of knowledge in developing methods for large-scale quantum systems and inspires me to mentor future scientists.”

Zhang was surprised to learn he had won the prize, as nominees may have up to eight years of research experience after finishing a doctoral degree and he was about four years past his PhD when he learned he had been nominated.

“This award was an unexpected but deeply appreciated honor,” he explained. “I believe recent breakthroughs in fractional quantum anomalous Hall effects played a significant role in earning this recognition, and I am grateful for the support and acknowledgment of my work in this exciting field.”

Zhang will accept the prize at the 35th IUPAP International Conference on Computational Physics (CCP2024) to be held in Thessaloniki, Greece, July 7-12. As part of his recognition, he’s invited to deliver a lecture. Though his summer agenda hadn’t included the conference, he said he “quickly organized my travel arrangements and prepared for the event.”

About IUPAP

The International Union of Pure and Applied Physics (IUPAP) was established more than a century ago in Brussels with 13 member countries, holding its first General Assembly in 1923 in Paris. That number has grown to 60 member countries, with the Union being the only international physics organization run by the physics community itself. IUPAP’s mission is “to assist in the worldwide development of physics, to foster international cooperation in physics, and to help in the application of physics toward solving problems of concern to humanity.”