Xu’s background in physics and experience with experimental chemistry allow him to test quantum theories in the real world. “While physicists and chemists study both materials, physicists tend to see them more as abstract equations, while chemists are concerned with their emerging properties,” Xu said. “Because I have a pure physics background and speak the language of chemistry, I can translate difficult theories into real space.”
With a few well-founded assumptions and some innovative techniques, Xu and his team bridge the gap between quantum physics and chemistry by testing theories with materials. First, they predict which materials can realize topological properties. The chemical formulas for the elements in such materials provide insufficient insight; Xu is also interested in their macroscopic properties.
“If I were to study water, steam and ice alone by going to their H20 comparison, I would learn nothing about their different properties.” said Xu. “As a chemist, I try to find certain elements and arrange them microscopically so that they can produce a topological property.”
Xu’s lab then tests current theories of chemical reactions against experimental data to expand the map of topological materials. Using specialized refrigerators in which atoms and molecules are cooled to temperatures just above absolute zero, making them highly controllable and more visible, Xu and his team test the flow of electrons through materials with currents.
They are also interested in the optical properties of materials and testing their interaction with light. The team fires photons at the materials and collects quantum mechanical topological data based on how light scatters, reflects and transmits. Xu has already provided strong evidence for theoretical particles that answer one of the most vexing problems in quantum science.
In a study published last year in Nature, Xu and his team set out to investigate the properties of axions, a theoretical elementary particle proposed by physicist Frank Wilczek. The Nobel laureate named it after a brand of laundry detergent because it “cleaned up” the complex, highly technical Strong Charge Parity problem in quantum chromodynamics by filling a gap between theory and observation.
In addition, one of the most tantalizing predictions about axion states is that we might be able to use them to control magnetization, which could revolutionize technology of all kinds, as magnetism and magnetic materials are at the core of many, many applications.
In a class of topological materials called axion insulators, Xu’s team attempted to simulate the behavior of the axion. They fabricated a dual-gated MnBi2Te4 device in an argon environment and measured its electrical and optical properties, discovering new avenues to detect and manipulate the rich internal structure of topological materials.
“We have discovered a real material that can support the axion insulator state,” said Xu. “We confirmed that it had the predicted properties, a strong coupling between electricity and magnetism.”
Xu has provided evidence for a theoretical particle and plans to investigate the spin properties of Weyl semimetals, a new state of matter with an unusual electronic structure that has deep analogies to particle physics and leads to unique topological properties.