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Unveiling quantum materials with neutrons

A conversation with quantum scientist Huibo Cao

The next great materials discovery may well come from the exotic interactions of electrons at a scale a million times smaller than a human hair. This is the scale of quantum physics, and it’s where ORNL’s Huibo Cao focuses his efforts.

Cao grew up in the Shanxi province of northern China, in a small village called Shuiquan. After earning a bachelor’s degree in physics from Shanxi University and a Ph.D. in condensed matter physics from the Institute of Physics, Chinese Academy of Sciences, in Beijing, he took a postdoctoral position at the Laboratoire Léon Brillouin at the Alternative Energies and Atomic Energy Commission–Saclay in France. 

He came to ORNL in 2009, working through Oak Ridge Associated Universities, and joined the lab’s staff in 2013 as an instrument scientist at the High Flux Isotope Reactor.

Cao won a 2018 Early Career Research Program award from DOE’s Office of Science for his proposal “Local Site Magnetic Susceptibility for Quantum Materials by Polarized Neutron Diffraction.” We talked with him about the project and about what drew him to a career in science. This is an edited transcript.

1. What are quantum materials?

Quantum materials are materials that exhibit properties or phenomena that cannot be understood by classical or semiclassical theory. In condensed matter physics, quantum materials can be defined as solids with exotic electron properties, unconventional spin orders, abnormal phonon behaviors, or other phenomena resulting from the subtle balance of competing interactions among quantum properties such as spin, orbital, charge and lattice degrees of freedom. 

Quantum materials include superconductors, which exhibit no electrical resistance; multiferroics, which exhibit both magnetic and electric polarizations; spin ices, whose magnetic moments show disorder like the disorder of protons in regular ice; spin liquids, in which electron spins are random; and a variety of other phenomena you can only find at the nanoscale. Understanding these systems requires us to detect the couplings of different variables and their subtle balance at atomic and subatomic scales. Neutrons are small quantum magnets and provide an ideal tool for detecting magnetic order and coupling.

2. How will you study quantum materials with this project?

This project uses polarized neutrons—in which spins are polarized in the same direction—to detect magnetic susceptibilities to applied fields and visualize magnetic density distributions. They can also determine the coupling of spin, orbital and lattice degrees of freedom. 

Polarized neutrons are more sensitive to spin components than non-polarized neutrons and provide a way to detect a field-induced spin response at each atomic site. This reveals a site’s magnetic strength from neighboring sites and the local environment.

In addition, by reconstructing maps of spin density in the whole lattice, we can directly show magnetic interaction paths and possible orbital order and hybridization—that is, the mixing of orbitals. 

With this information, we expect to build a complete Hamiltonian description of a quantum magnetic system describing the total energy in the system. 

If we succeed, we will be able to measure small crystals of quantum materials, as well as small crystals under high pressure, thereby speeding the study of quantum materials. Success will also enable us to study and calibrate a system with the same sample, avoiding the confusion of quantum behaviors from impurity and chemical disorder.

3. How will a new understanding of quantum materials be helpful?

Understanding the subtle balance of interactions among all the degrees of freedom in a given quantum candidate material can guide the design of, or search for, new, better or true quantum materials, and can help us explore more properties originating from quantum mechanics.

4.What attracted you to science?

I was attracted to science by the beauty of physics, and I was curious about the laws that governed the physical world. In high school, I was impressed by Newton's laws of motion. In college, Einstein and Schrödinger taught me about uncertainty. Now I am excited to observe the beauty of magnetism by neutrons and curious about the laws governing exotic states and fascinating phenomena.