Atomic dance gives rise to a magnet
Quantum materials hold the key to a future of lightning-fast, energy-efficient information systems. However, the problem with tapping their transformative potential is that, in solids, the vast number of atoms often drowns out the exotic quantum properties of electrons.
Rice University researchers in the lab of quantum materials scientist Hanyu Zhu found that when they move in circles, atoms can also work wonders: When the atomic lattice in a rare-earth crystal becomes animated with a corkscrew-shaped vibration known as a chiral phonon, the crystal is transformed into a magnet. The research was supported in part by the U.S. National Science Foundation through two research grants, including a CAREER award, and one of the authors is an NSF Graduate Research Fellow. The study is published in Science.
According to the study, exposing cerium fluoride to ultrafast pulses of light sends its atoms into a dance that momentarily enlists the spins of electrons, causing them to align with the atomic rotation. Since atoms rotate only in particular frequencies and move for a longer time at lower temperatures, additional frequency- and temperature-dependent measurements further confirm that magnetization occurs because of the atoms' collective chiral dance.
"The effect of atomic motion on electrons is surprising because electrons are so much lighter and faster than atoms," said Zhu. "Material properties would remain unchanged if atoms went clockwise or counterclockwise, i.e., traveled forward or backward in time ― a phenomenon that physicists refer to as time-reversal symmetry."
The idea that the collective motion of atoms breaks time-reversal symmetry is relatively recent. Chiral phonons have now been experimentally demonstrated in a few different materials, but exactly how they impact material properties is not well understood.
Zhu said, "We decided to focus on a fascinating phenomenon called spin-phonon coupling." Spin-phonon coupling plays an important part in real-world applications like writing data on a hard disk.
In their new experiments, Zhu and the team members had to find a way to drive a lattice of atoms to move in a chiral fashion. This required both choosing the right material and creating light at the right frequency to send its atomic lattice aswirl, with the help of theoretical computation from the collaborators.
In addition to the insights into spin-phonon coupling derived from the research findings, the experimental design and setup will help inform future research on magnetic and quantum materials.