![]() ![]() ![]() The idea that the collective motion of atoms breaks time-reversal symmetry is relatively recent. 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." "Electrons can usually adapt to a new atomic position immediately, forgetting their prior trajectory. "The effect of atomic motion on electrons is surprising because electrons are so much lighter and faster than atoms," said Zhu, Rice's William Marsh Rice Chair and an assistant professor of materials science and nanoengineering. Since atoms only rotate in particular frequencies and move for a longer time at lower temperatures, additional frequency- and temperature-dependent measurements further confirm that magnetization occurs as a result of the atoms' collective chiral dance. Though short-lived, the force that aligns the spins outlasts the duration of the light pulse by a significant margin. But in this instance, the chiral movement of the atomic lattice polarizes the spins inside the material as if a large magnetic field were applied." "Chirality - also called handedness because of the way in which left and right hands mirror each other without being superimposable - should not affect the energies of the electrons' spin. "Each electron possesses a magnetic spin that acts like a tiny compass needle embedded in the material, reacting to the local magnetic field," said Rice materials scientist and co-author Boris Yakobson. This alignment would otherwise require a powerful magnetic field to activate, since cerium fluoride is naturally paramagnetic with randomly oriented spins even at zero temperature. 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.Īccording to a study published in Science, 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.
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