Large effective magnetic fields from chiral phonons in rare-earth halides

• Published in: Science (American Association for the Advancement of Science) Vol. 382; no. 6671; pp. 698 - 702
• Main Authors: Luo, Jiaming, Lin, Tong, Zhang, Junjie, Chen, Xiaotong, Blackert, Elizabeth R., Xu, Rui, Yakobson, Boris I., Zhu, Hanyu
• Format: Journal Article
• Language: English
• Published: Washington The American Association for the Advancement of Science 10.11.2023
• Subjects: Cerium; Circular polarization; Energy efficiency; Halides; Magnetic fields; Materials science; Optoelectronics; Phonons; Rare earth elements
• ISSN: 0036-8075; 1095-9203; 1095-9203
• DOI: 10.1126/science.adi9601

Time-reversal symmetry (TRS) is pivotal for materials’ optical, magnetic, topological, and transport properties. Chiral phonons, characterized by atoms rotating unidirectionally around their equilibrium positions, generate dynamic lattice structures that break TRS. Here, we report that coherent chiral phonons, driven by circularly polarized terahertz light pulses, polarize the paramagnetic spins in cerium fluoride in a manner similar to that of a quasi-static magnetic field on the order of 1 tesla. Through time-resolved Faraday rotation and Kerr ellipticity, we found that the transient magnetization is only excited by pulses resonant with phonons, proportional to the angular momentum of the phonons, and growing with magnetic susceptibility at cryogenic temperatures. The observation quantitatively agrees with our spin-phonon coupling model and may enable new routes to investigating ultrafast magnetism, energy-efficient spintronics, and nonequilibrium phases of matter with broken TRS. The manipulation and control of the optoelectronic properties of a material finds application across a range of fields. However, doing so by applying electric or magnetic fields can be slow and not always practical. Luo et al . have shown that chiral phonons driven by ultrafast pulses of circularly polarized terahertz radiation can induce magnetic fields on the order of one tesla in the rare earth trihalide cerium fluoride (see the Perspective by Kaindl). Such control of spin-phonon coupling provides a route to on-demand ultrafast, large magnetic fields on an atomic scale that would be useful for both fundamental materials science and the development of energy-efficient spintronic devices. —Ian S. Osborne Terahertz-driven chiral phonons can induce large magnetic fields in the rare-earth trihalide cerium fluoride.