Observation of chiral phonons

• Published in: Science (American Association for the Advancement of Science) Vol. 359; no. 6375; pp. 579 - 582
• Main Authors: Zhu, Hanyu, Yi, Jun, Li, Ming-Yang, Xiao, Jun, Zhang, Lifa, Yang, Chih-Wen, Kaindl, Robert A., Li, Lain-Jong, Wang, Yuan, Zhang, Xiang
• Format: Journal Article
• Language: English
• Published: United States The American Association for the Advancement of Science 02.02.2018; American Association for the Advancement of Science (AAAS)
• Subjects: Asymmetry; Brillouin zones; Broken symmetry; Chemical reactions; Chemical speciation; Chirality; Circular dichroism; Condensed matter physics; Coupling; Data processing; Dichroism; Elementary particles; Energy efficiency; Hall effect; Handedness; Information processing; Infrared absorption; Lattice vibration; Lifts; Monolayers; Phonons; Quantum Hall effect; Selenides; Symmetry; Transport phenomena; Tungsten; Tungsten compounds
• ISSN: 0036-8075; 1095-9203; 1095-9203
• DOI: 10.1126/science.aar2711

Chirality is associated with the breaking of symmetry, often described as left- or right-handed behavior. Such asymmetry can be seen, for example, in the electronic responses of particular materials or the reactions between particular chemical species. Zhu et al. observed a chiral phonon mode in a monolayer of the transition metal dichalcogenide WSe 2 , detected spectroscopically as the circular dichroism of the phonon-assisted transition of holes. Phonon chirality could be used to control the electron-phonon coupling and/or the phonon-driven topological states of solids. Science , this issue p. 579 A chiral response of phonons is observed in a monolayer of tungsten diselenide. Chirality reveals symmetry breaking of the fundamental interaction of elementary particles. In condensed matter, for example, the chirality of electrons governs many unconventional transport phenomena such as the quantum Hall effect. Here we show that phonons can exhibit intrinsic chirality in monolayer tungsten diselenide. The broken inversion symmetry of the lattice lifts the degeneracy of clockwise and counterclockwise phonon modes at the corners of the Brillouin zone. We identified the phonons by the intervalley transfer of holes through hole-phonon interactions during the indirect infrared absorption, and we confirmed their chirality by the infrared circular dichroism arising from pseudoangular momentum conservation. The chiral phonons are important for electron-phonon coupling in solids, phonon-driven topological states, and energy-efficient information processing.