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Breakthroughs in the study of chiral phonons

A recent study by Professor Lifa Zhang of the School of Physics and Technology at NNU, and other researchers from the U.S. Department of Energy's Lawrence Berkeley National Laboratory, is shining a new light on chiral phonons. There is now evidence that there is a naturally occurring circular rotation in the structure of an atomically thin (2-D) material, which is the first experimental verification of the existence of chiral phonons theoretically found by Prof. Lifa Zhang and Qian Niu (PRL 115, 115502 (2015)).

Phonons in solids are usually regarded as the collective linear motion of atoms.  Chirality, however, has never been focused on before. Chirality is associated with the breaking of symmetry, often described as left- or right-handed behavior.

The team observed a chiral phonon mode in a monolayer of the transition metal dichalcogenide WSe2 and confirmed their chirality by the infrared circular dichroism arising from pseudoangular momentum conservation.

The chiral properties found in the study are likely to exist across a wide range of 2-D materials based on a similar patterning in their atomic structure. The phonon chirality could be used to control the electron-phonon coupling and/or the phonon-driven topological states of solids. The study could guide the design of materials to enhance phonon-based effects.

This study titled, “Observation of chiral phonon,” by Hanyu Zhu, Jun Yi, Ming-Yang Li, Jun Xiao, Lifa Zhang, Chih-Wen Yang, Robert A. Kaindl, Lain-Jong Li, Yuan Wang and Xiang Zhang, was recently published online in the journal of Science (Science359 (6375), 579-582. DOI:10.1126/science.aar2711).

Professor Zhang has spent years in the study of phononics and quantum heat conduction. He, together with other researchers provided a topological understanding of the phonon Hall effect (PHE) and found topological phase transition in PHE (PRL105, 225901 (2010)). Inspired by the PHE, they then found phonon angular momentum in magnetic systems with time reversal symmetries broken by spin-phonon interaction (PRL112, 085503 (2014)) and in non-magnetic crystals with inversion symmetry breaking, they found chiral phonons with valley contrasting circular polarization and proposed valley phonon Hall effect(PRL 115, 115502 (2015)).  All these findings have contributed to the development of phononics.