136th CEMS Colloquium

Title

Angular momentum and chirality of phonons

Abstract

The idea that phonons in crystals can carry angular momentum was already proposed more than 60 years ago [1]. In 2015, renewed attention was brought to this concept in the context of two-dimensional materials, leading to the introduction of the term chiral phonons [2]. Since then, the study of chiral phonons has rapidly expanded into various fields, including transport phenomena, nonlinear optics, ultrafast dynamics, and magnetism, forming a new and interdisciplinary research area. Notable physical phenomena such as the phonon Zeeman effect (lifting of degeneracy in phonon states), the Einstein-de Haas and Barnett effects (mechanical rotation induced by magnetization change and vice versa), and the phonon Hall effect (deflection of heat current in a magnetic field) have been theoretically predicted and experimentally observed. Understanding the origins of these effects demands a systematic and cross-disciplinary approach.

In this talk, we report our recent experimental observations of chiral phonons using circularly polarized Raman spectroscopy [3–5]. Furthermore, we propose a clarified terminology: the term true chiral phonons is reserved for phonons that satisfy the definition of chirality as given by Barron [6], while we introduce the term axial phonons to describe phonons that possess angular momentum but do not exhibit mirror asymmetry. This proposal aims to establish a unified and rigorous framework for discussing the angular momentum and chirality of phonons [7].

[1] V. S. Vonsovskii and M. S. Svirskii, Sov. Phys. Solid State 3, 1568 (1962).
[2] L. Zhang and Q. Niu, Phys. Rev. Lett. 115, 115502 (2015).
[3] K. Ishito, H. Mao, Y. Kousaka, Y. Togawa, S. Iwasaki, T. Zhang, S. Murakami, J. Kishine, and T. Satoh, Nat. Phys. 19, 35 (2023).
[4] K. Ishito, H. Mao, K. Kobayashi, Y. Kousaka, Y. Togawa, H. Kusunose, J. Kishine, and T. Satoh, Chirality 35, 338 (2023).
[5] G. Kusuno, T. Hayashida, T. Nagai, H. Watanabe, R. Oiwa, T. Kimura, and T. Satoh, arXiv:2505.22488 (2025).
[6] L. D. Barron, Molecular Light Scattering and Optical Activity, 2nd ed. (Cambridge Univ. Press, 2004).
[7] D. M. Juraschek, T. Satoh et al., to be published in Nat. Phys.

Contact

email: mnaoko@riken.jp
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