The TRR 227 kindly invites you to a special TRR 227 Seminar.

Speaker: Prof. Dr. Takuya Satoh, Department of Physics, Tokyo Institute of Technology, Tokyo, Japan & Quantum Research Center for Chirality, Institute for Molecular Science, Okazaki, Japan

Title:

Truly chiral phonons in chiral materials

Abstract:

Chirality is the structural properties of an object that differ from its mirror image. Chirality exists not only in physical objects such as crystals and molecules, but also in dynamic motion such as phonons. Recently, chiral phonons with pseudo-angular momentum (PAM) and angular momentum have been proposed [1] and observed [2]. However, chiral phonons that propagate while rotating in three-dimensional materials have not been reported. Here, we used circularly polarized Raman spectroscopy and first-principles calculations to identify chiral phonons in chiral crystals (trigonal α-HgS [3] and Te [4]). Circularly polarized Raman spectroscopy detected a peak splitting of the Γ₃ modes along the principal axis of α-HgS in the backscatter configuration. In the anti-Stokes spectrum, the observed splitting of the Γ₃ mode is a mirror image of the splitting in the Stokes spectrum. Furthermore, peak splitting occurs when the PAMs of the circularly polarized incident light and the scattered light are reversed. From calculations of phonon PAM, we verified the conservation law of PAM in Raman scattering by considering three-fold rotational symmetry. This approach made it possible to determine the chirality of crystals in a non-contact and non-destructive manner. We also evaluated the chirality of phonons using a measure with symmetry similar to that of an electrical toroidal monopole.

[1] L. Zhang and Q. Niu, Phys. Rev. Lett. 115, 115502 (2015)

[2] H. Zhu et al., Science 359, 579 (2018)

[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)