SPICE Workshop on Chiral Phonons, July 29th - 31st 2025
Hiroki Ueda
In this talk, I will present three spectroscopic studies that probe the nature of chiral phonons: (1) Resonant inelastic X-ray scattering (RIXS) with circular polarization to directly observe phonon chirality, (2) Polarized inelastic neutron scattering to test the magnetic character of chiral phonons, and (3) Circular THz-pump Kerr rotation-probe experiments to generate coherent chiral acoustic phonons and characterize the resulting phononic magnetic fields. (1) Using RIXS with circular polarization, we observed chiral phonons as a circular contrast in the phonon
excitation peaks in both the chiral crystal quartz [1] and the polar crystal LiNbO3 [2]. The angular momentum transfer between a circularly polarized X-ray photon and a chiral phonon imposes the selection rule in the RIXS process, enabling us to detect chiral phonons. Our demonstration clarifies the symmetry requirement for materials to show chiral phonons. (2) Polarized inelastic neutron scattering allows us to probe the magnetic character of chiral phonons through the spin-flip scattering channel at non-Γ points. Our preliminary results reveal a finite cross-section in the spin-flip channel. However, the lack of contrast between enantiomers with opposite handedness indicates that the magnetic scattering due to chiral phonons remains below the detection threshold. (3) A large phononic magnetic field was reported when resonantly driving the transverse optical (TO) soft mode in SrTiO3 by a circular THz pulse, i.e., circular phonons [3]. Below the antiferrodistortive transition temperature, we found that this excitation induces the coherent generation of a transverse acoustic (TA) mode through TO-TA coupling at finite momenta [4]. Thus, exciting the circular TO soft mode at the Γ point leads to the emergence of a coherent chiral TA mode at non-Γ points. Detailed investigation implies that the phononic magnetic fields produced by both the circular TO and the chiral TA modes are effective magnetic fields.
Reference
[1] H. Ueda, M. García-Fernández, S. Agrestini, C. P. Romao, J. van den Brink, N. A. Spaldin, K.-J. Zhou, and U. Staub, Nature 618, 946-950 (2023).
[2] H. Ueda, A. Nag, C. P. Romao, M. García-Fernández, K.-J. Zhou, and U. Staub, arXiv: 2504.03330.
[3] M. Basini, M. Pancaldi, B. Wehinger, M. Udina, V. Unikandanunni, T. Tadano, M. C. Hoffmann, A. V. Balatsky, and S. Bonetti, Nature 628, 534-539 (2024).
[4] G. Orenstein et al., Nat. Phys. 21, 961-965 (2025).