Chiral Phonons

by Hiroki Ueda (PSI)

Tuesday 9 Jun 2026, 14:00 → 15:00 Europe/Zurich

WHGA/121

Phonons with angular momentum have created an emerging field of chiral phononics.

Formally, chiral phonons are rotational phonon modes with finite momentum out of the

rotational plane, breaking improper rotational symmetry. Their intrinsic angular momentum

gives rise to various emergent phenomena, including magnetism, transport, and light-matter

interactions [1]. In this presentation, I will show our recent work on chiral phonons using X-

rays and time-resolved optical probes.

 

First, I will present direct demonstrations of chiral phonons in the chiral crystal quartz [2],

the polar crystal LiNbO3 [3], and the ferroelectric crystal BaTiO3 [4] using resonant inelastic

X-ray scattering (RIXS) with circularly polarized X-rays at the O K edge. Besides momentum

accessibility due to short wavelengths of X-rays, angular momentum transfer between

circularly polarized X-ray photons and chiral phonons imposes the selection rule in the RIXS

process, enabling chiral phonon observation via circular dichroism in phonon excitation peaks

at finite momentum. By ferroelectric polarization reversal in BaTiO3, we demonstrate in-situ

electrical switching of phonon chirality, opening pathways for electrical control of chiral

phonon-related emergent phenomena.

 

Second, we focus on low-energy acoustic phonons, which are more relevant to transport

properties than optical phonons due to their larger thermal population and group velocity. I will

present recent results of resonant diffuse scattering with circular polarization, analogous to

RIXS with circular polarization but without energy resolution, on the chiral crystal CsCuCl3.

A clear circular contrast on the tail of the polarization-independent resonant diffraction peak at

the Cu L3 edge exhibits sign reversal across the Γ point and upon switching the crystal chirality,

consistent with chiral acoustic phonons.

 

Finally, I will present possible observations of coherent chiral acoustic phonons in the non-

chiral crystal SrTiO3, using THz-pump, Kerr rotation-probe measurements. Deep in the

tetragonal phase, softening of the transverse-optical (TO) mode causes its coupling to the

transverse-acoustic (TA) phonon mode at finite out-of-plane momentum. This coupling enables

coherent generation of TA phonons when the TO mode is driven by THz pulses [5]. When the

TO mode is excited with circularly polarized THz pulses, slow oscillations consistent with

coherent rotational TA phonons are observed. Since the TO-TA coupling occurs at finite

momentum, the generated coherent TA mode is chiral, carrying finite phonon angular

momentum parallel to momentum.

 

[1] D. M. Juraschek, H. Ueda et al., Nat. Phys. 21, 1532-1540 (2025).

[2] H. Ueda et al., Nature 618, 946-950 (2023).

[3] H. Ueda et al, Nat. Commun. 17, 212 (2026).

[4] M. Grimes, H. Ueda et al., arXiv: 2603.06144.

[5] G. Oreenstein et al., Nat. Phys. 21, 961-965 (2025).