Atoms in a material are rarely still. They jiggle backwards and forwards in collective lattice vibrations known as phonons. Their motion can also carry a rotational element: in 2023, scientists at Paul Scherrer Institut (PSI) proved experimentally the existence of chiral phonons, which exhibit handedness, depending on which way they rotate.

Now, the same team of researchers, in collaboration with the ESRF, have shown that an applied electric field can control the handedness of these phonons.

Making chirality switchable

Whereas the researchers made their initial discovery in quartz, in the latest study, they created a tiny device out of the material barium titanate (BaTiO3). This material is ferroelectric, meaning that it possesses an electrical polarisation that can be reversed by an electric field. Collaborators in Taiwan created membranes of barium titanate just 40 nm thick and added miniature electrodes to create a tiny device, which was placed on a silicon substrate.

The researchers studied how the handedness of the atomic vibrations in the material changed as they flipped the electrical polarisation – back and forth. When they reversed the electrical polarisation, the handedness of the phonons reversed too.

Intriguingly, they found that the switched state persists after the electric field is removed. The electric field therefore provides a reliable way to control handedness of the phonons. Switching was achieved at room temperature and using a voltage of just 3 V – two factors that could help with future integration into devices.

Watching the handedness reverse

The team were able to read out the handedness of chiral phonons using circularly polarised X-rays at the ESRF. Here, the researchers used resonant inelastic X-ray scattering (RIXS), which allowed them to resolve phonon chirality by seeing how angular momentum is transferred between circularly polarised X-rays and the lattice. The excited chiral phonons carry only very little energy, which they acquire from the scattered X-rays. Resolving these tiny losses in the energy of the X-rays demands a spectrometer with very high energy resolution, which the researchers found at the ID32 beamline of the ESRF.

From fundamental questions of nature to future devices

Because chiral phonons include a swirling motion, they carry angular momentum. “We now know that phonon angular momentum is something that can be controlled by electricity. This opens a pathway towards phonon-based information technologies,” says Michael Grimes, first author of the paper from the PSI Center for Photon Sciences. Magnetism is also closely linked to angular momentum, through the spin and orbital motion of electrons.

Chiral phonons therefore have the potential to interact with and influence electronic and magnetic states. “The ability to control the handedness of the phonons could, in principle, provide a means to manipulate magnetic states and therefore the information encoded in them,” explains Urs Staub, physicist in the PSI Center for Photon Sciences who led the study.

As well as the practical implications, the topic is part of a wider question about the origins of chirality in life.

“Chiral phonons are fascinating because they touch on a fundamental question in nature, which is very poorly understood: why does handedness occur?” adds Staub. “Biology is handed - but why is a mystery. Whether magnetism plays – or played – a role in this is hotly debated. Chiral phonons connect the motion of atoms with magnetism.”

Reference:

Michael Grimes et al, Nature Materials, 07.09.2026 (online)
DOI: 10.1038/s41563-026-02737-w

Text adapted from the PSI article by Miriam Arrell.

Top image: The collective vibrations of atoms in a crystal are known as phonons. In chiral phonons, these vibrations include a rotational motion, giving them a left- or right-handed character. Researchers have now shown that this handedness can be switched using an electric field. © Paul Scherrer Institute PSI/Mahir Dzambegovic and Monika Bletry.