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.
Image Credit:© Paul Scherrer Institute / Mahir Dzambegovic and Monika Bletry
Scientific Frontline: Extended "At a Glance" Summary: Chiral Phonons
The Core Concept: Chiral phonons are collective, rotational atomic vibrations within a crystal lattice that exhibit distinct left-handed or right-handed characteristics.
Key Distinction/Mechanism: Unlike standard lattice vibrations, chiral phonons carry angular momentum. Their inherent handedness can be reliably reversed and maintained at room temperature by applying a low-voltage electrical field to a ferroelectric material.
Origin/History: Scientists at the Paul Scherrer Institute first experimentally proved the existence of chiral phonons in quartz in 2023. In September 2026, researchers successfully demonstrated the ability to control this handedness using thin membranes of \(BaTiO_3\).
Major Frameworks/Components:
- Ferroelectricity: Materials possessing an intrinsic electrical polarization that can be flipped using an applied electric field, allowing for the reversal of phonon chirality.
- Resonant Inelastic X-ray Scattering (RIXS): An advanced technique utilizing circularly polarized X-rays at a synchrotron facility to observe the transfer of angular momentum and resolve phonon handedness.
- Angular Momentum Coupling: The fundamental interaction linking the rotational motion of chiral phonons with magnetism through the spin and orbital dynamics of electrons.
Branch of Science: Condensed Matter Physics, Quantum Physics, Materials Science.
Future Application: Regulating phonon angular momentum via electricity establishes a foundation for advanced phonon-based information technologies, enabling novel methods to manipulate magnetic states and encode data.
Why It Matters: Beyond enabling future computing architectures, understanding chiral phonons offers profound fundamental insights into the connection between atomic motion, magnetism, and the unexplained prevalence of chirality in biological systems.
Atoms in a material are rarely still. They jiggle backward and forward in collective lattice vibrations known as phonons. Their motion can also carry a rotational element: in 2023, scientists at PSI experimentally proved the existence of chiral phonons, which exhibit handedness depending on which way they rotate.
Now, the same team of researchers has 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 barium titanate (\(\text{BaTiO}_3\)). This material is ferroelectric, meaning that it possesses an electrical polarization 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 polarization back and forth. When they reversed the electrical polarization, the handedness of the phonons reversed as well.
Intriguingly, they found that the switched state persists after the electric field is removed. The electric field, therefore, provides a reliable way to control the handedness of the phonons. Switching was achieved at room temperature using a voltage of just 3 V—two factors that could help with future integration into devices.
Watching the Handedness Reverse
The team was able to read out the handedness of chiral phonons using circularly polarized X-rays at the European Synchrotron Radiation Facility (ESRF) in Grenoble. Here, the researchers used a technique known as resonant inelastic X-ray scattering (RIXS), which allowed them to resolve phonon chirality by seeing how angular momentum is transferred between circularly polarized X-rays and the lattice.
"We now know that phonon angular momentum is something that can be controlled by electricity. This opens a pathway toward phonon-based information technologies," says Michael Grimes, first author of the paper from the PSI Center for Photon Sciences.
From Fundamental Questions of Nature to Future Devices
Because chiral phonons include a swirling motion, they carry angular momentum. 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, a physicist in the PSI Center for Photon Sciences who led the study.
In addition to 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."
Published in journal: Nature Materials
Title: Electric-field switching of g-wave phonon chirality in ferroelectric \(BaTiO_3\)
Authors: Michael Grimes, Hiroki Ueda, Clifford J. Allington, Carl P. Romao, Kurt Kummer, Puneet Kaur, Li-Shu Wang, Yao-Wen Chang, Jan-Chi Yang, Shih-Wen Huang, and Urs Staub
Source/Credit: Paul Scherrer Institute | Miriam Arrell
Edited by: Scientific Frontline
Reference Number: phy090726_01
