Nanotechnology (Commonwealth Union) – Understanding the nature of an atom plays a crucial role for scientists to understand the nature of matter and how it can be manipulated in everyday use.
Researchers at the Paul Scherrer Institute (PSI) have shown that an electric field can invert the handedness of atomic vibrations, called chiral phonons. Such unusual phonons provide a link between atomic motion and magnetism, and the ability to control their sense of rotation could open new doors for phonon-based information technologies.
Atoms in materials are always on the move . They vibrate together through the crystal lattice in patterns called phonons . Sometimes these vibrations also have a rotational component. PSI researchers experimentally verified the existence of chiral phonons in 2023. Chiral phonons have a distinct handedness that is set by the direction in which they rotate.
This handedness can now be altered by an externally applied electric field, as the researchers have demonstrated in their latest work.
The team’s original experiments were carried out using quartz. For the new study, however, the researchers developed a miniature device based on barium titanate (BaTiO₃). The material is ferroelectric, meaning its electrical polarisation can be switched when an electric field is applied.
Researchers in Taiwan fabricated extremely thin barium titanate membranes measuring only about 40 nanometres in thickness. They incorporated tiny electrodes into the membranes, creating a compact device that was then mounted on a silicon substrate.
The researchers investigated how the direction of rotation, or handedness, of atomic vibrations within the material responded when the electrical polarisation was switched repeatedly between two states. They found that reversing the electrical polarisation also caused the handedness of the phonons to switch.
Remarkably, the researchers discovered that the new state remained stable even after the applied electric field was turned off. This demonstrates that an electric field can provide a dependable means of controlling phonon handedness. The switching process worked at room temperature and required a voltage of only 3 V, characteristics that could make the approach more practical for incorporation into future electronic devices.
The researchers measured the handedness of the chiral phonons using circularly polarised X-rays at the European Synchrotron Radiation Facility (ESRF) in Grenoble. They employed a method called resonant inelastic X-ray scattering (RIXS), which enabled them to determine phonon chirality by examining the transfer of angular momentum between the circularly polarised X-rays and the crystal lattice.
Michael Grimes, the study’s first author from the PSI Center for Photon Sciences indicated that they now know that phonon angular momentum can be manipulated using electricity and this creates a potential route towards information technologies based on phonons.
Because chiral phonons involve a rotational, swirling form of atomic motion, they possess angular momentum. Magnetism is similarly associated with angular momentum, which arises from the spin and orbital movements of electrons.
This means that chiral phonons could potentially interact with electronic and magnetic properties and even modify them.
Urs Staub, a physicist at the PSI Center for Photon Sciences and the study’s lead researcher indicated that in principle, being able to switch the handedness of phonons could offer a way to control magnetic states and, consequently, the information stored within those states.
Beyond their potential technological applications, these findings also contribute to a broader scientific investigation into the fundamental origins of chirality in living systems.
“Chiral phonons are fascinating because they touch on a fundamental question in nature, which is very poorly understood: why does handedness occur?” said 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.”
