Three Volts Flipped Which Way the Atoms Spin Inside a Crystal, and It Stayed Flipped

Atoms in a crystal do not simply jiggle. In some materials the collective vibrations, called phonons, carry a rotational sense — the atoms trace tiny circles, and those circles go either clockwise or counterclockwise. Physicists call that handedness chirality, and until now it has been something you observe rather than something you set.

A team at the Paul Scherrer Institute in Switzerland has shown that a small electric field can flip it, and that the flip persists. Working with membranes of barium titanate just 40 nanometers thick, the researchers reversed the chirality of the material's phonons using about three volts at room temperature. When the field was removed, the vibrations kept their new handedness rather than relaxing back.

"We now know that phonon angular momentum is something that can be controlled by electricity," said Michael Grimes, the paper's first author. "This opens a pathway toward phonon-based information technologies."

Barium titanate is ferroelectric, which is the key to the result. A ferroelectric crystal carries a built-in electric polarization that can itself be switched by a field and then stays put — the same property that makes ferroelectric memory possible. By tying phonon chirality to that polarization, the group turned a fleeting vibrational property into something with a stable, addressable state, which is what any information-carrying degree of freedom has to have.

Seeing the effect at all required an unusual measurement. The team used resonant inelastic X-ray scattering with circularly polarized X-rays at the European Synchrotron Radiation Facility in Grenoble, a technique sensitive to the rotational character of lattice vibrations at the relevant energies. Chiral phonons were first demonstrated experimentally in quartz in 2023; the difference here is that the effect has been moved into a device-like ferroelectric film and put under electrical control.

The specific mode the researchers switched has what is called g-wave chirality, a description of the symmetry pattern the rotating atoms trace out — the same vocabulary physicists use to classify electron orbitals and superconducting order parameters. The paper, "Electric-field switching of g-wave phonon chirality in ferroelectric BaTiO3," appears in Nature Materials.

What this might eventually be good for is speculative but not vague. Phonons carry angular momentum, they interact with electron spins, and they move heat. A vibrational state that can be written electrically and holds its value is a candidate for carrying information between magnetic and electronic parts of a device, and for steering heat flow in materials where that currently cannot be controlled at all. Three volts and a 40-nanometer film is the kind of specification that engineers, not just spectroscopists, can work with.