Electric field switches chiral phonon
Researchers have demonstrated reversible electric control over chiral phonons, collective atomic vibrations with a left- or right-handed rotational
Scientists at the Paul Scherrer Institute (PSI) have shown that an electric field can control the handedness of chiral phonons. The research, published in Nature Materials, builds on the team's 2023 experimental proof of these collective atomic vibrations, which possess a left- or right-handed rotational character.
Making chirality switchable
The latest work used a ferroelectric material, barium titanate (BaTiO3). Its electric polarization can be reversed by an applied field. Collaborators in Taiwan fabricated membranes of the material just 40 nanometers thick. They added miniature electrodes to create a tiny device on a silicon substrate.
The team studied how the handedness of atomic vibrations changed as they flipped the electric polarization. Reversing the polarization also reversed the phonon handedness. Crucially, the switched state persists after removing the electric field. This provides a reliable control mechanism. Switching was achieved at room temperature with a voltage of just 3 volts, factors that could aid future device integration.
Watching the handedness reverse
The researchers read out the phonon chirality using circularly polarized X-rays at the European Synchrotron Radiation Facility (ESRF). They employed resonant inelastic X-ray scattering (RIXS). This technique resolves phonon chirality by tracking angular momentum transfer between the X-rays and the crystal lattice.
"We now know that phonon angular momentum is something that can be controlled by electricity," says Michael Grimes, first author of the paper at the PSI Center for Photon Sciences. "This opens a pathway toward phonon-based information technologies."
From fundamental questions to future devices
Chiral phonons carry angular momentum due to their swirling motion. Magnetism is also linked to angular momentum through electron spin and orbital motion. Therefore, chiral phonons could 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, the physicist at PSI who led the study.
The topic also connects to broader questions about chirality's origins in nature. "Chiral phonons are fascinating because they touch on a fundamental question in nature, which is very poorly understood: why does handedness occur?" adds Staub. He notes that biology is handed, but the reason remains a mystery. The potential role of magnetism is hotly debated. Chiral phonons link atomic motion with magnetism.
