Electric-field switching of g-wave phonon chirality in ferroelectric BaTiO3.

Lattice vibrations carrying angular momentum, known as chiral phonons, offer a route to couple lattice, electronic and magnetic degrees of freedom, but their deterministic control has remained elusive. Here we demonstrate reversible electric-field switching of phonon angular momentum in the technologically relevant ferroelectric BaTiO3. Using circularly dichroic resonant inelastic X-ray scattering, we directly resolve phonon chirality via angular momentum transfer between circularly polarized X-rays and the lattice. We observe a momentum-dependent dichroic response that switches with ferroelectric polarization, establishing a robust and non-volatile gyroelectric effect. The measured dichroism is in quantitative agreement with first-principles calculations. These results establish phonon angular momentum as an electrically controllable degree of freedom and provide a pathway towards phonon-based information and energy technologies.