Quantum timing, not power, controls atomic motion in a crystal

Researchers at ETH Zurich and collaborating institutions have demonstrated a way to surpass limitations on atomic motion within a class of materials called sliding ferroelectrics. The work reveals that conventional expectations of proportionality between force and displacement, Hooke’s law, break down under intense optical excitation, a phenomenon that typically limits how much a crystal lattice can be displaced. Instead of increasing the power of light pulses, the team found that precisely timing multiple pulses allows for markedly larger phonon amplitudes at a fixed energy level. This approach exposes a regime of anharmonic phonon coupling, opening new pathways for steering lattice motion in quantum materials. Van der Waals material tungsten ditelluride (WTe₂) exhibits a surprising response to optical excitation; conventional expectations of increasing atomic motion with pulse strength fail as the material approaches a saturation point. Researchers discovered that the amplitude of a key phonon mode, important for controlling ferroelectric polarization, actually diminishes under single-pulse excitation. This behavior deviates from the established principle that lattice-driving forces scale linearly with photo-excited carrier density, a finding with implications for materials designed to manipulate polarization. First-principles calculations attribute this nonlinear limitation…