Correlated terahertz phonon-ion interactions control ion conduction in a solid electrolyte

Ionic conduction in solids that exceeds 1 mS cm^-1 is predicted to involve coupled phonon-ion interactions in the crystal lattice. Here, we use theory and experiment to measure the possible contribution of coupled phonon-ion hopping modes which enhance Li⁺ migration in Li_0.5La_0.5TiO₃ (LLTO). The ab initio calculations predict that the targeted excitation of individual TiO₆ rocking modes greatly increases the Li⁺ jump rate as compared to the excitation of vibrational modes associated with heating. Experimentally, coherently driving TiO₆ rocking modes via terahertz (THz) illumination leads to a ten-fold decrease in the differential impedance compared to the excitation of acoustic and optical phonons. Additionally, we differentiate the ultrafast responses of LLTO due to ultrafast heating and THz-range vibrations using laser-driven spectroscopy (LUIS), finding a unique long-lived response for the THz-range excitation. These findings provide new insights into coupled ion migration mechanisms, indicating the important role of THz-range coupled phonon-ion hopping modes in enabling fast ion conduction at room temperature.