An Experimental Technique for Directly Measuring Ultrafast Ion Hopping and Its Inherent Many-Body Correlations

In superionic conductors, ion hopping involves a complex interplay of ion-phonon, ion-electron, and ion-ion correlations that is challenging to measure experimentally. In this presentation, we present a new experimental technique that can directly measure ultrafast ion hopping on its inherent picosecond and longer timescale. The technique works by measuring the time-resolved perturbation to a GHz impedance signal when potential ion-coupling interactions are driven with UV to THz irradiation. The use of high-bandwidth, real-time electronics allows synchronization of the impedance measurement to the ultrafast laser pulses for varying carrier frequencies. The result of the measurement is the relative strength of different correlations in the many-body ion hopping Hamiltonian. We demonstrate the technique on Li_0.5La_0.5TiO₃ (LLTO), a solid-state Li⁺conductor. The ultrafast laser-driven impedance measurements reveals that the dominant ion hopping mechanism in LLTO is through a coupled phonon-ion THz rocking mode. Although this higher frequency mode is less than one quarter of the overall phonon density of states, it leads to the majority of ion hops as compared to other optical and acoustic phonon modes. Metastable states lasting tens of minutes were also measured for ion-electron perturbations. Further work on extending the technique to measure ion-ion correlations is currently underway, but the technique is already generally applicable to any complex ion conducting system such as polymers, fuel cells, and membranes.