Most ultrafast pump-probe experiments average over many laser shots and discard the shot-to-shot variation in the sample response. In this work we kept that variation instead of averaging it away. Using time-resolved X-ray micro-diffraction at the Advanced Photon Source, we tracked the lattice response of a single grain of Li0.5La0.5TiO3, a solid-state lithium ion conductor, after each pump pulse.
We found that the lattice trajectories were not independent from shot to shot. Neighboring pump-probe events showed correlated lattice expansion and relaxation, with a correlation “length” of about 1500 laser shots. That length corresponds to an energy barrier of 0.4 ± 0.1 eV, close to the known activation energy for lithium ion migration in this material. A simple Markov-chain model reproduces the effect and points to lithium hopping as the microscopic origin.
We call this approach nonequilibrium noise correlation spectroscopy. It gives a way to extract hidden dynamical correlations from data that would normally be treated as noise, with applications beyond ionic conductors to any system with stochastic photoinduced dynamics.
Congratulations to Jackson, Alfred and the team. We are especially delighted that our article was chosen as an Editors’ Highlight.
The paper is now out in Nature Communications open access: https://www.nature.com/articles/s41467-026-72663-7
