Ultrafast Polaronic Lattice Distortions in Perovskite Oxides by Time-resolved Coherent Diffraction Imaging

Polarons generated by the interaction between the excited carriers and lattice in perovskite oxides in an ultrafast time scale are important for understanding the underlying science and practical applications. Such a polaron affects structural distortions and carrier transport in perovskite-oxides. Polarons are expected to form in one picosecond by non-thermal effect. It is crucial to obtain precise measurements on the generation and initial propagation of polarons before their development as phonons to explore the potential of materials. So far, polaron formation has been studied intensively by computational outcomes or by observation of the changes in diffuse scattering in films or bulk. However, there is no clear experimental observation or explanation for polaron formation and propagation behavior.

The recent advance of X-ray free electron laser (XFEL) opens the area of ultrafast structural dynamics with a few tens of femtoseconds time resolution due to its unique characteristics of X-rays. XFEL makes it possible to obtain critical information on the intermediate states or pathways during the phase transformation, which only measures the initial and final states with many existing techniques.

In this presentation, I will present a direct observation of initial polaron generation and relevant strain evolution in perovskite-oxide nanocrystals. The ultrafast lattice distortions by photoexcitation are measured by time-resolved Bragg coherent X-ray diffraction imaging with an optical laser pump and an X-ray pulse probe from an X-ray free electron laser. We observe a particular type of lattice distortion caused by excited carriers in the entire nanocrystal on the picosecond time scale. Atomic-scale motions linked to momentary internal structural distortions are observed, which are related to the polaronic phase transition.