Using light to control transient phases in quantum materials is an emerging route to engineer new properties and functionality, with both thermal and non-thermal phases observed out of equilibrium. Transient phases are expected to be heterogeneous, either through photo-generated domain growth or by generating topological defects, and this impacts the dynamics of the system. However, this nanoscale heterogeneity has not been directly observed. Here we use time- and spectrally resolved coherent X-ray imaging to track the prototypical light induced insulator-to-metal phase transition in vanadium dioxide on the nanoscale with femtosecond time resolution. We show that the early-time dynamics are independent of the initial spatial heterogeneity and observe a 200 fs switch to the metallic phase. A heterogeneous response emerges only after hundreds of picoseconds. Through spectroscopic imaging, we reveal that the transient metallic phase is a highly orthorhombically strained rutile metallic phase, an interpretation that is in contrast to those based on spatially averaged probes. Our results demonstrate the critical importance of spatially and spectrally resolved measurements for understanding and interpreting the transient phases of quantum materials.
@article{arxiv.2202.08585,
title = {Ultrafast X-ray imaging of the light-induced phase transition in VO2},
author = {Allan S. Johnson and Daniel Pérez-Salinas and Khalid M. Siddiqui and Sungwon Kim and Sungwook Choi and Klara Volckaert and Paulina E. Majchrzak and Søren Ulstrup and Naman Agarwal and Kent Hallman and Richard F. Haglund and Christian M. Günther and Bastian Pfau and Stefan Eisebitt and Dirk Backes and Francesco Maccherozzi and Ann Fitzpatrick and Sarnjeet Dhesi and Pierluigi Gargiani and Manuel Valvidares and Nongnuch Artrith and Frank de Groot and Hyeongi Choi and Dogeun Jang and Abhishek Katoch and Soonnam Kwon and Sang Han Park and Hyunjung Kim and Simon E. Wall},
journal= {arXiv preprint arXiv:2202.08585},
year = {2023}
}