Mesoscopic structural phase progression in photo-excited VO2 revealed by time-resolved x-ray diffraction microscopy.

Yi Zhu,Matthew J Highland,Stuart S P Parkin,Jaewoo Jeong,Eric M Dufresne,Mahesh G Samant,Haidan Wen,Il Woong Jung,John W Freeland,Paul G Evans,Donald A Walko,Qingteng Zhang,Pice Chen,Zhonghou Cai

doi:10.1038/srep21999

Abstract

Dynamical phase separation during a solid-solid phase transition poses a challenge for understanding the fundamental processes in correlated materials. Critical information underlying a phase transition, such as localized phase competition, is difficult to reveal by measurements that are spatially averaged over many phase separated regions. The ability to simultaneously track the spatial and temporal evolution of such systems is essential to understanding mesoscopic processes during a phase transition. Using state-of-the-art time-resolved hard x-ray diffraction microscopy, we directly visualize the structural phase progression in a VO2 film upon photoexcitation. Following a homogenous in-plane optical excitation, the phase transformation is initiated at discrete sites and completed by the growth of one lattice structure into the other, instead of a simultaneous isotropic lattice symmetry change. The time-dependent x-ray diffraction spatial maps show that the in-plane phase progression in laser-superheated VO2 is via a displacive lattice transformation as a result of relaxation from an excited monoclinic phase into a rutile phase. The speed of the phase front progression is quantitatively measured, and is faster than the process driven by in-plane thermal diffusion but slower than the sound speed in VO2. The direct visualization of localized structural changes in the time domain opens a new avenue to study mesoscopic processes in driven systems.

Highlights

M aR as the observation of a metal-like monoclinc phase[23] and inhomogenous onset transition time scales[18] stimulates futher microscopic investigation on the phase transition in VO2
Using a newly developed laser pumped x-ray diffraction imaging technique with 350 nm spatial resolution and 100 ps temporal resolution[39], we quantitatively studied the structural phase propagation during the photo-induced phase transition in a VO2 thin film (Fig. 1a)
The second stage, following the initial fs phase transition, is a process in which the lattice is superheated above the transition temperature within a few ps, as a result of electron-phonon coupling during which the absorbed radiation energy in electronic degree of freedom is transferred to the lattice

Summary

Introduction

M aR as the observation of a metal-like monoclinc phase[23] and inhomogenous onset transition time scales[18] stimulates futher microscopic investigation on the phase transition in VO2. Studies in the time[28,29,30,31] and space[18,32,33,34,35,36,37] domains have shown the dynamical and heterogeneous nature of the phase transition respectively; the microscopic processes including spatial progression of the phase transition, the energetics and dynamics of phase boundaries, and the characteristic length scale and speed of the transformation, are not yet known Quantitative characterization of these mesoscopic processes requires the visualization of the dynamical processes in appropriate time and space domains simultaneously[38]. The resulting high-temperature R phase propagates from a series of nucleation sites into regions of the M phase (Fig. 1b) This process is not driven by thermal diffusion since the quantitative measurement of the phase progression speed is faster than that is predicted by thermal diffusion. This experiment represents the first hard x-ray measurement with sub-ns and sub-μ m resolution that directly captures a mesoscopic structural phase transformation in correlated materials

Methods

Results

Conclusion