title: Mesoscopic structural phase progression in photo-excited VO2 revealed by time-resolved x-ray diffraction microscopy.
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.
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: Yi
authors.initials: Y
authors.lastname: Zhu
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: Zhonghou
authors.initials: Z
authors.lastname: Cai
authors.affiliation: Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
authors.forename: Pice
authors.initials: P
authors.lastname: Chen
authors.affiliation: Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
authors.forename: Qingteng
authors.initials: Q
authors.lastname: Zhang
authors.affiliation: Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Highland
authors.affiliation: Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: Il Woong
authors.initials: IW
authors.lastname: Jung
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: Donald A
authors.initials: DA
authors.lastname: Walko
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: Eric M
authors.initials: EM
authors.lastname: Dufresne
authors.affiliation: IBM Almaden Research Center, San Jose, California 95120, USA.
authors.forename: Jaewoo
authors.initials: J
authors.lastname: Jeong
authors.affiliation: IBM Almaden Research Center, San Jose, California 95120, USA.
authors.forename: Mahesh G
authors.initials: MG
authors.lastname: Samant
authors.affiliation: IBM Almaden Research Center, San Jose, California 95120, USA.|Max Planck Institute for Microstructure Physics, Halle 06120, Germany.
authors.forename: Stuart S P
authors.initials: SS
authors.lastname: Parkin
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: John W
authors.initials: JW
authors.lastname: Freeland
authors.affiliation: Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
authors.forename: Paul G
authors.initials: PG
authors.lastname: Evans
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: Haidan
authors.initials: H
authors.lastname: Wen