title: Initial Atomic Motion Immediately Following Femtosecond-Laser Excitation in Phase-Change Materials.
abstract: Despite the fact that phase-change materials are widely used for data storage, no consensus exists on the unique mechanism of their ultrafast phase change and its accompanied large and rapid optical change. By using the pump-probe observation method combining a femtosecond optical laser and an x-ray free-electron laser, we substantiate experimentally that, in both GeTe and Ge_{2}Sb_{2}Te_{5} crystals, rattling motion of mainly Ge atoms takes place with keeping the off-center position just after femtosecond-optical-laser irradiation, which eventually leads to a higher symmetry or disordered state. This very initial rattling motion in the undistorted lattice can be related to instantaneous optical change due to the loss of resonant bonding that characterizes GeTe-based phase change materials. Based on the amorphous structure derived by first-principles molecular dynamics simulation, we infer a plausible ultrafast amorphization mechanism via nonmelting.
authors.affiliation: Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan.
authors.forename: E
authors.initials: E
authors.lastname: Matsubara
authors.affiliation: Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan.
authors.forename: S
authors.initials: S
authors.lastname: Okada
authors.affiliation: Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Ichitsubo
authors.affiliation: Office of Society-Academia Collaboration for Innovation, Kyoto University, Kyoto 611-0011, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Kawaguchi
authors.affiliation: WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan.|Mathematics for Advanced Materials-OIL, AIST-Tohoku University, Sendai 980-8577, Japan.
authors.forename: A
authors.initials: A
authors.lastname: Hirata
authors.affiliation: Beijing Computational Science Research Center , Beijing 100084, People's Republic of China.
authors.forename: P F
authors.initials: PF
authors.lastname: Guan
authors.affiliation: Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Tokuda
authors.affiliation: The Institute of Scientific and Industrial Research, Osaka University, Osaka 567-0047, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Tanimura
authors.affiliation: Office of Society-Academia Collaboration for Innovation, Kyoto University, Kyoto 611-0011, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Matsunaga
authors.affiliation: WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan.|State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, People's Republic of China.
authors.forename: M W
authors.initials: MW
authors.lastname: Chen
authors.affiliation: Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan.
authors.forename: N
authors.initials: N
authors.lastname: Yamada