title: Femtosecond structural transformation of phase-change materials far from equilibrium monitored by coherent phonons.
abstract: Multicomponent chalcogenides, such as quasi-binary GeTe-Sb2Te3 alloys, are widely used in optical data storage media in the form of rewritable optical discs. Ge2Sb2Te5 (GST) in particular has proven to be one of the best-performing materials, whose reliability allows more than 10(6) write-erase cycles. Despite these industrial applications, the fundamental kinetics of rapid phase change in GST remain controversial, and active debate continues over the ultimate speed limit. Here we explore ultrafast structural transformation in a photoexcited GST superlattice, where GeTe and Sb2Te3 are spatially separated, using coherent phonon spectroscopy with pump-pump-probe sequences. By analysing the coherent phonon spectra in different time regions, complex structural dynamics upon excitation are observed in the GST superlattice (but not in GST alloys), which can be described as the mixing of Ge sites from two different coordination environments. Our results suggest the possible applicability of GST superlattices for ultrafast switching devices.
authors.affiliation: Division of Applied Physics, Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8573, Japan.
authors.forename: Muneaki
authors.initials: M
authors.lastname: Hase
authors.affiliation: Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 4, 1-1-1 Higashi, Tsukuba 305-8562, Japan.
authors.forename: Paul
authors.initials: P
authors.lastname: Fons
authors.affiliation: Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 4, 1-1-1 Higashi, Tsukuba 305-8562, Japan.
authors.forename: Kirill
authors.initials: K
authors.lastname: Mitrofanov
authors.affiliation: Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 4, 1-1-1 Higashi, Tsukuba 305-8562, Japan.
authors.forename: Alexander V
authors.initials: AV
authors.lastname: Kolobov
authors.affiliation: Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 4, 1-1-1 Higashi, Tsukuba 305-8562, Japan.
authors.forename: Junji
authors.initials: J
authors.lastname: Tominaga