@article{MoellenbeckHanischBlicharskiSchneideretal., author = {M{\"o}llenbeck, Simone and Hanisch-Blicharski, Anja and Schneider, Paul and Ligges, Manuel and Zhou, Ping and Kammler, Martin and Krenzer, Boris and Horn-von Hoegen, Michael}, title = {Ultra-fast Time-Resolved Electron Diffraction of Strongly Driven Phase Transitions on Silicon Surfaces}, series = {MRS Online Proceedings Library}, journal = {MRS Online Proceedings Library}, publisher = {Springer}, doi = {10.1557/PROC-1230-MM03-09}, abstract = {The dynamics of strongly driven phase transitions at surfaces are studied by ultra-fast time-resolved reflection high energy electron diffraction. The surfaces are excited by an intense fs-laser pulse (pump) and probed by an ultra-short electron pulse with variable time delay. The order-disorder phase transition from a c(4 {\~A}— 2) to a(2 {\~A}— 1) of the bare Si(001) surface shows a transient decrease of the intensity of the c(4 {\~A}— 2) spots which recovers on a time scale of a few hundred picoseconds indicating the excitation of the phase transition. On Si(111) a monolayer of Indium induces a (4 {\~A}— 1) reconstruction which undergoes a Peierls like phase transition to a (8 {\~A}— "2") reconstruction below 100 K. Upon laser excitation with a fluence of 3.4 mJ/cm 2 at a temperature of 72 K the phase transition was strongly driven. The (8 {\~A}— "2")-diffraction spots instantaneously disappears, while the intensity of the (4 {\~A}— 1)-spots increases. This increase of the (4 {\~A}— 1) spot intensity excludes an explanation by the Debye-Waller-Effect and is evidence for a true structural phase transition at a surface. {\^A}© 2010 Materials Research Society.}, language = {en} } @article{HanischBlicharskiKrenzerMoellenbecketal., author = {Hanisch-Blicharski, Anja and Krenzer, Boris and M{\"o}llenbeck, Simone and Ligges, Manuel and Zhou, Ping and Kammler, Martin and Horn-von Hoegen, Michael}, title = {Transient Cooling of Ultrathin Epitaxial Bi(111)-Films on Si(111) Upon Femtosecond Laser Excitation Studied by Ultrafast Reflection High Energy Electron Diffraction}, series = {MRS Online Proceedings Library}, journal = {MRS Online Proceedings Library}, publisher = {Springer}, doi = {10.1557/PROC-1172-T04-08}, pages = {1 -- 6}, abstract = {With time resolved ultrafast electron diffraction the cooling process across the interface between a thin film and the underlying substrate was studied after excitation with short laser pulses. From the exponential decay of the surface temperature evolution a thermal boundary conductance of 1430 W/(cm2K) is determined for a 9.7 nm thin Bi(111) film on Si(111). A linear dependence between laser fluence and initial temperature rise was measured for film-thicknesses between 2.5 nm and 34.5 nm. The ratio of initial temperature rise and laser fluence for different film-thicknesses is compared to a model taking multilayer optics into account. The data agree well with this model.}, language = {en} } @article{KrenzerHanischBlicharskiSchneideretal., author = {Krenzer, Boris and Hanisch-Blicharski, Anja and Schneider, Paul and Payer, Thomas and M{\"o}llenbeck, Simone and Osmani, O. and Kammler, Martin and Meyer, R. and Horn-von Hoegen, Michael}, title = {Phonon confinement effects in ultrathin epitaxial bismuth films on silicon studied by time-resolved electron diffraction}, series = {Physical Review B}, volume = {80}, journal = {Physical Review B}, number = {2}, publisher = {APS}, doi = {10.1103/PhysRevB.80.024307}, abstract = {The transient temperature evolution of ultrathin bismuth films, epitaxially grown on a silicon single crystal, upon femtosecond laser excitation is studied by time-resolved electron diffraction. The exponential decay of the film temperature is explained by phonon reflection at the interface, which results in a strongly reduced thermal conduction in the cross plane of the layered system. The thermal boundary conductance is found to be as low as 1273 W/(K cm2). Model calculations, including phonon confinement effects, explain the linear relationship between the observed film-temperature decay constant and the film thickness. Even for 2.5 nm thin films the phonon transmission probability across the interface is given by bulk properties. Our simulations show that phonon confinement effects are negligible for bismuth-film thicknesses larger than 1 nm.}, language = {en} }