@article{HanischBlicharskiJanzenKrenzeretal., author = {Hanisch-Blicharski, Anja and Janzen, A. and Krenzer, B. and Wall, S. and Klasing, F. and Kalus, A. and Frigge, Tim and Kammler, Martin and Horn-von Hoegen, Michael}, title = {Ultra-fast electron diffraction at surfaces: from nanoscale heat transport to driven phase transitions}, series = {Ultramicroscopy}, volume = {127}, journal = {Ultramicroscopy}, publisher = {Elsevier}, doi = {10.1016/j.ultramic.2012.07.017}, pages = {2 -- 8}, abstract = {Many fundamental processes of structural changes at surfaces occur on a pico- or femtosecond time scale. In order to study such ultra-fast processes, we have combined modern surface science techniques with fs-laser pulses in a pump-probe scheme. Reflection high energy electron diffraction (RHEED) with grazing incident electrons ensures surface sensitivity for the probing electron pulses. Utilizing the Debye-Waller effect, we studied the cooling of vibrational excitations in monolayer adsorbate systems or the nanoscale heat transport from an ultra-thin film through a hetero-interface on the lower ps-time scale. The relaxation dynamics of a driven phase transition far away from thermal equilibrium is demonstrated with the In-induced (8×2) reconstruction on Si(111). This surface exhibits a Peierls-like phase transition at 100K from a (8×2) ground state to (4×1) excited state. Upon excitation by a fs-laser pulse, this structural phase transition is driven into an excited (4×1) state at a sample temperature of 20K. Relaxation into the (8×2) ground state occurs after more than 150 ps.}, language = {en} } @article{WallKrenzerWippermannetal., author = {Wall, Simone and Krenzer, Boris and Wippermann, Stefan and Sanna, Simone and Klasing, Friedrich and Hanisch-Blicharski, Anja and Kammler, Martin and Schmidt, Wolf Gero and Horn-von Hoegen, Michael}, title = {Atomistic picture of charge density wave formation at surfaces}, series = {Physical review letters}, volume = {109}, journal = {Physical review letters}, number = {18}, publisher = {APS}, doi = {10.1103/PhysRevLett.109.186101}, abstract = {We used ultrafast electron diffraction and density-functional theory calculations to gain insight into the charge density wave (CDW) formation on In/Si(111). Weak excitation by a femtosecond-laser pulse results in the melting of the CDW. The immediate freezing is hindered by a barrier for the motion of atoms during the phase transition: The melted CDW constitutes a long-lived, supercooled phase and is strong evidence for a first-order transition. The freezing into the CDW is triggered by preexisting adsorbates. Starting at these condensation nuclei, the CDW expands one dimensionally on the In/Si(111) surface, with a constant velocity of more than 80 m/s.}, language = {en} } @article{StreubuehrKalusZhouetal., author = {Streub{\"u}hr, C. and Kalus, A. and Zhou, P. and Ligges, M. and Hanisch-Blicharski, Anja and Kammler, Martin and Bovensiepen, U. and Horn-von Hoegen, Michael and von der Linde, D.}, title = {Comparing ultrafast surface and bulk heating using time-resolved electron diffraction}, series = {Applied Physics Letters}, volume = {104}, journal = {Applied Physics Letters}, publisher = {American Institute of Physics}, doi = {10.1063/1.4872055}, pages = {1 -- 3}, abstract = {From measurements of the transient Debye-Waller effect in Bismuth, we determine the buildup time of the random atomic motion resulting from the electronic relaxation after short pulse laser excitation. The surface sensitive reflection high energy electron diffraction and transmission electron diffraction yield a time constant of about 12 ps and 3 ps, respectively. The different energy transfer rates indicate relatively weak coupling between bulk and surface vibrational modes. (C) 2014 AIP Publishing LLC.}, language = {en} } @inproceedings{ZhouStreubuehrKalusetal., author = {Zhou, P. and Streub{\"u}hr, C. and Kalus, A. and Frigge, Tim and Wall, S. and Hanisch-Blicharski, Anja and Kammler, Martin and Ligges, M. and Bovensiepen, U. and von der Linde, D. and Horn-von Hoegen, Michael}, title = {Ultrafast time resolved reflection high energy electron diffraction with tilted pump pulse fronts}, series = {EPJ Web of Conferences - XVIIIth International Conference on Ultrafast Phenomena}, volume = {41}, booktitle = {EPJ Web of Conferences - XVIIIth International Conference on Ultrafast Phenomena}, publisher = {EPJ}, doi = {10.1051/epjconf/20134110016}, abstract = {We present time-resolved RHEED from a laser excited Pb(111) surface using a pulse front tilter for the compensation of the velocity mismatch of electrons and light. The laser pulses with tilted fronts were characterized by a spatially resolving cross correlator. The response of the surface upon excitation was observed to be less than 2 ps.}, language = {en} } @article{FriggeWallKrenzeretal., author = {Frigge, Tim and Wall, S. and Krenzer, B. and Wippermann, St. and Sanna, S. and Klasing, F. and Hanisch-Blicharski, Anja and Kammler, Martin and Schmidt, W. G. and Horn-von Hoegen, Michael}, title = {Abstract - A Reply to the Comment by H. Shim et al.}, series = {Physical review letters}, volume = {111}, journal = {Physical review letters}, number = {14}, publisher = {American Physical Society}, doi = {10.1103/PhysRevLett.111.149602}, language = {en} } @article{FriggeKalusKlasingetal., author = {Frigge, Tim and Kalus, Annika and Klasing, Friedrich and Kammler, Martin and Hanisch-Blicharski, Anja and Horn-von Hoegen, Michael}, title = {Nanoscale Heat Transport in Self-Organized Ge Clusters on Si(001)}, series = {MRS Online Proceedings Library}, journal = {MRS Online Proceedings Library}, publisher = {Springer}, doi = {10.1557/opl.2013.148}, pages = {45 -- 50}, abstract = {Ultrafast time resolved transmission electron diffraction (TED) in a reflection geometry was used to study the cooling behavior of self-organized, well defined nanoscale germanium hut and dome clusters on Si(001). The clusters were heated in a pump-probe scheme by fs-laser pulses. The resulting transient temperature rise was then determined from the drop in diffraction intensity caused by the Debye-Waller effect. From a cooling time of τ=177 ps we estimated a strongly reduced heat transfer compared with homogeneous films of equivalent thickness.}, language = {en} } @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{KrenzerHanischBlicharskiSchneideretal., author = {Krenzer, B. and Hanisch-Blicharski, Anja and Schneider, P. and Payer, Th. and M{\"o}llenbeck, S. 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} } @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} }