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2.
Ultramicroscopy ; 127: 2-8, 2013 Apr.
Article in English | MEDLINE | ID: mdl-22975358

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.

3.
Rev Sci Instrum ; 82(3): 035111, 2011 Mar.
Article in English | MEDLINE | ID: mdl-21456793

ABSTRACT

The precise knowledge of the diffraction condition, i.e., the angle of incidence and electron energy, is crucial for the study of surface morphology through spot profile analysis low-energy electron diffraction (LEED). We demonstrate four different procedures to determine the diffraction condition: employing the distortion of the LEED pattern under large angles of incidence, the layer-by-layer growth oscillations during homoepitaxial growth, a G(S) analysis of a rough surface, and the intersection of facet rods with 3D Bragg conditions.

4.
Science ; 315(5812): 633-6, 2007 Feb 02.
Article in English | MEDLINE | ID: mdl-17272718

ABSTRACT

Intense femtosecond laser excitation can produce transient states of matter that would otherwise be inaccessible to laboratory investigation. At high excitation densities, the interatomic forces that bind solids and determine many of their properties can be substantially altered. Here, we present the detailed mapping of the carrier density-dependent interatomic potential of bismuth approaching a solid-solid phase transition. Our experiments combine stroboscopic techniques that use a high-brightness linear electron accelerator-based x-ray source with pulse-by-pulse timing reconstruction for femtosecond resolution, allowing quantitative characterization of the interatomic potential energy surface of the highly excited solid.

5.
Phys Rev Lett ; 96(6): 066101, 2006 Feb 17.
Article in English | MEDLINE | ID: mdl-16606012

ABSTRACT

The average strain state of Ge films grown on Si(111) by surfactant mediated epitaxy has been compared to the ordering of the interfacial misfit dislocation network. Surprisingly, a smaller degree of average lattice relaxation was found in films grown at higher temperature. On the other hand, these films exhibit a better ordered dislocation network. This effect energetically compensates the higher strain at higher growth temperature, leading to the conclusion that, apart from the formation of misfit dislocations, their ordering represents an important channel for lattice-strain energy relaxation.

6.
Microsc Microanal ; 10(1): 105-11, 2004 Feb.
Article in English | MEDLINE | ID: mdl-15306072

ABSTRACT

We have used in situ electron microscopy to observe the nucleation of Ge islands on lithographically patterned Si(001) mesas. Images were obtained at video rate during chemical vapor deposition of Ge, using a reflection electron microscopy geometry that allows nucleation to be observed over large areas. By comparing the kinetics of nucleation and coarsening on substrates modified by different annealing conditions, we find that the final island arrangement depends on the nature of the mesa sidewalls, and we suggest that this may be due to changes in diffusion of Ge across the nonplanar surface.


Subject(s)
Germanium/chemistry , Microscopy, Electron/methods , Silicon/chemistry , Crystallization , Microscopy, Electron/instrumentation , Quantum Dots , Surface Properties
7.
Phys Rev Lett ; 87(22): 225701, 2001 Nov 26.
Article in English | MEDLINE | ID: mdl-11736408

ABSTRACT

Time-resolved x-ray diffraction with ultrashort ( approximately 300 fs), multi-keV x-ray pulses has been used to study the femtosecond laser-induced solid-to-liquid phase transition in a thin crystalline layer of germanium. Nonthermal melting is observed to take place within 300-500 fs. Following ultrafast melting we observe strong acoustic perturbations evolving on a picosecond time scale.

8.
Phys Rev Lett ; 85(3): 586-9, 2000 Jul 17.
Article in English | MEDLINE | ID: mdl-10991346

ABSTRACT

Damping of impulsively generated coherent acoustic oscillations in a femtosecond laser-heated thin germanium film is measured as a function of fluence by means of ultrafast x-ray diffraction. By simultaneously measuring picosecond strain dynamics in the film and in the unexcited silicon substrate, we separate anharmonic damping from acoustic transmission through the buried interface. The measured damping rate and its dependence on the calculated temperature of the thermal bath is consistent with estimated four-body, elastic dephasing times (T2) for 7-GHz longitudinal acoustic phonons in germanium.

9.
Science ; 286(5443): 1340-2, 1999 Nov 12.
Article in English | MEDLINE | ID: mdl-10558985

ABSTRACT

Using ultrafast, time-resolved, 1.54 angstrom x-ray diffraction, thermal and ultrafast nonthermal melting of germanium, involving passage through nonequilibrium extreme states of matter, was observed. Such ultrafast, optical-pump, x-ray diffraction probe measurements provide a way to study many other transient processes in physics, chemistry, and biology, including direct observation of the atomic motion by which many solid-state processes and chemical and biochemical reactions take place.


Subject(s)
Germanium/chemistry , X-Ray Diffraction , Biochemistry/methods , Crystallization , Lasers , Temperature
10.
J Bacteriol ; 175(19): 6212-9, 1993 Oct.
Article in English | MEDLINE | ID: mdl-8407793

ABSTRACT

Escherichia coli has an iron(II) transport system (feo) which may make an important contribution to the iron supply of the cell under anaerobic conditions. Cloning and sequencing of the iron(II) transport genes revealed an open reading frame (feoA) possibly coding for a small protein with 75 amino acids and a membrane protein with 773 amino acids (feoB). The upstream region of feoAB contained a binding site for the regulatory protein Fur, which acts with iron(II) as a corepressor in all known iron transport systems of E. coli. In addition, a Fnr binding site was identified in the promoter region. The FeoB protein had an apparent molecular mass of 70 kDa in sodium dodecyl sulfate-polyacrylamide gel electrophoresis and was localized in the cytoplasmic membrane. The sequence revealed regions of homology to ATPases, which indicates that ferrous iron uptake may be ATP driven. FeoA or FeoB mutants could be complemented by clones with the feoA or feoB gene, respectively.


Subject(s)
Escherichia coli/metabolism , Genes, Bacterial , Iron/metabolism , Adenosine Triphosphatases/genetics , Aerobiosis , Amino Acid Sequence , Anaerobiosis , Animals , Base Sequence , Binding Sites , Biological Transport/genetics , Cattle , Chromosomes, Bacterial , Escherichia coli/genetics , Escherichia coli/growth & development , Genotype , Kinetics , Molecular Sequence Data , Open Reading Frames , Phenotype , Plasmids , Restriction Mapping , Sequence Homology, Amino Acid , Sequence Homology, Nucleic Acid
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