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Plenarvorträge - DPG-Tagungen

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Oberflächenphysik Montag<br />

I bzw. Typ II-Stufen [1] verglichen werden kann.<br />

Neben Tripelstufen werden auch Makrostufen gefunden. Diese Makrostufen<br />

verlaufen ebenfalls in -Richtungen. Makrostufen, die aus<br />

Tripelstufen des Typs I entstanden sind, sind erheblich steiler als solche,<br />

die von Stufen des Typs II herrühren. Dies lässt sich durch eine attraktive<br />

Wechselwirkung zwischen Tripelstufen des Typs I bei Stufenabständen <<br />

1 nm erklären [1].<br />

[1] Puchin et al., J. Phys.: Cond. Matt. 13 (2001) 2081<br />

O 14.40 Mo 18:00 Bereich C<br />

Au/Si(111): Local ordering of the (5 × 2) reconstruction — •C.<br />

Seifert, F.-J. Meyer-zu Heringdorf, and M. Horn-von Hoegen<br />

— Inst. für Laser- und Plasmaphysik, Universität Duisburg-Essen<br />

(Campus Essen)<br />

At Au coverages ΘAu < 1/3ML and substrate temperatures above<br />

300 ◦ C, Si(111) surfaces exhibit a striped (5×2) reconstruction, consisting<br />

of three different domains with an angle of 60 ◦ between them. Upon deposition<br />

of Au, each of the domains forms needles that grow predominantly<br />

in the 2-fold direction of the reconstruction. The resulting anisotropy is<br />

so dominating, that if one domain encounters another one with a different<br />

rotational orientation, the needle-like domain keeps growing, and can<br />

erode the other domains (1) or even completely intersect them (2). Here<br />

we discuss a combined High Resolution Low Energy Electron Diffraction<br />

(SPA-LEED) and Scanning Tunneling Microscopy (STM) experiment to<br />

understand the details of the (5 × 2) reconstruction. The (5 × 2) LEED<br />

pattern can be understood as a (5 × 1) pattern with elongated (5 × 2)<br />

spots that form stripes located at 50% Brillouin-Zone (BZ). The detailed<br />

shape of the stripes depends on the sample preparation temperature.<br />

Room temperature STM measurements combined with fourier analysis<br />

and simple simulations of the adatom arrangement indicate a local ordering<br />

of the gold adatoms.<br />

[1] T. Hasegawa, S. Hosaka, S. Hosoki Surf. Sc. 357-358 (1996), 858-862<br />

[2] F.-J. Meyer zu Heringdorf, R.M. Tromp, unpublished<br />

O 14.41 Mo 18:00 Bereich C<br />

GIXRD-measurements and -simulation of CaF2 on Si(111)<br />

— •Andreas Gerdes 1 , Carsten Deiter 1 , Eddy Patrick<br />

Rugeramigabo 1 , and Joachim Wollschläger 2 — 1 Institut für<br />

Festkörperphysik, Universität Hannover, Appelstr. 2 — 2 Institut für<br />

physikalische Chemie und Festkörperphysik, Universität Bremen, im<br />

Nord-Westen-Bremens NW2<br />

Due to the small lattice mismatch at room temperature CaF2 is a<br />

good candidate to study growth processes on Si(111). Further the system<br />

semiconductor/insulator promises interesting features for electronic<br />

devices on the nanoscale. We grew CaF2 films of different thickness at<br />

T = 500 ◦ C - 600 ◦ C and investigated them by means of GIXRD. Intensity<br />

oscillations due to the finite CaF2 films thickness show that the films<br />

are very homogeneous.<br />

O 14.42 Mo 18:00 Bereich C<br />

GIXRD and AFM Investigations of CaF2/Si Multilayers —<br />

•Carsten Deiter 1 , Andreas Gerdes 1 , Eddy Patrick Rugeramigabo<br />

1 , Joachim Wollschläger 2 , Cunrang Wang 3 , Bernd<br />

Müller 3 , and Karl Hofmann 3 — 1 Institut für Festkörperphysik,<br />

Universität Hannover, D-30167 Hannover — 2 Institut für Angewandte<br />

und Physikalische Chemie, Universität Bremen, D-28359 Bremen<br />

— 3 Institut für Halbleiterbauelemente und Werkstoffe, Universität<br />

Hannover, D-30167 Hannover<br />

Semiconductor/insulator multilayers with few nanometer thick films<br />

offer the opportunity to fabricate nanoelectronic devices with very interesting<br />

features. Due to small lattice mismatch the system CaF2/Si(111)<br />

is a good combination for growth of films with high morphological and<br />

electronical quality.<br />

We have investigated Si/CaF2/Si(111) structures by means of GIXRD<br />

and AFM. Si films (thickness 4nm) and clusters are deposited on CaF2<br />

(thickness 3nm) at temperatures between 800K and 1000K with and<br />

whithout deposition of additional Sb as a surfactant.<br />

O 14.43 Mo 18:00 Bereich C<br />

Growth and morphology of Co films on Pd(111) — •M. Przybylski,<br />

M. Wasniowska, W. Wulfhekel, and J. Kirschner —<br />

Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, 06120 Halle<br />

Ultrathin Co films were grown on an atomically clean and flat surfaces<br />

of Pd(111). Scanning Tunneling Microscopy (STM) shows that at 300K<br />

the growth starts by a formation of double-layer patches of a hexagonal<br />

shape. The Moiré pattern due to the 8.7% lattice mismatch of Co and<br />

Pd was clearly seen. At 170K, mobility of Co atoms is strongly reduced<br />

resulting in a more dense packing of the double-layer patches. At the coverage<br />

of ≈2ML of Co grown at 170K almost the whole surface (95%) is<br />

covered with the double-layer patches, in comparison to only 70% if the<br />

growth proceeds at 300K. In the latter case higher layers nucleate when<br />

the patches approach the size of 50nm. After annealing at 420K, the STM<br />

image shows drastically changed morphology of the film. A monolayer of<br />

Co appears which is continuous and covers 98% of the sample area. This<br />

is associated with changes of the LEED pattern: after annealing only<br />

single spots of hexagonal symmetry, typical for Pd(111), are seen with<br />

no superstructure spots, which were clearly visible before annealing.<br />

O 14.44 Mo 18:00 Bereich C<br />

X-ray study of the surface dynamics at the charge density<br />

wave transition in NbSe2 — •Bridget Murphy 1 , Jochim Stettner<br />

1 , Martin Müller 1 , Ingo Grotkopp 1 , Peter Boesecke 2 ,<br />

Till H. Metzger 2 , Michael Krisch 2 , Herwig Requardt 2 , and<br />

Werner Press 1,3 — 1 Institut für Experimentelle und Angewandte<br />

Physik, Christian-Albrechts-Universität Kiel, Leibnizstraße 19, 24098<br />

Kiel — 2 ESRF, BP 220, 38043 Grenoble Cedex 9, France — 3 Institut<br />

Laue-Langevin, BP 156, 38043 Grenoble Cedex 9, France<br />

2H-NbSe2 is a van der Waals bonded layered structure and displays a<br />

Peierls distortion resulting in a charge density wave transition at 33K.<br />

We investigated this transition using grazing incidence X-ray diffraction<br />

and inelastic X-ray scattering. The elastically scattered intensity of the<br />

satellite reflection, which is proportional to the square of the amplitude<br />

of the modulation, as well as the respective diffuse scattering have been<br />

measured for different incidence angles in order to carry out a direct comparison<br />

between the surface and bulk behaviour. We found that the temperature<br />

dependence of the surface charge density wave satellite clearly<br />

varies from that in the bulk and that the surface transition appears to be<br />

continuous. Moreover, the surface CDW transition occurs 1.4 K above<br />

that of the bulk. We conclude that we likely observe the unusual case<br />

of a ’surface transition’. In order to improve the understanding of the<br />

dynamics phonon dispersion curves at room temperature and around the<br />

phase transition have been determined by inelastic X-ray scattering.<br />

O 14.45 Mo 18:00 Bereich C<br />

Die Allotropie von Antimon und die Ostwaldsche Stufenregel —<br />

•B. Stegemann 1,2 , T. M. Bernhardt 2 , B. Kaiser 1 und K. Rademann<br />

1 — 1 Institut für Chemie, Humboldt-Universität zu Berlin, Brook-<br />

Taylor-Str. 2, D-12489 Berlin — 2 Institut für Experimentalphysik, Freie<br />

Universität Berlin, Arnimallee 14, D-14195 Berlin<br />

Das Element Antimon existiert in mehreren festen Modifikationen (Allotropie).<br />

Eine bisher unbekannte, feste Antimonmodifikation, bestehend<br />

aus periodisch angeordneten, undissoziierten Sb4-Clustern, konnte durch<br />

kontrollierte Abscheidung von Sb4-Clustern aus der Gasphase auf geeigneten<br />

Substraten erzeugt und mittels Rastertunnelmikroskopie nachgewiesen<br />

werden. Diese Modifikation weist einen metastabilen Charakter<br />

auf und wandelt sich bei Erreichen einer kritischen Größe in die amorphe<br />

Phase um. In Abhängigkeit von der Bedeckung und der Temperatur<br />

führt dann ein weiterer Phasenübergang zur Ausbildung der energetisch<br />

günstigsten, kristallinen Antimonmodifikation.<br />

Die stufenweisen Umwandlungen dieser allotropen Modifikationen sind<br />

irreversibel (monotrop) und können mit der Ostwaldschen Stufenregel<br />

erklärt werden, gemäß der ein in mehreren energetischen Zuständen vorkommendes<br />

chemisches System nicht direkt in den thermodynamisch stabilen<br />

Gleichgewichtszustand übergeht, sondern stufenweise über metastabile<br />

Zwischenzustände dorthin gelangt.<br />

O 14.46 Mo 18:00 Bereich C<br />

Orientational and positional phase transitions in geometric confined<br />

systems of alkanes - a numerical study — •Frank Oliver<br />

Pfeiffer und Heiko Rieger — Theoretische Physik, Universität des<br />

Saarlandes, 66041 Saarbrücken, Deutschland<br />

A classical phenomenological atom-atom potential is used to study alkane<br />

chains in a single 2D hexagonal pore via the Monte Carlo method.<br />

We find different phases characterized by orientational and positional order<br />

and discuss the influence of (1) the surface structure of the pore wall<br />

and (2) the pore diameter on the correlation functions. Our calculations<br />

for the confinement are compared to the bulk scenario and to recent<br />

experiments on n-alkanes in nano-pores by Huber and Knorr.

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