|Aufstellung der Publikationen der Arbeitsgruppe 2008|
Aufgeführt sind Arbeiten, die im Jahr 2008 publiziert
und zur Veröffentlichung eingereicht oder angenommen worden sind.
Sonderdrucke oder Preprints können von den Autoren oder im Sekretariat
des Lehrstuhls angefordert werden (Postanschrift: Humboldt-Universität
zu Berlin, Institut für Physik, Physik der Grenzflächen und
dünnen Schichten, Newtonstraße 15, 12489 Berlin, Tel.: (+49
30) 20 93 - 78 91, Fax: - 78 99).
Fast atoms with keV energies are scattered under a grazing angle of incidence from a clean and flat LiF(001) surface. For scattering along low index azimuthal directions within the surface plane ("axial surface channeling") we observe pronounced peak structures in the angular distributions for scattered projectiles which are attributed to "supernumerary rainbows". This phenomenon can be understood in framework of quantum scattering only and is observed here up to projectile energies of 20 keV. We demonstrate that the interaction potential and, in particular, its corrugation for fast atomic projectiles at surfaces can be derived with high accuracy.
2. K. Czerski, A. Huke, L. Martin, N. Targosz, D. Blauth, A. Gorska, P. Heide, and H. Winter
Observation of the enhanced electron screening in metallic environments is of fundamental importance for the understanding of strongly coupled astrophysical plasmas. Experimental screening energies determined by different groups for many metals are much larger than the theoretical predictions. However, a comparison between experimental and theoretical data is rather ambiguous because of contributions of systematic errors in the experiments. One of the most important problems is the uncertainty resulting from oxidation of the target surface during the measurements. Here, we present results of the first ultra-high vacuum (UHV) experiment s studying d+d nuclear reactions in a deuterized Zr target for which the experimental discrepancies are especially large. The total cross sections and angular distributions of the 2H(d,p)3H and 2H(d,n)3He reactions have been measured using a deuteron beam of energies between 8 and 30 keV provided by an electron cyclotron resonance (ECR) ion source with excellent long term stability. The cleanness of the target surface has been assured by combining Ar sputtering of the target and Auger spectroscopy. In an on-line analysis method, the homogeneity of the implanted deuteron densities could be monitored also . The resulting screening energy for Zr confirms the large value obtained in a previous experiment at poorer vacuum conditions.
Fast Ne atoms with energies from 3 to 70 keV are scattered under a grazing angle of incidence from a flat and clean LiF(001) surface. For scattering along low index directions (axial surface channeling) we observe pronounced peaks in the angular distributions of scattered projectiles which can be attributed to rainbow scattering. On the basis of classical trajectory calculations we derive from the rainbow angles the effective interaction potential of Ne atoms in front of LiF(001) and the underlying interatomic pair potentials in the eV energy range. Fair agreement with the data is found for individual pair potentials calculated from Hartree-Fock wave functions where the charge state of Li and F in the ionic crystal is taken into account.
The ionization of xenon Rydberg atoms excited to the lowest states in the n=20 Stark manifold at Si(100) surfaces possessing a robust (~ 10Å) native oxide layer is investigated. The data show that a sizeable fraction of the incident atoms are ionized relatively far from the surface through enhanced tunneling due to the presence of localized stray fields at the surface associated with surface charging or with surface inhomogeneities. A simple model is presented to justify this assertion.
The formation of doubly excited He atoms during impact of He2+ ions with energies from 64 to 500 eV on a clean and oxygen covered Ni(110) surface is studied via Auger electron spectroscopy. We find that the electron spectra are affected by the adsorption of oxygen on the surface which can be related in a clear-cut manner to shifts of the target work function. Our data are in conflict with recent work on this system where changes in the electron spectra with target temperature are attributed to a high local electron spin polarization of the Ni(110) surface. We demonstrate that the data does not probe surface magnetism but modifications of the work function caused by surface contaminations.
The structure of the ordered p(2×1) and p(3×1) phases of adsorbed oxygen as well as the formation of ultrathin NiO(100) layers on a Ni(110) single crystal are investigated by grazing scattering of fast hydrogen atoms. Via ion beam triangulation based on the detection of the number of emitted electrons, we obtain direct information on the structure of oxygen adsorbates and ultrathin nickel oxide layers. For oxidation using atomic instead of molecular oxygen, the gas exposure can be reduced by almost two orders of magnitude. We compare the experimental results with computer simulations based on classical projectile trajectories for grazing scattering of fast hydrogen atoms and test structure models for adsorbed oxygen and NiO(100).
The ionization of noble gas atoms with keV energies scattered under grazing angles of incidence of about 2° from atomically clean and flat metal surfaces is assigned to a two-electron Auger ionization process. Auger ionization, the inverse process of Auger neutralization, is a dynamical process that converts kinetic energy from the projectlile to electronic excitations. In this work, we present first realistic calculations of Auger ionization rates. In molecular dynamics simulations for grazing scattering of He atoms from Al(111), we demonstrate that our theoretical results are in quantitative accord with threshold behavior and angular dependencies of measured ion fractions.
9. M. Gruyters and D. Schmitz
"Microscopic Nature of Ferro- and Antiferromagnetic Interface Coupling of Uncompensated Magnetic Moments in Exchange Bias Systems"
Phys. Rev. Lett. 100 (2008) 077205
Exchange bias in layered CoO/Fe structures is investigated by x-ray resonant magnetic reflectivity (XRMR) measurements. Element-specific hysteresis loops are obtained from x-ray magnetic circular dichroism effects in the XRMR spectra. Evidence is provided for the existence of different types of uncompensated moments in the antiferromagnetic material. Explanations are given for the microscopic nature of these moments and the complex exchange interactions that determine the magnetization reversal in exchange bias systems.
10. T.Bernhard and H. Winter
Electron emission induced by H+ and He+ ions with energies up to 30 keV is studied for grazing impact on a clean and flat Cu(001) surface by making use of electron diffraction effects. Depending on the azimuthal orientation of low index directions in the crystal surface with respect to the projectile beam we observe intensity spots in the angular distributions of emitted electrons. The data is analyzed in the framework of electron diffraction at the ordered surface and reveals interesting details on the emission and coherence mechanisms for the excitation of conduction electrons by fast atomic projectiles in the surface region of a metal.
11. S. Wethekam, D. Valdés, R. C. Monreal, and H. Winter
The ionization of xenon Rydberg atoms excited to the lowest-lying state in the n=17 and 20 Stark manifolds at Au(111) surfaces is investigated as a function of the angle of incidence. Analysis of the data points to the presence of localized stray fields at the surface associated with surface inhomogeneities, which modify the atom-surface separation at which ionization occurs. A simple model is presented to justify this assertion and its implications are discussed.
13. S. Wethekam, M. Busch, and H. Winter
The formation of doubly exited states of He atoms during collisions of He2+ ions with energies between 60 eV and 1 keV with a Ni(110) surface is studied via Auger electron spectroscopy. We observe that the electron spectra from autoionisation of doubly excited states of 2s2, 2s2p, 2p2 configurations show a pronounced dependence on the coverage of the target surface with oxygen. For a controlled O2 adsorption on the Ni(110) surface we can explain the resulting changes in the electron spectra by the modification of the work function of the target surface. Thermal desorption and dissolution into the bulk of surface contaminations at elevated temperatures provides an alternative interpretation of recent work where the local electron spin
14. A. Schüller and H. Winter
15. M. Busch, S. Wethekam, and H. Winter
The formation of doubly excited states of He atoms during impact of He2+ ions with projectile energies of 60−1000 eV under near-grazing angles of incidence of 5° −20° on clean and adsorbate-covered Ni(110) surfaces is studied by means of Auger electron spectroscopy. Pronounced dependencies of electron spectra from autoionization of atoms in doubly excited 2s2, 2s2p and 2p2 configurations on the coverage of the target surface with adsorbates are observed. These are directly related to work function changes, which are studied for the controlled adsorption of oxygen. Changes of the electron spectra on the target temperature are found for adsorbate-covered surfaces only, which puts recent interpretations of similar electron spectra in terms of a high local electron spin polarization of Ni(110) into question by an alternative interpretation based on thermal desorption or dissolution into bulk of surface contaminations. The formation of doubly excited states is studied for the oxygen p(2×l) and p(3×l) superstructures on Ni(110) in order to provide well-defined experimental data for theoretical investigations.
16. A. Schüller, M. Busch, S. Wethekam, and H. Winter
Fast atoms with energies from 500 eV up to several keV are scattered under a grazing angle of incidence from a Fe(110) surface covered with defined superstructures of sulphur or oxygen atoms. For scattering along low index azimuthal directions of the superstructures within the surface plane under "axial surface channeling" we observe defined diffraction patterns in the angular distributions for scattered projectiles. From the analyses of those patterns we derive the widths of low indexed axial channels and the corrugation of the interaction potential across these channels. This allows us to estimate the positions of adsorbed atoms on the Fe(110) surface.
17. S. Wethekam, M. Busch, R.C. Monreal and H. Winter
The survival of ions during grazing scattering of keV He+ ions from a clean Ni(110) surface is studied as function of target temperature. We observe ion fractions in the scattered beams of typically 10-3 which show a slight increase with temperature of the target surface. From computer simulations of projectile trajectories we attribute this enhancement for ion fractions to effects of thermal vibrations of lattice atoms on the survival of ions in their initial charge state. Based on concepts of Auger neutralization, we discuss the role of the spin polarization of target electrons on charge transfer. We do not find corresponding signatures in our data and conclude that in the present case of Ni(110) the spin polarization has to be small.
18. A. Schüller and H. Winter
For scattering of fast atoms under axial channeling conditions from surfaces diffraction effects have been observed in the distributions for scattered projectiles. Basic features concerning this recently observed new phenomenon in ion/atom surface scattering will be demonstrated for collisions of light atoms, ions, and molecules with well ordered clean and adsorbate covered surfaces of insulators and metals. The potential of this method for the investigation of structures of surfaces is discussed.
19. T. Bernhard, J. Seifert, and H. Winter
Recent developments in studies on the structure of surfaces based on ion beam triangulation are discussed. We will outline recent experimental progress in the application of this method which is closely related to the detection of the number of emitted electrons per incident ion during scattering under surface channeling conditions. Key features are the pronounced change of electron emission for the projectile beam aligned along a low index crystallographic direction in the surface plane ("axial surface channeling") and the interpretation of data in terms of classical trajectory computer simulations. As a representative example we will discuss here studies on the structure of the low temperature c(8x2) Mn/Cu(001) phase.
20. M. Busch, A. Schüller, S. Wethekam, and H. Winter
Fast atoms with energies from 350 eV up to 1.5 keV are scattered under a grazing angle of incidence from a clean and flat Ni(110) surface. For scattering under "axial surface channeling" conditions, we observe – as reported recently for insulator and semiconductor surfaces – defined diffraction patterns in the angular intensity distributions for scattered fast 3He and 4He atoms. We have investigated the domain of scattering conditions where decoherence phenomena are sufficiently small in order to observe for metal targets quantum scattering of fast atomic projectiles. As a consequence, fast atom diffraction appears to be a general technique with a wide range of applicability in surface science.