|Aufstellung der Publikationen der Arbeitsgruppe 2003|
Aufgeführt sind Arbeiten, die im Jahr 2003 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).
Total target currents for grazing scattering of keV protons from a crystal target are used to investigate the structure of surfaces and ultrathin films. This current shows pronounced maxima whenever the azimuthal incidence angle coincides with close-packed rows of atoms in the surface and subsurface layers. The real-space method is applied to study monolayer and bilayer films of Mn and of CoMn epitaxially grown on a Cu(001) surface.
He+ ions as well as neutral He atoms with keV energies are scattered under a grazing angle of incidence from a clean and atomically flat Ag(111) surface. From a comparison of ion fractions observed after scattering of He+ ions and He atoms we find for energies below some keV small but defined fractions of ions that have survived the complete scattering event with the surface. This feature allows us to clear up the microscopic interaction scenario for Auger neutralization of He+ ions at a Ag(111) surface. The Auger neutralization rates are 2 to 3 orders of magnitude smaller than conventional rates derived from experiments for He+-metal systems and agree with recent calculations.
R. Pentcheva, K. A. Fichthorn, M. Scheffler, T. Bernhard, R. Pfandzelter,
and H. Winter
We present a combined theoretical and experimental study of island nucleation and growth in the deposition of Co on Cu(001) - a prototype for understanding heteroepitaxial growth involving intermixing. Experimentally ion scattering is employed. Using density-functional theory, we obtain energy barriers for the various elementary processes and incorporate these into a kinetic Monte Carlo program to simulate the heteroepitaxial growth. Both the simulations and the experiments show a unique N-shape dependence of the island density on temperature that stems from the interplay and competition of the different processes involved.
The energy loss of slow ions scattered in front of a Al(111) surface is explained using a theoretical description based on scattering theory. The explicit inclusion of gradient corrections to account for a nonuniform electron density at the surface provides good agreement with the measured data over a wide range of distances (electron densities) for different ions with atomic number Z1 <= 20.
Ion beam triangulation is a real-space technique to probe structures of surfaces and ultrathin films. In-plane arrangement of surface atoms is provided from enhanced kinetic electron emission yields for the transition from planar to axial surface channeling. The technique is applied to clean (001) surfaces of fcc Cu and bcc Fe as well as ultrathin films of Mn and Co, epitaxially grown on these surfaces at different temperatures.
Coincident measurements of projectile energy loss and kinetic electron emission yield for grazing scattering of 150 eV/amu to some keV/amu neutral hydrogen and helium atoms from an atomically clean and flat Al(111) surface allow us to correlate electron emission and inelastic interaction mechanisms at a metal surface. Our data show evidence for a threshold behavior of kinetic electron emission which is interpreted by energy transfer in binary encounters of projectiles in the electron selvage of a quasi-free electron gas. Contributions of electron emission to projectile energy loss are found to be negligibly small.
S. Lederer, A. Mertens, H. Winter, F. Aumayr, HP. Winter, and V. Staemmler
In coincident studies on projectile energy loss and number distributions of emitted electrons for scattering of hydrogen atoms from an atomically clean and flat LiF(001) surface we derive probabilities for emission of electrons and production of surface excitons near their respective kinematic thresholds. We analyze our data in terms of electron transfer in binary atomic collisions with one collision partner being embedded at the anion site of an ionic crystal and derive information on the energy defects in these collisions.
He atoms and He+ ions are scattered with keV energies under a grazing angle of incidence from an atomically flat and clean Ag(111) surface. We have measured charge fractions of specularly reflected beams and studied the threshold behaviour for ionization of projectiles in terms of kinematically induced Auger ionization. From comparison of data for neutral and ionized projectiles we could show that precise studies on the kinematic onset of ionization can be performed with neutral projectiles. Small but defined fractions of ions survive the scattering event with the surface which affects the evaluation of data close to the threshold owing to a background of the signals for ions.
Grazing scattering of 25 keV He atoms is applied to study island nucleation and growth of Co on Cu(001) in real time and real space. With decreasing growth temperature, we observe, starting at 470 K, step-flow growth, layer growth, bilayer growth, and, below 210 K, reentrant layer growth. The density of nucleated islands shows a complex "N-shaped" dependence on temperature, which is at variance with standard nucleation theory. Results are explained on the basis of recent density-functional calculations.
We report on the emission of spin-polarized electrons during grazing and oblique impact of 2 - 100 keV H+, He+, Ne+, and Ar+ ions upon a magnetized Fe(100) surface. A combined analysis of spin state and energy of emitted electrons elucidates processes occuring in inelastic ion-surface scattering like electron cascading or plasmon-assisted electron emission. We find evidence for a significant enhancement of the spin moment at the topmost layer of Fe(100).
A. G. Borisov, V. Sidis, P. Roncin, A. Momeni, H. Khemliche, A. Mertens, and H. Winter
For slow F+ ions (v < 0.05 a.u.) scattered from a clean and flat LiF(001) surface under a grazing angle of incidence large fractions of negative F- ions have recently been observed in the reflected beam, while for neutral F0 projectiles no negative F- ions are produced in the same velocity range [P. Roncin et al., Phys. Rev. Lett. 89, 043201 (2002)]. From detailed studies on projectile energy loss and charge fractions, the conclusion was drawn that the F- ions are formed from F+ via a simultaneous capture of two electrons from adjacent F- sites at the surface. We present a theoretical description of the double-electron-capture process leading to F- formation from F+ projectiles grazingly scattered from the LiF(001) surface. We use quantum chemistry calculations to determine the relevant Hamiltonian matrix and close-coupling solution of the time-dependent Schrödinger equation. The theoretical results are in good agreement with experimental observations.
The coincident detection of projectile energy loss with the number of emitted electrons for the scattering of atoms from atomically clean and flat insulator and metal surfaces under grazing angles of incidence allows one to identify the relevant electron excitation and emission processes. From detailed studies for the scattering of H and He atoms from a LiF(001) as well as from an Al(111) surface we find evidence for clearly different interaction mechanisms. For not too high projectile velocities with respect to kinematic thresholds, electron emission for the insulator target is understood by an electronic promotion mechanism with the formation of H- ions as dominant precursor, whereas for the metal target electronic excitations are governed by direct energy transfer in binary collisions with the atomic projectiles. Based on these features we reveal a microscopic understanding for the well established, but so far poorly understood property that electron emission induced by atom or ion impact on insulator targets is more efficient than for metals.
We present a simple classical free electron gas model for the threshold behaviour of the kinetic electron emission caused by collisions of atomic projectiles with a metal. In our model we assume that electron emission close to the threshold is dominated by an energy transfer in binary collisions between atomic projectiles and a free electron gas. From conservation laws and phase space considerations we derive the threshold behaviour for the kinetic electron emission and the mean electron energy transfer, which compare well with recent experimental studies on electron emission from the selvage of atomically clean and flat Al(111) under grazing impact of light neutral atoms.
Single-electron excitation in He+-He collisions is investigated in the 30-50 keV range, where the excitation cross-sections have resonance-like maxima. By measuring anticrossing spectra, we show that the 1s5l levels of He I with l$2 are populated effectively and highly coherently in this energy range. The superposition states of the excited elektron resulting from the collision process are identified as the parabolic Stark states with the largest electric dipole moments. The results are explained with the Paul-trap model and saddle dynamics.
We present an analysis of data obtained in coincident studies on projectile energy loss and number distributions of emitted electrons for scattering of hydrogen atoms from an atomically clean and flat LiF(001) surface. The emission of electrons, production of surface excitons, and formation of negative ions is explained by a model of electron capture by projectiles at fluorine lattice sites where the formation of negative hydrogen ions is the common precursor for the electronic processes of relevance here. We describe the interactions in terms of concepts for gas phase collisions and derive probabilities for the detachment of negative ions during the collision with the insulator surface.
He+ ions and He atoms with keV energies are scattered under a grazing angle of incidence from an atomically flat and clean Cu(111) surface. From the comparison of measured charge fractions for specularly reflected charged and neutral projectiles we find evidence for the survival of incident ions from neutralization during the complete scattering event. The survival of ions can only be understood, if the neutralization proceeds around the distance of closest approach, i.e. at the jellium edge. Thus neutralization of ions proceeds closer to the surface than derived in most former evaluations of experimental data where response phenomena are approximated by the concept of classical image charge interactions. From the analysis of our data we derive Auger neutralization rates which compare well with recent theoretical work.
Cs ions with energies ranging from 10 keV to 1.8 MeV are scattered under a grazing angle of incidence from a flat and clean Cu(111) surface. The observed fractions of Cs atoms in the scattered beams and their dependence on projectile velocity are well described by a model of kinematically assisted resonant charge transfer between projectile and two-dimensional surface-state continuum of the target surface. A comparison with calculations for a target represented by the electronic structure of a free-electron metal shows neutral fractions which are enhanced for the Cu(111) by more than one order of magnitude. This is the strongest effect of the projected band gap of a metal surface on the charge transfer observed so far.
18. H. Winter, K. Maass, S. Lederer, HP. Winter, and F.
Fast He+ ions and He0 atoms are scattered under grazing incidence from a clean and flat Al(111) surface. The target current recorded as function of azimuthal rotation with respect to the surface normal is enhanced for scattering along low-index crystallographic directions in the surface plane. From coincident studies on the number of electrons emitted per ion impact we discovered the interaction mechanism. In contrast to earlier interpretations our data reveal that the increase of apparent total yield for surface channeling along low-index directions is due to penetration of the surface layer by a small fraction of projectiles which produce a substantially larger number of electrons.
Csq+ ions with charge q = 1 to 17 and energies ranging from 75 keV to 1.8 MeV are scattered under a grazing angle of incidence from a flat and clean Cu(111) surface. Charge state distributions of scattered ions are recorded as function of projectile velocity and reveal dominant contributions of charge q = 1 over the complete range of velocities/energies studied here. For higher final charge states (up to q = 5) we observe a dependence on projectile velocity which is attributed to specific features of charge transfer in grazing ion surface scattering.
20. T. Bernhard, R. Pfandzelter, and H. Winter
Structure and growth of monolayer and bilayer films of Mn on Cu(001) are studied by grazing scattering of fast He+ ions and protons. A (quasi-)hexagonal arrangement is observed for the c(8x2) Mn monolayer grown at low temperatures (< 270 K). At higher temperatures the structure of a c(2x2) Mn monolayer is in accordance with a buckled CuMn surface alloy and a c(12x8) structure at a coverage of about 1.7 monolayers.
21. T. Bernhard, Z. L. Fang, and H. Winter
H+ and He+ ions with an energy of 25 keV are scattered under a grazing angle of incidence from a clean and flat Cu(001) surface. For specific azimuthal orientations of the crystal surface with respect to low index directions in the surface plane we observe the ion induced emission of electrons with a conventional LEED (Low Energy Electron Diffraction) setup. By operating the instrument in an energy dispersive mode we find intensity distributions of emitted electrons which can unequivocally be ascribed to diffraction effects at the target surface.
The growth and structure of ultrathin Rh films on Fe(100) are studied by grazing scattering of 50 keV He projectiles, incident along "random" and low-index surface lattice directions. Oscillations in the specular intensity for scattered ions indicate initial layer-by-layer growth, followed by multilayer growth on top of the two-monolayer film. Growth temperatures below about 400 K result in a persistent, though rough layer growth up to higher coverages. From distinct maxima in the target current as a function of the azimuthal incidence angle, we deduce that growth is epitaxial and pseudomorphic.
Using cobalt oxide/ferromagnet bilayers with extraordinarily strong magnetic coupling the nature of the interfacial exchange interaction in exchange bias systems has been investigated. For Fe, Co and Ni as ferromagnetic materials, an almost linear temperature dependence of the exchange bias field and a common blocking temperature 175 K have been found. Additionally, an inverse proportionality between the bias field and the thickness of the ferromagnetic layer has been confirmed. The average coupling energies at T = 10 K have been determined to 0.90, 1.02 and 0.80 erg/cm2 for CoO/Fe, CoO/Co and CoO/Ni bilayers, respectively. The presently observed behavior strongly deviates from recent theoretical mean-field analysis and other experimental investigations on antiferromagnetic/ferromagnetic bilayers.
24. M. Gruyters
Magnetoresistance and magnetic hysteresis loop measurements have been used to investigate the strong interfacial coupling in cobalt oxide/ferromagnet bilayers at low temperatures. For the biased state, an asymmetry in the magnetization reversal processes has been found. It is characterized by domain wall motion as the dominant mechanism for the decreasing field branch and an appreciable amount of magnetization rotation for the increasing field branch. The observed behavior is different from other exchange bias systems.
25. M. Gierlings, M. Gruyters, D. Riegel, M. J.
Prandolini, T. Funk, and W. D. Brewer
We report the first observation of the effects of exchange bias on the nuclear spin polarization and induced magnetic moments at the magnetic/nonmagnetic interfaces in Co/Au(x)/CoO trilayer systems using low temperature nuclear orientation (LTNO). This technique allows us to determine simultaneously the average alignment of the nuclear moments for the two radioactive probe isotopes 198Au and 60Co with respect to an external magnetic field axis. The total average Au g-ray anisotropy measured was found (i) to decrease with increasing Au thickness, indicating that large hyperfine fields are restricted to the interfacial Au layers and (ii) to be canted away from the applied field axis even when the Co layers are magnetically saturated. This canting was found to originate at the CoO/Au interface as could be shown from comparative measurments on CoO/Au/CoO trilayers containing two AFM CoO/Au interfaces and on a Co/Au/Co trilayer with two FM Co/Au interfaces. In the case of CoO/Au/CoO, the observed canting was found to be dependent on the Au layer thickness.
26. R. Kirsch, M. J. Prandolini, M. Gierlings, M.
Gruyters, W. D. Brewer, D. Riegel
Single Fe impurities were implanted in an Er single crystal and found to occupy both substitutional and interstitial sites, below a temperature of 200 K. The local susceptibility of Fe on both sites follows a Curie-Weiss law and exhibits a positive local Curie constant, indicating an antiferromagnetic coupling between the Fe and the surrounding Er moments. The corresponding nuclear spin relaxation rates follow a Korringa law as a function of temperature, confirming the dominance of local magnetism and the formation of local moments on each of the sites occupied by Fe.
27. Z. L. Fang, T. Bernhard, and H. Winter
Electron emission for impact of 25 keV H+ and He+ ions on a Cu(001) surface under a grazing angle of incidence is studied with a LEED setup. For specific energies of emitted electrons and crystallographic orientations of the target surface, pronounced diffraction spots are observed which can be related to the two-dimensional reciprocal lattice of the target surface and analyzed in terms of a modified Ewald construction.
Single-electron excitation of He atoms by 30 and 50 keV He+ impact was investigated using anticrossing spectroscopy. The measured anticrossing spectra give evidence for the assumption that in the intermediate energy range, where the excitation cross-sections have resonance-like maxima, excitation proceeds via Paul-trap promotion. The electron to be excited gains energy by transiently riding during the collision on the saddle of the two-center potential of the (He+)2 molecular core.
29. A. Schüller, G. Adamov, S. Wethekam, K. Maass,
A. Mertens, and H. Winter
He and N atoms are scattered with keV energies under a grazing angle of incidence from clean and flat Ag(111) and Al(111) surfaces. For incidence along low index crystallographic directions in the surface plane, atomic projectiles are steered by rows of atoms ("axial surface channeling") giving rise to characteristic rainbows in their angular distribution. From the analysis of this effect we derive effective scattering potentials which reveal pronounced dynamical effects. We attribute our observation to the embedding energy for penetration of atoms in the electron gas of a metal.
30. H. Winter
The electronic interaction mechanisms during grazing scattering of hyper-thermal hydrogen atoms from a LiF(001) surface is studied via projectile-electron coincidences. This type of translation energy spectroscopy applied to atom-surface scattering allows one to investigate in detail the relevant electronic processes. It turns out that the transient formation of negative ions during the scattering process is the precursor for electronic excitations of the solid as well as for emission to vacuum. The specific nature of electron transfer in terms of a local capture event from the anion sites of the ionic crystal plays a decisive role in the efficient formation of negative ions. Our studies unravel the complete reaction paths during the scattering process and provide a consistent interpretation for the at first glance surprisingly large total electron emission yields for impact of atoms/ions on the surface of ionic crystals.
Structure and growth of monolayer and bilayer films of Mn and CoMn on
Cu(0 0 1) are studied by grazing scattering of fast He+ ions and protons.
A (quasi-)hexagonal arrangement is observed for the c(8 × 2) Mn
monolayer grown at low temperatures (<270 K). Capping of the film by
an additional Mn layer results in a complex, layer dependent film structure,
whereas capping with Co results in a pseudomorphic square film. At high
temperature, the structure of a c(2 × 2) Mn monolayer is in accordance
with a buckled CuMn surface alloy.