Structure and Dynamics of Heterogeneous Systems

The following sections are included:

• PREFACE

• CONTENTS

Quantum chemical ab initio cluster calculations were performed for the adsorption of small molecules on metal oxide surfaces. Two systems were studied in detail: The adsorption of N₂ on the (110) surface plane of TiO₂ (rutile) and the adsorption of CO on the polar (0001) surface of Cr₂O₃. In both cases a full five-dimensional potential for the interaction of a single molecule with the respective surface was calculated. For N₂/TiO₂ (110) the minimum was found for the end-on adsorption of N₂ atop a coordinately unsaturated surface Ti atom, with an adsorption energy of (35 ± 5) kJ/mol. In the case of CO/Cr₂O₃ (0001) the CO molecule is adsorbed strongly tilted (almost side-on) along a line connecting two Cr³⁺ ions at the surface; the calculated adsorption energy is 22 kJ/mol. In conjunction with empirical pair potentials for the N₂/N₂ and CO/CO interaction in the gas phase, Monte Carlo simulations were carried out to determine adsorption isotherms and the geometric structure of adsorbed monolayers.

The results of two ab initio molecular dynamics (AIMD) simulations of mediumsized organic molecules are described, one in the (electronic) ground state, the other in the excited state. Gradients obtained from non-empirical calculation of the electronic wave function are used to drive the motions of the nuclei by a numerical first-order approximation of Newton's equations. In the first simulation, the relaxation of a monomethine dye from the planar transition state to the helically twisted minimum energy geometry is studied, a reaction which involves only the electronic ground state of the molecule. In the second example, we follow the isomerization about the central double bond of the penta-3,5-dien-1-iminium molecular ion in an electronically excited state, a reaction which serves as a model for the initial step in the visual process of the vertebrate eye. Both simulations yield reaction trajectories that agree with classical expectations and with results previously obtained by others.

We report ab initio calculations of the ground and several excited states of protonated 11-cis-and all-trans-retinal Schiff base, the chromophores of the visual pigment rhodopsin and its photoproduct, bathorhodopsin, respectively. A supramolecule in which a formate anion and water have been attached to the Schiff base is described as a first attempt to model part of the complex environment of the chromophore. For highly coiled geometries of the retinals CD spectral data have been calculated. The comparison with experimental data suggests that a revision of the presently accepted structures may be necessary.

We present results of ab initio total energy calculations and molecular-dynamics simulations of dye molecules on the NaCl(100) surface. The investigations concentrate on the flat dye molecules trimethin, [C₁₉H₁₇N₂O₂]⁺, which form sandwich-like structures if closely packed, and the cyanine molecule monomethin, [C₂₁H₂₃N₂]⁺, which shows a typical stereochemical deformation due to two repulsive methyl groups. The molecular-dynamics simulations are able to reproduce the experimentally observed configurations of the charged dye molecules on surfaces.

The vacancy formation and migration energy of Al as well as the migration energies of Cu and Zn impurities were calculated at T = 0K using the Local-Density Approximation based on ultrasoft pseudopotentials. Diffusion processes were studied in supercells with 32 and 108 lattice sites. Furthermore the effect of Cu and Zn atoms on the activation energy for diffusion jumps of Al atoms in an ordered Al-Cu-Zn system was investigated. In each case relaxation of atoms around the defect was taken into account by relaxing all atoms in the supercell.

No abstract received.

We shortly review the surface processes relevant in epitaxy and discuss the conditions for the realization of different growth modes . We discuss the stability of 3-dimensional and 2-dimensional islands in terms of macroscopic thermodynamics and derive the critical island size . We introduce the kinetic equations governing epitaxy in the limit when island nucleation is the growth determining mechanism and discuss the important parameters. Finally, we discuss the action of surfactants, specifically of As, for homo-epitaxy and Ge hetero-epitaxy on Si(111). We present results of ab initio calculations for the rates of diffusion and incorporation of single Si, Ge, and Sn ad-atoms on As-terminated Si(111) and discuss growth models for these group-IV elements on Si(111):As.

The atomistics of metal on metal growth is studied using time resolved STM, SPALEED and kinetic Monte-Carlo simulations (KMC). A comparison of the growth behavior seen with STM and KMC is used to show the influence of the fundamental diffusion steps on the morphology of the growing surface in the homoepitaxial system Fe on Fe(110). Especially the influence of a Schwoebel-Ehrlich barrier and an anisotropy in edge diffusion are shown. Details of the transition from a pseudomorphic layer to the relaxed bulk like structure is shown in the examples Fe on W(110). The transition proceeds stepwise and extends over several layers. It involves different metastable arrangements of the layer depending on the deposition temperature.

We use a scanning tunneling microscope (STM) capable of imaging the growing layer during MBE-growth at high temperatures. This method (MBSTM) opens the possibility to follow MBE growth processes dynamically on the atomic scale and gives access to the evolution of specific features during growth. The influence of surface reconstructions on growth kinetics can be studied directly. For the case of growth of Si islands on Si(111) we find lateral growth of rows of the width of the 7×7 reconstruction unit cell at the edges of two-dimensional islands. This leads to a kinetic stabilization of magic island sizes. The evolution of size and shape of individual {105} faceted Ge islands (hut cluster) on Si(001) is measured during growth. A slower growth rate is observed when an island grows to larger sizes. This behavior can be explained by kinetically self-limiting growth.

Using low-temperature scanning tunnelling microscopy we have investigated confinement of electronic surface states to hexagonal islands and other nanoscale structures on Ag(111). Local spectroscopy and spatial maps of the differential conductance are analyzed using simple models of the electronic structure. Moreover, we studied electron scattering from isolated Ce ad-atoms on Ag(111). dI/dV tunnelling spectra reveal a characteristic antiresonance around the Fermi energy which is interpreted a Fano interference between delocalized Ag states and the localized Kondo resonance.

Using an empirical tight binding potential, we study the strain dependence of hopping and exchange diffusion barriers for Platinum on the unreconstructed Pt(100) surface. Also, adatom binding energies and Ehrlich-Schwoebel barriers are calculated as a function of substrate strain ∊. By analyzing the above quantities, we predict that for the growth of Pt on Pt(100) Layer by Layer growth should be improved under compressive strain.

This paper is an extension to a previous article by Scheffler and Wolfs.⁶ We study the rate of energy dissipation due to inelastic collisions in a charged granular gas. One observes that the electrostatic repulsion of two particles is effectively reduced by nearest neighbor interactions in a dense granular gas. We study the radial distribution function for dense systems, which leads to a better expression for the reduced energy barrier.

In this article kinetic Monte Carlo simulations for molecular beam epitaxy (MBE) and pulsed laser depositon (PLD) are compared. It will be shown that an optimal pattern conservation during MBE is achieved for a specific ratio of diffusion to deposition rate. Further on pulsed laser deposition is presented as an alternative way to control layer by layer growth. First results concerning the island density in the sub-monolayer regime are shown.

Semiconducting nanocrystallites like PbS exhibit electronic and optical properties greatly differing from those observed in the bulk material due to quantum size effects. By decreasing the diameter of PbS nanoparticles to about 1 nm the optical band gap increases significantly by a factor of 10 with respect to the bulk material. In this paper we investigate with ab initio methods the dependence of the optical band gap on the size of the system.

No abstract received.

This paper addresses the structural and thermodynamical properties of Ar₅₅ clusters. A description of thermodynamical and structural properties is given. Caloric curves for the micro-canonical and the canonical ensemble have been obtained by molecular dynamics simulations and histogram methods. The structure of the clusters is investigated by defining basins of minima on the potential energy surface with a disconnectivity tree. Further, an approximation of the system's partition function is discussed.

In this article we review the present understanding of the interrelation between the energetic stability of surface alloys and magnetism, magnetic ordering, the choice of substrate and surface orientation. We discuss trends for 3d-alloys on a Cu(100) surface and specifically present ab initio results for the formation energy and the interdiffusion energy of Mn surface alloy films on the Cu(100), (110) and (111) surface and on the Ag(100) and Ni(100) surfaces. Results based on the density-functional theory elucidate the origin of the existence of thermodynamically stable surface alloys. Although in detail, the results depend on the selected substrate/alloy combinations, the trends presented here have a universal character. Recently, several of these surface alloys have been found experimentally. The large structural corrugation of the surface alloy atoms and their enhanced magnetic moment contribute to the stabilization of the alloys. This will be discussed as a result of a large magneto-volume effect. Total energy and force calculations are carried out using the full–potential linearized augmented plane–wave (FLAPW) method in film geometry.

We report on the preparation of multilayered Co₈₀Fe₂₀(t)/Al₂O₃(30 Å) films by ion-beam sputtering, on their static magnetization, and on tunnel magnetoresistance (TMR) in the current-in-plane (CIP) and current-perpendicular-to-plane (CPP) geometries. It is found that Co₈₀Fe₂₀ layers are discontinuous in Al₂O₃ matrix at thicknesses t ≤ 18 Å. In the CIP case, the thickness t can be optimized to give maximum TMR (^~6.5 % for t = 10 Å) or maximum initial slope of TMR with field (^~24 %/kOe for t = 13 Å) at room temperature. Magnetization data show that ferromagnetism onsets at t ≥ 13 Å, indicating that magnetic percolation precedes electrical percolation in this system. The temperature dependence of TMR was found quite different for the two geometries: fairly strong in the CIP case and weak in the CPP case. To explain these features we propose a model taking into account the significant differences specifics of short-range magnetic correlations within and across the layers.

We have studied the structure of monolayers of iron on a Cu(001) substrate. By means of molecular-dynamics simulations in combination with embedded-atom potentials for the description of the interactions between the atoms, we have investigated the growth of bcc domains in the iron film with decreasing temperature. It turns out that the bcc domains become larger with decreasing temperature, but even at lowest temperatures there remains a significant amount of fcc iron. In contrast to this, cylindrical iron islands transform nearly completely to the bcc structure at low temperatures. This different behavior is discussed in the frame of experimental deposition techniques which strongly influences the final structure of the grown films.

Different coverages of Sn on Si(111) have been investigated by means of ab initio molecular dynamics simulations. The resulting structure, density of states and Schottky barrier have been determined for the relaxed configurations and are compared with experimental results.

We present the preparation of (001) oriented (Fe/Pd) multilayer films on sapphire substrates. The samples are grown on specifically prepared Pd buffer layers, which are pre-seeded with a thin Fe layer. Structural and topographical investigations utilizing in-situ RHEED and STM and ex-situ XRD clearly evidence epitaxial growth along the [001] direction throughout more than 30 Fe/Pd bilayers.

We have prepared thin Ni_xAl_{1 − x}, alloy films in the concentration range 0.56 ≤ x ≤ 0.70 by co-sputtering from element targets. Structural investigations utilizing TEM and XRD reveal that the films consist of ultra-fine grains with d_grain ≃ 10nm. Similar to bulk material the nano-grained Ni-Al samples undergo a structural phase change from (ordered) bcc to (ordered) fcc with increasing Ni content, which is, however, shifted towards higher Ni concentrations. From measurements of the electrical resistivity we find no indication for a martensitic phase transition for x = 0.635 and x = 0.65.

Microscopic and mesoscopic models have been applied in order to describe the deposition of thin films onto solid surfaces by Energetic Cluster Impact (ECI). On the atomic scale, Molecular Dynamic (MD) Simulations of the impact of a single cluster onto the surface have been carried out. These simulations reveal, that the local slope of the surface is diminished by a downhill particle current induced by the impact. Introduction of this downhill current into a continuum description of the development of the film profile leads to the Edwards-Wilkonson-Equation. The parameters of this equation can be estimated from the MD-results. The analytic solution of the Edwards-Wilkinson-Equation gives the development in time of the power spectrum and the roughness of the film. The results are in good agreement with Atomic-Force-Microscopy- measurements (AFM) on copper films deposited by ECI.

Structure and growth of epitaxial Sn films on InSb(001), Cu(001) and fcc-Fe/Cu(001) substrates were investigated by reflection high-energy electron diffraction (RHEED) and ¹¹⁹Sn Mössbauer spectroscopy. The Sn films grow epitaxially in the α-Sn(001) phase up to 1100 Å thickness on InSb, up to 5.5 ML on Cu(001), and up to 2.7 ML on fcc-Fe/Cu(001). Various surface reconstructions as a function of Sn coverage have been observed. The in-plane lattice parameter of the α-Sn overlayer was studied as a function of coverage.

A computational model of thermally activated magnetisation reversal in magnetic materials is described. The dynamic magnetic properties of the material are simulated using the Langevin formalism which is essentially the Landau-Lifshitz-Gilbert equation of motion augmented by a random thermal field whose properties are determined by the fluctuation-dissipation theorem. The model can include anisotropic crystalline, magnetostatic (dipole) and exchange interactions. The simulation is used to calculate pulsed-field magnetisation and remanence curves which are obtained by applying a field pulse to a system in the remanent magnetization state. These simulations show an increased non-local damping of the system when interactions are included which potentially explains the difference in effective damping constants between ferromagnetic resonance (FMR) and magnetisation reversal experiments. Application of the Langevin equation leads to a magnetic response in the form of correlated magnetisation fluctuations (spin waves).

No abstract received.

In this contribution we review the different strategies used to determine the magnetic anisotropy properties. For bulk transition metals partial satisfactory results have been obtained within ab-initio schemes. For low-dimensional system, semiempirical tight-binding Hamiltonian provides an efficient framework to get the differences of energy associated with different orientations of the magnetization, and to describe the orbital magnetism. Different “technical” difficulties encountered in both approaches are discussed, as the “irregular” behavior of the Magnetic Anisotropy Energy, in the k-space and in the real-space. In a near future, realspace ab-initio and semi-empirical methods should gain a larger interest. Another effort should be done on a fully ab-initio determination of the orbital moments.

We discuss briefly various measuring techniques and review the basic concepts of new micro-SQUID technique which allows one to study single nanometer-sized magnetic particles at low temperature. In the case of sufficiently small particles, the magnetization reversal occurs by uniform rotation and the measurement of the angular dependence of the magnetization reversal yields the effective magnetic anisotropy. The influence of the temperature on the magnetization reversal is discussed. Probabilities of switching, switching field distributions, and telegraph noise measurements are proposed to check the predictions of the Néel-Brown theory describing thermal activated magnetization reversal. At very low temperature, we will see how quantum effects can be revealed.

We study a classical, anisotropic Heisenberg antiferromagnet in a field by Monte Carlo methods. We calculate the transverse and longitudinal antiferromagnetic order parameters as function of temperature and axial magnetic field in order to obtain the phase diagram in the field-temperature plane consisting of antiferromagnetic-, spin-flop-, and saturated paramagnetic phase. For zero temperature the critical fields can be calculated analytically. Crossing the spin flop phase by varying the temperature, the magnetic specific heat shows two peaks.

A three-dimensional layered Ising-Antiferromagnet with a ferromagnetic intra-layer coupling to z neighbors, zJ > 0, and an antiferromagnetic interlayer coupling to z′ neighbors, z′J′ < 0, is investigated by Monte Carlo simulations on a hexagonal lattice. The physical nature of the anomalous temperature bahavior of the sublattice magnetizations, which is found for certain values of r = zJ/z′J′ and z′ in magnetic fields is explained in terms of successive phase transitions. They take place on the ferromagnetic 2-dimensional spin-down sublattice at , smeared by a finite stabilizing molecular field, and on both antiferromagnetically coupled sublattices at .

We investigate the thermally activated magnetization switching in a classical Heisenberg spin chain driven by an external magnetic field. For small system sizes we expect that the magnetic moments rotate coherently, while in the case of larger system sizes the magnetization reversal is proposed to be due to soliton-antisoliton nucleation. We compare Monte Carlo simulations with the direct integration of the Landau-Lifshitz-Gilbert equation of motion with Langevin dynamics as well as with asymptotic solutions for the escape rates following from the Fokker-Planck equation, finding agreement for low temperatures and high damping. We also discuss deviations in the intermediate temperature regime.

We analyze interface motion in the random field Ising model with quenched disorder by means of Monte Carlo simulations. In the absence of thermal fluctuations a depinning transition occurs at some critical threshold field. We study the interface motion in the vicinity of the threshold field by varying the temperature as well as the driving field. It turns out that thermal fluctuations yield a rounded transition which can be characterized by a critical exponent.