E. Wimmer

THEORY OF SURFACE ELECTRONIC STRUCTURE.

Publisher Summary This chapter describes a newly developed, highly accurate, and unified method for calculating surfaces, the full-potential linearized augmented plane wave (FLAPW) method. This is a unified method in that it can easily treat not only simple metals and semiconductors but also transition-metal surfaces. It is also unified in the sense that it is capable of treating molecular absorption on surfaces and also the extreme limit of the isolated molecule and the clean surface. In this approach, which represents a major advance in that the local density equations are solved without any shape approximations to the electronic potential or electronic charge density, a new technique for solving Poissons equation for a general charge density and potential has been implemented. The chapter discusses the general theoretical framework for the FLAPW calculations: the thin-slab structural model for surfaces and the local-(spin)-density-functional approximation and the limitations of each of these. It presents the FLAPW methodology a selection of applications with examples that illustrate its applicability to a variety of surface and interface phenomena.

Total energy all-electron theory of surface structural, electronic, and magnetic properties

In the last decade, intense experimental efforts using advanced techniques for sample preparation and characterization have provided a vast quantity of data. These developments have challenged present theoretical understanding and have encouraged the development of theoretical methods interpreting the phenomena observed. Our total energy all‐electron local density theoretical approach, implemented as the full potential linearized augmented plane wave (FLAPW) method, is described and shown to have high precision (to 10−9 in the total energy) and stability for describing the structural, electronic, and magnetic properties of (i) free surfaces [including surface relaxation and reconstruction, e.g., W(001)], (ii) interface phenomena in Au/Cr/Au(001) sandwiches [e.g., prediction of the enhanced magnetic moment on the interface Cr site], (iii) catalytic promotion and poisoning of molecular dissociation on surfaces [e.g., CO + K or S/Ni(001)]. The three examples just cited illustrate the present predictive power...

Magnetism at surfaces and interfaces

Abstract The current state-of-the-art of ab-initio calculations of the magnetic structures of surfaces and interfaces is highlighted by presenting results obtained with the recently developed full-potential linearized augmented plane wave method for thin films. In particular, spin density maps, (induced) magnetic moments and hyperfine-fields are presented for the clean metal surfaces Fe(001), Ni(001) and Pt(001). The magnetic moments on an interface are discussed for the prototypical case Ni/Cu.

Generation of H−, D− ions on composite surfaces with application to surface/plasma ion source systems

We review some salient features of the experimental and theoretical data pertaining to hydrogen negative ion generation on minimum work function composite surfaces consisting of Cs/transition metal substrates. Cesium or hydrogen ion bombardment of a cesium‐activated negatively biased electrode exposed to a cesium–hydrogen discharge results in the release of hydrogen negative ions. These ions originate through desorption of hydrogen particles by incident cesium ions, desorption by incident hydrogen ions, and by backscattering of incident hydrogen. Each process is characterized by a specific energy and angular distribution. The calculation of ion formation in the crystal selvage region is discussed for different approximations to the surface potential. An ab initio, all‐electron, local density functional model for the composite surface electronics is discussed.

Electronic structure and magnetism of surfaces, interfaces and modulated structures (superlattices)

Recent developments in the study of surfaces and interfaces of metals and of artificial materials such as bimetallic sandwiches and modulated structures are described. Key questions include the effects on magnetism of reduced dimensionality and the possibility of magnetically “dead” layers. These developments have stimulated an intensified theoretical effort to investigate and describe the electronic and magnetic structure of surfaces and interfaces. One notable success has been the development of a highly accurate full-potential all-electron method (the FLAPW method) for solving the local spin density equations self-consistently for a single slab geometry. We describe here this advanced state of ab initio calculations in determining the magnetic properties of transition metal surfaces such as those of the ferromagnetic metals Ni(001) and Fe(001) and the Ni/Cu(001) interface. For both clean Fe and Ni(001) we find an enhancement of the magnetic moments in the surface layer. The magnetism of surface and interface Ni layers on Cu(001) (no “dead” layers are found) is described and compared to the clean Ni(001) results. Finally, the role ofμSR experiments in answering some of the questions raised in these studies will be discussed.

Cs/transition metal composite surfaces: First principles calculations of high Z, low work function systems

Some salient features of the experimental and theoretical data pertaining to hydrogen negative ion generation on minimum‐work‐function composite surfaces consisting of Cs/transition metal substrates are reviewed. Cesium or hydrogen ion bombardment of a cesium‐activated negatively‐biased electrode exposed to a cesium‐hydrogen discharge results in the release of hydrogen negative ions. These ions originate through desorption of hydrogen particles by incident cesium ions, desorption by incident hydrogen ions, and by backscattering of incident hydrogen. Each process is characterized by a specific energy and angular distribution. The calculation of ion formation in the crystal selvage region is discussed for different approximations to the surface potential. Results of ab initio, all‐electron, local density function calculations for the composite surface electronics of Cs on W(001) and Mo(001) are presented and discussed.

Local Density Approach to Surfaces and Adsorbed Layers

We show that the local density problem for the thin film geometry can be solved with high accuracy by employing our all-electron full-potential linearized augmented-plane-wave (FLAPW) method. This is achieved by removing all shape approximations in the charge density and the potential and by using a highly flexible variational basis set. Its utility even for treating molecules on surfaces is demonstrated for the severe test case of the (nearly) free O2 molecule where the FLAPW method is found to be at least as accurate as state-of-the art localized-basis molecular methods. The high accuracy and precision of the FLAPW approach for metallic surfaces is shown by our finding that for the interpretation of surface sensitive photoemission measurements of the Al-2p core levels both a surface induced core level shift of 120 meV to reduced binding energies and a 38 meV crystal field splitting are important. Based on all-electron local density calculations we explain the lowering of the work function of a transition metal surface [W(001)] upon deposition of a Cs over-layer by a multiple dipole formation. Finally, we demonstrate for a graphite monolayer that local density total energies give excellent descriptions of equilibrium geometries and discuss the overestimation of local-density cohesive energies due to an incomplete treatment of correlation effects in the free atom.