The electronic structure of solids studied using angle resolved photoemission spectroscopy

Abstract

In the past two decades, photoelectron spectroscopy has emerged as an indispensable tool for studying the electronic properties of solids and their surfaces. 1'2 This is because it provides the most direct and complete method of measuring all the occupied electronic states in a solid, both core and valence. This review focuses upon angle-resolved photoemission (ARP), 35 a variant which has proven particularly useful in studying the valence electronic structure of crystalline solids since it provides the only method of directly measuring valence band dispersion relations. For a student of solid state physics ARP is often quite captivating, since it brings to life the simple one-electron models for the electronic structure of crystals. The aficionados stay involved because of the breadth of possible applications and the multitude of interesting physical problems to which ARP can be applied. ARP provides unique information about the occupied electronic states and bonding in solids. The application of ARP to understand electronic structure and its impact upon bonding is at the heart of this review. We have endeavored to write a topic-oriented rather than a technique-oriented review. Apart from a brief tutorial in the next few pages which covers enough basics to get started, the phenomenology of ARP is introduced slowly and by way of example as the need arises. In this way, the application of the technique to various problems can be easily understood. Our review is by no means complete; a more exhaustive survey will soon appear. 6 Rather, we want to discuss the application of ARP to a few current problems in solid state physics and chemistry which will continue to be important in the future: the interplay between localized and delocalized electronic phenomena, 7'8 the influence of electron correlation on one-electron theory, and the interactions between high energy and low energy phenomena in solids, to name a few. A closely related technique, angle-resolved inverse photoemission, provides complementary information about the unoccupied electronic manifold. Inverse photoemission has been adequately reviewed recently, 9'10 and we make no systematic attempt to include discussion of it here. ARP can be applied to a large variety of systems, including adsorbate or thin film covered metals and semiconductors (and even insulators in some cases). It provides information most easily about two-dimensional (2D) or quasi-two-dimensional electronic states localized at surfaces or associated with ultrathin films, but it has also been successfully applied to many bulk, three dimensional (3D) systems. Our own prejudice toward 2D surface states may be apparent from the choice of material to be included, but we have discussed several 3D systems as well. We have minimized the discussion of chemisorption or adsorbate systems except when the phenomena involved are germane to our subject. The review addresses these issues in the following fashion. In the remainder of this section, we review the