Core-level Spectroscopy in Condensed Systems describes how recent improvement of various experimental methods, together with new light and x-ray sources, have provided fresh information about the electronic states and atomic structures of a wide variety of materials. The topics coveredrange from the high-energy spectroscopy of bulk electronic states of rare-earth and transition metals and compounds, including high T superconductors, to recent developments in photoelectron diffraction and other surface problems, all with emphasis on theoretical aspects.

Core level spectroscopy has become a powerful tool in the study of electronic states in solids. From fundamental aspects to the most recent developments, Core Level Spectroscopy of Solids presents the theoretical calculations, experimental data, and underlying physics of x-ray photoemission spectroscopy (XPS), x-ray absorption spectroscopy (XAS), x

For several years, core level spectroscopies and other, c\osely related, electron spectroscopies have provided very useful information about the atomic composition, the geometric structure, and the electronic structure of condensed matter. Recently, these spectroscopies have also been used for the study of magnetic properties; such studies have a great potential to extend our knowledge and understanding of magnetic systems. This volume collects the lectures presented at the NATO Advanced Study Institute on "Core Level Spectroscopies for Magnetic Phenomena: Theory and Experiment" held at the Ettore Majorana Centre, Erice, Sicily, on 15 to 26 May 1994. The topics considered at the ASI covered a wide range of subjects involving the use of core-level and related spectroscopies to study magnetic phenomena. There are a large and growing number of applications of these spectroscopies to the study of magnetic materials; an important objective of the ASI was to stimulate further growth. The topics covered at the ASI can be placed into three general groups: 1) fundamental principles of core level spectroscopies; 2) basic aspects of magnetic phenomena; and, 3) the combination of the two previous topics embodied in applications of the spectroscopies to magnetism. In all three groups, theoretical interpretations as weH as experimental measurements were presented, often both of these aspects were covered in a single lecture or series oflectures. The theoretical treatments ofthe spectroscopies as weH as of the magnetic phenomena help to establish a framework for understanding many of the experimental measurements on magnetic materials.

The experimental discovery of the fractional quantum Hall effect (FQHE) at the end of 1981 by Tsui, Stormer and Gossard was absolutely unexpected since, at this time, no theoretical work existed that could predict new struc tures in the magnetotransport coefficients under conditions representing the extreme quantum limit. It is more than thirty years since investigations of bulk semiconductors in very strong magnetic fields were begun. Under these conditions, only the lowest Landau level is occupied and the theory predicted a monotonic variation of the resistivity with increasing magnetic field, depending sensitively on the scattering mechanism. However, the ex perimental data could not be analyzed accurately since magnetic freeze-out effects and the transitions from a degenerate to a nondegenerate system complicated the interpretation of the data. For a two-dimensional electron the positive background charge is well separated from the two gas, where dimensional system, magnetic freeze-out effects are barely visible and an analysis of the data in the extreme quantum limit seems to be easier. First measurements in this magnetic field region on silicon field-effect transistors were not successful because the disorder in these devices was so large that all electrons in the lowest Landau level were localized. Consequently, models of a spin glass and finally of a Wigner solid were developed and much effort was put into developing the technology for improving the quality of semi conductor materials and devices, especially in the field of two-dimensional electron systems.

Introduction to Solid-State Theory is a textbook for graduate students of physics and materials science. It also provides the theoretical background needed by physicists doing research in pure solid-state physics and its applications to electrical engineering. The fundamentals of solid-state theory are based on a description by delocalized and localized states and - within the concept of delocalized states - by elementary excitations. The development of solid-state theory within the last ten years has shown that by a systematic introduction of these concepts, large parts of the theory can be described in a unified way. This form of description gives a "pictorial" formulation of many elementary processes in solids, which facilitates their understanding.