EXAFS of Disordered Systems

Abstract

The basic theoretical foundation for condensed matter physics is well known. The Schrödinger equation of electrons and nuclei with the Coulomb interaction and relativistic corrections such as the spin-orbit interaction contains all of the basic physics. The complication in applying this basic knowledge is the large number of particles involved, of the order of 10²³. To reduce this problem to a manageable size, simplifying assumptions or approximations are required. Usually, the initial assumption is the structure of the matter. For example, the knowledge that crystalline solids have a periodic array of atoms permits the exploitation of the periodicity to reduce the number of particles from 10²³down to the order of 1, namely, the ion cores and the valence electrons in a unit cell. In disordered systems the structure is more complicated since the long-range periodicity is absent, but knowledge of structure is still a basic requirement for any detailed understanding of their properties. For crystalline samples, diffraction, using sources with wavelengths of the order of 1Å, is the extremely productive technique by which to determine the long-range order. Sources typically used for diffraction are x-rays, neutrons, and electrons.