9319246 Haydock This is a proposal based generically on the elctronic structure of disordered materials; the disorder may be glassy, it may be in the form of a disordered overlayer or it may simply be the presence of a surface. Tight binding techniques will be used to examine these systems implemented primarily via the recursion method. ( 1) In glassy systems like amorphous graphitic carbon, the validity of an approximately conserved momentum-like quantum number, resulting from the preservation of local real-space order, will be studied. (2) The reason why metal atoms randomly distributed on a semiconductor surface produce metallic contacts at low surface coverage (and the degree of localization with coverage) will be studied. (3) The linear and non-linear collective response of electrons to external fields (both above and below the plasmon threshold) at metal surfaces will be studied to understand the anisotropy of second harmonic generation. %%% This is a numerically intensive proposal to study several aspects of disorder in solids. The disorder may be at the level of glassy 3D structure; at the level of randomly placed atoms on an otherwise ordered crystalline semiconductor substrate; or even at the level of the presence of a surface in an crystal. In the first case, the project looks for certain spectroscopic invariants due to the glassy structure. In the second case, the work involves explaining why metal atoms deposited at low coverages on a semiconductor surface give rise to conductivity. And in the third case, the work tries to understand why systems which we expect to be very isotropic have a very anisotropic response when their surfaces scatter externally impinging laser light. ***
