6 - Elementary excitations

Summary

Introduction

The preceding chapters have focused entirely on the equilibrium free energy state of an isolated clean crystal surface. Unfortunately, many of the most interesting conceptual (and commercial) issues in surface physics intimately involve the interaction of a solid surface with foreign matter. If the interaction is strong, it is necessary to treat the surface and the foreign material as a single combined system. This is the subject of Part 2 of this book. However, if the interaction is weak, the surface merely responds to the external perturbation while retaining its individual identity. In fact, any real experiment designed to probe the properties of even an isolated surface invariably perturbs the system and invokes a characteristic response. This response is determined by the low-lying excited states of the system.

For example, consider an experiment designed to determine the binding energy and dispersion of an electronic surface state. In practice, one uses photoemission spectroscopy to measure the kinetic energy and propagation vector of an electron ejected from the sample into the vacuum. However, what one actually measures is the energy and relative momentum of an excited electronic state (with a finite lifetime) that consists of 10²³ electrons in the presence of the surface-localized hole left behind by the photoelectric event. A priori, this could be a horribly complex state of the interacting many-body system.