Silicon Vacancy: A Possible "Anderson Negative-U" System

This Letter analyzes the electron states associated with the silicon vacancy and proposes a distortion model for the surrounding atoms that is derived from self-consistent calculations for single-particle states and from model calculations of elastic restoring forces. The predicted level structure is such that $V^{+}$ is, contrary to currently accepted ideas, a metastable state which decays either to $V^0$ or to $V^{++}$, depending on the Fermi energy.