Evidence for correlation effects in the hyperfine structure of the negative vacancy in silicon

Recently published experimental data on the hyperfine structure of the negative vacancy in silicon are discussed. They contradict the prediction of one-electron theories that the contact terms of atoms lying in certain symmetry planes should vanish. A first possible explanation consisting of an inverted order of the one-electron levels is shown to require a much-too-large distortion coupling. The influence of many-electron effects is then worked out in some detail and shown to provide a correct order of magnitude for these contact terms when using parameters determined previously for $V^0$. This demonstrates (i) that a coherent picture of many-electron effects is obtained for the different charge states of the vacancy, (ii) that many-electron effects have little influence on the ground state, confirming Watkins's view that the Jahn-Teller effect dominates this state, and (iii) that the many-electron effects are indeed at the origin of these small nonvanishing contact terms.