Extended interstitials in silicon and germanium

Using different classical interaction models for Si and Ge, we study self-interstitial structures. Besides the usually studied well-localized configurations we find new configurations in the densely packed planes. For the Stillinger-Weber model these are the energetically most favored ones. Some of these configurations exhibit quasilocalized vibrations with low frequencies. We expect these or similar defect structures to play an important role in amorphization by irradiation.