THE LOST SYMMETRY OF GE(111)-C(2X8)

Using the scanning tunneling microscope in the constant current topography mode, we observe, for different bias voltages, that not only the two rest-atoms but also the two adatoms in the unit cell of the Ge(111)-c(2 x 8) reconstruction are not equivalent. Furthermore, earlier assumed mirror symmetry in the unit cell is lost in agreement with recent calculations [N. Takeuchi el al., Phy. Rev. Lett. 69, 648 (1992)]. The asymmetry is much more pronounced in the local density of electronic states than in the real shift of atomic coordinates. By increasing the tunneling current up to 40 nA, asymmetric features are strongly enhanced to the extent that observed protrusions can hardly be associated with atomic positions.