Periodic lattice distortion accompanying the(3×3)charge-density-wave phase of Sn/Ge(111)

Surface x-ray diffraction has been used to determine the periodic lattice distortion (PLD) accompanying the charge-density-wave (CDW) formation in the α phase of Sn on Ge(111). Scanning tunneling microscopy observations of the CDW show a $\sim 0.5-\AA$ vertical charge corrugation in the image of the Sn atoms, but the measured vertical ripple in the PLD is almost zero, $\sim 0.04\pm 0.04\AA .$ The PLD occurs almost exclusively in the outermost Ge layer, where a 0.22-Å lateral motion of the three Ge atoms is associated with the Sn atom with largest amplitude in the CDW. Surprisingly, Sn ion cores move in the opposite direction from valence electrons. These results will be discussed in light of the recent models put forward to explain the CDW formation.