Periodic lattice distortion accompanying the (3x3) 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 alpha phase of Sn on Ge(111). Scanning tunneling microscopy observations of the CDW show a similar to 0.5 Angstrom vertical charge corrugation in the image of the Sn atoms, but the measured vertical ripple in the PLD is almost zero, similar to 0.04 +/- 0.04 Angstrom. The PLD occurs almost exclusively in the outermost Ge layer, where a 0.22-Angstrom 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. [S0163-1829(98)01708-1].