Nature of the low-temperature 3 x 3 surface phase of Pb/Ge(111)

The structure of the low-temperature 3 X 3 surface phase of Pb/Ge(111) has been determined by surface x-ray diffraction experiments. Analysis of the d ata reveals that the Pb layer exhibits a vertical corrugation of similar to 0.4 Angstrom, affecting also the two top Ge layers. Significant distortion s are found down to the third Ge layer. This structural model supports that the 3 X 3 ground state can be best described as a cooperative distortion s tabilized by a gain in electronic energy. [S0031-9007(99)08749-9].