Nature of the Low-Temperature3×3Surface Phase of Pb/Ge(111)

The structure of the low-temperature $3\times 3$ surface phase of Pb/Ge(111) has been determined by surface x-ray diffraction experiments. Analysis of the data reveals that the Pb layer exhibits a vertical corrugation of $\sim 0.4Å$, affecting also the two top Ge layers. Significant distortions are found down to the third Ge layer. This structural model supports that the $3\times 3$ ground state can be best described as a cooperative distortion stabilized by a gain in electronic energy.