Symmetry breaking and atomic displacements in the 3×3 surface phase of Pb/Ge(111)

The atomic structure of the low-temperature 3×3 surface phase of Pb/Ge(111) has been analyzed using the surface X-ray diffraction technique. The atomic displacements found in this phase give rise to a structure distorted both in the surface plane and in the vertical direction. This structure is compared with the 3×3R30° parent phase found at room temperature. Several of the symmetry elements present in the ideal, flat 3×3R30° phase, are absent in the 3×3 phase. The origin and extent of the symmetry breaking at low temperature is discussed in detail.