J. Alvarez

The structure of the Ge(001)-(2 × 1) reconstruction investigated with X-ray diffraction

Abstract The atomic structure of the room temperature (2 × 1) reconstruction of Ge(001) has been investigated with X-ray diffraction measurements by collecting an extensive and accurate set of diffracted intensities. The structural model that best agrees with the data consists of a buckled array of disordered dimers: there is a 0.5 probability of finding one of the two dimer orientations (positive and negative tilt angles) in any unit cell. The dimer tilt angle is found to be 15.6 ± 0.6°, its bond length is expanded by 4% compared to the bulk bond length, the reconstruction is found to extend to eight atomic layers and the maximum distortion of the bonds has been found to be less than 6%. The model is compared with similar models, and the dynamical and ordered models and the dynamics of the dimer flipping are discussed.

Vibrational Anisotropy of a CO Monolayer on Ni(110)

We have determined the structure of CO/Ni(110) with high accuracy using surface X-ray diffraction. CO molecules are bound in short-bridge sites, tilted alternately to form a lattice with p2mg symmetry. The atomic positions agree well with previous LEED results, but the increased sensitivity permits accurate anisotropic vibrational parameters to be determined as well. We find vibrations which are substantially larger parallel to the surface plane than perpendicular, implying the existence of a low-frequency vibrational mode not seen in an earlier EELS study.

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

Abstract 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 × 3 R 30° parent phase found at room temperature. Several of the symmetry elements present in the ideal, flat 3 × 3 R 30° phase, are absent in the 3×3 phase. The origin and extent of the symmetry breaking at low temperature is discussed in detail.