Morphology of monatomic step edges on vicinal Si(001)

The roughness of monatomic A- and B-type step edges on 0.5° misoriented Si(001) has been analyzed on an atomic scale with scanning tunneling microscopy. On small length scales, measured along the step edge (<200 Å), one-dimensional random-walk behavior is observed for both types of step edges. For the rough B-type step edge we also found evidence for waviness of the edge. The period of this wave is about 100 dimer-row spacings (≊750–800 Å). The energetic step-step interaction and entropic repulsion, which both scale as 1/L2 (L is the average terrace length), are estimated to be about 0.03–0.06 and 0.2 meV per dimer-row spacing, respectively, for a 0.5° misoriented surface. These interactions are approximately three orders of magnitude smaller compared to the kink formation energies which are 0.1–0.2 eV. Despite the weak strength of energetic and entropic step-step interactions, these long-range interactions have a profound effect on the step-edge morphology, e.g., the distribution of terrace lengths and the long-range waving of the rough B-step edge

OXYGEN CONCENTRATION OF Eu1 Ba2 Cu3 O7-x IN VACUUM : AN ATOM PROBE STUDY II

Atom Probe mass analysis using a wide acceptance angle instrument was used to measure the oxygen content and metallic stoichiometry of the near-surface region of the superconducting ceramic oxide Eu1 Ba2 Cu3 O7-x (x≈0.1) after exposure at 85K and room temperature in vacuum. An oxygen depleted layer formed by H2 imaging must be removed before bulk concentrations are obtained. Room temperature holding in vacuum overnight (≈18h) then depletes the surface of oxygen to a depth of greater than 4 layers (1.2nm). However, after holding the specimen at 85K for up to 3h either with or without an applied field, no detectable loss of oxygen occurred. Therefore, for short time vacuum exposures at liquid nitrogen temperatures and below, no oxygen loss is expected, however, significant oxygen loss occurs for 18h vacuum exposures at room temperature

ON THE INTERPRETATION OF FIELD ION MICROSCOPY (FIM) IMAGES OF ASYMMETRICAL SPECIMENS OF "1,2,3"-TYPE HIGH-Tc SUPERCONDUCTORS

Field Ion Microscopy (FIM) specimens of "1,2,3"-type high-Tc superconducting materials, typically have an asymmetrical shape. This results from the anisotropic properties, which are involved in the preparation techniques. As a consequence of this shape, the magnification and resolution depend on the direction in the FIM image. Assuming that the elliptical poles in published micrographs are due to tip shape, we calculate an asymmetry in the magnification as large as a factor of four. Therefore, the current image interpretation is revisited quantitatively, keeping in mind the magnification asymmetry. This strong effect enhances the striped appearance of the images, and has not been fully recognized so far

Fluctuations of monatomic steps on Si(001)

The motion of monatomic steps on Si(001) is studied on an atomic scale at elevated temperatures with scanning tunneling microscopy. The kinks in the step edges move in units of two dimers along the monatomic A-type step edge and perpendicular to the monatomic B-type step edge. The overall time dependencies of the equilibrium step fluctuations of A- and B-type step edges were found to be both proportional to t0.6±0.1. The fluctuations of long kinks in the B-type step edge are, however, much larger and exhibit initially a linear t dependence, i.e., one-dimensional random-walk behavior. Both time dependencies can be understood in terms of the Langevin equation

FIELD ION MICROSCOPE IMAGE SIMULATIONS FOR ICOSAHEDRAL Al-Mn

New computer simulations of FIM images for the icosahedral phase of Al-Mn, using the Moore-Ranganathan-shell method, are presented and compared to the experimentally observed images. The closest agreement found thus far is for a cubic model having a 3.32 nm unit cell edge and containing an icosahedral inner atomic motif, with all Mn and only some of the Al atoms included in the simulated images. (An octahedral-motif decoration of a perfect 3-D Penrose model was also tested for comparison.) This surprising result can be understood as the result of inner-cell motif domination over the parent cubic features, due to the very large unit cell size

Field-ion-microscopy contradiction of the quasicrystal model based on twinning of a cubic crystal

Comparison of computer simulations of field-ion images with experiments clearly shows that twinning of a cubic crystal is not compatible with experimental field-ion microscopic observations of quasicrystals.

THE ATOMIC STRUCTURE OF THE ICOSAHEDRAL PHASE PROBED BY FIELD ION MICROSCOPY COMPARED WITH COMPUTER SIMULATIONS

Field ion images have been simulated of fifteen different decorations of the rhombohedral cells of the three dimensional Penrose packing in order to achieve a best fit with FIM experiment. The decorations having the correct number of atoms per unit cell result in strong fivefold poles in the FIM image. In contrast, the experimental images show two- and threefold poles stronger than fivefold ones