Termination of Single Crystal Bi2Se3 Surfaces Prepared by Various Methods

Bismuth Selenide (Bi2Se3) is a topological insulator with a two-dimensional layered structure that enables clean and well-ordered surfaces to be prepared by cleaving. Although some studies have demonstrated that the cleaved surface is terminated with Se, as expected from the bulk crystal structure, other reports have indicated either a Bi- or mixed-termination. Low energy ion scattering (LEIS), low energy electron diffraction (LEED) and x-ray photoelectron spectroscopy (XPS) are used here to compare surfaces prepared by ex situ cleaving, in situ cleaving, and ion bombardment and annealing (IBA) in ultra-high vacuum (UHV). Surfaces prepared by in situ cleaving and IBA are well ordered and Se-terminated. Ex situ cleaved samples could be either Se-terminated or Bi-rich, are less well ordered and have adsorbed contaminants. This suggests that a chemical reaction involving atmospheric contaminants, which may preferentially adsorb at surface defects, could contribute to the non-reproducibility of the termination.Comment: 31 pages, 10 figures, 1 tabl

Surface Structure of In Situ Cleaved Single Crystal Bi2Se3 Measured by Low Energy Ion Scattering

Bismuth Selenide is a two-dimensional topological insulator material composed of stacked quintuple layers (QL). The layers are held together by a weak van der Waals force that enables surface preparation by cleaving. Low energy ion scattering experiments (LEIS) show that Bi2Se3 cleaved under ultra-high vacuum (UHV) has a Se-terminated structure that is consistent with cleaving between QLs. Comparison of experimental data to molecular dynamics simulations confirms the Se-termination and provides an estimate of the surface relaxation.Comment: 23 pages, 6 figures, 1 tabl

Characteristics of dividing and combining flows

In the present thesis, some aspects of dividing and combining flows are studied. The first part of the study considers the discharge characteristics of flow past a two-dimensional lateral slot located in a rectangular conduit. Existing theoretical solutions to this problem are based on free streamline theory. The test results provide data to verify the predictions of the theoretical model related to the dependence of the slot discharge coefficient on discharge ratios and the ratio of the kinetic energy to the total energy of the flow approaching the slot. The test results validate the model predictions. A theoretical expression for the outflow through a rectangular lateral weir located in a circular open channel is derived. The theoretical weir discharge coefficient is expressed as a function of the parameters relating the geometry of the weir and the channel, and a velocity parameter that is a function of the dimensionless flow depth, the weir sill height and the approach Froude number. Experimental results are presented to verify the proposed theoretical model. The characteristics of dividing flows in closed rectangular conduits are studied in the next section. The contraction coefficient and the loss coefficients of branching flows are determined using results of an existing model dealing with a two-dimensional lateral outlet fitted with an external barrier. For dividing closed conduit flows, two different procedures are used to obtain the variation of an energy loss coefficient and the contraction coefficient with the discharge ratio. Detailed velocity and pressure distribution profiles are presented to describe the flow processes. In the final part of the thesis, the characteristics of combining flows past 90\sp\circ junctions of a rectangular closed conduit are presented. Detailed pressure and velocity distribution data are obtained to understand the flow processes. Both the mean and fluctuating components of the velocity field are determined in the two flow sections immediately downstream of the junction. Simple empirical models are developed to determine the transfer of momentum and the contraction coefficient. Experimental data are used to verify the predicted results of the proposed model