Browsing by Subject "scanning tunneling microscopy"

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Open Access
Adsorption site of alkali metal overlayers on Si(001) 2 × 1
(1992) Batra, I. P.; Çıracı, Salim
The alkali metal semiconductor interfaces are currently being investigated by a variety of tools. Most studies to date at half a monolayer coverage have shown preference for either a quasi-hexagonal (H) site or a long-bridge (B) site. At this coverage one-dimensional chain structure for K on Si(001) 2 × 1 have now been confirmed by scanning tunneling microscopy (STM). The data, however, is consistent with either of the two sites. STM investigations at low coverages suggested that alkali metals like K and Cs occupy a novel site, Y, which is a bridge site between two Si atoms belonging to different dimers along the dimer row [110] direction. The total energy calculations for this new Y site, discovered by STM, have shown that it is indeed a site of (local) energy minimum. The ability of the surface silicon atoms, which are not adjacent to the alkali metal atom, to buckle makes the Y site a competitive adsorption site. We deduce the nature of bonding between alkali metals and Si using the STM data. It is concluded that the bond is substantially ionic in nature. © 1992.
Open Access
Atomic-scale tip-sample interactions and contact phenomena
(1992) Çıracı, Salim
Tip-sample interactions become crucial owing to increased overlap at small tip-sample separation. The potential barrier collapses before the point of maximum attraction on the apex of the tip, but the effective barrier may remain significant owing to the strong confinement of current-carrying states to the constriction between tip and sample. At such separations the perpendicular tip force is still attractive and determined by ion-ion repulsion and redistribution of electronic charge. Electronic states are modified by the tip-induced perturbation of the potential in the vicinity of the tip. Self-consistent calculations reveal that local properties, such as elastic deformation, effective height and width of the tunneling barrier, electronic states and attractive tip force are site-dependent and reversible on the atomic scale. Numerical results suggest a relation between the perpendicular tip force and barrier height as a function of separation. A mechanical contact is formed with relatively strong bonds at separation near the point of zero force gradient. Whether the effective potential can collapse and hence the first channel can open to allow a transition from tunneling to ballistic conduction, and whether the conductance can show quantized steplike changes with increasing plastic deformation depends on material properties. © 1992.
Open Access
Ballistic transport and tunneling in small systems
(1990) Tekman, A Erkan
Ballistic transport and tunneling of electrons in mesoscopic systems have become one of the most important subjects of condensed matter physics. The quantum point contacts and scanning tunneling microscope form the basic experimental tools in this area and have been used for understanding many features of small systems. In this work ballistic transport and tunneling in small systems are investigated theoretically. Ballistic transport through narrow constrictions is investigated for a variety of configurations. It is found that for a uniform constriction the conductance is quantized in units of the quantum of conductance (2e^/A) for long channels. The interference of waves in the constriction gives rise to the resonance structure superimposed on the quantized steps. The lack of the resonance structure in the experimental results are attributed to temperature effects and/or adiabatic transport due to tapering of the constriction. It is shown that elastic scattering by an impurity distorts the quantization of conductance. Novel resonant tunneling effects due to formation of bound states are predicted for an attractive impurity or a local widening at the center of the constriction. It is shown that the probing in scanning tunneling microscopy have very much in common with narrow constrictions. The transition from tunneling to point contact regime is explained by the vanishing effective potential barrier as a result of tip-sample interaction. For noble and simple metals it is conjectured that lateral position dependent interaction between the tip and sample leads to corrugation of the potential barrier and in turn to atomic corrugation observed by scanning tunneling microscopy. The focused field emission of electrons from point sources is analyzed in a systematical way. The effective barrier due to the lateral confinement and nonadiabatic transport through the horn-like opening are found to be responsible for focusing. The nonequilibrium nature of transport is investigated by use of Keldysh Green’s function technique. The effects of elastic and inelastic scattering are analyzed in a strictly one-dimensional geometry. The features of voltage and current probes are studied and the Landauer formulae are examined for multiprobe measurements.