Nabil M. Amer

Simultaneous measurement of lateral and normal forces with an optical‐beam‐deflection atomic force microscope

An atomic force microscope capable of measuring, simultaneously yet separately, lateral (‘‘frictional’’) and normal forces is described. A direction‐dependent feature, absent in topological images, is found when scanning stepped surfaces of NaClu2009(001) in ultrahigh vacuum. A simple model is presented to account for this observation.

Flux creep characteristics in high‐temperature superconductors

We describe the voltage‐current characteristics of YBa2Cu3O7−δ epitaxial films within the flux creep model in a manner consistent with the resistive transition behavior. The magnitude of the activation energy, and its temperature and magnetic field dependences, are readily derived from the experimentally observed power law characteristics and show a (1−T/Tc)3/2 type of behavior near Tc. The activation energy is a nonlinear function of the current density and it enables the determination of the shape of the flux line potential well.

Optical‐beam‐deflection atomic force microscopy: The NaCl (001) surface

We have imaged, in ultrahigh vacuum, the (001) surface of NaCl using an optical‐beam‐deflectin force microscope operating in the short‐range repulsive regime. The design and performance characteristics of the microscope are given, and the observed atomic corrugations are compared with those deduced from He‐atom scattering experiments.

Flux creep in Bi2Sr2CaCu2O8 epitaxial films

We incorporate the experimentally deduced flux line potential well structure into the flux creep model. Application of this approach to the resistive transition in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} epitaxial films explains the power law voltage-current characteristics and the nonlinear current dependence of the activation energy. The results cannot be accounted for by a transition into a superconducting vortex-glass phase.We incorporate the experimentally deduced flux line potential well structure into the flux creep model. Application of this approach to the resistive transition in Bi2Sr2CaCu2O8 epitaxial films explains the power law voltage‐current characteristics and the nonlinear current dependence of the activation energy. The results cannot be accounted for by a transition into a superconducting vortex‐glass phase.

Epitaxial growth of Fe on Au(111): a scanning tunneling microscopy investigation

Abstract The epitaxial of Fe on Au(111) has been studied with scanning tunneling microscopy. At low coverage the iron nucleates forming polygonal islands whose spacing is determined by the underlying Au(111) zigzag reconstruction geometry. The islands are one atomic layer high and grow laterally with increasing coverage. At three monolayers the coherent (fcc) layer growth is disrupted and a transition to the bcc iron structure occurs. We relate our results to published magnetic measurements.

Scanning tunneling microscopy of surfactant-mediated epitaxy of Ge on Si(111): strain relief mechanisms and growth kinetics

Abstract The role of strain relief and kinetics in surfactant-mediated epitaxial growth of Ge on Si(111) was studied with scanning tunneling microscopy. For coverages of up to ≈ 20 ML the images reveal a variety of strain relief mechanisms which include trench formation and increasing surface roughness. Additionally, at 10 ML, the onset of a periodic surface undulation is observed which coincides with the injection of misfit dislocations at the Ge-Si interface . For coverages larger than 20 ML, the Ge epilayer grows as defect-free atomically-flat large terraces.

A contactless method for investigating the thermal properties of thin films

The photothermal deflection technique has been extended as a contactless method to investigate thermal transport in thin films. A theoretical model is developed which quantitatively describes the transport behavior, and is shown to be in excellent agreement with experimental results. This approach yields the thermal diffusivity directly and in a spatially-resolved manner.

Light‐induced defects in hydrogenated amorphous silicon alloys

The effect of light illumination on gap state absorption of hydrogenated amorphous silicon (a‐Si:H) alloys has been investigated using photothermal deflection spectroscopy. The alloys studied were the large gap materials a‐SiC:H and a‐SiO:H and the narrow gap a‐SiGe:H and a‐Ge:H. The results indicate a direct relationship between the gap energy and defect formation. As the gap opens, the number of metastable defects increases; whereas for the narrow gap materials, significantly fewer defects are observed. This behavior is consistent with the interpretation of defect formation by electron‐hole recombination.

Atomic resolution scanning tunneling microscopy with a gallium arsenide tip

A scanning tunneling microscope which uses a gallium arsenide tip has been successfully constructed. Atomic resolution is demonstrated by the imaging of the Si(111)‐7×7 surface in ultrahigh vacuum. Details of the tip preparation are given and the tip tunneling current characteristics are discussed.

Time‐resolved surface expansion of metals under picosecond laser illumination

Picosecond photothermal displacement experiments were performed to resolve the rise time of the actual surface expansion of laser‐illuminated metals. A rise time of ∼100 ps was resolved for Ni, in agreement with the predictions of a hydrodynamic model. The applicability of the photothermal effect to time‐resolved scanning probe microscopies is addressed.