Some Publications by Nabil M. Amer

Bert Voigtländer, Gerhard Meyer, and Nabil M. Amer, "Epitaxial growth of thin magnetic cobalt films on Au(111) studied by scanning tunneling microscopy"(pdf) Physical Review Letters B.44.10354 (1991). The initial nucleation and the subsequent growth of Co on Au(111) have been studied with scanning tunneling microscopy. At low coverage the cobalt nucleates forming polygonal islands whose location and spacing are determined by the underlying Au(111) zigzag reconstruction geometry. The islands are two atomic layers high and grow laterally with increasing coverage. A model for the nucleation and growth is proposed and a comparison with the growth of other metals on gold is given. We also qualitatively relate our structural results to published magnetic measurements.

P. Mårtensson, G. Meyer, N. M. Amer, E. Kaxiras, and K. C. Pandey, "Evidence for trimer reconstruction of Si(111) √3 × √3 -Sb: Scanning tunneling microscopy and first-principles theory"(pdf) Physical Review Letters B.42.7230 (1990). Scanning-tunneling-microscopy images of the Si(111) √3 × √3 -Sb surface show a trigonal lattice of protrusions, with a characteristic dependence on bias voltage. When probing filled surface states, the protrusions consist of three topographic maxima, while probing empty surface states gives a single maximum. These observations are interpreted in terms of an Sb-trimer model, the structure of which is obtained through first-principles calculations. Charge-density contours show that the three maxima seen when probing filled states can be directly related to the positions of the Sb-trimer atoms.

E. Zeldov, N. M. Amer, G. Koren, A. Gupta, R. J. Gambino, and M. W. McElfresh "Optical and electrical enhancement of flux creep in YBa2Cu3O7-δ epitaxial films ", Phys. Rev. Lett. 62, 3093 (1989) We report two novel flux-creep-related phenomena in YBa2Cu3O7-δ films at elevated currents in presence of magnetic fields: a sharp onset of nonequilibrium optical response and a current-dependent thermally activated electrical resistivity. In this regime the activation energy of the resistivity scales uniquely with current density as log(J0/J) with J0≊3×106 A/Cm2. This nonlinear current dependence is significantly different from the predictions of the standard flux-creep model. At currents ≲7×103 A/cm2 the resistivity is current independent and the optical response is purely bolometric.

E. Zeldov, N. M. Amer, G. Koren, and A. Gupta, "Nonbolometric optical response of YBa2Cu3O7-δ epitaxial films "(pdf) Phys. Rev. B 39, 9712 (1989). The optical response of high-quality epitaxial YBa2Cu3O7-δ films on LaGaO3 and SrTiO3 substrates is reported. A careful analysis of the results reveals a strong nonbolometric response at temperatures below the onset of the superconducting transition. This finding is in contrast with recently published reports which attribute the optical response of epitaxial films to a bolometric effect only. We interpret this response as due to photoenhanced flux creep in the superconducting films.

Andrew Skumanich and Nabil M. Amer, Evidence for the intrinsic nature of light-induced defects in undoped a-Si:H Phys. Rev. B 37, 8465 (1988) Absorption measurements using photothermal deflection spectroscopy were performed on a series of a-Si:H films with various levels of typical impurities to investigate their role in light-induced defect formation. It was found that at low concentrations (<5 at. %) the light-induced defect creation rate is independent of impurity concentration for oxygen, nitrogen, and carbon, which agrees with electron-spin-resonance results.

Marjorie A. Olmstead and Nabil M. Amer, Temperature dependence of the Si and Ge (111)2×1 surface-state optical absorption Phys. Rev. B 33, 2564 (1986) The temperature dependence of the dangling-bond optical absorption of cleaved silicon and germanium (111)2×1 surfaces has been studied using photothermal displacement spectroscopy. The polarization-dependent surface-state absorption was utilized as an order parameter to monitor the Ge(111)2×1→c-2×8 phase transition in real time. The temperature dependence of the Si(111)2×1 surface optical gap in the range 300–450 K was also measured. Unlike the corresponding bulk case, no shift of the surface gap was observed. Reasons for this temperature independence are discussed.

Gerhard Meyer and Nabil M. Amer, Simultaneous measurement of lateral and normal forces with an optical-beam-deflection atomic force microscope Appl. Phys. Lett. -- November 12, 1990 -- Volume 57, Issue 20, pp. 2089-2091 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 NaCl (001) in ultrahigh vacuum. A simple model is presented to account for this observation. Applied Physics Letters is copyrighted by The American Institute of Physics.