928 research outputs found
Composition profiling InAs quantum dots and wetting layers by atom probe tomography and cross-sectional scanning tunnelling microscopy
This study compares cross-sectional scanning tunnelling microscopy (XSTM) and
atom probe tomography (APT). We use epitaxially grown self-assembled InAs
quantum dots (QDs) in GaAs as an exemplary material with which to compare these
two nanostructural analysis techniques. We studied the composition of the
wetting layer and the QDs, and performed quantitative comparisons of the indium
concentration profiles measured by each method. We show that computational
models of the wetting layer and the QDs, based on experimental data, are
consistent with both analytical approaches. This establishes a link between the
two techniques and shows their complimentary behaviour, an advantage which we
exploit in order to highlight unique features of the examined QD material.Comment: Main article: 8 pages, 6 figures. Appendix: 3 pages, 5 figure
Complete Wetting of Pits and Grooves
For one-component volatile fluids governed by dispersion forces an effective
interface Hamiltonian, derived from a microscopic density functional theory, is
used to study complete wetting of geometrically structured substrates. Also the
long range of substrate potentials is explicitly taken into account. Four types
of geometrical patterns are considered: (i) one-dimensional periodic arrays of
rectangular or parabolic grooves and (ii) two-dimensional lattices of
cylindrical or parabolic pits. We present numerical evidence that at the
centers of the cavity regions the thicknesses of the adsorbed films obey
precisely the same geometrical covariance relation, which has been recently
reported for complete cone and wedge filling. However, this covariance does not
hold for the laterally averaged wetting film thicknesses. For sufficiently deep
cavities with vertical walls and close to liquid-gas phase coexistence in the
bulk, the film thicknesses exhibit an effective planar scaling regime, which as
function of undersaturation is characterized by a power law with the common
critical exponent -1/3 as for a flat substrate, but with the amplitude
depending on the geometrical features.Comment: 12 page
Separately contacted electron-hole double layer in a GaAs/AlxGa1−xAs heterostructure
We describe a method for creating closely spaced parallel two-dimensional electron and hole gases confined in 200 Å GaAs wells separated by a 200 Å wide AlxGa1−xAs barrier. Low-temperature ohmic contacts are made to both the electrons and holes, whose densities are individually adjustable between 10^(10)/cm^2 to greater than 10^(11)/cm^2
Breakdown of the Two-Step Model in K-Shell Photoemission and Subsequent Decay Probed by the Molecular-Frame Photoelectron Angular Distributions of CO_2
We report results of measurements and of Hartree-Fock level calculations of molecular-frame photoelectron angular distributions (MFPADs) for C 1s photoemission from CO2. The agreement between the measured and calculated MFPADs is on average reasonable. The measured MFPADs display a weak but definite asymmetry with respect to the O+ and CO+ fragment ions at certain energies, providing evidence for an overlap of gerade and ungerade final ionic states giving rise to a partial breakdown of the two-step model of core-level photoionization and its subsequent Auger decay
Dissociative photoionization of the NO molecule studied by photoelectron-photon coincidence technique
Low-energy photoelectron–vacuum ultraviolet (VUV) photon coincidences have been measured using synchrotron radiation excitation in the inner-valence region of the nitric oxide molecule. The capabilities of the coincidence set-up were demonstrated by detecting the 2s−1 → 2p−1 radiative transitions in coincidence with the 2s photoelectron emission in Ne. In NO, the observed coincidence events are attributed to dissociative photoionization with excitation, whereby photoelectron emission is followed by fragmentation of excited NO+ ions into O+ + N* or N+ + O* and VUV emission from an excited neutral fragment. The highest coincidence rate occurs with the opening of ionization channels which are due to correlation satellites of the 3σ photoionization. The decay time of VUV photon emission was also measured, implying that specific excited states of N atoms contribute significantly to observed VUV emission
A novel, lineage-primed prestalk cell subtype involved in the morphogenesis of D-discoideum
Dictyostelium morphogenesis requires the tip, which acts as an organizer and conducts orchestrated cell movement and cell differentiation. At the slug stage the tip region contains prestalk A (pstA) cells, which are usually recognized by their expression of reporter constructs that utilize a fragment of the promoter of the ecmA gene. Here, using the promoter region of the o-methyl transferase 12 gene (omt12) to drive reporter expression, we demonstrate the presence, also within the pstA region, of a novel prestalk cell subtype: the pstVA cells. Surprisingly, a sub-population of the vegetative cells express a pstVA: GFP marker and, sort out to the tip, both when developing alone and when co-developed with an excess of unmarked cells. The development of such a purified GFP-marked population is greatly accelerated: by precocious cell aggregation and tip formation with accompanying precocious elevation of developmental gene transcription. We therefore suggest that the tip contains at least two prestalk cell subtypes: the developmentally-specified pstA cells and the lineage-primed pstVA cells. It is presumably the pstVA cells that play the dominant role in morphogenesis during the earlier stages of development. The basis for the lineage priming is, however, unclear because we can find no correlation between pstVA differentiation and nutrient status during growth or cell cycle position at the time of starvation, the two known determinants of probable cell fate
Photoelectron Angular Distributions for Two-photon Ionization of Helium by Ultrashort Extreme Ultraviolet Free Electron Laser Pulses
Phase-shift differences and amplitude ratios of the outgoing and
continuum wave packets generated by two-photon ionization of helium atoms are
determined from the photoelectron angular distributions obtained using velocity
map imaging. Helium atoms are ionized with ultrashort extreme-ultraviolet
free-electron laser pulses with a photon energy of 20.3, 21.3, 23.0, and 24.3
eV, produced by the SPring-8 Compact SASE Source test accelerator. The measured
values of the phase-shift differences are distinct from scattering phase-shift
differences when the photon energy is tuned to an excited level or Rydberg
manifold. The difference stems from the competition between resonant and
non-resonant paths in two-photon ionization by ultrashort pulses. Since the
competition can be controlled in principle by the pulse shape, the present
results illustrate a new way to tailor the continuum wave packet.Comment: 5 pages, 1 table, 3 figure
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