Composition of the “GaAs” quantum dot,grown by droplet epitaxy

Self-assembled strain-free quantum dot (QD) structures were grown on AlGaAs surface by the droplet epitaxal method. The QDs were developed from pure Ga droplets under As pressure. The QDs were investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Both techniques show that the QDs are very uniform in size and their distribution on the surface is also homogeneous. The high resolution cross-sectional TEM investigation shows perfect lattice matching between the QD and the substrate, and also the faceting of the side walls of QD can be identified exactly by lattice planes. Analytical TEM (elemental mapping by EELS) unambiguously identifies the presence of Al in the QD.     

Precipitation from homogeneous solution by photochemical action: determination of thorium

Thorium was precipitated from homogeneous solution by photochemical reduction of periodate to iodate in a solution containing thorium and perchloric acid, by means of a 2537 A low-pressure mercury vapour lamp. For weighing, the precipitate was redissolved, precipitated once as thorium hydroxide, and finally as thorium oxalate, which was ignited to thorium dioxide. Quantitative results were obtained in the range 35-180 mg of thorium.     

Quantized dispersion of two-dimensional magnetoplasmons detected by photoconductivity spectroscopy

We find that the long-wavelength magnetoplasmon, resistively detected by photoconductivity spectroscopy in high-mobility two-dimensional electron systems, deviates from its well-known semiclassical nature as uncovered in conventional absorption experiments. A clear filling-factor dependent plateau-type dispersion is observed that reveals a so far unknown relation between the magnetoplasmon and the quantum Hall effect.     

Korrosion

Ohne Zusammenfassung     

Analysis of the pion form factor in the space-like and time-like region

We discuss an analytic parametrization of the pion form factor which is only weakly model dependent. Its model dependence comes from a preference of those analytic functions that are smooth in a certain well-defined sense. Different solutions for different values ofχ ² corresponding to different degrees of smoothness can be constructed. All available space-like and time-like data are taken into account simultaneously. Our approach allows conclusions on an incompatibility of space-like with time-like data sets and on the significance of the occurrence of higher coupled resonances. We discuss the asymptotic behaviour which is consistent with QCD and give the relevant low energy and resonance parameters.     

Thermal conductance of ballistic point contacts

We study the thermal conductance of ballistic point contacts. These contacts are realized as few nanometer long pillars in so-called air-gap heterostructures (AGHs). The pillar length is orders of magnitude smaller than the mean free path of the phonons up to room temperature. Because of the small dimension and the low density of the pillars, the thermal conductance of the AGHs is several orders of magnitude reduced in comparison to bulk structures. The measurement results are in quantitative agreement with a simple model that is based on the Boltzmann transport equation.     

Structural and magnetic properties of Ni2MnIn Heusler thin films grown on modulation-doped InAs heterostructures with metamorphic buffer

This paper reports on the morphological, structural, magnetic, and magneto-optic properties of Ni₂MnIn Heusler films grown on InAs-high electron-mobility transistor structures (HEMT) with metamorphic buffers for spintronic applications. Similar to our previous results on the Ni₂MnIn/InAs (001) system, the Heusler layer is found to have a (110) orientation relative to the (001) InAs-HEMT surface. We observe almost equal spin-polarizations for Heusler films on (001) InAs-HEMT as well as on (001) InAs. In addition, we find further support for interfacial intermixing previously reported for the Ni₂MnIn/InAs (001) system. On the other hand, the Heusler/InAs-HEMT system shows distinct morphologic, structural, and magnetic properties as compared to the Ni₂MnIn/InAs (001) system. In particular, more rapid and complex plastic deformation effects resulting in a high surface density of pin-holes in the Heusler films are found. We report on complex mutual deformation effects between the Heusler films and the underlying InAs-HEMT structure. Furthermore, a hysteresis loop squareness close to 1 for a 50 nm Heusler film on InAs-HEMT is observed. We tentatively associate these phenomena with the higher mismatch strain of the Ni₂MnIn/InAs-HEMT system compared to Ni₂MnIn films grown on (001) InAs.     

Formation and dissolution of InAs quantum dots on GaAs

In situ reflection high energy electron diffraction (RHEED) has been used to study the time evolution during self-assembled molecular beam epitaxy (MBE) growth of InAs quantum dots on GaAs. Using a special data acquisition technique, two characteristic time constants are determined very precisely: the time t_c up to the first appearance of InAs dots and the time t_f it takes to complete the 2D–3D transition of all islands. Surprisingly, we find that t_c increases with temperature which disagrees with a thermally activated process. In contrast to this, t_f behaves Arrhenius-like and an activation energy of E_f0.39 eV is determined. Furthermore, the sum t_c+t_f does not depend significantly on temperature and corresponds to an InAs coverage of 2.0 monolayers. A second focus of this paper is the study of dissolution of InAs dots after interruption of the As flux. From the experiments, an activation energy of 3.2 eV for desorption of In located on top of the wetting layer is determined, whereas direct desorption from the wetting layer corresponds to an activation energy of 3.4 eV.