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We present low-temperature magnetotransport measurements on selectively grown Sb2Te3-based topological insulator ring structures. These devices display clear Aharonov-Bohm oscillations in the conductance originating from phase-coherent transport around the ring. The temperature dependence of the oscillation amplitude indicates that the Aharonov-Bohm oscillations originate from ballistic transport along the ring arms. We attribute these oscillations to the topological surface states. Further insight into the phase coherence is gained by comparing with similar Aharonov-Bohm-type oscillations in topological insulator nanoribbons exposed to an axial magnetic field. Here, quasi-ballistic phase-coherent transport is confirmed for closed-loop topological surface states in the transverse direction enclosing the nanoribbon. In contrast, the appearance of universal conductance fluctuations indicates phase-coherent transport in the diffusive regime, which is attributed to bulk carrier transport. Thus, it appears that even in the presence of diffusive p-type charge carriers in Aharonov-Bohm ring structures, phase-coherent quasi-ballistic transport of topological surface states is maintained over long distances.

Both a pH‐sensitive thin‐film sensor and a thick‐film reference electrode have been integrated into a hybrid sensor module that is suitable for (bio‐)chemical analysis in aqueous solutions. The pH‐sensitive thin‐film sensor consists of a capacitive layer structure of Al/p‐Si/SiO2/Al2O3and is prepared by semiconductor‐compatible process steps. The thick‐film reference electrode of p‐Si/Ag/AgCl is printed by conventional screen‐printed technique onto the same silicon substrate. Such combination of the thin‐film sensor and the thick‐film reference allows the realization of a miniaturized sensor hybrid without an external macroscopic reference electrode. Due to its set‐up, the sensor hybrid shows a response characteristic to both H+‐ and Cl−‐ions and can be employed in well‐defined analytes, like for biomedical applications. The utility of the new thin‐film / thick‐film sensor hybrid is demonstrated for the determination of the H+‐ and Cl−‐ion concentration in aqueous solutions.

A software feedback control system for the constant‐current‐mode operation of the light‐addressable potentiometric sensor (LAPS) was developed. The constant‐current‐mode LAPS (CLAPS) is suitable for online monitoring and recording of changes in the pH value or the ion concentration. An enzyme LAPS was fabricated by adsorptive immobilization of penicillinase on the pH‐sensitive layer of Ta2O5. This sensor was operated in the constant‐current‐mode and the detection limit for penicillin G was found to be at least as low as 100 µM.

Neurodegeneration in Alzheimer's disease (AD) is associated with the appearance of dystrophic neuronal growth profiles that most likely reflect an impairment of neuronal reorganization. This process of morphodysregulation, which eventually goes awry and becomes a disease itself, might be triggered either by a variety of life events that place an additional burden on the plastic capability of the brain or by genetic pertubations that shift the threshold for decompensation. This paper summarizes recent evidence that impairment of the p21ras intracellular signal transduction, which is is mediated by a hierarchy of phosphorylation signals and eventually results in loss of differentiation control and an attempt of neurons to re-enter the cell-cycle, is critically involved in this process. Neurodegeneration might thus be viewed as an alternative effector pathway of those events that in the dividing cell would lead to cellular transformation. This hypothesis might be of heuristic value for the development of a therapeutic strategy.

Abstract: In this paper a number of optical spectroscopic methods for investigating surface electronic structure are discussed, including reflectance techniques, ellipsometry, surface photoconductivity and surface photovoltage spectroscopy. In addition to electron scattering techniques and UV-photoemission, optical spectroscopic methods have contributed much in recent times to the understanding of electronic surface states on solids. A discussion and comparison is given of the nature and significance of information obtained by these methods and exemplary experimental results are presented to illustrate the contribution of the optical techniques to the present knowledge about surface states. The relation between information obtained from optical measurements and electron spectroscopy is considered.

Summary Human visual cortex was studied using NADPH-diaphorase histochemistry and nitric oxide synthase immunohistochemistry. Large, strongly stained, sparsely spined non-pyramidal cells (average soma diameter: 16 × 16 µm) occur in layers H–VI, but are commonest in layers II–III. Small weakly stained multipolar cells (average soma diameter 3.6 × 4 µm, stellate like cells) in layers II–VI are concentrated in layer IV of areas 17 and 18. The density of these cells, measured with a computer assisted microscopy system is less in area 18 than 17. Large, strongly stained, predominantly horizontal cells (average soma diameter 12 × 19 µm) are localized in the underlying white matter. Axons of the large, strongly NADPH-diaphorase positive cells are thin and unbranched with fine boutons. These axons ascend to layer I. The large, strongly stained cells in layers II–VI we identify as Martinotti neurons. In layer I parallel unbranched positive fibres with some fine boutons run horizontally and build dense axonal plexuses together with the axons of Martinotti neurons. Axons of presumed extrinsic origin are morphologically different from NADPH-diaphorase positive intrinsic fibres. They show thick varicosities running in different directions and forming a network in layers III–VI. Basket like formations of these fibres were frequently observed in layers IV, V and VI. Other fibres seem to innervate blood vessels. Nitric oxide synthase was also demonstrated immunohistochemically by a polyclonal rabbit nitric oxide synthase antiserum. The morphology and distribution of the immunostained cells correspond with those seen with NADPH-diaphorase histochemistry. Double labelling experiments confirm the colocalization of NADPH-diaphorase and nitric oxide synthase in all demonstrated cells. Immunohistochemical demonstration of glial fibrillary acidic protein has shown that astrocytes are not involved in the NADPH-diaphorase/NOS system in the human visual cortex.

ALTERATIONS in the expression of both the β-amyloid precursor protein (APP) and nitric oxide synthase (NOS) might be involved in neurodegenerative conditions and/or in the neuronal response to injury. We have investigated the relationship between the increased expression of β-amyloid precursor protein (APP) and the reactive changes in the expression of isoforms of nitric oxide synthase (NOS) in neurons and glial cells after small electrolytic lesions placed to the cerebral cortex. An increase in the expression of APP in both neurons and glial cells was detected 4 days post-operation. The inducible NOS (iNOS) was observed in macrophages or glial cells surrounding the lesion site. No major changes in constitutive NOS (cNOS) were found. APP immunoreactivity was not co-localized with either iNOS or cNOS at this survival time. At longer survival times (8 and 12 days post-lesion), a reactive increase in the expression of cNOS in cortical pyramidal neurons was seen in addition to the elevated expression of iNOS in astrocytes. The reactive expression of cNOS was confined to a subset of neurons also showing a high expression of APP. The present results suggest a relationship between reactive changes in the expression of APP and cNOS during the neuronal response to injury.

We investigated the localization of the constitutive isoform of nitric oxide synthase (cNOS) in the rat visual cortex by electron microscopy, using a pre-embed-ding immunohistochemical method. NOS-immunoreactivity was demonstrated in very few somata, dendrites and axon terminals of nerve cells, and also in a few glial cells. The morphological characteristics of labelled glial cells enabled us to identify them as astrocytes. In comparison to the enzyme activity in neurons, the glial cells show a very weak reactivity which was not detectable at the light microscopical level. Immunoreactivity was located in the perikaryon and in the processes of astrocytes. Qualitative analyses with the electron microscope indicate that NOS-positive astroglial cells constitute a very small proportion in the rat visual cortex, whereas the great majority of astrocytes is NOS-negative. An unexpected observation was the localization of NOS immunoreactivity in astroglial sheath-like structures around some synaptic junctions. These results suggest that NO may act very locally at some synapses, and that specialized glial cells are involved in the NO-mediated mechanisms.

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To take advantage of the benefits of gaseous precursors for the growth of AlGaAs, a reduction of the carbon and oxygen uptake from the sources is necessary. Therefore, new precursors have been developed recently to overcome these problems. An entirely new approach is the use of an intramolecular saturated precursor, where the coordinative saturation against oxygen is realised by means of a double ring structure. 1-(3-dimethylaminopropyl)-1-ala-cyclohexan (APAH) is a representative of this class of precursors. In metalorganic molecular beam epitaxy (MOMBE) the suitability of APAH for the growth of AlxGa1-xAs layers (0?x?1) was investigated in combination with TEGa and arsine. For the grown layers morphology and crystallinity are found to be excellent, whereas the background carrier concentrations are in the range of p=1017to 1019cm-3. SIMS measurements clearly identify carbon as the main acceptor. Other than carbon, nitrogen is also incorporated from APAH. Despite this, APAH is useful for particular heterostructure applications as demonstrated by quantum well structures with excellent interfaces. Therefore, in MOMBE of AlGaAs, APAH is a suitable Al precursor for applications where p-type doping of 1017cm-3or above is required or acceptable (e.g., in HBT).

A new method for the fabrication of color-sensitive Si-photodiodes is presented. Color sensitivity was achieved by using porous silicon multilayer stacks which act as interference filters if the formation parameters are controlled carefully. These filters were integrated in the upper, p+-type part of a p+n-junction. As expected, the spectral response of the photodiodes was determined by the transmission spectra of the filters, while the porous silicon had no significant influence on the electrical characteristics. The great advantage of this method over conventional ones is that it makes very cheap, fast filter fabrication requiring no expensive deposition process possible.