Aharonov-Bohm Interference and Phase-Coherent Surface-State Transport in Topological Insulator Rings.

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.

Phase-coherent loops in selectively-grown topological insulator nanoribbons.

We succeeded in the fabrication of topological insulator (Bi0.57Sb0.43)2Te3 Hall bars as well as nanoribbons by means of selective-area growth using molecular beam epitaxy. By performing magnetotransport measurements at low temperatures information on the phase-coherence of the electrons is gained by analyzing the weak-antilocalization effect. Furthermore, from measurements on nanoribbons at different magnetic field tilt angles an angular dependence of the phase-coherence length is extracted, which is attributed to transport anisotropy and geometrical factors. For the nanoribbon structures universal conductance fluctuations were observed. By performing a Fourier transform of the fluctuation pattern a series of distinct phase-coherent closed-loop trajectories are identified. The corresponding enclosed areas can be explained in terms of nanoribbon dimensions and phase-coherence length. In addition, from measurements at different magnetic field tilt angles we can deduce that the area enclosed by the loops are predominately oriented parallel to the quintuple layers.

Electron Interference in Hall Effect Measurements on GaAs/InAs Core/Shell Nanowires.

We present low-temperature magnetotransport measurements on GaAs/InAs core/shell nanowires contacted by regular source-drain leads as well as laterally attached Hall contacts, which only touch parts of the nanowire sidewalls. Low-temperature measurements between source and drain contacts show typical phase coherent effects, such as universal conductance fluctuations in a magnetic field aligned perpendicularly to the nanowire axis as well as Aharonov-Bohm-type oscillations in a parallel aligned magnetic field. However, the signal between the Hall contacts shows a Hall voltage buildup, when the magnetic field is turned perpendicular to the nanowire axis while current is driven through the wire using the source-drain contacts. At low temperatures, the phase coherent effects measured between source and drain leads are superimposed on the Hall voltage, which can be explained by nonlocal probing of large segments of the nanowire. In addition, the Aharonov-Bohm-type oscillations are also observed in the magnetoconductance at magnetic fields aligned parallel to the nanowire axis, using the laterally contacted leads. This measurement geometry hereby directly corresponds to classical Aharonov-Bohm experiments using planar quantum rings. In addition, the Hall voltage is used to characterize the nanowires in terms of charge carrier concentration and mobility, using temperature- and gate-dependent measurements as well as measurements in tilted magnetic fields. The GaAs/InAs core/shell nanowire used in combination with laterally attached contacts is therefore the ideal system to three-dimensionally combine quantum ring experiments using the cross-sectional plane and Hall experiments using the axial nanowire plane.

Anisotropic phase coherence in GaAs/InAs core/shell nanowires.

Low-temperature transport in nanowires is accompanied by phase-coherent effects, which are observed as modulation of the conductance in an external magnetic field. In the GaAs/InAs core/shell nanowires investigated here, these are h/e flux periodic oscillations in a magnetic field aligned parallel to the nanowire axis and aperiodic universal conductance fluctuations in a field aligned perpendicularly to the nanowire axis. Both electron interference effects are used to analyse the phase coherence of the system. Temperature-dependent measurements are carried out, in order to derive the phase coherence lengths in the cross-sectional plane as well as along the nanowire sidewalls. It is found that these values show a strong anisotropy, which can be explained by the crystal structure of the GaAs/InAs core/shell nanowire. For nanowires with a radius as low as 45 nm, flux periodic oscillations were observed up to a temperature of 55 K.

Quantum dots in InAs nanowires induced by surface potential fluctuations.

Back-gated InAs nanowire field-effect transistors are studied focusing on the formation of intrinsic quantum dots, i.e. dots not intentionally defined by electrodes. Such dots have been studied before, but the suggested explanations for their origin leave some open questions, which are addressed here. Stability diagrams of samples with different doping levels are recorded at electron temperatures below 200 mK, allowing us to estimate the number and size of the dots as well as the type of connection, i.e. in series or in parallel. We discuss several potential physical origins of the dots and conclude that they are most probably induced by potential fluctuations at the nanowire surface. Additionally, we show that via gate voltage and doping, the samples can be tuned to different regimes of Coulomb blockade.

Supercurrent in Bi4Te3 Topological Material-Based Three-Terminal Junctions.

In this paper, in an in situ prepared three-terminal Josephson junction based on the topological insulator Bi4Te3 and the superconductor Nb the transport properties are studied. The differential resistance maps as a function of two bias currents reveal extended areas of Josephson supercurrent, including coupling effects between adjacent superconducting electrodes. The observed dynamics for the coupling of the junctions is interpreted using a numerical simulation of a similar geometry based on a resistively and capacitively shunted Josephson junction model. The temperature dependency indicates that the device behaves similar to prior experiments with single Josephson junctions comprising topological insulators' weak links. Irradiating radio frequencies to the junction, we find a spectrum of integer Shapiro steps and an additional fractional step, which is interpreted with a skewed current-phase relationship. In a perpendicular magnetic field, we observe Fraunhofer-like interference patterns in the switching currents.

Flux periodic oscillations and phase-coherent transport in GeTe nanowire-based devices.

Despite the fact that GeTe is known to be a very interesting material for applications in thermoelectrics and for phase-change memories, the knowledge on its low-temperature transport properties is only limited. We report on phase-coherent phenomena in the magnetotransport of GeTe nanowires. From universal conductance fluctuations measured on GeTe nanowires with Au contacts, a phase-coherence length of about 280 nm at 0.5 K is determined. The distinct phase-coherence is confirmed by the observation of Aharonov-Bohm type oscillations for parallel magnetic fields. We interpret the occurrence of these magnetic flux-periodic oscillations by the formation of a tubular hole accumulation layer. For Nb/GeTe-nanowire/Nb Josephson junctions we obtained a critical current of 0.2 µA at 0.4 K. By applying a perpendicular magnetic field the critical current decreases monotonously with increasing field, whereas in a parallel field the critical current oscillates with a period of the magnetic flux quantum confirming the presence of a tubular hole channel.

Strain relaxation and ambipolar electrical transport in GaAs/InSb core-shell nanowires.

The growth, crystal structure, strain relaxation and room temperature transport characteristics of GaAs/InSb core-shell nanowires grown using molecular beam epitaxy are investigated. Due to the large lattice mismatch between GaAs and InSb of 14%, a transition from island-based to layer-like growth occurs during the formation of the shell. High resolution transmission electron microscopy in combination with geometric phase analyses as well as X-ray diffraction with synchrotron radiation are used to investigate the strain relaxation and prove the existence of different dislocations relaxing the strain on zinc blende and wurtzite core-shell nanowire segments. While on the wurtzite phase only Frank partial dislocations are found, the strain on the zinc blende phase is relaxed by dislocations with perfect, Shockley partial and Frank partial dislocations. Even for ultrathin shells of about 2 nm thickness, the strain caused by the high lattice mismatch between GaAs and InSb is relaxed almost completely. Transfer characteristics of the core-shell nanowires show an ambipolar conductance behavior whose strength strongly depends on the dimensions of the nanowires. The interpretation is given based on an electronic band profile which is calculated for completely relaxed core/shell structures. The peculiarities of the band alignment in this situation implies simultaneously occupied electron and hole channels in the InSb shell. The ambipolar behavior is then explained by the change of carrier concentration in both channels by the gate voltage.

Age-dependent changes of glyoxalase I expression in human brain.

Increased modification and crosslinking of proteins by advanced glycation end products (AGEs) is a characteristic feature of aging, and contributes to the formation of many of the lesions of neurodegenerative diseases including neurofibrillary tangles and amyloid plaques in Alzheimer's disease. Therefore, defense mechanisms against AGE formation or detoxification of their precursors such as the glyoxalase system are of particular interest in aging research. Thus, we investigated the age-dependent protein expression, the activity as well as the RNA level of glyoxalase I in Brodmann area 22 (auditory association area of superior temporal gyrus) of the human cerebral cortex. Our immunohistochemical results demonstrate the localization of glyoxalase I in neurons, predominantly pyramidal cells, as well as in astroglia, located predominantly in the subpial region. The number of glyoxalase I expressing neurons and astroglia increases with age, with a peak at approximately 55 years, and progressively decreases thereafter. These results were confirmed by biochemical investigations in total brain tissue, where the RNA, the protein level as well as the activity of glyoxalase I enzyme were analyzed in different age groups. In conclusion, the increase in glyoxalase I expression up to the age of 55 may be a compensatory mechanism against high oxoaldyde levels and the accumulation of AGEs. However, the decline of glyoxalase expression and activity in old age, possibly caused by impairment in transcription or/and translation, may subsequently lead to increased levels of reactive carbonyl compounds, followed by protein crosslinking, inflammation, oxidative stress and neuronal degeneration.

N-Channel field-effect transistors with floating gates for extracellular recordings.

A field-effect transistor (FET) for recording extracellular signals from electrogenic cells is presented. The so-called floating gate architecture combines a complementary metal oxide semiconductor (CMOS)-type n-channel transistor with an independent sensing area. This concept allows the transistor and sensing area to be optimised separately. The devices are robust and can be reused several times. The noise level of the devices was smaller than of comparable non-metallised gate FETs. In addition to the usual drift of FET devices, we observed a long-term drift that has to be controlled for future long-term measurements. The device performance for extracellular signal recording was tested using embryonic rat cardiac myocytes cultured on fibronectin-coated chips. The extracellular cell signals were recorded before and after the addition of the cardioactive isoproterenol. The signal shapes of the measured action potentials were comparable to the non-metallised gate FETs previously used in similar experiments. The fabrication of the devices involved the process steps of standard CMOS that were necessary to create n-channel transistors. The implementation of a complete CMOS process would facilitate the integration of the logical circuits necessary for signal pre-processing on a chip, which is a prerequisite for a greater number of sensor spots in future layouts.

Angle-dependent magnetotransport in GaAs/InAs core/shell nanowires.

We study the impact of the direction of magnetic flux on the electron motion in GaAs/InAs core/shell nanowires. At small tilt angles, when the magnetic field is aligned nearly parallel to the nanowire axis, we observe Aharonov-Bohm type h/e flux periodic magnetoconductance oscillations. These are attributed to transport via angular momentum states, formed by electron waves within the InAs shell. With increasing tilt of the nanowire in the magnetic field, the flux periodic magnetoconductance oscillations disappear. Universal conductance fluctuations are observed for all tilt angles, however with increasing amplitudes for large tilt angles. We record this evolution of the electron propagation from a circling motion around the core to a diffusive transport through scattering loops and give explanations for the observed different transport regimes separated by the magnetic field orientation.

Quantum Transport and Nano Angle-resolved Photoemission Spectroscopy on the Topological Surface States of Single Sb2Te3 Nanowires.

We report on low-temperature transport and electronic band structure of p-type Sb2Te3 nanowires, grown by chemical vapor deposition. Magnetoresistance measurements unravel quantum interference phenomena, which depend on the cross-sectional dimensions of the nanowires. The observation of periodic Aharonov-Bohm-type oscillations is attributed to transport in topologically protected surface states in the Sb2Te3 nanowires. The study of universal conductance fluctuations demonstrates coherent transport along the Aharonov-Bohm paths encircling the rectangular cross-section of the nanowires. We use nanoscale angle-resolved photoemission spectroscopy on single nanowires (nano-ARPES) to provide direct experimental evidence on the nontrivial topological character of those surface states. The compiled study of the bandstructure and the magnetotransport response unambiguosly points out the presence of topologically protected surface states in the nanowires and their substantial contribution to the quantum transport effects, as well as the hole doping and Fermi velocity among other key issues. The results are consistent with the theoretical description of quantum transport in intrinsically doped quasi-one-dimensional topological insulator nanowires.

Immunochemical crossreactivity of antibodies specific for "advanced glycation endproducts" with "advanced lipoxidation endproducts".

Antibodies against advanced glycation endproducts (AGEs) are used for their immunohistological localization in tissues, for example in Alzheimer's disease (AD) or diabetes. Many monoclonal and polyclonal antibodies have been used, and their specificity is unknown in most cases. Increased radical production, leading to the formation of lipid-derived reactive carbonyl species, such as malondialdehyde (MDA), acrolein, and glyoxal, is a characteristic aspect of age-related diseases like Alzheimer's disease or diabetic polyneuropathy. These reactive carbonyl species are able to modify proteins, resulting in AGE related structures, termed "advanced lipoxidation products" (ALEs). In this study, the monoclonal carboxymethyllysine-specific antibody 4G9 and the polyclonal AGE-antibody K2189 were tested for their immunoreactivity towards these carbonyl-derived protein modifications. To investigate which carbonyl-modified amino acid side chains are specifically recognized by these antibodies, peptide membranes were incubated with glyoxal, MDA and acrolein. As model proteins, microtubuli associated protein tau (MAP-tau), beta-amyloid, human serum albumin and chicken egg albumin were incubated likewise. It was found that both antibodies detected reaction products of these carbonyl compounds on lysine- and arginine residues and for the protein modification, it was found that some epitopes might not be detected. In conclusion, AGE-antibodies might not only detect sugar-derived AGEs but also structures derived from lipid peroxidation products (serving as markers of oxidative stress).

Methylglyoxal, glyoxal, and their detoxification in Alzheimer's disease.

The accumulation of advanced glycation end products (AGEs) in brains with Alzheimer's disease (AD) has been implicated in the formation of insoluble deposits such as amyloid plaques and neurofibrillary tangles. AGEs are also known to activate glia, resulting in inflammation and neuronal dysfunction. As reactive intermediates of AGE formation, neurotoxic reactive dicarbonyl compounds such as glyoxal and methylglyoxal have been identified. One of the most effective detoxification systems for methylglyoxal and glyoxal is the glutathione-dependent glyoxalase system, consisting of glyoxalase I and glyoxalase II. In this study, we have determined the methylglyoxal and glyoxal levels in the cerebrospinal fluid of AD patients compared to healthy controls. Methylglyoxal levels in AD patients were twofold higher than in controls, but this difference was not significant due to the large intergroup variations and the small sample size. However, the concentrations of both compounds were five to seven times higher in CSF than in plasma. We also investigated the glyoxalase I level in AD and healthy control brains. The number of glyoxalase I- positive neurons were increased in AD brains compared to controls. Our findings suggest that glyoxalase I is upregulated in AD in a compensatory manner to maintain physiological methylglyoxal and glyoxal levels.

Resolving ambiguities in nanowire field-effect transistor characterization.

We have modeled InAs nanowires using finite element methods considering the actual device geometry, the semiconducting nature of the channel and surface states, providing a comprehensive picture of charge distribution and gate action. The effective electrostatic gate width and screening effects are taken into account. A pivotal aspect is that the gate coupling to the nanowire is compromised by the concurrent coupling of the gate electrode to the surface/interface states, which provide the vast majority of carriers for undoped nanowires. In conjunction with field-effect transistor (FET) measurements using two gates with distinctly dissimilar couplings, the study reveals the density of surface states that gives rise to a shallow quantum well at the surface. Both gates yield identical results for the electron concentration and mobility only at the actual surface state density. Our method remedies the flaws of conventional FET analysis and provides a straightforward alternative to intricate Hall effect measurements on nanowires.

Detection of nitric oxide synthase (NOS) immunoreactive neurons in the human septal area: a matter of method?

There is a remarkable discrepancy between biochemical and cell morphological findings with regard to the presence of NADPH diaphorase/neuronal nitric oxide synthase (NOS) in the primate septal area. Whereas considerable concentrations of neuronal nitric oxide synthase and high enzyme activities have been measured in postmortem human septal nuclei, histochemical studies were either unable to detect any nitric oxide synthase immunoreactivity in primate septal neurons, or found only a very few nitrergic neurons in this region. This study aimed to investigate the possible presence of nitrergic neurons in human the septal region in greater detail. After having studied a total of 16 postmortem human brains we conclude that the immunohistochemical demonstration of nitric oxide synthase in human septal neurons is largely dependent on the mode of tissue handling: in brain specimens which were fixed en-bloc with paraffin and embedded in paraplast, nitric oxide synthase immunoreactivity is barely detectable, whereas a satisfying immunostaining is obtained on free-floating frozen sections after an immersion-fixation with 4% paraformaldehyde and 0.5% glutaraldehyde, followed by sucrose protection of the specimens. We show herein that there are indeed nitric oxide synthase-containing neurons in the human septum, thus supporting results from previous biochemical studies.

Aberrant expression of NOS isoforms in Alzheimer's disease is structurally related to nitrotyrosine formation.

Various isoforms of the nitric oxide (NO) producing enzyme nitric oxide synthase (NOS) are elevated in Alzheimer's disease (AD) indicating a critical role for NO in the pathomechanism. NO can react with superoxide to generate peroxynitrite, a process referred to as oxidative stress, which is likely to play a role in AD. Peroxynitrite in turn, nitrates tyrosine residues to form nitrotyrosine which can be identified immunohistochemically. To study the potential structural link between the increased synthesis of NO and the deposition of nitrotyrosine in AD, we analyzed the expression of neuronal NOS (nNOS), inducible NOS (iNOS) and endothelial NOS (eNOS) in AD and control brain, and compared the localization with the distribution of nitrotyrosine. Nitrotyrosine was detected in neurons, astrocytes and blood vessels in AD cases. Aberrant expression of nNOS in cortical pyramidal cells was highly co-localized with nitrotyrosine. Furthermore, iNOS and eNOS were highly expressed in astrocytes in AD. In addition, double immunolabeling studies revealed that in these glial cells iNOS and eNOS are co-localized with nitrotyrosine. Therefore, it is suggested that increased expression of all NOS isoforms in astrocytes and neurons contributes to the synthesis of peroxynitrite which leads to generation of nitrotyrosine. In view of the wide range of isoform-specific NOS inhibitors, the determination of the most responsible isoform of NOS for the formation of peroxynitrite in AD could be of therapeutic importance in the treatment of Alzheimer's disease.

Degeneration of beta-amyloid-associated cholinergic structures in transgenic APP SW mice.

Cholinergic dysfunction is a consistent feature of Alzheimer's disease, and the interrelationship between beta-amyloid deposits, inflammation and early cholinergic cell loss is still not fully understood. To characterize the mechanisms by which beta-amyloid and pro-inflammatory cytokines may exert specific degenerating actions on cholinergic cells ultrastructural investigations by electron microscopy were performed in brain sections from transgenic Tg2576 mice that express the Swedish double mutation of the human amyloid precursor protein and progressively develop beta-amyloid plaques during aging. Both light and electron microscopical investigations of the cerebral cortex of 19-month-old transgenic mice revealed a number of pathological tissue responses in close proximity of beta-amyloid plaques, such as activated microglia, astroglial proliferation, increased number of fibrous astrocytes, brain edema, degeneration of nerve cells, dendrites and axon terminals. Ultrastructural detection of choline acetyl transferase (ChAT)-immunostaining in cerebral cortical sections of transgenic mice clearly demonstrated degeneration of ChAT-immunoreactive fibres in the environment of beta-amyloid plaques and activated glial cells suggesting a role of beta-amyloid and/or inflammation in specific degeneration of cholinergic synaptic structures.

Flux quantization effects in InN nanowires.

InN nanowires, grown by plasma-enhanced molecular beam epitaxy, were investigated by means of magnetotransport. By performing temperature-dependent transport measurements and current measurements on a large number of nanowires of different dimensions, it is proven that the carrier transport mainly takes place in a tube-like surface electron gas. Measurements on three representative nanowires under an axially oriented magnetic field revealed pronounced magnetoconductance oscillations with a periodicity corresponding to a single magnetic flux quantum. The periodicity is explained by the effect of the magnetic flux penetrating the coherent circular quantum states in the InN nanowires, rather than by Aharonov-Bohm type interferences. The occurrence of the single magnetic flux quantum periodicity is attributed to the magnetic flux dependence of phase-coherent circular states with different angular momentum quantum numbers forming the one-dimensional transport channels. These phase coherent states can exist because of the almost ideal crystalline properties of the InN nanowires prepared by self-assembled growth.