1,171 research outputs found
Impact of Many-Body Effects on Landau Levels in Graphene
We present magneto-Raman spectroscopy measurements on suspended graphene to
investigate the charge carrier density-dependent electron-electron interaction
in the presence of Landau levels. Utilizing gate-tunable magneto-phonon
resonances, we extract the charge carrier density dependence of the Landau
level transition energies and the associated effective Fermi velocity
. In contrast to the logarithmic divergence of at
zero magnetic field, we find a piecewise linear scaling of as a
function of charge carrier density, due to a magnetic field-induced suppression
of the long-range Coulomb interaction. We quantitatively confirm our
experimental findings by performing tight-binding calculations on the level of
the Hartree-Fock approximation, which also allow us to estimate an excitonic
binding energy of 6 meV contained in the experimentally extracted
Landau level transitions energies.Comment: 10 pages, 6 figure
Computer simulation of crystallization kinetics with non-Poisson distributed nuclei
The influence of non-uniform distribution of nuclei on crystallization
kinetics of amorphous materials is investigated. This case cannot be described
by the well-known Johnson-Mehl-Avrami (JMA) equation, which is only valid under
the assumption of a spatially homogeneous nucleation probability. The results
of computer simulations of crystallization kinetics with nuclei distributed
according to a cluster and a hardcore distribution are compared with JMA
kinetics. The effects of the different distributions on the so-called Avrami
exponent are shown. Furthermore, we calculate the small-angle scattering
curves of the simulated structures which can be used to distinguish
experimentally between the three nucleation models under consideration.Comment: 14 pages including 7 postscript figures, uses epsf.sty and
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Crustal flow around the Eastern Himalayan Syntaxis in western Yunnan, China
Abstract HKT-ISTP 2013
A
Generalized conductance sum rule in atomic break junctions
When an atomic-size break junction is mechanically stretched, the total
conductance of the contact remains approximately constant over a wide range of
elongations, although at the same time the transmissions of the individual
channels (valence orbitals of the junction atom) undergo strong variations. We
propose a microscopic explanation of this phenomenon, based on Coulomb
correlation effects between electrons in valence orbitals of the junction atom.
The resulting approximate conductance quantization is closely related to the
Friedel sum rule.Comment: 4 pages, 1 figure, appears in Proceedings of the NATO Advanced
Research Workshop ``Size dependent magnetic scattering'', Pecs, Hungary, May
28 - June 1, 200
Integrated impedance bridge for absolute capacitance measurements at cryogenic temperatures and finite magnetic fields
We developed an impedance bridge that operates at cryogenic temperatures
(down to 60 mK) and in perpendicular magnetic fields up to at least 12 T. This
is achieved by mounting a GaAs HEMT amplifier perpendicular to a printed
circuit board containing the device under test and thereby parallel to the
magnetic field. The measured amplitude and phase of the output signal allows
for the separation of the total impedance into an absolute capacitance and a
resistance. Through a detailed noise characterization, we find that the best
resolution is obtained when operating the HEMT amplifier at the highest gain.
We obtained a resolution in the absolute capacitance of
6.4~aF at 77 K on a comb-drive actuator, while maintaining
a small excitation amplitude of 15~. We show the magnetic field
functionality of our impedance bridge by measuring the quantum Hall plateaus of
a top-gated hBN/graphene/hBN heterostructure at 60~mK with a probe signal of
12.8~.Comment: 7 pages, 5 figure
Cell specific quantitative iron mapping on brain slices by immuno-µPIXE in healthy elderly and Parkinson’s disease
Iron is essential for neurons and glial cells, playing key roles in neurotransmitter synthesis, energy production and myelination. In contrast, high concentrations of free iron can be detrimental and contribute to neurodegeneration, through promotion of oxidative stress. Particularly in Parkinson's disease (PD) changes in iron concentrations in the substantia nigra (SN) was suggested to play a key role in degeneration of dopaminergic neurons in nigrosome 1. However, the cellular iron pathways and the mechanisms of the pathogenic role of iron in PD are not well understood, mainly due to the lack of quantitative analytical techniques for iron quantification with subcellular resolution. Here, we quantified cellular iron concentrations and subcellular iron distributions in dopaminergic neurons and different types of glial cells in the SN both in brains of PD patients and in non-neurodegenerative control brains (Co). To this end, we combined spatially resolved quantitative element mapping using micro particle induced X-ray emission (mu PIXE) with nickel-enhanced immunocytochemical detection of cell type-specific antigens allowing to allocate element-related signals to specific cell types. Distinct patterns of iron accumulation were observed across different cell populations. In the control (Co) SNc, oligodendroglial and astroglial cells hold the highest cellular iron concentration whereas in PD, the iron concentration was increased in most cell types in the substantia nigra except for astroglial cells and ferritin-positive oligodendroglial cells. While iron levels in astroglial cells remain unchanged, ferritin in oligodendroglial cells seems to be depleted by almost half in PD. The highest cellular iron levels in neurons were located in the cytoplasm, which might increase the source of non-chelated Fe3+, implicating a critical increase in the labile iron pool. Indeed, neuromelanin is characterised by a significantly higher loading of iron including most probable the occupancy of low-affinity iron binding sites. Quantitative trace element analysis is essential to characterise iron in oxidative processes in PD. The quantification of iron provides deeper insights into changes of cellular iron levels in PD and may contribute to the research in iron-chelating disease-modifying drugs
Cell specific quantitative iron mapping on brain slices by immuno-μPIXE in healthy elderly and Parkinson’s disease
Iron is essential for neurons and glial cells, playing key roles in neurotransmitter synthesis, energy production and myelination. In contrast, high concentrations of free iron can be detrimental and contribute to neurodegeneration, through promotion of oxidative stress. Particularly in Parkinson’s disease (PD) changes in iron concentrations in the substantia nigra (SN) was suggested to play a key role in degeneration of dopaminergic neurons in nigrosome 1. However, the cellular iron pathways and the mechanisms of the pathogenic role of iron in PD are not well understood, mainly due to the lack of quantitative analytical techniques for iron quantification with subcellular resolution. Here, we quantified cellular iron concentrations and subcellular iron distribution in dopaminergic neurons and different types of glial cells in the SN both in brains of PD patients and in non-neurodegenerative control brains (Co). To this end, we combined spatially resolved quantitative element mapping using micro particle induced X-ray emission (μPIXE) with nickel-enhanced immunocytochemical detection of cell type-specific antigens allowing to allocate element-related signals to specific cell types. Distinct patterns of iron accumulation were observed across different cell populations. In the control (Co) SNc, oligodendroglial and astroglial cells hold the highest cellular iron concentration whereas in PD, the iron concentration was increased in most cell types in the substantia nigra except for astroglial cells and ferritin-positive oligodendroglial cells. While iron levels in astroglial cells remain unchanged, ferritin in oligodendroglial cells seems to be depleted by almost half in PD. The highest cellular iron levels in neurons were located in the cytoplasm, which might increase the source of non-chelated Fe3+, implicating a critical increase in the labile iron pool. Indeed, neuromelanin is characterised by a significantly higher loading of iron including most probable the occupancy of low-affinity iron binding sites. Quantitative trace element analysis is essential to characterise iron in oxidative processes in PD. The quantification of iron provides deeper insights into changes of cellular iron levels in PD and may contribute to the research in iron-chelating disease-modifying drugs
Fluorescent Nanozeolite Receptors for the Highly Selective and Sensitive Detection of Neurotransmitters in Water and Biofluids
The design and preparation of synthetic binders (SBs) applicable for small biomolecule sensing in aqueous media remains very challenging. SBs designed by the lock-and-key principle can be selective for their target analyte but usually show an insufficient binding strength in water. In contrast, SBs based on symmetric macrocycles with a hydrophobic cavity can display high binding affinities but generally suffer from indiscriminate binding of many analytes. Herein, a completely new and modular receptor design strategy based on microporous hybrid materials is presented yielding zeolite-based artificial receptors (ZARs) which reversibly bind the neurotransmitters serotonin and dopamine with unprecedented affinity and selectivity even in saline biofluids. ZARs are thought to uniquely exploit both the non-classical hydrophobic effect and direct non-covalent recognition motifs, which is supported by in-depth photophysical, and calorimetric experiments combined with full atomistic modeling. ZARs are thermally and chemically robust and can be readily prepared at gram scales. Their applicability for the label-free monitoring of important enzymatic reactions, for (two-photon) fluorescence imaging, and for high-throughput diagnostics in biofluids is demonstrated. This study showcases that artificial receptor based on microporous hybrid materials can overcome standing limitations of synthetic chemosensors, paving the way towards personalized diagnostics and metabolomics
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