195 research outputs found
Experimental evidence for the formation of stripe phases in Si/SiGe
We observe pronounced transport anisotropies in magneto-transport experiments
performed in the two-dimensional electron system of a Si/SiGe heterostructure.
They occur when an in-plane field is used to tune two Landau levels with
opposite spin to energetic coincidence. The observed anisotropies disappear
drastically for temperatures above 1 K. We propose that our experimental
findings may be caused by the formation of a unidirectional stripe phase
oriented perpendicular to the in-plane field.Comment: 4 pages, 3 figure
Valley splitting of Si/SiGe heterostructures in tilted magnetic fields
We have investigated the valley splitting of two-dimensional electrons in
high quality Si/SiGe heterostructures under tilted magnetic fields.
For all the samples in our study, the valley splitting at filling factor
() is significantly different before and after the
coincidence angle, at which energy levels cross at the Fermi level. On both
sides of the coincidence, a linear density dependence of on the
electron density was observed, while the slope of these two configurations
differs by more than a factor of two. We argue that screening of the Coulomb
interaction from the low-lying filled levels, which also explains the observed
spin-dependent resistivity, is responsible for the large difference of
before and after the coincidence.Comment: REVTEX 4 pages, 4 figure
Nanoimprinted superlattice metallic photonic crystal as ultraselective solar absorber
A two-dimensional superlattice metallic photonic crystal (PhC) and its fabrication by nanoimprint lithography on tantalum substrates are presented. The superior tailoring capacity of the superlattice PhC geometry is used to achieve spectrally selective solar absorption optimized for high-temperature and high-efficiency solar-energy-conversion applications. The scalable fabrication route by nanoimprint lithography allows for a high-throughput and high-resolution replication of this complex pattern over large areas. Despite the high fill factor, the pattern of polygonal cavities is accurately replicated into a resist that hardens under ultraviolet radiation over an area of 10 mm². In this way, cavities of 905 nm and 340 nm width are achieved with a period of 1 μm. After pattern transfer into tantalum via a deep reactive ion-etching process, the achieved cavities are 2.2 μm deep, separated by 85–95 nm wide ridges with vertical sidewalls. The room-temperature reflectance spectra of the fabricated samples show excellent agreement with simulated results, with a high spectral absorptance approaching blackbody absorption in the range from 300 to 1900 nm and a steep cutoff. The calculated solar absorptivity of this superlattice PhC is 96% and its thermal transfer efficiency is 82.8% at an operating temperature of 1500 K and an irradiance of 1000 kW/m².United States. Army Research Office (W911NF-13-D-0001)United States. Department of Energy (DE-SC0001299
Screening Breakdown on the Route toward the Metal-Insulator Transition in Modulation Doped Si/SiGe Quantum Wells
Exploiting the spin resonance of two-dimensional (2D) electrons in SiGe/Si
quantum wells we determine the carrier-density-dependence of the magnetic
susceptibility. Assuming weak interaction we evaluate the density of states at
the Fermi level D(E_F), and the screening wave vector, q_TF. Both are constant
at higher carrier densities n, as for an ideal 2D carrier gas. For n < 3e11
cm-2, they decrease and extrapolate to zero at n = 7e10 cm-2. Calculating the
mobility from q_TF yields good agreement with experimental values justifying
the approach. The decrease in D(E_F) is explained by potential fluctuations
which lead to tail states that make screening less efficient and - in a
positive feedback - cause an increase of the potential fluctuations. Even in
our high mobility samples the fluctuations exceed the electron-electron
interaction leading to the formation of puddles of mobile carriers with at
least 1 micrometer diameter.Comment: 4 pages, 3 figure
Interaction effects at the magnetic-field induced metal-insulator transition in Si/SiGe superlattices
A metal-insulator transition was induced by in-plane magnetic fields up to 27
T in homogeneously Sb-doped Si/SiGe superlattice structures. The localisation
is not observed for perpendicular magnetic fields. A comparison with
magnetoconductivity investigations in the weakly localised regime shows that
the delocalising effect originates from the interaction-induced spin-triplet
term in the particle-hole diffusion channel. It is expected that this term,
possibly together with the singlet particle-particle contribution, is of
general importance in disordered n-type Si bulk and heterostructures.Comment: 5 pages, 3 figures, Solid State Communications, in prin
Valley splitting in a Si/SiGe quantum point contact
We present the theory and measurement of valley splitting in a quantum point
contact (QPC) in a modulation doped Si/SiGe heterostructure. Our measurements
are performed on a submicron Schottky-gated device. An effective mass theory is
developed for a QPC formed in a quantum well, grown on a miscut substrate. Both
theory and experiments include a perpendicular magnetic field. Our results
indicate that both QPC and magnetic confinement can enhance the valley
splitting by reducing the spatial extent of the electronic wavefunction.
Consequently, the valley splitting can be much larger than the spin splitting
for small magnetic fields. We also observe different valley splittings for
different transverse modes in the QPC, supporting the notion that when steps
are present at the quantum well interface, the spatial extent of the
wavefunction plays a dominant role in determining the valley splitting.Comment: 23 pages, 14 figure
New adipokines vaspin and omentin. Circulating levels and gene expression in adipose tissue from morbidly obese women
<p>Abstract</p> <p>Background</p> <p>Vaspin and omentin are recently described molecules that belong to the adipokine family and seem to be related to metabolic risk factors. The objectives of this study were twofold: to evaluate vaspin and omentin circulating levels and mRNA expression in subcutaneous and visceral adipose tissues in non-diabetic morbidly obese women; and to assess the relationship of vaspin and omentin with anthropometric and metabolic parameters, and other adipo/cytokines.</p> <p>Design</p> <p>We analysed vaspin and omentin circulating levels in 71 women of European descent (40 morbidly obese [BMI ≥ 40 kg/m<sup>2</sup>] and 31 lean [BMI ≤ 25]). We assessed vaspin and omentin gene expression in paired samples of visceral and subcutaneous abdominal adipose tissue from 46 women: 40 morbidly obese and 6 lean. We determined serum vaspin and plasma omentin levels with an Enzyme-Linked Immunosorbent Assay and adipose tissue mRNA expression by real time RT-PCR.</p> <p>Results</p> <p>Serum vaspin levels in the morbidly obese were not significantly different from those in controls. They correlated inversely with levels of lipocalin 2 and interleukin 6. Vaspin mRNA expression was significantly higher in the morbidly obese, in both subcutaneous and visceral adipose tissue.</p> <p>Plasma omentin levels were significantly lower in the morbidly obese and they correlated inversely with glucidic metabolism parameters. Omentin circulating levels, then, correlated inversely with the metabolic syndrome (MS). Omentin expression in visceral adipose tissue was significantly lower in morbidly obese women than in controls.</p> <p>Conclusions</p> <p>The present study indicates that vaspin may have a compensatory role in the underlying inflammation of obesity. Decreased omentin circulating levels have a close association with MS in morbidly obese women.</p
Size and surface charge of gold nanoparticles determine absorption across intestinal barriers and accumulation in secondary target organs after oral administration
It is of urgent need to identify the exact physico-chemical characteristics which allow maximum uptake and accumulation in secondary target organs of nanoparticulate drug delivery systems after oral ingestion. We administered radiolabelled gold nanoparticles in different sizes (1.4-200 nm) with negative surface charge and 2.8 nm nanoparticles with opposite surface charges by intra-oesophageal instillation into healthy adult female rats. The quantitative amount of the particles in organs, tissues and excrements was measured after 24 h by gamma-spectroscopy. The highest accumulation in secondary organs was mostly found for 1.4 nm particles; the negatively charged particles were accumulated mostly more than positively charged particles. Importantly, 18 nm particles show a higher accumulation in brain and heart compared to other sized particles. No general rule accumulation can be made so far. Therefore, specialized drug delivery systems via the oral route have to be individually designed, depending on the respective target organ
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