1,145 research outputs found
Aluminum arsenide cleaved-edge overgrown quantum wires
We report conductance measurements in quantum wires made of aluminum
arsenide, a heavy-mass, multi-valley one-dimensional (1D) system. Zero-bias
conductance steps are observed as the electron density in the wire is lowered,
with additional steps observable upon applying a finite dc bias. We attribute
these steps to depopulation of successive 1D subbands. The quantum conductance
is substantially reduced with respect to the anticipated value for a spin- and
valley-degenerate 1D system. This reduction is consistent with
disorder-induced, intra-wire backscattering which suppresses the transmission
of 1D modes. Calculations are presented to demonstrate the role of strain in
the 1D states of this cleaved-edge structure.Comment: Submitted to Applied Physics Letter
Electrochemical machining of stainless steel microelements with ultrashort voltage pulses
An electrochemical pulse technique enables the fabrication of three-dimensional microelements from stainless steel. The method is based on the application of ultrashort (nanosecond) voltage pulses, whereupon electrochemical reactions are locally confined with submicrometer precision. Employing properly shaped tool electrodes enables the machining of freestanding cantilevers or microstructures directly to a metal sheet. Due to gentle removal of the material, the grain structure of the material is revealed without any chemical or mechanical modifications. This is demonstrated by measuring the vibration frequency of a cantilever, and agrees well with the value derived from the bulk material properties
Nanoscale Electrochemistry
The lateral extension of electrochemically induced surface modifications is usually determined by the macroscopic size of the electrodes and the diffusion length of the reacting species. To overcome this constraint, we conducted an electrochemical reaction far from equilibrium. We applied short voltage pulses ( ≤100ns, up to ±4V) to a scanning tunneling microscope tip while imaging a Au(111) surface in concentrated electrolytes. They lead either to hole formation by anodic dissolution of the Au or to cathodic deposition of Cu islands (in the Cu2+ containing electrolyte), both of nanometer extension
Supersymmetric black holes in 2D dilaton supergravity: baldness and extremality
We present a systematic discussion of supersymmetric solutions of 2D dilaton
supergravity. In particular those solutions which retain at least half of the
supersymmetries are ground states with respect to the bosonic Casimir function
(essentially the ADM mass). Nevertheless, by tuning the prepotential
appropriately, black hole solutions may emerge with an arbitrary number of
Killing horizons. The absence of dilatino and gravitino hair is proven.
Moreover, the impossibility of supersymmetric dS ground states and of
nonextremal black holes is confirmed, even in the presence of a dilaton. In
these derivations the knowledge of the general analytic solution of 2D dilaton
supergravity plays an important role. The latter result is addressed in the
more general context of gPSMs which have no supergravity interpretation.
Finally it is demonstrated that the inclusion of non-minimally coupled
matter, a step which is already nontrivial by itself, does not change these
features in an essential way.Comment: 30 pages, LaTeX, v2: mayor revision (rearranged title, shortened
abstract, revised introduction, inserted section from appendix to main text,
added subsection with new material on non-SUGRA gPSMs, added clarifying
remarks at some places, updated references); v3: corrected minor misprints,
added note with a new referenc
A leed analysis of the (2×1)H-Ni(110) structure
A monolayer of H atoms adsorbed on Ni(110) below 180 K forms a (2×1) structure. The unit cell exhibits a glide symmetry plane and contains two adsorbed atoms. Based on a quantitative comparison between experimental and calculated LEED I/V spectra using standard R-factors the following structure was derived: On the clean Ni(110) surface the separation between the first two atomic layers, d12, is contracted by 8.5%±1.5% with respect to the bulk value; those between the second and third and the third and fourth layer, d23 and d34, are expanded by 3.5%±1.5% and 1%±1.5%, respectively—in agreement with recent other results. In the presence of the H adlayer the contraction of d12 is reduced to 4.5%±1.5%, while the expansion of d23 is not affected within the limits of accuracy. The third interlayer spacing d34 returns to its bulk value. The H atoms occupy threefold-coordinated sites formed by two Ni atoms from the first layer and one Ni atom from the second layer which confirms previous more qualitative conclusions based on He diffraction and vibrational spectroscopy. The bond lengths between H and its neighbouring Ni atoms were determined to be equal, namely 1.72±0.1 Å
Scanning tunneling microscopy observations on the reconstructed Au(111) surface: Atomic structure, long-range superstructure, rotational domains, and surface defects
MicroRNA-24 regulates vascularity after myocardial infarction
BACKGROUND: Myocardial infarction leads to cardiac remodeling and development of heart failure. Insufficient myocardial capillary density after myocardial infarction has been identified as a critical event in this process, although the underlying mechanisms of cardiac angiogenesis are mechanistically not well understood. METHODS AND RESULTS: Here, we show that the small noncoding RNA microRNA-24 (miR-24) is enriched in cardiac endothelial cells and considerably upregulated after cardiac ischemia. MiR-24 induces endothelial cell apoptosis, abolishes endothelial capillary network formation on Matrigel, and inhibits cell sprouting from endothelial spheroids. These effects are mediated through targeting of the endothelium-enriched transcription factor GATA2 and the p21-activated kinase PAK4, which were identified by bioinformatic predictions and validated by luciferase gene reporter assays. Respective downstream signaling cascades involving phosphorylated BAD (Bcl-XL/Bcl-2-associated death promoter) and Sirtuin1 were identified by transcriptome, protein arrays, and chromatin immunoprecipitation analyses. Overexpression of miR-24 or silencing of its targets significantly impaired angiogenesis in zebrafish embryos. Blocking of endothelial miR-24 limited myocardial infarct size of mice via prevention of endothelial apoptosis and enhancement of vascularity, which led to preserved cardiac function and survival. CONCLUSIONS: Our findings indicate that miR-24 acts as a critical regulator of endothelial cell apoptosis and angiogenesis and is suitable for therapeutic intervention in the setting of ischemic heart disease. [KEYWORDS: Animals, Apoptosis/drug effects, Arterioles/pathology, Capillaries/pathology, Cell Hypoxia, Cells, Cultured/drug effects/metabolism, Collagen, Drug Combinations, Drug Evaluation, Preclinical, Endothelial Cells/ metabolism/pathology, GATA2 Transcription Factor/biosynthesis/genetics, Gene Expression Profiling, Heart Failure/etiology, Heme Oxygenase-1/biosynthesis/genetics, Laminin, Male, Mice, Mice, Inbred C57BL, MicroRNAs/antagonists & inhibitors/genetics/ physiology, Myocardial Infarc
Electron-correlation effects in appearance-potential spectra of Ni
Spin-resolved and temperature-dependent appearance-potential spectra of
ferromagnetic Nickel are measured and analyzed theoretically. The Lander
self-convolution model which relates the line shape to the unoccupied part of
the local density of states turns out to be insufficient. Electron correlations
and orbitally resolved transition-matrix elements are shown to be essential for
a quantitative agreement between experiment and theory.Comment: LaTeX, 6 pages, 2 eps figures included, Phys. Rev. B (in press
Numerical study of a first-order irreversible phase transition in a CO+NO catalyzed reaction model
The first-order irreversible phase transitions (IPT) of the Yaldran-Khan
model (Yaldran-Khan, J. Catal. 131, 369, 1991) for the CO+NO reaction is
studied using the constant coverage (CC) ensemble and performing epidemic
simulations. The CC method allows the study of hysteretic effects close to
coexistence as well as the location of both the upper spinodal point and the
coexistence point. Epidemic studies show that at coexistence the number of
active sites decreases according to a (short-time) power law followed by a
(long-time) exponential decay. It is concluded that first-order IPT's share
many characteristic of their reversible counterparts, such as the development
of short ranged correlations, hysteretic effects, metastabilities, etc.Comment: 17 pages, 10 figure
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