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51.
We discuss equilibrium shapes of crystals attached to walls. Optimal shapes for different configurations of walls are found and the minimality of the overall surface tension is proven with the help of a simple geometrical argument based on the isoperimetric inequality and monotonicity. Stability results in the form of Bonnesen inequalities are obtained in the two-dimensional case. 相似文献
52.
Ulrich Dirnagl Ute Lindauer Andreas Them Walter Pfister Karl Max Einh upl Arno Villringer 《Micron (Oxford, England : 1993)》1993,24(6):611-622
The location of the brain within the cranium has prevented in vivo studies with microscopical resolution for a long time. Cranial window techniques provide microscopical access to the brain surface, but subsurface structures cannot be visualized with conventional microscopy. Confocal microscopy with its increased depth of penetration and optical sectioning capabilities is ideally suited for the investigation of thick biological specimens. Due to the scanning process, however, temporal resolution is limited, a significant disadvantage in the in vivo setting. In this article we demonstrate that confocal laser scanning microscopy can be utilized to study brain cortex microvascular morphology, capillary hemodynamics, leukocyte behaviour and intracellular ion concentrations in anesthetized rats through a closed cranial window. Three different confocal microscopes are compared: a Biorad MRC-600 with multiline Kr/Ar-Laser (488/568/647 nm), a Noran Odyssey acousto-optic scanning microscope with multiline Ar-Laser (458/488/514/529 nm) and a Biorad Viewscan DVC-250 slit scanning microscope with Ar-Laser (488/514 nm). With all microscopes a Zeiss × 40 water immersion objective, n.a. 0.75 is used. A Laser-Doppler flowmeter continuously measures regional cerebral blood flow in the area of microscopical investigation. As fluorescent dyes we used: fluorescein sodium as blood plasma marker (given I.V.); rhodamine 6G to label leukocytes (given I.V.); and the AM-esters of BCECF (pH-sensitive), Fluo-3 and Calcium-Green (both calcium-sensitive) as intracellular ion-concentration markers (loaded via superfusion over the cranial window). With this setup, we are able to study the flow dynamics in the capillary network of the cortex (erythrocyte flow velocities and flux rates), the behaviour of leukocytes in capillaries and postcapillary veins (plugging of capillaries, adhesion to the endothelium, extravasation into the parenchyma), and intracellular changes of [H+] and [Ca2+] under physiological and pathophysiological conditions (cerebral ischemia and meningitis). The comparison between the conventional CLSM (Biorad 600) and the real time CLSMs revealed that the increase in temporal resolution afforded by the real time instruments is offset by a reduction in spatial resolution and, most importantly, in the signal to noise ratio, resulting in a lower depth of penetration into the tissue and necessitating frame averaging. 相似文献
53.
We report an experimental demonstration of a heterodyne polarization rotation measurement with a noise floor 4.8 dB below the optical shot noise by use of classically phase-locked quantum twin beams emitted above threshold by an ultrastable type II Na:KTP cw optical parametric oscillator. We believe that this is the largest noise reduction achieved to date in optical phase-difference measurements. 相似文献
54.
A new concept for continuous measurements on microchips is presented. A PMMA (polymethylmethacrylate) based capillary electrophoresis chip with integrated conductivity detection is combined with a second chip, which undertakes the task of fluid handling and electrical connections. The combination of electrokinetic and hydrodynamic flows allows long-term continuous stable analyses with good reproducibilities of migration time and peak heights of analytes. The two-chip system is characterized in terms of stability and reproducibility of separation and detection of small ions. Relative standard deviations of <1% and 3% respectively for retention times and peak heights during long-term measurements can be achieved. The new system combines simple handling and automated analysis without the need for refilling, cleaning or removal of the separation chip after one or several measurements. 相似文献
55.
We report luminescence spectra with spatial and spectral resolution which show the predominant effect of lithium on optical and electrical properties of annealed ZnTe. Lithium is identified in photoluminescence spectra at 5 K whereas SEM visualizes the diffusion of this impurity. We can also observe the effect of high impurity concentration of luminescence with the apparition of excitons bound to LiLi pairs. 相似文献
56.
Helmut Pfister 《manuscripta mathematica》1973,10(3):273-287
Let E be a real or complex linear space, s and t linear topologies on E, ts. We define some kind of mixed topology m(s, t) such that tm(s, t)s. The linear topology m(s, t) is characterized by a theorem of Banach-Steinhaus type and we show that the condition t=m(s, t) covers various concepts of barrelledness. Secondly we show that s and m(s, t) induce the same uniformity on certain subsets of E, if there is a base of s-neighbourhoods of O which are t-closed; this theorem will be a generalization of a well known theorem of A. Grothendieck on (DF)-spaces. 相似文献
57.
N. Magnea D. Bensahel J.L. Pautrat K. Saminadayar J.C. Pfister 《Solid State Communications》1979,30(5):259-263
Capacitance, photoluminescence and infra-red absorption experiments have been performed on copper doped ZnTe single crystals. Comparative measurements on samples treated at various temperatures show clearly that Cu introduces a single acceptor level with high solubility. Spectroscopic results allow to identify the persistent acceptor “a” at 149 meV as copper on substitutional zinc site confirming that native defects are not responsible for the p-type conductivity of undoped ZnTe. 相似文献
58.
The mathematical problem of the existence of solutions for Newtonian dust stars is formulated, and proofs for the non-existence of such stars as isolated objects are given. It is however shown that such stars can be stabilized by reasonable exterior (strained) matter. Comments are made on the problem of Einsteinian rotating dust stars. 相似文献
59.
Quantum Zeno and anti-Zeno paradoxes 总被引:1,自引:0,他引:1
Continuous observation of a time independent projection operator is known to prevent change of state (the quantum Zeno paradox).
We discuss the recent result that generic continuous measurement of time dependent projection operators will in fact ensure
change of state: an anti-Zeno paradox. 相似文献
60.
Haelewaters Danny Richard F. Comont Serena Y. Zhao Donald H. Pfister 《科学通报(英文版)》2014,59(5-6):528-532
This study intended to find data on obligate ectoparasitic Laboulbeniales (Fungi, Ascomycota) on Chinese Harmonia axyridis (Coleoptera, Coccinellidae). After having screened over four thousand dried specimens of H. axyridis and close relatives, we present the first (historical) record of Chinese H. axyridis infected with Hesperomyces virescens. We suggest that H. virescens is a historically globally distributed species and hypothesize that (native) infection was lost when H. axyridis was introduced in North America. 相似文献