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31.
Using a new experimental technique, based on the scattering of ultrasounds, we perform a direct measurement of particle velocities, in a fully turbulent flow. This allows us to approach intermittency in turbulence from a dynamical point of view and to analyze the Lagrangian velocity fluctuations in the framework of random walks. We find experimentally that the elementary steps in the walk have random uncorrelated directions but a magnitude that is extremely long range correlated in time. Theoretically, a Langevin equation is proposed and shown to account for the observed one- and two-point statistics. This approach connects intermittency to the dynamics of the flow. 相似文献
32.
Kempa K Zhou Y Engelbrecht JR Bakshi P Ha HI Moser J Naughton MJ Ulrich J Strasser G Gornik E Unterrainer K 《Physical review letters》2002,88(22):226803
We show theoretically that in quantum wells subjected to a strong magnetic field the intersubband current peaks at magnetic field values, which reveal the underlying specific intersubband scattering mechanism. We have designed and grown a superlattice structure in which such current oscillations are clearly visible, and in which the transition from the purely single-electron to the mixed single- and two-electron scattering regimes can be observed by tuning the applied voltage bias. The measurements were conducted in ultrahigh magnetic fields (up to 45 T) to obtain the full spectrum of the current oscillations. 相似文献
33.
Treussart F Alléaume R Le Floc'h V Xiao LT Courty JM Roch JF 《Physical review letters》2002,89(9):093601
We studied intensity fluctuations of a single photon source relying on the pulsed excitation of the fluorescence of a single molecule at room temperature. We directly measured the Mandel parameter Q(T) over 4 orders of magnitude of observation time scale T by recording every photocount. On time scale of a few excitation periods, sub-Poissonian statistics is clearly observed and the probablility of two-photons events is 10 times smaller than Poissonian pulses. On longer times, blinking in the fluorescence, due to the molecular triplet state, produces an excess of noise. 相似文献
34.
Multi-echo spiral and echo-planar (EPI) imaging sequences were compared in functional imaging experiments at 3 Tesla. Both sequence types allow calculation of the effective transversal relaxation time T(2)* and the initial signal intensity I(0). These parameters can be used in evaluation of the functional signal with respect to inflow effects and other vascular sources. Prior to functional magnetic resonance imaging (fMRI) experiments T(2)* measurements in the human brain were performed with single- and multi-echo FLASH (fast low angle shot) and compared with EPI und spiral imaging sequences. These experiments resulted in T(2)* values ranging from 42.9 to 53.8 ms in a ROI including white and gray matter and CSF in a prefrontal brain region, and allowed validation of the quantitative results of the fast single-shot techniques. In functional experiments with motor stimulation mean absolute T(2)* increases during stimulation of 1.1 +/- 0.6 ms and 1.4 +/- 0.9 ms were found with multi-echo EPI and spiral imaging, respectively, averaged over the activated pixels. In addition, absolute T(2)* values and the size of activated areas obtained with both sequences are comparable. In these investigations spiral imaging allowed higher spatial resolution due to more efficient use of available gradient performance. 相似文献
35.
Kolbitsch C Lorenz IH Hörmann C Hinteregger M Löckinger A Moser PL Kremser C Schocke M Felber S Pfeiffer KP Benzer A 《Magnetic resonance imaging》2002,20(7):535-541
Conflicting results reported on the effects of hyperoxia on cerebral hemodynamics have been attributed mainly to methodical and species differences. In the present study contrast-enhanced magnetic resonance imaging (MRI) perfusion measurement was used to analyze the influence of hyperoxia (fraction of inspired oxygen (FiO2) = 1.0) on regional cerebral blood flow (rCBF) and regional cerebral blood volume (rCBV) in awake, normoventilating volunteers (n = 19). Furthermore, the experiment was repeated in 20 volunteers for transcranial Doppler sonography (TCD) measurement of cerebral blood flow velocity in the middle cerebral artery (CBFV(MCA)). When compared to normoxia (FiO2 = 0.21), hyperoxia heterogeneously influenced rCBV (4.95 +/- 0.02 to 12.87 +/- 0.08 mL/100g (FiO2 = 0.21) vs. 4.50 +/- 0.02 to 13.09 +/- 0.09 mL/100g (FiO2 = 1.0). In contrast, hyperoxia diminished rCBF in all regions (68.08 +/- 0.38 to 199.58 +/- 1.58 mL/100g/min (FiO2 = 0.21) vs. 58.63 +/- 0.32 to 175.16 +/- 1.51 mL/100g/min (FiO2 = 1.0)) except in parietal and left frontal gray matter. CBFV(MCA) remained unchanged regardless of the inspired oxygen fraction (62 +/- 9 cm/s (FiO2 = 0.21) vs. 64 +/- 8 cm/s (FiO2 = 1.0)). Finding CBFV(MCA) unchanged during hyperoxia is consistent with the present study's unchanged rCBF in parietal and left frontal gray matter. In these fronto-parietal regions predominantly fed by the middle cerebral artery, the vasoconstrictor effect of oxygen was probably counteracted by increased perfusion of foci of neuronal activity controlling general behavior and arousal. 相似文献
36.
Moser J Boscá F Lovell WW Castell JV Miranda MA Hye A 《Journal of photochemistry and photobiology. B, Biology》2000,58(1):13-19
Carprofen is a non-steroidal antiinflammatory drug with marked photosensitising properties. In order to elucidate the mechanisms underlying the phenomenon of drug-protein photobinding, mixtures of the drug and human serum albumin were irradiated under different experimental conditions. After irradiation and subsequent gel-filtration chromatography of the photomixture, the eluting protein fraction was analysed by means of fluorescence spectroscopy. The formation of drug-protein photoadducts could be evidenced by the characteristic emission properties of the carbazole chromophore. The photobinding of the drug to human serum albumin appears to involve the formation of aryl radicals resulting from carbon-halogen photocleavage. This mechanistic interpretation is supported by the observed variations in the intensity of the fluorescence spectra, which can be correlated with the lower quantum yield emission of chlorocarbazoles as compared to non-halogenated analogues. The results from laser flash photolysis studies are also in agreement with this proposal. 相似文献
37.
Diode-pumped passively mode-locked Nd:YAG laser with 10-W average power in a diffraction-limited beam 总被引:2,自引:0,他引:2
Spühler GJ Paschotta R Keller U Moser M Dymott MJ Kopf D Meyer J Weingarten KJ Kmetec JD Alexander J Truong G 《Optics letters》1999,24(8):528-530
We present a passively mode-locked Nd:YAG laser with 10.7-W average output power in a diffraction-limited beam. Stable self-starting mode locking with a pulse duration of 16 ps and a pulse energy of 120 nJ is obtained with a semiconductor saturable-absorber mirror. The laser is directly side pumped with two 20-W diode bars. Single-pass frequency doubling in an external 5-mm-thick KTP crystal yields 3.2-W average power at 532 nm. 相似文献
38.
Thomas Moser 《Compositio Mathematica》1999,117(2):123-152
Let X be an arbitrary variety over a finite field k and p=char k,n N. We will construct a complex of étale sheaves on X together with trace isomorphism from the highest étale cohomology group of this complex onto Z/pnZ such that for every constructible Z/pnZ-sheaf on X the Yoneda pairing is a nondegenerate pairing of finite groups. If X is smooth, this complex is the Gersten resolution of the logarithmic de Rham–Witt sheaf introduced by Gros and Suwa. The proof is based on the special case proven by Milne when the sheaf is constant and X is smooth, as well as on a purity theorem which in turn follows from a theorem about the cohomological dimension of Ci-fields due to Kato and Kuzumaki. If the existence of the Lichtenbaum complex is proven, the theorem will be the p-part of a general duality theorem for varieties over finite fields. 相似文献
39.
Flossmann G Folk R Moser G 《Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics》1999,60(1):779-791
We compare theoretical results for the shear viscosity calculated in one-loop order within the field-theoretical method of the renormalization-group theory with experiments. Our expressions describe the nonasymptotic crossover in both temperature and density, and allow us to consider effects of finite gravitation and finite frequency at which the experiments are performed. In doing so we treat the critical exponent x(eta) of the shear viscosity as an independent parameter, keeping the one-loop value of the Kawasaki amplitude fixed. Within our model we also consider the temperature and density dependence of the thermal diffusion including gravitational effects. 相似文献
40.
Zusammenfassung Bei den Arzneistoffen Dapson und Ethambutoldihydrochlorid liegen außergewöhnlich komplizierte Polymorphieverhältnisse vor. In beiden Fällen wurden vier Modifikationen nachgewiesen, beiDapson existiert außerdem noch ein Hydrat. Das Dapson-Anhydrat liegt als enantiotrope Mod. III vor, die bei 80° in Mod. II übergeht. Diese Form ist meist bis zu ihrem Schmelzpunkt beständig. Mod. I, die aus der Schmelze kristallisiert, ist ebenfalls mit Mod. II enantiotrop. Außerdem besteht zwischen Mod. I und Mod. III Enantiotropie. Die Umwandlung Mod. IIIMod. II erfolgt spontan und unabhängig von Keimen, so daß der Schmelzpunkt von Mod. III nicht bestimmt werden kann. Bei Mod. IV konnte keine Enantiotropie mit einer anderen Kristallform nachgewiesen werden.
Ethambutoldihydrochlorid weist nur enantiotrope Umwandlungen durch Umklappen ganzer Kristalle in das Gitter der anderen Modifikation auf, so daß nur der Schmelzpunkt der Hochtemperaturform bestimmbar ist. Die Tieftemperaturform besteht aus Mod. II, Mod. IV ist bei Raumtemperatur metastabil, während die instabile Mod. III nur ein ganz schmales Existenzgebiet zwischen ca. 40 und 45° besitzt, da sie sowohl mit Mod. IV als mit Mod. I enantiotrop ist.
On the polymorphism of dapsone and ethambutol dihydrochloride
Summary There are extraordinarily complicated polymorphism conditions in the drugs dapsone and ethambutol chloride. In both cases, four modifications were detected, and in the case of dapsone there exists a hydrate in addition. The dapsone anhydrate is present as enantiotropic Mod. III, which transforms into Mod. II at 80°. This form is mostly stable up to its melting point. Mod. I, which crystallizes from the melt, is likewise enantiotropic with Mod. II. In addition, there is enantiotropy between Mod. I and Mod. III. The transformation of Mod. IIIMod. II takes place continuously and independently from nuclei, so that the melting point of Mod. III cannot be determined. In Mod. IV, no enantiotropy with another crystal form could be detected.Ethambutol dihydrochloride only displays enantiotropic transformations by folding over of entire crystals into the lattice of the other modification, so that only the melting point of the high temperature form can be determined. The low temperature form consists of Mod. II, Mod. IV is metastable at room temperature, whereas the unstable Mod. III only possesses a quite narrow range of existence between about 40 and 45°, since it is enantiotropic both with Mod. IV and with Mod. I.相似文献