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101.
We study a stochastic neural-network model in which neurons and synapses change with a priori probability p and 1–p, respectively, in the limit p0. This implies neuron activity competing with fast fluctuations of the synaptic connections—in fact, random oscillations around values given by a learning (for example, Hebb's) rule. The consequences for the system performance of a dynamics constantly checking at random the set of memorized patterns is thus studied both analytically and numerically. We describe various nonequilibrium phase transitions whose nature depends on the properties of fluctuations. We find, in particular, that under rather general conditions locally stable mixture states do not occur, and pattern recognition and retrieval processes are substantially improved for some classes of synaptic fluctuations. 相似文献
102.
We report what is believed to be the first observation of a nonlinear mode in a cylindrical nonlinear Fabry-Perot cavity. The field enhancement from cavity buildup, as well as the large chi((3)) optical nonlinearity that is due to resonantly excited (85)Rb vapor, allows the nonlinear mode to form at low incident optical powers of less than 1 mW. The mode is observed to occur for both self-focusing and self-defocusing nonlinearity. 相似文献
103.
The fluorimetric properties of the oxime, thiosemicarbazone, azine and 2-pyridylhydrazone of 3-hydroxypyridine-2-aldehyde have been studied; quantum yields have been measured in ethanol-water and ethanol-dimethylformamide media. Two kinetic methods are described for the determination of trace amounts of manganese(II), based on its catalytic effect on the oxidation of the thiosemicarbazone and of the azine by hydrogen peroxide. The reactions are followed by measuring the rate of change of the fluorescence. The calibration graph is linear over the manganese range 5-50 ng ml with a precision of +/- 3.0% for the thiosemicarbazone, and 0.4-0.9 mug ml with a precision of +/-2.0% for the azine. Interferences by foreign ions have been investigated. 相似文献
104.
Refraction indices at room temperature and the temperature dependence of the spontaneous birefringence have been measured in ferroelastic lead orthophosphate. The 180°C phase transition in this material has been determined as a thermodynamic first order transition. 相似文献
105.
106.
R. Torres M. Jadraque M. Martín 《Applied Physics A: Materials Science & Processing》2004,79(4-6):1057-1060
The mechanism of laser ablation of 2Cu(CO3)·Cu(OH)2 at 308 nm is investigated by time-of-flight mass spectroscopy at laser fluences in the range of 0.07 to 0.6 Jcm-2. The neutral and ion composition of the plume reveals the presence of Cu, Cu2O, CuO, H2O and CO2, which appear as free species or forming clusters. Plume composition is compatible with a thermal ablation mechanism which involves chemical processes similar to those reported for the purely thermal decomposition of the target. Velocity distributions of neutral species in the plume are measured and fitted to shifted Maxwell–Boltzmann time distributions; the best fitting yields flow velocities of 0.07 cms-1 for CO2 and Cu and 0.04 cms-1 for Cu2, which is compatible with an expansion regime characterized by weak interaction of the ejected particles. PACS 82.30.Nr; 78.70.-g 相似文献
107.
Relative importance of hydrogen bonding and coordinating groups in modulating the zinc-water acidity
Mareque-Rivas JC Prabaharan R de Rosales RT 《Chemical communications (Cambridge, England)》2004,(1):76-77
The presence of second-sphere -NH(2) groups in the proximity of a zinc(ii)-bound water molecule enhances its acidity by ca. 2 pK(a) units. 相似文献
108.
Mareque-Rivas JC Torres Martín de Rosales R Parsons S 《Chemical communications (Cambridge, England)》2004,(5):610-611
Amino H-bond donors adjacent to a zinc(II) centre increase the affinity of phosphates to the zinc(II) centre. 相似文献
109.
Subphthalocyanines: tuneable molecular scaffolds for intramolecular electron and energy transfer processes 总被引:1,自引:0,他引:1
González-Rodríguez D Torres T Guldi DM Rivera J Herranz MA Echegoyen L 《Journal of the American Chemical Society》2004,126(20):6301-6313
A series of four subphthalocyanine-C(60) fullerene dyads have been prepared through axial functionalization of the macrocycle with m-hydroxybenzaldehyde and a subsequent dipolar cycloaddition reaction. The subphthalocyanine moiety has been peripherally functionalized with substituents of different electronic character, namely fluorine or iodine atoms and ether or amino groups, thus reaching a control over its electron-donating properties. This is evidenced in cyclic voltammetry experiments by a progressive shift to lower potentials, by ca. 200 mV, of the first oxidation event of the SubPc unit in the dyads. As a consequence, the energy level of the SubPc(*)(+)-C(60)(*)(-) charge-transfer state may be tuned so as to compete with energy transfer deactivation pathways upon selective excitation of the SubPc component. For instance, excitation of those systems where the level of the radical pair lies high in energy triggers a sequence of exergonic photophysical events that comprise (i) nearly quantitative singlet-singlet energy transfer to the C(60) moiety, (ii) fullerene intersystem crossing, and (iii) triplet-triplet energy transfer back to the SubPc. On the contrary, the stabilization of the SubPc(*)(+)-C(60)(*)(-) radical pair state by increasing the polarity of the medium or by lowering the donor-acceptor redox gap causes charge transfer to dominate. In the case of 1c in benzonitrile, the thus formed radical pair has a lifetime of 0.65 ns and decays via the energetically lower lying triplet excited state. Further stabilization is achieved for dyad 1d, whose charge-transfer state would lie now below both triplets. The radical pair lifetime consequently increases in more than 2 orders of magnitude with respect to 1c and presents a significant stabilization in less polar solvents, revealing a low reorganization energy for this kind of SubPc-C(60) systems. 相似文献
110.
We show theoretically that a chirped quasi-phase-matching nonlinear crystal structure can significantly enhance the axial resolution in quantum optical coherence tomography by increasing the spectral width of the generated entangled photon pairs. For pulsed pumps we show how the pump-pulse duration affects the maximum resolution attainable. 相似文献