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831.
We find a renormalized "time-dependent diffusion coefficient," D(t), for pulsed excitation of a nominally diffusive sample by solving the Bethe-Salpeter equation with recurrent scattering. We observe a crossover in dynamics in the transformation from a quasi-1D to a slab geometry implemented by varying the ratio of the radius, R, to the length, L, of the cylindrical sample with reflecting side walls and open ends. Immediately after the peak of the transmitted pulse, D(t) falls linearly with a nonuniversal slope that approaches an asymptotic value for R/L>1. The value of D(t) extrapolated to t=0 depends only upon the dimensionless conductance g for R/L<1 and only upon kl(0) for R/L>1, where k is the wave vector and l(0) is the bare mean free path. 相似文献
832.
We have observed angular anisotropy in weak localization of light from highly scattering, orientationally ordered, nematic liquid crystals. This demonstration of angular anisotropy in a multiple-scattering interference phenomenon was facilitated by a light scattering instrument with extraordinary angular resolution. The measured anisotropies were consistent with a simple model of coherent backscattering generalized for propagation-direction dependent mean free paths. 相似文献
833.
Background
Delays between actions and their outcomes severely hinder reinforcement learning systems, but little is known of the neural mechanism by which animals overcome this problem and bridge such delays. The nucleus accumbens core (AcbC), part of the ventral striatum, is required for normal preference for a large, delayed reward over a small, immediate reward (self-controlled choice) in rats, but the reason for this is unclear. We investigated the role of the AcbC in learning a free-operant instrumental response using delayed reinforcement, performance of a previously-learned response for delayed reinforcement, and assessment of the relative magnitudes of two different rewards. 相似文献834.
Cheung EY Kitchin SJ Harris KD Imai Y Tajima N Kuroda R 《Journal of the American Chemical Society》2003,125(48):14658-14659
A three-component molecular cocrystal material has been prepared by a solvent-free route involving mechanical grinding of the pure phases of the individual components. This material is not accessible from conventional solution-state crystallization procedures. Due to the fact that the grinding procedure intrinsically leads to a microcrystalline powder sample, the use of powder X-ray diffraction data is essential for structure determination. This work emphasizes the scope and utility of ab initio structure solution directly from powder X-ray diffraction data for carrying out structural characterization of new materials prepared via the solid-state grinding route, leading to the opportunity to establish structure-property relationships for such materials. 相似文献
835.
836.
A. Albrecht S.K. Cheung K.S. Leung C.K. Wong 《Computational Optimization and Applications》2001,19(2):179-208
This paper deals with the minimization of local forces in two-dimensional placements of flexible objects within rigid boundaries. The objects are disks of the same size but, in general, of different materials. Potential applications include the design of new amorphous polymeric and related granular materials as well as the design of package cushioning systems. The problem is considered on a grid structure with a fixed step size w and for a fixed diameter of the discs, i.e., the number of placed disks may increase as the size of the placement region increases. The near-equilibrium configurations have to be calculated from uniformly distributed random initial placements. The final arrangements of disks must ensure that any particular object is deformed only within the limits of elasticity of the material. The main result concerns -approximations of the probability distribution on the set of equilibrium placements. Under a natural assumption about the configuration space, we prove that a run-time of n+logO(1)(1/} is sufficient to approach with probability 1 – the minimum value of the objective function, where depends on the maximum of the escape depth of local minima within the underlying energy landscape. The result is derived from a careful analysis of the interaction among probabilities assigned to configurations from adjacent distance levels to minimum placements. The overall approach for estimating the convergence rate is relatively independent of the particular placement problem and can be applied to various optimization problems with similar properties of the associated landscape of the objective function. 相似文献
837.
838.
Yak-Fa Cheung 《Tetrahedron letters》1979,20(40):3809-3810
N-Bromosuccinimide was shown to oxidize aldehydes to the corresponding acid bromides. 相似文献
839.
Linder B.P. En W.G. Cheung N.W. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1998,26(6):1628-1634
The effect of wells and substrate type on gate oxide charging damage during plasma processing, and more specifically plasma immersion ion implantation, is modeled. The simulation combines the equations governing the plasma currents and integrated circuit device models to determine the gate oxide stressing voltage during implantation. Depending on the substrate type and the surface potential (Vs), a depletion region may exist, reducing the gate oxide voltage, and hence the gate oxide damage. In addition, well structures, by the nature of their capacitance, modulate Vs, altering the oxide stressing voltage. For most PIII implant conditions, gate oxides with p-type channel doping will be damaged more than those oxides with n-type channel doping. Experimental results confirm the substrate and well effects on plasma charging damage 相似文献
840.
Jones E.C. Cheung N.W. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1997,25(1):42-52
Plasma immersion ion implantation (PIII) is a large-area doping technique that can provide very high implant current at very low implant energy. Multiple ion species, plasma conditions, and implanter current-voltage waveforms in PIII lead to an exponential implant profile which is different from conventional implant profiles. A methodology is developed for using in situ measurements of the implanter current (I) and implant voltage (V) to derive an energy spectrum for a single implant pulse. The advantage of this technique is that a per-pulse profile may be determined experimentally, even in the presence of substrate etching, without any need for an implant model. If the ion species concentrations in the plasma are known, the energy spectrum found from the ion current and voltage waveforms can be used to construct a per-pulse implant profile. If the ion species distribution is not known a priori for a multispecies plasma, secondary ion mass spectroscopy (SIMS) data from an implanted sample can be used to estimate the ion species distribution and calibrate the IV-generated profile within a factor of two. Data from 1-5 kV, 2.5-5 kHz BF3 PIII implants are used to demonstrate the concept. The implant profile for a single pulse can then be used to project the final implant profile and total implanted dose as a function of implant time, Pm pulse frequency, and substrate etching. In this work, an estimated secondary electron yield function is used to separate the total implant current into ion and secondary electron current components. The 25% dose variation error introduced by this effective secondary electron yield function could be avoided in a system which can measure ion current directly 相似文献