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Moldovan L Melinte S Bayot V Papadakis SJ De Poortere EP Shayegan M 《Physical review letters》2000,85(20):4369-4372
We report very low temperature ( T) thermopower and resistivity ( rho) measurements on variable-density, two-dimensional hole systems confined to GaAs quantum wells. As the hole density is lowered from 1.49x10(11) cm(-2) to 0.14x10(11) cm(-2), the system crosses from an insulating ( drho / dT less, similar0) to a metallic regime ( drho / dT>0) and finally displays insulating behavior ( drho / dT<0). Diffusion thermopower shows a striking sign reversal in a narrow range of density in the metallic regime, suggesting a qualitative change in the conduction or the scattering mechanism. 相似文献
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It is common to schedule project activities first, then the utilization of equipment and its operating crew is planned based on such schedule. Real world experiences indicate that activity scheduling can be heavily impacted by the resources needed. In particular, if a project requires highly specialized and expensive equipment type, then one needs to take into account the schedule and cost of such resources in developing the project schedule. 相似文献
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It is shown theoretically that ultrashort terahertz electromagnetic pulses can be generated in resonant systems containing a quantum semiconductor superlattice. This effect is due to the compression of radiation incident on a cavity as a result of ultrafast self-modulation of the cavity Q factor upon cyclic switching of the superlattice from the screening to the transparent state in a pump field. 相似文献
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Intense laser waves can form a time-dependent gate, which transmits or reflects particles depending on their initial phases. When faced by a relativistic electron beam, such a barrier slices it by randomly scattering all but some particles, which nearly conserve their velocity. Subfemtosecond or attosecond periodic electron bunches are then formed downstream and can be used, for example, to generate coherent x rays via Thomson backscattering of the laser light. 相似文献
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EP Sheretov VS Gurov MV Dubkov OV Korneeva 《Rapid communications in mass spectrometry : RCM》1999,13(16):1699-1702
In this article we compare the classical monopole mass filter of von Zahn and the monopole mass filter with a hyperbolic V-shaped electrode. The experimental results and those of computer simulation for both mass spectrometers are presented. We show that the replacement of a conventional 90 degrees V-shaped electrode by an electrode with a hyperbolic profile substantially improves the peak shape of any given mass, and increases the mass resolution by a factor of 3-4 and the abundance sensitivity by a factor of 100. The potential of high analytical performance combined with electroforming techniques for electrode manufacture indicate future practical uses of such instruments. Copyright 1999 John Wiley & Sons, Ltd. 相似文献
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We show that dynamical localization of electrons in a thin semiconductor superlattice irradiated by a TM electromagnetic wave can lead to the formation of multistable states of the high-frequency field, which is accompanied by significant distortions of the reflection characteristics of superlattices. This effect is caused by the phenomenon of nonlinear plasma resonance and the related significant increase in the component of an alternating electric field, parallel to the superlattice axis, in the case where the real part of the corresponding component of the tensor of nonlinear dielectric permittivity becomes zero. It is found that the generation efficiency of odd harmonics of the radiation increases considerably under resonance conditions. 相似文献
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A Supply Chain (SC) requires undertaking considerable number of activities covering the flow of information and goods among multiple production and distribution cells over several tiers. The successful implementation of a SC hinges on the optimum integration and synchronization of these activities. 相似文献
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V. L. Bratman I. E. Dodin D. A. Lukovnikov 《International Journal of Infrared and Millimeter Waves》1999,20(6):991-1007
Realization of an effective single-mode oscillator based on the stimulated emission of radiation by relativistic electrons moving along a periodically corrugated waveguide, is theoretically shown to be possible in the regime, when the corrugation period is much larger than the operating wavelength. 相似文献