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91.
V.B. Timofeev A.V. Larionov J. Zeman G. Martinez V.I. Falko M. Bayer A. Forchel 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(1):39-43
Magnetooptical studies performed on a wide InGaAs/GaAs single quantum well indicate that optically non-active (dark) excitons
with total angular momentum play the role of a reservoir for the creation of free multiparticle excitonic complexes. After analyzing the magnetic field
evolution of the circularly polarized components of the low energy structure appearing in the main excitonic luminescence
line we assign this feature to the excitonic trion formation. The binding energy of the excitonic trions was estimated to
be of the order of 1 meV.
Received: 29 October 1997 / Received in final form: 20 February 1998 / Accepted: 21 February 1998 相似文献
92.
This article summarizes the basic concepts of multiplication and provides some evidence that the traditional third‐grade curriculum and instruction emphasizing memorization of multiplication facts produces much less understanding of the basic concepts of multiplication than a standards‐based curriculum and instruction emphasizing construction of number sense and meaning for operations. This study also describes a collection of assessment tasks that provided meaningful evidence of children's understandings of basic multiplication concepts, including understandings of the relationships between multiplication and addition. 相似文献
93.
Susumu Saito Koichiro Umemoto Steven G. Louie Marvin L. Cohen 《Solid State Communications》2004,130(5):335-339
Using the ab initio pseudopotential total-energy method and the density-functional theory, we study the energetics of face-centered-cubic Cs3C60 which is a material of great interest as a possible high transition-temperature superconductor. At the optimized lattice constant the volume per C60 is found to be smaller than the the most-stable hexagon-coordination A15 phase, while the total energy of the fcc phase is about 0.9 eV higher than the A15 phase. These results indicate that a low-temperature and high-pressure synthesis method might be a possible way to produce the fcc Cs3C60 phase. In addition, it is also found that the A15 Cs3C60 should show a phase transformation from a hexagon-coordination phase to a pentagon-coordination phase under hydrostatic pressure. 相似文献
94.
95.
We present a first-principles investigation of the phonon-induced electron self-energy in graphene. The energy dependence of the self-energy reflects the peculiar linear band structure of graphene and deviates substantially from the usual metallic behavior. The effective band velocity of the Dirac fermions is found to be reduced by 4%-8%, depending on doping, by the interaction with lattice vibrations. Our results are consistent with the observed linear dependence of the electronic linewidth on the binding energy in photoemission spectra. 相似文献
96.
Marvin A. Hamstad 《Experimental Mechanics》1986,26(1):7-13
The technique of acoustic emission has two broad applications areas. The first is nondestructive evaluation. The second is
as a tool in studies or research which are not fundamentally directed towards acoustic emission. It is this second application
with which we are concerned here. Acoustic emission is a very useful tool in this role because of its high sensitivity, real-time
capability, volume-monitoring approach, and sensitivity to any process or mechanism which generates sound waves. This paper
presents a comprehensive review of areas where acoustic emission (AE) has been used for materials studies on composite materials.
The following fields, among others, will be covered: (1) time-dependent composite properties, (2) impact studies, (3) correlation
of AE with stress level, (4) application to matrix cure studies, (5) relationship of AE-detected damage to other measures
of damage, (6) studies of the effects of matrix material, (7) application to differences in second phase, (8) interface studies,
(9) AE and dimensional stability, (10) AE applied to orientation studies, and (11) environmental effects. This review will
emphasize the roles that AE can play as a tool for the materials scientist: (1) discovery of damage mechanisms, (2) characterization
of damage progression with increasing time or stress, (3) optimization of fabrication variables, and (4) reduction in the
numbers of test specimens required in various studies. 相似文献
97.
This study shows how data envelopment analysis (DEA) can be used to reduce vertical dimensionality of certain data mining databases. The study illustrates basic concepts using a real-world graduate admissions decision task. It is well known that cost sensitive mixed integer programming (MIP) problems are NP-complete. This study shows that heuristic solutions for cost sensitive classification problems can be obtained by solving a simple goal programming problem by that reduces the vertical dimension of the original learning dataset. Using simulated datasets and a misclassification cost performance metric, the performance of proposed goal programming heuristic is compared with the extended DEA-discriminant analysis MIP approach. The holdout sample results of our experiments shows that the proposed heuristic approach outperforms the extended DEA-discriminant analysis MIP approach. 相似文献
98.
A resin-bound nitroso compound sequestered a single unexpected component from crude plant seed extracts. Several plants, including Piper nigrum, Eugenia caryophyllata, and Pimenta dioica, were extracted with organic solvent in the presence of a nitroso-containing resin. The nitroso resin selectively sequestered a single compound, β-caryophyllene, via a chemo- and regioselective ene reaction. The ene product was released from the resin, and proper selection of the solid-phase linker and cleavage cocktail allowed concomitant further transformation of the primary ene product to a novel functionalized polycycle. Preliminary studies indicate that the new hydroxylamine-containing natural product derivatives have antibiotic activity. 相似文献
99.
A dielectric distributed Bragg reflector (DBR) formed by four pairs of hydrogenated amorphous silicon/silicon nitride layers is used as the back reflector in thin-film silicon solar cells. The DBR was designed to perform in a broad wavelength range with the peak reflectance at 600 nm. The DBR was fabricated at low substrate temperature (172 °C) and applied at the rear side of flat and textured amorphous silicon single-junction solar cells in both superstrate (pin) and substrate (nip) configurations. The spectral response and electrical I–V characteristics were measured. Solar cells with optimized DBR exhibit an enhanced external quantum efficiency in the long wavelength range and the electrical performance is comparable to solar cells having conventional Ag back reflector. 相似文献
100.
Inorganic scintillators: today and tomorrow 总被引:6,自引:0,他引:6
Marvin J. Weber 《Journal of luminescence》2002,100(1-4):35-45
The past half-century has witnessed the discovery of many new scintillator materials and numerous advances in our understanding of the basic physical processes governing the operation of inorganic scintillators. These developments are reviewed briefly, but then the question arises—what about today and tomorrow? Have we exhausted luminescence phenomena and the periodic table in our search for improved scintillator materials? Properties of both intrinsic and activated scintillator materials, crystalline and amphorous, are considered. Several fundamental limits of scintillator performance are examined together with the prospects for discovering better scintillators guided by first-principles theoretical calculations of the processes active in scintillation. 相似文献