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排序方式: 共有110条查询结果,搜索用时 31 毫秒
1.
J. Kircher M. Cardona M. Alouani S. Gopalan O. Jepsen O. K. Andersen
E. Kaldis
J. Karpinski S. Rusieki 《Physica C: Superconductivity and its Applications》1992,200(3-4):413-417We report results of ellipsometric measurements of the dielectric tensor of YBa2Cu4O8 as well as calculations employing density functional theory in the local density approximation (LDA). For the computation of the one-electron eigenvalues and states the linear-muffin-tin-orbital method (LMTO) was used. Measured and calculated results show good agreement. In particular, the calculation predicts different band structures for YBa2Cu3O7 and YBa2Cu4O8 in the vicinity of the Fermi energy which lead to significant differences in the optical spectra. One such difference is a region of small ε2 in the near infrared which is a fingerprint of the high (relative to the top of the highest fully occupied band) Fermi energy as compared to the situation in YBa2Cu3O7. Our experimental results confirm the theoretical findings. 相似文献
2.
Herein, we outline a method that is able to generate truly minimal basis sets that accurately describe either a group of bands, a band, or even just the occupied part of a band. These basis sets are the so-called NMTOs, muffin-tin orbitals of order N. For an isolated set of bands, symmetrical orthonormalization of the NMTOs yields a set of Wannier functions that are atom-centered and localized by construction. They are not necessarily maximally localized, but may be transformed into those Wannier functions. For bands that overlap others, Wannier-like functions can be generated. It is shown that NMTOs give a chemical understanding of an extended system. In particular, orbitals for the pi and sigma bands in an insulator, boron nitride, and a semimetal, graphite, will be considered. In addition, we illustrate that it is possible to obtain Wannier-like functions for only the occupied states in a metallic system by generating NMTOs for cesium. Finally, we visualize the pressure-induced s-->d transition. 相似文献
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5.
Tue Tjur 《Computational Statistics》2011,26(3):371-379
It is a trivial observation that the computers have changed the way statistics is practiced. But has it also changed the theory
of statistics and the way we teach it? I think yes—even if the changes appear to be surprisingly small in some contexts. This
is an attempt to give a more detailed answer, based on experiences from my own corner of the world from 1964 till now. 相似文献
6.
Andersen Sune K. Johannessen Tue Mosleh Majid Wedel Stig Tranto Janne Livbjerg Hans 《Journal of nanoparticle research》2002,4(5):405-416
Flame-generated aerosol particles of Al2O3 were deposited by gas filtration on two types of porous and ceramic tubes of -Al2O3 with mean pore diameters of 450 and 2700nm, respectively. The particles were aggregates with average mobility diameters in the range of 30–100nm and primary particle diameters of 4–8nm. The particles are characterized by differential mobility analysis, transmission electron microscopy, and by their specific surface area. The deposited membranes are characterized by gas permeability measurements, scanning electron microscopy, and by their pore size distribution from nitrogen capillary condensation. The particles form a distinct, homogeneous membrane layer with a porosity of 90% on top of the substrate surface and only penetrate slightly into the substrate structure. The mean pore sizes of the deposited membranes determined by nitrogen condensation agree approximately with those determined by gas permeation and the specific surface area. The mean pore diameter varies in the range of 30–70nm. The gas permeability of the deposited membranes is related to the specific surface area but influenced by the high porosity. The mean pore size and the permeability of the membranes are almost independent of the substrate structure.The development of a membrane with uniform properties is preceded by a short initial period in which the deposited particles, with an equivalent membrane thickness of roughly 2m, have a significantly lower permeability than the ultimately developed uniform membrane layer. This effect is particularly significant for the aerosol particles with the lowest mean size, probably due to particles deposited in the pore mouths of the substrate.The particles and the deposited membranes are X-ray amorphous but retain their specific surface area on heating to even high temperatures. When the membranes are heated to 1473K for 10h, X-ray diffraction shows a mixture of - and -alumina, accompanied by a partial disintegration of the membrane and a considerable loss of surface area. 相似文献
7.
M. Maglietta E. Zanazzi U. Bardi F. Jona D.W. Jepsen P.M. Marcus 《Surface science》1978,77(1):101-108
A LEED intensity analysis of the c(2 × 2) structure obtained upon adsorption of oxygen gas on Co {00l} is reported. Three structural models have been tested on the basis of a total of 13 LEED spectra for three angles of incidence and one azimuth. The correct model has oxygen atoms chemisorbed in the four-fold symmetrical hollows formed by four adjacent substrate atoms. Atomic arrangement and metal-oxygen distances are analogous to those found in Ni{001}c(2 × 2)O. 相似文献
8.
The transmission characteristics of an air-GaAs interface and the transient absorption and index spectra of the thin, photoexcited surface layer are investigated subsequent to excitation by a femtosecond laser pulse. We find that the total phase change and transmission of a terahertz (THz) probe pulse are dominated by interface effects. This observation has important implications in the interpretation of THz time-domain spectroscopy data of absorbing media. We also observe that the THz pulse apparently arrives at the detector as much as 60 fs earlier when it is transmitted through an optically excited GaAs wafer. This effect is fully explained in terms of a frequency-dependent transmission and phase shift at the air-GaAs interface and is not associated with superluminal propagation. 相似文献
9.
W. Simarew S. G. Menefee O. R. Overman A. Gorbatschewa H. Chr Jorgensen I. V. Hunt A. Konoplew H. Fincke C. Ehrlich R. Trommsdorff M. Asdrubali S. Hadwen R. Gwatkin F. K. Neave H. D. Kay W. R. Graham R. Aschaffenburg A. Jepsen P. Madelung Ch. Paley H. Kreker A. M. Williams May Davies S. B. Thomas G. M. Phillips W. M. Ashton G. D. Simons J. Davies B. F. Thomas L. Little H. A. Sirks G. Sjöström P. Jaulmes L. Poujol und J. A. B. Smith 《Fresenius' Journal of Analytical Chemistry》1943,126(7):282-286
Ohne Zusammenfassung 相似文献
10.
The scope of the palladium-catalyzed carboamination reaction for the synthesis of 2-substituted pyrrolidines, piperidines, and morpholines was investigated. Formation of a 2,4-disubstituted pyrrolidine proceeded in high yield but with a diastereoisomeric ratio of only 2:5, favoring the cis-isomer. The diastereoselectivity is hence significantly smaller than that observed previously in the formation of 2,3- and 2,5-disubstituted pyrrolidines. The yields of substituted piperidines and morpholines were lowered by competing Heck arylation reactions. Both the N-substituent and the choice of phosphine ligand for the palladium-catalyzed reaction were determining for the outcome. 相似文献