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231.
A.B. Henriques L.C.D. Gonçalves N.F. Oliveira Jr. S.M. Shibli P.L. Souza B. Yavich 《Zeitschrift für Physik B Condensed Matter》1997,104(3):457-461
The Shubnikov-de Haas spectra of GaAs samples with a periodical doping with Si were measured at 4.2 K using fields of 0–14 T. From the Shubnikov-de Haas spectrum the quantum mobilities associated with individual minibands were estimated for each sample. The sheet density of Si atoms in each doping plane was fixed at approximately 2.5 × 1012 cm?2 in all samples and the doping period was varied in the range 40–1000 Å. The quantum mobilities obtained experimentally are compared with theoretical calculations using the random phase approximation to describe the screened interaction between electrons and charged impurities. The theory describes qualitatively the results of the experiment, i.e. an increase of the quantum mobility with the index of the miniband, and a decrease in the mobility in all minibands when the doping period is made shorter (with a weak maximum at intermediate values of the doping period). Various possible improvements of the model are suggested. 相似文献
232.
I.O. Mazali A.G. Souza Filho B.C. Viana J. Mendes Filho O.L. Alves 《Journal of nanoparticle research》2006,8(1):141-148
In this paper we report the synthesis and characterization of TiO2 nanocrystal dispersed into a porous Vycor glass. We have obtained very small TiO2 nanocrystals in the anatase form. The nanocrystal size is controlled via the mass increment only thus preventing the growth through the coalescence process. The nanocrystal size was monitored through transmission electron microscope and Raman scattering. The␣coalescence control is attributed due to the obtention of nanocrystals dispersed into the host and to the terminal bonds present in the porous which act as an anchor thus resulting in a low diffusion of the nanocrystals through the porous network. 相似文献
233.
Rafael de Souza Monteiro Lílian W. C. Paes José Walkimar de M. Carneiro Donato A. G. Aranda 《Journal of Cluster Science》2008,19(4):601-614
The adsorption of CO on Fe, Pt and Co clusters was modeled by the DFT approach using the B3LYP and the BPW91 functionals together
with the LANL2DZ and the 6-31G(d) basis set. These calculations show that although CO adsorbs more strongly on Pt than on
either Fe or Co, the dissociation energy on Fe and on Co is lower than the corresponding dissociation energy on Pt. Therefore,
the activation energy for dissociation is not determined by the adsorption energy. Additionaly, the CO bond distances also
do not show any correlation to the adsorption energy. 相似文献
234.
Humberto César Chaves Fernandes Idalmir de Souza Queiroz Júnior 《International Journal of Infrared and Millimeter Waves》1996,17(8):1391-1402
The Transverse Transmission Line (TTL) is used in the theory of the overlay shielded microstrip lines considering the superconductor strip on two semiconductor regions. The superconductor effect is included with the boundary condition of the surface impedance, that is related to the complex conductivity of the material, calculated from the advanced two-fluid model. Applying the moment method the complex propagation constant of the structure, including the phase constant and the attenuation constant, is obtained. Results are presented for the complex propagation constant, versus the frequency and the temperature, of this overlay superconducting microstrip lines. 相似文献
235.
Humberto César Chaves Fernandes Eduardo Armorim Martins de Souza Idalmir de Souza Queiroz Jr 《International Journal of Infrared and Millimeter Waves》1995,16(1):185-200
In this study, the effect of the metallization thickness in finlines on semiconcuctor substrate is researched. The propagation parameters are computed to measure the inluence of the metallization. The theory and numerical results are presented to the propagation constant and characteristic impedance of the bilateral and unilateral finlines. The full wave analysis of the transverse transmission line — TTL method is used to determine the electromagnetic fields of the structure in Fourier transform domain — FTD. Applying the suitable boundary conditions, the moment method and expanding the fields in a set of suitable bases functions, a homogeneous matrix system is obtained and the propagation constant is computed. The characteristic impedance is obtained using the relation of the voltage in the slot and the transmitted power by the structure.Computational programs are developed to obtain numerical results to the propagation parameters composed by the propagation constant and characteristic impedance.This work received financial support by CAPES and CNPq. 相似文献
236.
José Carlos de Souza Kiihl 《Rendiconti del Circolo Matematico di Palermo》1980,29(3):411-419
In this paper we develop a technique to study the homomorphisma: MU * (B U1)→M U*?2 (B U1) defined by assigning to the class off: M→B U 1 the class off oi: N→B U1, wherei: N→M is the submanifold dual tof*(γ1)?f*(γ1), and γ1→B U is the 3 universal line boundle. So that we can present a (σn), where σnis the class of the classifying map of the canonical line boundle overC P n, in terms of the σi’s and chosen generators of Π★(M U). 相似文献
237.
Mössbauer X-ray diffraction and electron microscopy studies revealed that segregation and ordering occurred in the metal particles of the Antarctic L6 chondrite ALHA 76009. The ordered crystal structure of AuCu type (tetrataenite) was detected coexisting with the disordered 50-50 taenite. 相似文献
238.
239.
In the light of Einstein's equations a system only containing two scalar fields is considered: One is of long range and attractive, the other is of short range and repulsive. The sources of these fields are taken to be nonsingular and spherically symmetric. All components of the energy-momentum tensor are continuous. A static solution of the equations is obtained in the weak-field approximation. The source of the gravitational field shows a finite concentration on the center of symmetry and dilutes monotonically to zero outwards. A Schwarzschild-type gravitation is found at infinity. 相似文献
240.
Whei Oh Lin Edson de Souza Coutinho 《Monatshefte für Chemie / Chemical Monthly》1983,114(11):1231-1235
The title compounds are obtained directly on reaction of 6-chloroacetamidopiperonal with substituted phenylhydrazines. Piperonal was used as starting material. 相似文献