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91.
H. Wang X.-Y. Kuang A.-J. Mao X.-F. Huang 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,56(1):1-5
We perform ab initio calculations using a pseudo-potential plane-wave method
based on density functional theory, within the local density approximation and generalized gradient approximation, in order
to determine and predict the pressure dependence of structural and elastic properties of spinel compounds: MgAl2O4, MgGa2O4 and MgIn2O4. The
results are in agreement with the available experimental data and other theoretical calculations. 相似文献
92.
The electronic properties of one monolayer of Au atoms on polar ZnO surfaces are examined by first-principles slab calculations. It is found that an Au ad-layer on top of the surface is energetically more favourable than other gold diffused cases, and Au capping layer on the ZnO polar surfaces may modify the growing properties of ZnO nanostructures by enhancing the binding energy. 相似文献
93.
A.V. Tsvyashchenko L.N. Fomicheva O.I. Kochetov A. Velichkov M. Budzynski G.K. Ryasny 《Solid State Communications》2007,142(11):664-669
The time-differential perturbed angular correlations technique (TDPAC) has been employed for measuring the parameters of hyperfine interactions in earlier known RAl3 compounds, synthesized at high pressure (8 GPa) and high temperature, where R = La, Ce, Sm, Gd, Tb, Dy, Ho, Er, Yb and Lu. The 111Cd(111In) radioactive atom was used as a probe nucleus. The X-ray method has revealed that with the increase in the atomic number of a rare-earth element R, the obtained RAl3 high-pressure phases crystallize, respectively, into orthorhombic, hexagonal and cubic structures. It has been found that in the compounds containing R=La, Ce, Sm and Gd, a deviation from earlier known structural types and the formation of new ones is observed, which is associated with the change of the stoichiometric composition of the said compounds. The results of the PAC measurements have confirmed the deviation from the predetermined stoichiometric composition 1R:3Al for the compounds LaAl3, CeAl3, SmAl3 and GdAl3 and have verified the RAl3 stoichiometric structure for the other high-pressure phases obtained in this work. 相似文献
94.
A new compound UPd2Sb was prepared and studied by means of X-ray diffraction, magnetization, electrical resistivity, magnetoresistivity, thermoelectric power and specific heat measurements. The phase crystallizes with a cubic structure of the MnCu2Al-type (s.g. ). It orders antiferromagnetically at TN=55 K and exhibits a modified Curie-Weiss behaviour with reduced effective magnetic moment at higher temperatures. The electrical resistivity behaves in a manner characteristic of systems with strong electronic correlations, showing Kondo effect in the paramagnetic region and Kondo-like response to the applied magnetic field. The Seebeck coefficient exhibits a behaviour expected for scattering of conduction electrons on a narrow quasiparticle band near the Fermi energy. The low-temperature electronic specific heat in UPd2Sb is moderately enhanced being about 81 mJ/mol K2. 相似文献
95.
Masato Ohmukai Nobutomo Uehara Tetsuya Yamasaki Yasuo Tsutsumi 《Czechoslovak Journal of Physics》2004,54(7):781-784
We have investigated the effects of chemical etching on Raman spectra of porous silicon. The as-anodized porous silicon consisted
mainly of crystalline silicon, as indicated by the Raman spectra. The background in the spectrum was strong, indicating that
the porous silicon surface was rough due to the presence of pores. When chemical etching was performed five times, the Raman
spectrum revealed the presence of spherically shaped nanocrystalline silicon whose diameter was around 3.5 nm. Further chemical
etching, however, extinguished the nanocrystallites, in addition to smoothing the surface morphology. 相似文献
96.
Yu.D. Glinka A.S. Zyubin A.M. Mebel S.H. Lin L.P. Hwang Y.T. Chen 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,16(1):279-283
Photoluminescence (PL) from composites of 7- and 15-nm sized silica nanoparticles (SNs) and mesoporous silicas (MSs) induced
by 266- (4.66-) and 532-nm (2.33-eV) laser light has been studied at room temperature. The multiband PL from MSs in the range
of 1.0-2.1 eV is evidenced to originate from isolated bulk and surface non-bridging oxygens (NBOs) and from NBOs combined
with variously placed 1-nm sized pore wall oxygen vacancies (OVs). The nature and diversity of NBO light-emitters are confirmed
by ab initio calculations. The PL from SNs exhibits only a short wavelength part of the bands (1.5-2.1 eV) originated from isolated bulk
and surface NBOs. This fact indicates that the highly OV-bearing structures occur only in extremely thin (∼ 1 nm) silica layers.
The similarity of spectroscopic properties of silica-based nanoscale materials to those of surface-oxidized silicon nanocrystals
and porous silicon, containing silica-passivating layers of the same width, is discussed.
Received 20 November 2000 相似文献
97.
We report on the optical property investigation of SiGe nanocrystals (NCs) prepared by electrochemical anodization (ECA) of SiGe layer grown by ultrahigh vacuum chemical vapor deposition (UHVCVD). At room temperature, SiGe NCs with higher Ge content demonstrate a redshift of the photoluminescence (PL) peak compared to Si NCs. It was found that the surface chemical composition, density, and the size of the SiGe NCs were very sensitive to the annealing conditions. Various spectroscopy measurements such as PL, FTIR, and XPS have been carried out to reveal the mechanism of the PL peak transition. The results indicated that the PL peak position was determined by two major factors, namely, interface state density and the size of SiGe NCs. It was shown that the higher the interface state density, the more significant the redshift of the peak position. While the smaller the size of the SiGe NCs, the more significant the quantum size effects become, resulting in the blueshift of the PL peak position. 相似文献
98.
First-Principles Study of Orthorhombic Perovskites MgSiO3 up to 120 GPa and Its Geophysical Implications
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High-pressure behaviour of orthorhombic MgSiO3 perovskite crystal is simulated by using the density functional theory and plane-wave pseudopotentials approach up to 120 GPa pressure at zero temperature. The lattice constants and mass density of the MgSiO3 crystal as functions of pressure are computed, and the corresponding bulk modulus and bulk velocity are evaluated. Our theoretical results agree well with the high-pressure experimental data. A thermodynamic method is introduced to correct the temperature effect on the O-K first-principles results of bulk wave velocity, bulk modulus and mass density in lower mantle PIT range. Taking into account the temperature corrections, the corrected mass density, bulk modulus and bulk wave velocity of MgSiO3-perovskite are estimated from the first-principles results to be 2%, 4%, and 1% lower than the preliminary reference Earth model (PREM) profile, respectively, supporting the possibility of a pure perovskite lower mantle model. 相似文献
99.
S.G. Magalh aes A.A. Schmidt A. Theumann B. Coqblin 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,30(4):419-425
The Kondo lattice model has been analyzed in the presence of a random inter-site interaction among localized spins with non
zero mean J0 and standard deviation J. Following the same framework previously introduced by us, the problem is formulated in the path integral formalism where
the spin operators are expressed as bilinear combinations of Grassmann fields. The static approximation and the replica symmetry
ansatz have allowed us to solve the problem at a mean field level. The resulting phase diagram displays several phase transitions
among a ferromagnetically ordered region,a spin glass one, a mixed phase and a Kondo state depending on J0, J and its relation with the Kondo interaction coupling JK. These results could be used to address part of the experimental data for the CeNi
1 - x
Cu
x compound, when x ⩽ 0.8.
Received 24 June 2002 Published online 31 December 2002 相似文献
100.
Genetically controlled MRI contrast mechanisms and their prospects in systems neuroscience research 总被引:1,自引:0,他引:1
Application of MRI contrast agents to neural systems research is complicated by the need to deliver agents past the blood-brain barrier or into cells, and the difficulty of targeting agents to specific brain structures or cell types. In the future, these barriers may be wholly or partially overcome using genetic methods for producing and directing MRI contrast. Here we review MRI contrast mechanisms that have used gene expression to manipulate MRI signal in cultured cells or in living animals. We discuss both fully genetic systems involving endogenous biosynthesis of contrast agents, and semi-genetic systems in which expressed proteins influence the localization or activity of exogenous contrast agents. We close by considering which contrast-generating mechanisms might be most suitable for applications in neuroscience, and we ask how genetic control machinery could be productively combined with existing molecular agents to enable next-generation neuroimaging experiments. 相似文献