共查询到20条相似文献,搜索用时 109 毫秒
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The second-order degree of coherence of pseudo-thermal light and coherence time are experimentally studied via the Hanbruy-Brown-Twiss (HBT) scheme. The system consists of two non-photon-number-resolving single- photon-counting modules (SPCMs) operating in the Geiger mode. We investigate the coherence time of the incident beam for different spot sizes on a ground glass a~d speeds of a rotating ground glass. The corresponding coherence time can be obtained from Gaussian fitting for the measured second-order degree of coherence. The results show that the coherence time of measured pseudo-thermal light depends on the spot sizes and the rotating speeds of the ground glass. The maximum value of the second-order degree of coherence is reduced as the rotating speed decreases. This result can be well explained by the model of mixed thermal and coherent fields with different ratios. 相似文献
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Icosahedral iron clusters are synthesized and characterized in our group by using experimental methods.But the measurements of magnetic properties still face difficulties:the sizes of the clusters and the amount of product.Therefore theoretical methods are employed to study these issues.In this paper,icosahedral iron clusters containing 13,55,147,309 atoms are calculated by ab initio techniques.After the structural relaxation,the magnetism of the clusters is found to be similar to that of face centred cubic(FCC) iron with stronger ferromagnetism on the surface.But the central atom of each cluster is exceptional,which has a smaller magnetic moment.We also study the electronic structural properties in the clusters for a better explanation of the magnetism.It is suggested that in small clusters,the magnetic properties still depend on the local structural arrangement. 相似文献
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One of the long-standing controversial arguments in protein folding
is Levinthal's paradox. We have recently proposed a new nucleation
hypothesis and shown that the nucleation residues are the most
conserved sequences in protein. To avoid the complicated effect of
tertiary interactions, we limit our search for structural codes to
the nucleation residues. Starting with the hypotheses of secondary
structure nucleation and conservation of residues important for
folding, we have analysed 762 folds classified as unique by SCOP.
Segments of 17 residues around the top 20% conserved amino acids are
analysed, resulting in approximately 100 clusters each for the main
secondary structure classes of helix, sheet and coil. Helical
clusters have the longest correlation range, coils the shortest (four
residues). Strong specific sequence-structure correlation is observed
for coil but not for helix and sheet, suggesting a mapping
relationship between the sequence and the structure for coil. We
propose that the central sequences in these clusters form `structural
codes', a useful basis set for identifying nucleation sites, protein
fragments stable in isolation, and secondary structural patterns in
proteins (particularly turns and loops). 相似文献
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Observation of the Pinning-Induced Crystal-Hexatic-Glass Transition in Two-Dimensional Colloidal Suspensions 下载免费PDF全文
《中国物理快报》2021,(10)
Identification of the glass formation process in various conditions is of importance for fundamental understanding of the mechanism of glass transitions as well as for developments and applications of glassy materials. We investigate the role of pinning in driving the transformation of crystal into glass in two-dimensional colloidal suspensions of monodisperse microspheres. The pinning is produced by immobilizing a fraction of microspheres on the substrate of sample cells where the mobile microspheres sediment. Structurally, the crystal-hexatic-glass transition occurs with increasing the number fraction of pinning ρ_(pinning), and the orientational correlation exhibits a change from quasi-long-range to short-range order at ρ_(pinning) = 0.02. Interestingly, the dynamics shows a nonmonotonic change with increasing the fraction of pinning. This is due to the competition between the disorder that enhances the dynamics and the pinning that hinders the particle motions. Our work highlights the important role of the pinning on the colloidal glass transition, which not only provides a new strategy to prevent crystallization forming glass, but also is helpful for understanding of the vitrification in colloidal systems. 相似文献
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Formation and evolution mechanisms of large-clusters during rapid solidification process of liquid metal Al 总被引:1,自引:0,他引:1
LIU Rangsu DONG Kejun LIU Fengxiang ZHENG Caixing LIU Hairong & LI Jiyong Department of Applied Physics Hunan University Changsha China College of Materials Science Engineering Hunan University Changsha China College of Chemistry Chemical Engineering Hunan University Changsha China 《中国科学G辑(英文版)》2005,48(1):101-112
A molecular dynamics simulation study has been performed for the formation and evolution characteristics of nano-clusters in a large-scale system consisting of 400000 atoms of liquid metal Al. The center-atom method combined with pair-bond analysis technique and cluster-type index method (CTIM) has been applied here to describe the structural configurations of various basic clusters. It is demonstrated that both the 1551 bond-type and the icosahedral cluster (12 0 12 0) constructed by 1551 bond-types are dominant among all the bond-types and cluster-types, respectively, in the system and play a critical role in the microstructure transitions of liquid metal Al. The nano-clusters (containing up to 150 atoms) are formed by the combination of some middle and small clusters with distinctly different sizes, through mutual competition by unceasing annex and evolution in a seesaw manner (in turn of obtaining and losing), which do not occur as the multi-shell structures accumulated with an atom as the center and 相似文献
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High-quality indium-tin-oxide (ITO) films are deposited on p-type Czochralski silicon and 7059 Coming glass by direct-current magnetron sputtering at various temperatures. The structural, electrical and optical properties of the ITO films are investigated as functions of the substrate temperature. A comparison between the characteristics of the ITO films on silicon and Coming glass is presented. The results show that for the ITO film on silicon,the nucleation begins from room temperature; the resistivity reaches a maximum value at 75℃; the reflectivity increases with increasing temperature; when temperature is above 125℃, the ITO grows in a three-dimensional manner and forms a granular structure. However, for the ITO film on glass, it is still in an amorphous state at 75℃. Moreover, both the resistivity and the reflectivity decrease with increasing temperature. Above 125℃, the ITO grows in a two-dimensional manner and forms a domain structure. 相似文献
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The China Accelerator Driven Subcritical System (China-ADS) project, which is a strategic plan and aims to design and build an ADS demonstration facility, has been proposed and launched actively in China. Injector Ⅱ as one of the parallel injectors of China-ADS, and is prompted by the Institute of Modern Physics (IMP). In this paper, a new scheme with full period lattice structure for the SC section is proposed. In the new scheme, there are sixteen periods, with one superconducting solenoid and one superconducting cavity included in each period. All of the elements are contained in four eryomodules. The dreadful influence of the mismatch caused by period structural change can be avoided, and the beam quality is favorable. In addition, this new scheme has certain advantages in reducing the project's difficulty and construction risk. The details of the design and beam dynamic simulation for the full period lattice structure are given in this paper. 相似文献
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The China Accelerator Driven Subcritical System (China-ADS) project, which is a strategic plan and aims to design and build an ADS demonstration facility, has been proposed and launched actively in China. Injector Ⅱ as one of the parallel injectors of China-ADS, and is prompted by the Institute of Modern Physics (IMP). In this paper, a new scheme with full period lattice structure for the SC section is proposed. In the new scheme, there are sixteen periods, with one superconducting solenoid and one superconducting cavity included in each period. All of the elements are contained in four cryomodules. The dreadful influence of the mismatch caused by period structural change can be avoided, and the beam quality is favorable. In addition, this new scheme has certain advantages in reducing the project's difficulty and construction risk. The details of the design and beam dynamic simulation for the full period lattice structure are given in this paper. 相似文献
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基于Voronoi几何分形法, 分析了理想二十面体团簇和ZrCu二元金属玻璃中各种团簇的结构特点, 提出了一种判定金属玻璃原子结构中二十面体类团簇的方法. 并选取三个ZrCu 非晶成分作为研究对象, 基于Voronoi团簇, 利用该方法提取了各种构型团簇, 证实其中四种构型团簇的基本几何结构与理想二十面体相似, 并具有同样近似于理想二十面体的高致密度、高规则度和高五次对称性, 因此可称之为二十面体类团簇. 此类二十面体类团簇可作为金属玻璃的主要结构单元, 普遍存在于非晶结构中; 二十面体类团簇及其连接能包含几乎所有的原子, 从而形成非晶结构. 研究结果提供了一种新的团簇判定方法, 有助于从微观结构层面分析合金中的非晶形成机理. 相似文献
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Thin films of cadmium sulfide have been deposited on glass substrates and the structural properties of films have been investigated
using scanning electron microscopy and X-ray diffraction techniques. The films consist of domains (groups of grains) and weakly
bound grain clusters. The structural parameters of grains, domains and clusters and the effect of film thickness on these
parameters are reported. From the measurement of lattice constants in CdS films and in free CdS clusters, it has become evident
that the films on glass substrates have a tensile strain along their planes. The effect of thermal annealing on the partial
relaxation of the strain is discussed.
Received: 29 January 2001 / Accepted: 30 January 2001 / Published online: 3 May 2001 相似文献
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Local structures play a crucial role in glass formation and properties. In addition to topological short-range order, the geometric property of site symmetry is another important but less known characteristic of local structures. It is shown that the observed sharp increase of glass forming ability of Ce70-xAl10Cu20Cox upon Co addition is correlated with a dramatic increase of Al site symmetry, as reflected by decreasing quadrupole frequency measured by 27Al NMR. The result is consistent with the structure model of Al-centered icosahedral clusters as the predominant structural building blocks. 相似文献
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Structural and electronic properties of Bin (n = 2-14) clusters from density-functional calculations
J. M. Jia G. B. Chen D. N. Shi B. L. Wang 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2008,47(3):359-365
The structural and electronic properties of Bin
(n = 2-14) clusters have been systematically studied using gradient-corrected
density-functional theory. For each cluster size, a number of structural
isomers were constructed and optimized to search for the lowest-energy
structure. The competition of several structural patterns such as cages,
superclusters, and layered structures leads to the alternating appearance of
these configurations as global minima. Although the tendency of Bi to form
puckered-layer structures is already well-known, the electronic states of
Bin clusters are still far from that of the bulk. As well, a remarkable
even-odd atom number oscillation is observed in the structural and
electronic properties of the clusters, implying that the stability of
Bin clusters is mainly dominated by the electron shell effect rather
than by geometrical packing. The theoretically calculated values for
electron affinities agree well with available experimental data. 相似文献
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We report a hybrid atomic packing scheme comprised of a covalent-bond-mediated "stereochemical" structure and a densely packed icosahedron in a bulk metallic glass Pd40Ni40P20. The coexistence of two atomic packing models can simultaneously satisfy the criteria for both the charge saturation of the metalloid element and the densest atomic packing of the metallic elements. The hybrid packing scheme uncovers the structural origins of the excellent glass forming ability of Pd40Ni40P20 and has important implications in understanding the bulk metallic glass formation of metal-metalloid alloys. 相似文献
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S. E. Kichanov A. Kh. Islamov S. A. Samoilenko D. P. Kozlenko A. V. Belushkin V. S. Gurin G. P. Shevchenko E. E. Trusova L. A. Bulavin B. N. Savenko 《Journal of Surface Investigation: X-ray, Synchrotron and Neutron Techniques》2014,8(1):98-103
The features of the formation of Ce-Ti-O complex oxide nanoclusters in a silicate glass are studied by means of the small-angle neutron scattering technique. It is found that bounded regions of density fluctuation of the glass material are formed in the initial glass matrix without the addition of titanium and cerium oxides. These regions could serve as nucleation centers for oxide clusters of cerium and titanium upon their introduction into the matrix. The calculated average size of these inhomogeneities does not exceed 30 ± 1 nm, and their surface volume equals 0.72 ± 3 nm3. A structural mechanism for Ce-Ti-O oxide formation in a silicate glass, in which the nanoclusters are formed within a bounded region of glass material inhomogeneities at low concentrations of the initial cerium oxide (CeO2), is proposed. At high cerium oxide concentrations, oxide nanocluster growth occurs predominantly on the surface of these inhomogeneities. This leads to a sharp change in the nanocluster sizes and their fractal dimension. 相似文献
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Robert Morel Ariel BrenacCéline Portemont Thierry DeutschLucien Notin 《Journal of magnetism and magnetic materials》2007
Magnetic anisotropy has been measured in multiply twinned, icosahedral cobalt clusters. It is found that the low-temperature magnetization of deposited cluster layers is well defined with the Stoner–Wohlfarth model by averaging over clusters with a range of anisotropy energy. Anisotropy energy calculation based on Néel's pair model shows that the icosahedral structure and the layer-by-layer growth of the clusters induce oscillations of the magnetic anisotropy as a function of the filling of the outer surface of the particle. The magnetization measurement at room temperature indicates a weakly correlated cluster glass, as deduced from the approach to saturation that is well described with 2D random anisotropy model. 相似文献
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T. Ritschel P. J. Kuntz L. Zülicke 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2005,33(3):421-432
The geometric structure and bonding properties of medium-sized ArnH+
clusters (n = 2–35), in which a proton is wrapped up in a number of Ar
atoms, are investigated by applying a diatomics-in-molecules (DIM) model
with ab-initio input data generated by means of multi-reference
configuration-interaction (MRCI) computations. For the smaller complexes,
n = 2–7, cross-checking calculations employing the coupled-cluster approach
(CCSD) with the same one-electron atomic basis set as for the input data
calculations (aug-cc-pVTZ from Dunning), show good agreement thus justifying
the extension of the DIM study to larger n.
Local minima of the multi-dimensional potential-energy surfaces (PES) are
determined by combining a Monte-Carlo sampling followed, for each generated
point, by a steepest-descent optimization procedure. For the electronic
ground state of the ArnH+ clusters, the global minimum (corresponding
to the most stable structure of the cluster) as well as secondary minima are
found and analyzed. The structural and energetic data obtained reveal the
building-up regularities for the most stable structures and make it possible
to formulate a simple increment scheme. The low-lying excited states are
also calculated by the DIM approach; they all turn out to be globally
repulsive. 相似文献