M_2Ga(M=Cu,Ag,Au)二元合金小团簇的从头算研究(英文)

根据原子与分子反应静力学和群论理论推导出二元合金小团簇M2Ga(M=Cu,Ag,Au)的离解极限和基态电子态.采用量子力学从头算方法及小核实赝势和相应的基组,对二元合金小团簇M2Ga进行几何优化,得到M2Ga基态(2B2)为弯曲平面型结构,并与单元团簇M3(M=Cu,Ag,Au)相似.计算得到合金团簇比单元团簇稳定,其中Au2Ga体系最稳定.同时给出谐振频率、力常数,并且分析得到电子相关效应使得团簇更加紧凑、稳定性增强.     

Hot and solid gallium clusters: too small to melt

A novel multicollision induced dissociation scheme is employed to determine the energy content for mass-selected gallium cluster ions as a function of their temperature. Measurements were performed for Ga(+)(n) (n=17 39, and 40) over a 90-720 K temperature range. For Ga+39 and Ga+40 a broad maximum in the heat capacity-a signature of a melting transition for a small cluster-occurs at around 550 K. Thus small gallium clusters melt at substantially above the 302.9 K melting point of bulk gallium, in conflict with expectations that they will remain liquid to below 150 K. No melting transition is observed for Ga+17.     

镓原子集团结构的第一性原理分子动力学研究

用第一性原理分子动力学方法和模拟退火技术研究了镓原子集团(n≤8)的平衡结构,并与可得到的实验数据进行了比较。随着原子数的增加,发现在n=5时结构发生了从二维到三维的变化。所有集团可看作为由镓分子和原子链两种结构单元所构成,这两种结构单元分别出现在固相α-Ga和亚稳相β-Ga中。 关键词：     

Why do gallium clusters have a higher melting point than the bulk?

Density functional molecular dynamical simulations have been performed on Ga17 and Ga13 clusters to understand the recently observed higher-than-bulk melting temperatures in small gallium clusters [Phys. Rev. Lett. 91, 215508 (2003)]]. The specific-heat curve, calculated with the multiple-histogram technique, shows the melting temperature to be well above the bulk melting point of 303 K, viz., around 650 and 1400 K for Ga17 and Ga13, respectively. The higher-than-bulk melting temperatures are attributed mainly to the covalent bonding in these clusters, in contrast with the covalent-metallic bonding in the bulk.     

从头计算对GanNm团簇的结构与稳定性的研究

用B3LYP-DFT方法对GanN2(n=1~7)和GanN(n=2~8)团簇的结构与稳定性进行了研究.在6-31G*水平上进行了结构优化和频率分析,得到了GanN2(n=1~7)和GanN(n=2~8)团簇的基态结构.在GanN(n=2~8)团簇的基态几何结构中,N原子处在分子结构的中心;在GanN2(n=1~3)团簇中,N—N键比Ga—N键强;在GanN2(n=4~7)团簇中存在Ga3N单元和Ga4N单元.在GanN2(n=1~7)和GanN(n=2~8)团簇中,Ga4N2,Ga6P2,Ga3N,Ga5N和Ga7N较其它团簇稳定.     

Absence of magnetic order in Yb3Ga5O12: relation between phase transition and entropy in geometrically frustrated materials

From muon spin relaxation spectroscopy experiments, we show that the sharp peak (lambda-type anomaly) detected by specific heat measurements at 54 mK for the ytterbium gallium garnet compound, Yb3Ga5O12, does not correspond to the onset of a magnetic phase transition, but to a pronounced building up of dynamical magnetic pair correlations. Beside the lambda anomaly, a broad hump is observed at higher temperature in the specific heat of this garnet and other geometrically frustrated compounds. Comparing with other frustrated magnetic systems we infer that a ground state with long-range order is reached only when at least 1/4-1/3 of the magnetic entropy is released at the lambda transition.     

Pressure-driven "molecular metal" to "atomic metal" transition in crystalline Ga

We present the ultrasonic study of gallium (Ga I) under high pressure up to 1.7 GPa, including the measurements of the density and elastic properties during phase transitions to Ga II and to a liquid state. The observed large drop of both bulk and shear moduli (by 30% and 55%, correspondingly) during the phase transition to Ga II, as well as the increase of the Poisson's ratio from typically "covalent" ( approximately 0.22) to "metallic" ( approximately 0.32) values, experimentally testifies to the coexistence of a molecular and metallic behavior in Ga I and to the disappearance of the "covalency" during the transition to Ga II. A high value of the pressure derivative of the bulk modulus for Ga I and the increase in the Poisson's ratio can be associated with the weakening of the covalency in compressed Ga I and considered as a precursor of the transition to normal metal.     

Forbidden optical transitions between impurity levels in silicon and gallium phosphide

Experimental estimates are made of absorption cross sections for forbidden optical transitions from the ground state to long-lived excited states of P, As, Sb, In, and Ga impurities in silicon and Te impurities in gallium phosphide. The results can be used to predict the possibility of long-wavelength stimulated emission being excited as a result of the population inversion of long-lived impurity states in these materials.     

不同势下铱团簇结构和熔化行为的分子动力学模拟

采用分子动力学方法及淬火技术,结合Gupta势和Sutton-Chen势,模拟研究了包含13,14,55,56,147及148个原子数的铱团簇的熔化行为. 结果表明,Gupta势和Sutton-Chen势对所研究的Ir团簇的基态几何结构和熔化行为给出了基本一致的描述：两种不同势给出了完全相同的基态几何结构;两种势给出的Ir团簇熔点及预熔化区间随团簇尺寸的变化关系基本一致;对于小Ir_n团簇（n=13,14）两类势均表现出比热峰值相对于均方根键长涨落饱和值滞后的现象. 但是 关键词： 铱团簇 分子动力学 Gupta势和Sutton-Chen势 熔化     

第一性原理对GanP-m阴离子团簇结构及其光电子能谱的研究

本文利用密度泛函理论(DFT)对GanP-(n=2-7)和GanP2-(n=1-6)阴离子团簇的几何结构、电子态及稳定性进行了研究.在B3LYP/6-31G*水平上进行了结构优化和频率分析,得到了GanP-(n=2-7)和GanP2-(n=1-6)团簇的基态结构.这些阴离子团簇的几何结构随着n的增大,在n=5时由平面结构转化为立体结构;在GanP2-(n=1-6)团簇中,P-P比Ga-P容易成键;在GanP-(n=2-7)和GanP2-(n=1-6)阴离子团簇中,Ga3P2-,Ga4P2-,Ga5P2-和Ga6P-的基态结构最稳定.     

Structures and Electronic Properties ofCuN (N≤ 13) Clusters

A systematic study on the structures and electronic properties of copper clusters has been performed using the density functional theory. In the calculation, there are many isomers near the ground state for small copper clusters. Our results show that the three-dimensional isomers of copper clusters start from Cu₇ cluster and then show a tendency to form more compact structures. The results of the formation energy and the second derivative of binding energy with cluster size show that besides N = 8, N=11is also a magic number. Furthermore, it is the first time to find that the ground state of 11-atom clusters is a biplanar structure as same as the 13-atom cluster. The clear odd-even alternation as cluster size for the formation energy indicates the stability of electronic close shell existed in the range studied.     

Melting Transition of Small Aluminium Clusters Al11-20

Heat capacities of small aluminium clusters A111-20 are investigated using MD simulation with empirical many- body Gupta potential. The heat capacities of some clusters A111, A112, A113 and A119 show well-defined peaks while the heat capacities of Alls-ls indicate a gradual melting transition. The spectra of isomers obtained by quenches along the MD trajectory give good interpretation for those results.     

Cohesive properties of mercury clusters in the ground and excited states

Optimized structures and cohesive energies of small mercury clusters (Hg_N; N = 3–7, 13, 19) are calculated with the spin-orbit diatomics-in-molecules method. The theory takes into account the effect of s-p mixing which tends to enhance the binding energies in the ground state. It is shown that excimer clusters have significantly short optimum bond lengths and their atomic geometries differ considerably from those in the ground state. Excitation energy gap depends sensitively on both cluster size and nearest-neighbor separation. Numerical results are compared with other theories and experiments.     

Quantitative method for describing heterogeneity in plutonium-gallium alloys

Abstract

A new method for describing gallium (Ga) heterogeneity in plutonium–gallium (Pu–Ga) alloys in terms of two parameters, d_x and m₀, is presented in this paper. The parameters are derived from a modified Boltzmann function. The resulting sigmoidal curve function is fitted to a grain radius bearing curve, derived from quantitative Ga distribution map data. Describing the heterogeneity in terms of the derived parameters enables the effect of different parameters on the rate of homogenisation of the Ga to be quantitatively characterised. This quantification will allow us to determine the relationships that material properties and heat treatment conditions have on the rate of homogenisation of the Ga. Preliminary results presented here indicate a strong log–log correlation between the heat treatment time at a given temperature and the resulting heterogeneity, and also indicates that the combined iron (Fe) and nickel (Ni) content in the alloy affects the homogenisation rate.     

自旋冰材料R2Ti2O7（R=Dy，Ho）的单晶生长和低温物性研究

本文首先对R2Ti2O7（R=Dy,Ho）单晶的生长条件进行了探索,经过多次尝试,得到了较理想的生长条件,并获得了高品质的单晶．磁化率和比热测量证实了这些单晶材料具有自旋冰基态．另外,通过对不同条件下退火的Dy2Ti2O7单晶低温热传导的深入研究,发现R2Ti2O7单晶具有较小的声子峰,并且退火条件的变化对峰值的影响很...     

Site-selective electron nuclear double resonance in the exchange-narrowed regime of the ESR in conducting solids

Electron nuclear magnetic double resonance on conduction electrons reveals the hyperfine interaction hidden by the fast electron spin exchange. We used the Overhauser shift technique to investigate the electron spin density of the conduction band of gallium oxide, beta-Ga(2)O(3). Due to the monoclinic structure, the conduction band of beta-Ga(2)O(3) is anisotropic and it is dominated by contributions from the two nonequivalent Ga sites. The large quadrupole couplings of the two gallium isotopes (69)Ga and (71)Ga (both with I = 3/2) are completely resolved in our double-resonance experiments. This resolved quadrupole interaction allows the determination of the electric field gradients at both gallium sites with high precision and high sensitivity. The resolved quadrupole splitting is the key to the site-selected determination of the hyperfine interaction. The concepts behind these double-resonance techniques are rather general and should be applicable in similar semiconductor systems.     

71Ga,27Al and23Na solid state MAS NMR of β- and β″-aluminogallates

Two series of compounds with sodium β- and β″-alumina structure have been studied in which the aluminium is partly replaced by gallium. The level of substitution has been varied fromx(Ga/Ga+Al)=0.3 to 0.8. The approximately quantitative analysis of aluminium and gallium NMR of these aluminogallates has allowed us to determine a gallium site occupancy for the different values ofx. For sodium, several sites have been shown and a correlation between gallium substitution and sodium positioning has been proposed. The structures are affected by the changes in cation size from aluminium to gallium. The sites occupancy has been determined and allows one to understand the distortions, due to gallium insertion, in the structures.     

铟团簇结构和电子性质的密度泛函研究

用密度泛函方法研究了Inn(n=2-7)团簇的稳定结构和电子性质。结果表明：自旋多重度对结构的影响不大;对于基态结构,n≤5时为平面结构,n≥6时为立体结构,n=6为结构转变点;平均结合能曲线随团簇尺寸增大逐渐平缓;能隙、结合能的二阶差分和电离势随团簇尺寸的变化趋势完全一致,均反映出In4团簇的基态结构较为稳定,具有较强的非金属性。     

铟团簇结构和电子性质的密度泛函研究

用密度泛函方法研究了In_n(n=2～7)团簇的稳定结构和电子性质.结果表明:自旋多重度对结构的影响不大;对于基态结构,n≤5时为平面结构,n≥6时为立体结构,n=6为结构转变点;平均结合能曲线随团簇尺寸增大逐渐平缓;能隙、结合能的二阶差分和电离势随团簇尺寸的变化趋势完全一致,均反映出In_4团簇的基态结构较为稳定,具有较强的非金属性.     

生长条件对Ga掺杂ZnO薄膜微观结构及光致发光性能的影响

利用热氧化法在不同参数条件下生长了Ga掺杂范围较宽的ZnO薄膜, 研究了ZnO薄膜的表面微观结构和光致发光性能. 研究表明: Ga以Ga³⁺存在并掺入ZnO晶格取代Zn²⁺, Ga的掺入改变了ZnO薄膜中的缺陷类型及浓度、化学计量比、薄膜表面结晶质量, 进而影响了薄膜的光致发光性能. 随着热氧化温度升高, Ga掺杂量增大, ZnO薄膜的晶粒尺寸增大, 尺寸更均一, 紫外光与可见光强度比增大. 随着热氧化时间延长, Ga掺杂量降低, ZnO薄膜的晶粒尺寸均一性变差, 紫外光与可见光强度比减小.