(PbSn)n(n=1～10)合金团簇稳定性及金属性的研究

采用基于密度泛函理论(DFT)的第一性原理研究了合金团簇(PbSn)n(n=1～10)的稳定性及金属性.结果显示:同比例铅锡合金团簇具有与单质铅、锡团簇相同的幻数尺寸,但结构特点却与单质团簇不同,铅、锡原子间首先各自形成单质团簇,然后再结合成合金团簇,且表现出松散型与密堆积型的有机结合.另外,(PbSn)n(n=1～10)团簇构型在N=14处发生突变:类似于密堆积构型向层状转变,同时HOMO-LUMO能隙具有先增大后减小的趋势,并出现微弱的半导体向金属转变的行为,而且团簇原子总数为20的HOMO-LUMO能隙较相同尺寸单质团簇的还低,说明合金团簇具有比单质团簇更好的导电性.     

PbnSn(n=1～19)合金团簇的结构稳定性及其半导体-金属性转变

基于密度泛函理论(DFT),采用广义梯度近似(GGA)计算了PbxSn(n=1～19)合金团簇的结构演化、结合能和电子结构.结果表明Sn原子的掺杂增强了Pbn团簇的稳定性,其碎裂行为也很好地符合了已有的实验结果.随着PbnSn合金团簇尺寸的增加,其几何结构由类似于密堆积构型向松散构型演化,具体的转变尺寸为总原子数N=14.PbnSn团簇的HOMO-LUMO能隙呈现出先增加后降低的趋势,在6原子时达到最大值1.6 eV,表现出显著的半导体性质和尺寸依赖.当N≥7时,能隙振荡减小,呈现出微弱的半导体性向金属性的转变行为.     

PbnSn(n=1-19)合金团簇的结构稳定性及其半导体—金属性转变

基于密度泛函理论（DFT）,采用广义梯度近似（GGA）计算了PbnSn(n=1-19)合金团簇的结构演化、结合能和电子结构。结果表明Sn原子的掺杂增强了Pbn团簇的稳定性,其碎裂行为也很好地符合了已有的实验结果。随着PbnSn合金团簇尺寸的增加,其几何结构由类似于密堆积构型向松散构型演化,具体的转变尺寸为总原子数N=14。PbnSn团簇的HOMO-LUMO能隙呈现出先增加后降低的趋势,在6原子时达到最大值1.6eV,表现出显著的半导体性质和尺寸依赖。当N≥7时,能隙振荡减小,呈现出微弱的半导体性向金属性的转变行为。     

(CuSe)_n(n=1-10)团簇结构稳定性与磁性的理论研究

本文采用包含广义梯度近似的密度泛函理论对铜、硒二元团簇(CuSe)_n(n=1-10)的结构稳定性与磁性质进行系统的研究,发现:(CuSe)_n(n=1-10)二元团簇的原子总数为8时几何结构出现由平面向立体的转变行为,且Cu3Se3为"幻数"构型.同时,(CuSe)_n(n=1-10)团簇的平均结合能比同尺寸单质铜团簇的大,因此混合团簇(CuSe)_n(n=1-10)具有比单质铜团簇更强的稳定性;另外,根据(CuSe)_n(n=1-10)团簇的二阶差分能和HOMO-LUMO能隙随尺寸增加的演化趋势可获知,Cu8Se8的结构极其稳定.最后,对团簇磁性的研究发现,铜硒团簇展现出比较特别的磁性,其中,Cu3Se3的总磁矩为3μB,且磁性主要来源于硒原子,而当n=4,6,8和10时,(CuSe)_n(n=1-10)团簇磁性出现淬灭现象.     

Rhn、Con(n=3～56)团簇的结构特性研究

采用半经验的Gupta多体势结合遗传算法对Rhn、Con(n=3~56)团簇的基态结构特性进行了系统的研究.除在n=18~40尺寸范围内有少数团簇的构型不同,两种团簇具有相似的几何结构.在Rhn、Con(n=3~56)团簇的生长中,存在类Ih构型与类fcc构型之间的竞争,对于n≤24,两种团簇都从紧致密堆积结构过渡为类二十面体构型,Rh38及Co38为具有Oh对称性的类fcc构型,从n=39开始,铑团簇和钴团簇都呈现出明显的Ih生长模式.两种团簇的平均束缚能随原子数目的增加而增大,且在所研究的尺寸范围内铑团簇的平均结合能高于钴团簇.Rhn、Con(n=3~56)团簇具有相同的幻数序列:n=13,19,23,38,55.     

InnAsn (n=1-20)小团簇几何结构与电子性质的第一性原理研究

基于密度泛函理论的第一性原理方法,采用B3LYP下的赝势基组LanL2DZ,研究了InnAsn (n=1-20)团簇的基态几何结构、相对稳定性、电子性质及其振动光谱. 结果表明,当n=5-11时团簇的基态构型为层状结构; 当n=12-20时团簇的基态构型为笼状结构. 团簇平均结合能、二阶能量差分和HOMO-LUMO能隙均在n=9,12,18出现极大值,说明In9As9、In12As12和In18As18为幻数团簇. 另外,HOMO-LUMO能隙的计算结果表明InnAsn (n=1-20)团簇具有宽带隙半导体特征.     

(MgC)n (n=1~10)团簇结构稳定性及金属性第一性原理研究

采用基于密度泛函理论的第一性原理对同比例二元团簇(MgC)n (n=1~10)的结构稳定性和金属性进行比较系统的研究。结果显示：当原子总数N=2, 4, 6, 8, 12时,二元团簇的结构为平面形状,其余为对称性较低的三维构型,且Mg3C3为幻数结构。另外,在二元团簇的形成过程中,镁、碳原子并不是均匀分布于空间中,而是首先形成各自倾向的构型,再结合成团簇。同时,(MgC)n (n=1~10)二元团簇的基态结构在原子总数N=10时发生突变,由平面形向三维结构转变。此外,最高占据轨道和最低未占据轨道间的能隙值除N=8外呈逐渐降低的趋势,说明在同比例二元团簇随尺寸增加的演化中出现半导体性与金属性间的相互转变行为。     

(MgC)n (n=1~10)团簇结构稳定性及金属性第一性原理研究

采用基于密度泛函理论的第一性原理对同比例二元团簇(MgC)n (n=1~10)的结构稳定性和金属性进行比较系统的研究。结果显示：当原子总数N=2, 4, 6, 8, 12时,二元团簇的结构为平面形状,其余为对称性较低的三维构型,且Mg3C3为幻数结构。另外,在二元团簇的形成过程中,镁、碳原子并不是均匀分布于空间中,而是首先形成各自倾向的构型,再结合成团簇。同时,(MgC)n (n=1~10)二元团簇的基态结构在原子总数N=10时发生突变,由平面形向三维结构转变。此外,最高占据轨道和最低未占据轨道间的能隙值除N=8外呈逐渐降低的趋势,说明在同比例二元团簇随尺寸增加的演化中出现半导体性与金属性间的相互转变行为。     

In_nAs_n(n=1～20)小团簇几何结构与电子性质的第一性原理研究

基于密度泛函理论的第一性原理方法,采用B3LYP下的赝势基组LanL2DZ,研究了In_nAs_n（n=1～20）团簇的基态几何结构、相对稳定性、电子性质及其振动光谱.结果表明,当n=5～11时团簇的基态构型为层状结构;当n=12～20时团簇的基态构型为笼状结构.团簇平均结合能、二阶能量差分和HOMO-LUMO能隙均在n=9,12,18出现极大值,说明In₉ As₉、In₁₂ As₁₂和In₁₈ As₉18)为幻数团簇.另外,HOMO-LUMO能隙的计算结果表明In_nAs_n（n=1～20）团簇具有宽带隙半导体特征.     

PbnSn(n=1-19)合金团簇电偶极矩和极化率的研究

利用基于密度泛函理论的有限电场方法对PbnSn(n=1-19)团簇的电偶极矩和极化率进行研究。PbnSn(n=1-19)合金团簇的高对称性抑制其电偶极矩的产生。在团簇原子总数N＜14时，团簇的电偶极矩与能隙表现出相反的变化趋势（同一尺寸下，高能隙与低电偶极矩相对应），然而在14≤N≤19范围内，由于构型生长方式的转变，能隙与电偶极矩展现出相同的演化趋势。研究所得的极化率与实验值在部分尺寸相符合较好，但总体上与实验值还有一定的差别，这可能是实验中包含混合态或激发态的缘故。     

Neutron pick-up (n,2 n *) reaction

Direct²n^* contributions (²n^*-singlet n-n final state interaction) to the (n,2n) reaction cross sections are estimated.     

(n,n)-(2n, 0)碳纳米管异质结的扭转力学特性

相近直径的锯齿型和扶手椅型碳纳米管可以共轴组合形成5-7碳环交替出现的柱形对称异质结. 本文利用分子动力学方法研究了直径相近且等长锯齿型和扶手椅型碳纳米管形成的(n, n)-(2n, 0)结在扭转过程中的扭矩和轴向应力随扭转角度的变化规律以及应力传递过程. 研究发现, (n, n)-(2n, 0)结扭转应变在达弹性限度内不会产生轴向应力, 该效应对基于碳纳米管扭转特性的纳米振荡器件的设计具有重要意义.     

Lowering operators for the reduction U(n)↓SO(n)

A complete set of lowering operators is constructed for the reduction U(n)↓SO(n). In an irreducible representation of the unitary group U(n), every vector of maximal weight with respect to the subgroup SO(n) can be obtained using the lowering operators. The formula for the lowering operators is obtained using graphical techniques.     

Measurement of cross sections for the reactions 241Am(n, 2n) and 241Am(n, 3n)

The cross sections for the reactions ²⁴¹Am(n, 2n)²⁴⁰Am and ²⁴¹Am(n, 3n)²³⁹Am are measured for several neutron-energy values in the range 13.42–14.86 MeV. An upper limit on the cross section for the reaction ²⁴¹Am(n, α)²³⁸Np is also obtained.     

14MeV中子硒同位素反应截面的测量

在14MeV中子能区用活化法测得了⁸²Se(n,2n)^81m,gSe, ⁷⁶Se(n,2n)⁷⁵Se,⁷⁸Se(n,p)⁷⁸As,⁷⁶Se(n,p)⁷⁶As,⁷⁴Se(n,p)⁷⁴As和⁸⁰Se(n,α)⁷⁷Ge反应截面值, 以⁹³Nb(n,2n)^92mNb反应截面为中子注量标准, 并且将测量值和收集到的文献值进行了比较.     

Bound States in n Dimensions (Especially n = 1 and n = 2)

 We stress that in contradiction with what happens in space dimensions n ≥ 3, there is no strict bound on the number of bound states with the same structure as the semi-classical estimate for a large coupling constant. We give, in two dimensions, examples of weak potentials with one or infinitely many bound states. We derive bounds for one and two dimensions which have the “right” coupling-constant behaviour for large coupling. Received November 22, 2001; accepted for publication November 28, 2001     

Local state probabilities for solvable restricted solid-on-solid models:A n,D n,D n (1) , andA n (1)

The local state probabilities (LSPs) are exactly computed for four hierarchies of solvable lattice models. They are restricted solid-on-solid (RSOS) models whose local states and their adjacent conditions are specified by Dinkin diagrams of typesA _n,D _n,D _n ⁽¹⁾ andA _n ⁽¹⁾ . The LSPs are expressed in terms of modular functions characterized by branching identities among the theta functions. Their automorphic properties are used to study the critical behaviors. Some fine structures are found in the spectrum of the critical exponents.     

Cross sections for the (n,n′), (n,p) and (n, 2n) reactions on 107Ag and 109Ag isotopes

The cross sections for (n, n′), (n, p) and (n, 2n) reactions on the target nuclei ¹⁰⁷Ag and ¹⁰⁹Ag were measured by the activation technique in the neutron energy range 13 to 18 MeV. The results are interpreted in terms of the compound nucleus and precompound emission models.     

THE SYSTEMATIC CALCULATIONS OF (n,2n) AND (n,3n) REACTION CROSS SECTIONS FOR MEDIUM-HEAVY NUCLEI AT 14.5MeV

The (n,2n) and (n,3n) reaction processes have been studied by mechanism analysis of the neutron-induced nuclear reactions,formulae for evaluating the reduced cross section have been given.The cross sections of 14.5MeV (n,2n),(n,3n) reactions have been calculated according to the systematic method for Z=58—83 about 53 nuclei.A good agreement between the results of the systematic calculations and the experimental data has been reached.     

Pressure-tuned collapse of the Mott-like state in Ca(n+1)Ru(n)O(3n+1) (n=1,2): Raman spectroscopic studies

We report a Raman scattering study of the pressure-induced collapse of the Mott-like phases of Ca3Ru2O7 (T(N)=56 K) and Ca2RuO4 (T(N)=110 K). The pressure dependence of the phonon and two-magnon excitations in these materials indicate (i) a T approximately 0 pressure-induced collapse of the antiferromagnetic (AF) insulating phase above P(*) approximately 55 kbar in Ca3Ru2O7 and P(*) approximately 5-10 kbar in Ca2RuO4, (ii) a remarkable insensitivity of the exchange interaction to pressure in both systems, and (iii) evidence for persistent AF correlations above the critical pressure of Ca2RuO4, suggestive of phase separation involving AF insulator and ferromagnetic metal phases.