Nb2H的电子结构和相互作用

应用基于密度泛函理论的赝势-平面波方法研究了Nb2H的电子结构和H原子占据点之间的关系.计算结果表明:由4个Nb近邻构成的四面体中心点(T点)为H的稳定俘获点,而由6个Nb近邻构成的八面体中心点(O点)则为相互作用势的极大值点,是不稳定点.相邻T点间存在低能量通道,具有鞍点(S点)结构.在T点及近邻低能通道上,H的1s能级展宽较弱,Nb的4d,5s带部分向下延展,与H带杂化后形成孤立带.当H由T向O移动时,孤立的杂化带向金属价带靠近,带隙逐渐消失.H处于完整宿主晶格中的T点时,布里渊区Г点处的光学声子结构色散较强,而在远离Г点处的色散很弱.H在T点邻域及T-T通道上的电子结构稳定性和声子的色散关系表明,H倾向于以稳定负离子形式存在.     

纤锌矿BeO掺Cd的电子结构与能带特性研究

采用基于密度泛函理论平面波赝势方法, 对纤锌矿BeO掺Cd的Be_1-xCd_xO合金进行电子结构与能带特性研究. 结果表明: Be_1-xCd_xO的价带顶始终由O 2p电子态决定, 而导带底由Be 2s和Cd 5s的电子态决定.随着Be_1-xCd_xO合金的Cd掺杂量增加, Cd 4d与O 2p的排斥效应逐渐加强, 同时Be_1-xCd_xO的带隙逐渐变小, 出现"直接-间接-直接"的带隙转变. 为了使理论值与实验值相一致, 对Be_1-xCd_xO带隙进行修正, 并分析了纤锌矿BeO-ZnO-CdO三元合金的带隙和弯曲系数与晶格常数的关系.     

高性能MgB2长线材制备及性能表征

采用原位粉末装管工艺，分别以Mg粉(99.5%)，无定形B粉(99.9%)为原料，以纳米SiC(10—30nm)作为掺杂材料制备铁基MgB₂线.首先将已混合的原料在丙酮介质中球磨，真空干燥后，将粉末填入铁管内，然后通过孔型轧制、旋锻和拉拔等冷加工工艺得到11m长外径Ф1.75mm铁基MgB₂超导线.用扫描电镜，电子能谱，X射线衍射仪和超导量子干涉仪测试发现，样品微观结构整齐，晶粒大小均匀，内部仅含微量MgO，T_C(onset)=35.1K，ΔT_C＝5.3K.纳米SiC掺杂后，其中C造成MgB₂晶格畸变，形成有效磁通钉扎中心，C元素在MgB₂中分布均匀.标准四引线测试结果表明，11m线均分10段后，各点的J_c(4.2K,10T)均超过1.0×10⁴A/cm^{2，最高值达到1.2×104A/cm2.在10—18T范围各点临界电流值分布均匀，变化率小于10%.}     

Formation of rippled surface morphology during Si/Si （100） epitaxy by ultrahigh vacuum chemical vapour deposition

The Si epitaxial films are grown on Si (100) substrates using pure Si2H6 as a gas source using ultrahigh vacuum chemical vapour deposition technology. The values of growth temperature T_g are 650 ℃, 700 ℃, 730 ℃, 750 ℃, and 800 ℃. Growth mode changes from island mode to step-flow mode with T_g increasing from 650 ℃ to 700 ℃. Rippled surface morphologies are observed at T_g = 700 ℃, 730 ℃, and 800 ℃, but disappear when T_g = 750 ℃. A model is presented to explain the formation and the disappearance of the ripples by considering the stability of the step-flow growth.     

Cd掺杂纤锌矿ZnO电子结构的第一性原理研究

采用密度泛函理论结合投影缀加波方法，对掺杂Cd导致ZnO禁带宽度下降的机理进行了研究. 通过对掺杂前后电子能带结构，态密度以及分态密度的计算和比较，发现Cd_xZn_1-xO价带顶端(VBM)始终由O-2p占据；而导带顶端(CBM)则由Cd-5s与Zn-4s杂化轨道控制. 随着掺杂浓度的增加，决定带隙宽度的CBM的位置下降，同时VBM的位置上升，从而导致了带隙的变窄，出现了红移现象. 此外，Cd掺杂会使晶胞发生膨胀，这种张应变也是导致Cd     

α-, β-和γ-Si3N4 高压下的电子结构和相变: 第一性原理研究

本文采用第一性原理框架下的赝势平面波方法结合振动类德拜模型研究了α,β和γ-Si₃N₄在高温下的点阵常数,弹性常数和弹性模量.研究发现三种同质异相体的体模量都很高.β-Si₃N₄在低温下表现出脆性,在高温下则表现出延展性.γ-Si₃N₄在低温和高温下都是脆性的共价化合物.β → γ 相变的相界斜率为正值,说明在较高温度时合成γ-Si₃N₄所需的压强也较高.α → γ 相变的相界可以表示成 P=16.29- 1.835-10^-2 T+9.33945-10^-5T²-2.16759-10^-7T³+2.91795-10^-10T⁴.本文还分析了Si₃N₄同质异相体在高压下的态密度和能带.在α-Si₃N₄中主要是Si-s, p和N-s,p的轨道杂化对晶体的稳定性起作用.α和β-Si₃N₄都具有Γ_V-Γ_C类型的间接带隙(分别是4.9~eV和4.4~eV)而γ-Si₃N₄具有直接带隙(3.9~eV). 研究还发现α-Si₃N₄和β-Si₃N₄的价带顶分别沿着Γ-M和Γ-A方向.本文的计算结果和已有的实验数据是一致的.     

Eu掺杂TbMnO3多晶材料的介电性质

采用固相反应法制备了Tb_0.8Eu_0.2MnO₃多晶材料.对样品的X射线衍射（XRD）分析表明Eu³⁺固溶于TbMnO₃中.测量了样品在低温(100 K ≤T≤ 300 K)和低频下(200 Hz≤f≤100 kHz)的复介电性质.在此温度区间内发现了两个介电弛豫峰.经分析认为低温峰（T≈170 K）起源于局域载流子漂移引起的偶极子极化效应,而高温峰（T≈290 K）则是由离子电导产生的边界和界面层的电容效应引起的.电阻率的测量显示在低温下（T≈230 K）存在明显的导电机制转变.     

New aspects on URu2Si2 and CeTIn5 (T=Rh, Ir, Co) observed by high pressure NMR and NQR

NMR and NQR studies on two interesting systems (URu₂Si₂, CeTIn₅) were performed under high pressure. (1) URu₂Si₂: In the pressure range 3.0 to 8.3 kbar, we have observed new ²⁹Si NMR signals arising from the antiferromagnetic (AF) region besides the previously observed ²⁹Si NMR signals which come from the paramagnetic (PM) region in the sample. This gives definite evidence for spatially-inhomogeneous development of AF ordering below T ₀ of 17.5 K. The volume fraction is enhanced by applied pressure, whereas the value of internal field (∼91 mT) remains constant up to 8.3 kbar. In the AF region, the ordered moment is about one order of magnitude larger than 0.03 μB. (2) CeTIn₅: The pressure and temperature (T) dependences of nuclear spin-lattice relaxation rate 1/T ₁ of ¹¹⁵In in CeTIn₅ have shown that the superconductivity (SC) occurs close to an AF instability. From the T dependences of 1/T ₁ and Knight shift below T _c. CeTIn₅ has been found to exhibit non-s wave (probable d wave) SC with even parity and line nodes in the SC energy gap.     

Al2O3Hx(x=1—3)分子团簇的结构与光谱研究

用密度泛函理论(DFT)的B3lyp方法在6-311++g(d,p)水平上对Al₂O₃H_x(x=1—3)分子的几何构型, 电子结构, 振动频率等性质进行了系统研究. 并给出了它们可能基态结构的总能量(E_T), 零点能(E_z), 摩尔热容(C_v), 标准熵(S), 原子化能(ΔE_m), 垂直电离能(IP)及垂直电子亲和能(E_A). Al₂O₃H和Al₂O₃H₂分子可能的基态的几何构型都为平面结构. Al₂O₃H₃的两个可能为基态的几何构型都是在立体Al₂O₃(D_3h)的几何结构基础上加三个氢原子构成. 这三个分子的能量最低结构为Al₂O₃H(²A′)C_s, Al₂O₃H₂(¹A′) C_s, Al₂O₃H₃ (²A) C₁.     

Nuclear spin-lattice and nuclear spin-spin relaxation time measurements in EuB6 at low temperatures using the spin-echo technique

Experimental results on the nuclear spin-lattice and nuclear spin-spin relaxation times in the ferromagnetic EuB₆ at temperatures below 4·2 K are presented using the external magnetic field,H _ext, in the range of 0 ⩽H _ext ⩽ 10 kG. Nuclear spin-spin relaxation time computed on the basis of the Suhl-Nakamura process turns out to be 3·2μs, which compares well with the experimental value 11·1μs obtained with the 10 kG magnetic field at 1·7 K. It is found that in the ferromagnetic EuB₆,T ₁ is approximately 5 × 10³ times larger thanT ₂ at 1·7 K with the 10 kG magnetic field. Thus the effect ofT ₁ onT ₂ can be neglected. From the experimental value ofT ₂, the value of the homogeneous line broadening is found to be 14 kHz. The corresponding value obtained from the cw method is 175 kHz. This evidently shows the presence of the inhomogeneous line broadening in the cw NMR.     

Influence of impurity hydrogen on the structure and properties of bulk Li and pressure effects

The structure and properties of a 16-atom body-centered cubic lithium cell with an interstitial hydrogen atom are studied using a pseudopotential-plane-wave method within the density functional theory at 0 K and high pressures. The host lattice is dramatically distorted by the introduction of H. Although the hydrogen atom is stable at the tetragonal site in perfect bcc host lattice, it favors the octahedral site formed by six non-equivalent Li atoms after full relaxation of the cell, showing P4/mmm symmetry within the pressures ranging from 0 to 6 GPa. The lattice ratio (a/c) changes irregularly with external pressure at about 3 GPa. The hydrogen band lies in the bottom of the valence band, separated by a gap from the metallic bands, illustrating the electronegativity of hydrogen. High reflectivity in the low frequency area induced by the impurity hydrogen is observed when only interband transitions are taken account of. A dip in reflectivity due to parallel band transitions is observed at ∼0.4 eV. Another dip at ∼4.3 eV appears when external pressure increases over 4 GPa.     

Half-metallicity and magnetism of the quaternary inverse full-Heusler alloy Ti<Subscript>2-x</Subscript>M<Subscript>x</Subscript>CoAl (M = Nb,V) from the first-principles calculations

Using the first-principles calculations based on density functional theory, we investigate the more d-electrons doping effects on the electronic structure and magnetism of the parent inverse Heusler alloy Ti₂CoAl by the substitution of Nb and V atoms for Ti(A) and Ti(B) atoms locating at the two inequivalent sublattices. The Ti₂CoAl is half-metallic with Fermi level near the top of the minority-spin valence band and hence its spin-polarization is easily reduced by the spin-flip excitation. Our total energy calculations show that the V/Nb doping at the Ti(A)/Ti(B) site is energetically favorable compared with the Ti(B)/T(A) site due to the lower total energy. Our band structure calculations indicate that for the V doped compounds, half-metallicity can be well retained regardless of doping sites and percentages except for the case of Ti(A)-site doping with x = 1, while for Nb doped compounds, the half-metallicity persists only in Ti(B)-site doping with different percentages. For the doped compounds with half-metallicity, the Fermi level shifts from the top of minority-spin valence band to the bottom of minority-spin conduction band with increasing content of x, and typically, the doped compounds (V in Ti(A) and Ti(B) sites at x = 0.75 and 0.5, respectively; Nb in Ti(B) site at x = 0.5), whose Fermi levels are adjusted to the expected positions to effectively inhibit the spin-flip excitation are promising candidates for spintronics applications.     

Dangling bond modulating the electronic and magnetic properties of zigzag SiGe nanoribbon

First-principles nonmagnetic calculations reveal a metallic character in zigzag SiGe nanoribbons (ZSiGeNRs) regardless of their width. The partial DOS projected onto the Si and Ge atoms of ZSiGeNR shows that a sharp peak at the Fermi level is derived from the edge Si and Ge atoms. The charge density contours show the Si–Ge bond is covalent bond, while for the Si–H bond and Ge–H bond, the valence charges are strongly accumulated around H atoms due to their stronger 1 s potential and the higher electronegativity of 2.20 than that of 1.90 for Si atom and 2.01 for Ge atom, so that a significant charge transformation from Si or Ge atoms to H atoms and thus an ionic binding feature. Spin–polarization calculations show that the band structures of ZSiGeNR are modified by the dangling bonds. Compared with perfect ZSiGeNR which is a ferrimagnetic semiconductor, the bands of the ZSiGeNRs with bare Si edge, bare Ge edge, and bare Si and Ge edges shift up and nearly flat extra bands appear at the Fermi level. The ZSiGeNR with bare Si edge or bare Ge edge is a ferrimagnetic metal, while ZSiGeNR with bare Si and Ge edges is a nonmagnetic metal.     

压力下替位氢对金属锂能带结构的影响

 利用第一原理的平面波-赝势密度泛函方法,研究了零温下体心立方（bcc）金属锂由于氢的替位掺杂和外界压力的改变所引起的电子结构变化。结果表明：掺杂体系仍然呈金属性,但是由于氢原子俘获了一个金属自由电子,具有了离子属性,使得费米面以下出现了孤立能带和带隙,最低金属价带偏离了自由电子带,形成鞍型带底,其它价带和导带均出现不同程度的简并解除和带型畸变。在压力的作用下,带隙加宽,费米面下的孤立带趋向于一条直线,成为一条能级。     

硅烯饱和吸附碱金属原子的第一性原理研究

基于密度泛函理论的第一性原理计算, 研究了硅烯饱和吸附碱金属元素原子的稳定性、微观几何结构和电子性质, 并与纯硅烯及其饱和氢化结构进行了对比分析. 研究发现复合物SiX(X=Li, Na, K, Rb)的形成能都是负的, 相对于纯硅烯来说可以稳定存在. Bader电荷分析表明, 电荷从碱金属原子转移至硅原子. 从成键方式来看, 硅烯与氢原子形成共价键, 而与碱金属原子之间形成的键主要是离子性成键, 但还存在部分共价关联成分. 能带计算表明, 锂原子饱和吸附在硅烯形成的复合物SiLi是直接带隙的半导体, 带隙大小为0.34 eV. 其他碱金属饱和吸附在硅烯上形成的复合物都表现为金属性.     

H掺杂α-Fe2O3的第一性原理研究

α-Fe₂O₃是一种重要的磁性半导体材料, 在电子器件中应用广泛, 具有重要的研究意义. 本文基于密度泛函理论, 采用GGA+U方法, 应用第一性原理对间隙H掺杂前后的六方相α-Fe₂O₃的晶格常数、态密度、Bader 电荷分布进行了计算分析. 研究了U值对结果的影响, 发现U=6 eV时, 体相α-Fe₂O₃的晶胞平衡体积、Fe原子磁矩、带隙值与实验值最符合. 在选取合适U值后, 第一性原理计算结果表明, H掺杂后, 间隙H部分被氧化, 其最近邻的Fe 和O部分被还原, H和O有一定程度的成键. 在费米面附近, 出现了新的杂化能级, 杂化能级扩展了价带顶的宽度, 同时导带底下移, 引起带隙减小, 表明H掺杂是一种有效的能带结构调控方法.     

Electronic structure of cubic uranium compounds

Self-consistent cellular multiple scattering techniques and photoemission energy distribution curves obtained at 20<hv<80 eV are used to study the density of states of UN and US. The calculations are based on a model using a finite cluster of atoms in a condensed-matter-like boundary potential. The main results refer to the mixing of thes, p, d, andf-states of uranium into a valence and a conduction band. Thef-states form orbitals with the ligands, within the valence and conduction bands. In the nitride the amount off character in the valence band is only 0.3 electrons and thef electrons are in two resonant levels (of each spin) in the conduction band. Only the first of these levels is occupied for the local, alternate from atom to atom, majority spin. In the sulfide the amount off character in the valence band is 0.59 electrons and the rest of thef-levels are in a resonance state (of majority spin) at the beginning of the conduction band. The conduction band is mainly of localized uranium 6d character. The theoretical results compare favorably with the photoemission data reported here.     

Doping effects on metallic and semiconductor single-wall carbon nanotubes

In this paper we investigate nitrogen- and boron-doped zigzag and armchair single-wall carbon nanotubes (SWNTs) with theoretical models based on the density functional theory. We take into account nitrogen and boron doping for two isomers in which substitutive atoms are on opposite sides of the tube, but only in one isomer the impurity sites are symmetrical with respect to the diameter. The band structures show a strong hybridization with impurity orbitals that change the original band structure. Although the two isomers of armchair SWNT exhibit the same formation energy, their band structures are different. Indeed asymmetrical isomers are gapless and exhibit a crossing of valence and conduction bands at k?=?π/c, leading to metallic SWNTs. Band structures of symmetrical isomers, on the other hand, exhibit an energy gap of 0.4?eV between completely filled valence and empty conduction bands. We use density of charge in order to understand this difference. In zigzag SWNT an impurity band is introduced in the energy gap and for N doping this band is just partially occupied in such a way that the electronic behaviour is reversed from semiconductor to metallic. Whereas for a given isomer armchair SWNT shows similar behaviours of N- and B-doped structures, B-doped zigzag SWNTs present different band structure and occupation compared to the N-doped case.     

金属Nb中H的量子行为研究

采用基于密度泛函原理的赝势平面波方法,计算了H在Nb晶体中的势场.通过求解H在该势场中的Schrdinger方程,得到了H的振动状态.计算结果表明,H的基态和第一激发态是局域的,第二激发态是非局域的,可以在T点之间迁移以及做4T/6T环运动. 关键词： 第一性原理 金属氢化物 量子振动     

Resonant photoemission and absorption spectroscopy of the Cu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>TiSe<Subscript>2</Subscript> compound

Single crystals of the Cu _x TiSe₂ compound with x = 0.05, 0.09, and 0.33 have been grown. Resonance photoelectron Cu 3p-3d and 2d-3d spectra of the valence bands, the spectra of the core levels, and the L absorption spectra for titanium and copper have been obtained. It is shown that the degree of oxidation of titanium atoms is +4 and the state of copper atoms is close to the state of free copper ions. It is found that the spectra of the valence bands obtained under the Cu 3p and 2p resonance conditions radically differ. For the spectra in the Cu 2p excitation regime, several bands corresponding to different decay channels of the excited state are observed. According to calculations of the density of states, the 3d states of copper are filled incompletely; the occupancy of the 3d band of copper is 9.5 electrons per atom.