强激光照射对2H-SiC晶体电子特性的影响

使用基于密度泛函微扰理论的第一原理赝势法, 计算了纤锌矿结构2H-SiC晶体在强激光照射下的电子特性, 分析了其能带结构和电子态分布. 计算结果表明: 2H-SiC平衡晶格参数a 和c随电子温度T_e的升高逐渐增大; 电子温度在0–2.25 eV范围内时, 2H-SiC仍然是间接带隙的半导体晶体, 当T_e超过2.25 eV达到2.5 eV以上时, 2H-SiC变为直接带隙的半导体晶体; 随着电子温度升高, 导带底和价带顶向高能量或低能量方向发生了移动, 当电子温度T_e大于3.5 eV以后, 价带顶穿越费米能级; 电子温度T_e在0–2.0 eV变化时, 带隙随电子温度升高而增大; T_e在2.0–3.5 eV范围变化时, 带隙随电子温度升高而快速地减少, 表明2H-SiC晶体的金属性随电子温度T_e的继续升高而增强. 在T_e =0, 5.0 eV 处, 计算了2H-SiC晶体总的电子态密度和分波态密度. 电子结构表明T_e =0 eV 时, 2H-SiC 是一个带隙为2.3 eV的半导体; 在T_e =5.0 eV时, 带隙已经消失而呈现出金属特性, 表明当电子温度升高时晶体的共价键变弱、金属键增强, 晶体经历了一个熔化过程, 过渡到金属状态.     

强激光照射对6H-SiC晶体电子特性的影响

使用基于密度泛函微扰理论的第一性原理赝势法, 模拟研究了纤锌矿6H-SiC晶体在强激光照射下电子特性的变化. 研究结果表明, 电子温度T_e在升高到3.89 eV及以上后, 6H-SiC由间接带隙的晶体变为直接带隙的晶体; 带隙值随电子温度T_e升高先是增大后又快速减小, 当电子温度T_e大于4.25 eV以后, 带隙已经消失而呈现出金属特性.     

In2O3晶体电子结构和光吸收机理研究

为了得到准确的In₂O₃晶体电子结构, 本文分别采用GGA, GGA+U, HSE06的方法计算了电子结构, 并进行了G₀W₀修正, 通过比较计算结果, 得到HSE06+G₀W₀方法计算得到的禁带宽带最接近实验结果. 在此基础上使用Hedin的G₀W₀近似方法和Bethe-Salpeter方程计算得到了In₂O₃晶体的光学性质, 计算结果与实验结果吻合很好, 在此基础上通过对准粒子能带结构、光学跃迁矩阵和光学吸收谱的分析, 给出了In₂O₃晶体的光学跃迁机理.     

Structures,stabilities,and magnetic properties of the Fe_nAu(n=1-12) clusters

The configurations,stabilities,electronic,and magnetic properties of Fe_nAu(n = 1–12) clusters are investigated systematically by using the relativistic all-electron density functional theory with the generalized gradient approximation.The substitutional effects of Au in Fe_(n+1)(n = 1,2,4,5,10–12) clusters are found in optimized structures which keep the similar frameworks with the most stable Fe_(n+1)clusters.And the growth way for Fe_nAu(n = 6–9) clusters is that the Au atom occupies a peripheral position of Fen cluster.The peaks appear respectively at n = 6 and 9 for Fen Au clusters and at n = 5 and 10 for Fe_(n+1)clusters based on the size dependence of second-order difference of energy,implying that these clusters possess relatively high stabilities.The analysis of atomic net charge Q indicates that the charge always transfers from Fe to Au atom which causes the Au atom to be nearly non-magnetic,and the doped Au atom has little effect on the average magnetic moment of Fe atoms in Fen Au cluster.Finally,the total magnetic moment is reduced by 3 μB for each of Fen Au clusters except n = 3,11,and 12 compared with for corresponding pure Fe_(n+1) clusters.     

High field magnetoresistance of CeAl2 at low temperatures

We have measured the magnetoresistance ρ(H,T₀) of the magnetic Kondo lattice CeAl₂ at temperatures T₀ in the range 0.035 – 1.3 K, well below the Néel temperature T_N=3.8 K, in magnetic fields H up to 145 k0e. The ρ vs H curve exhibits a rapid decrease between H=45–65 kOe corresponding to the metamagnetic transition at H=H_M. The resistivity then levels out to a value which depends only weakly on field and temperature. For HM a small positive magnetoresistance was observed with the derivative dρ/dH increasing as the temperature is lowered. The present results are compared with the ρ(H,T₀) data obtained for the CePb₃ magnetic Kondo lattice, where the decrease in the resistivity at HH_M was considered earlier to be evidence of field induced superconductivity.     

The electronic structures of epitaxial CrSi2 film prepared on Si(111) substrate

The valence band (VB) electronic structures of CrSi₂ were studied by synchrotron radiation photoemission. Overall features of the VB photoemission spectra measured at room temperature (RT) and 20 K by using synchrotron radiation (photon energy, hν=20–120 eV) were similar. Two characteristic emissions were observed corresponding to the bonding and the nonbonding Cr-d partial density of states (PDOS) in the CrSi₂. The onset of the VB photoemission measured at 20 K was located at about 0.32 eV below Fermi level, due to the energy band gap of CrSi₂ more than 0.32 eV.     

熔石英中过氧缺陷及中性氧空位缺陷的几何结构、电子结构和吸收光谱的准粒子计算

本文使用密度泛函理论研究了熔石英中peroxy linkage(POL)缺陷和中性氧空位(NOV)缺陷的几何结构,电子结构以及光学性质.采用自洽的准粒子GW计算结合求解Bathe-Salpeter方程的多体理论,研究了缺陷引起的电子结构和光学吸收谱的变化.首先研究了无缺陷非晶结构的电子结构与吸收谱,得到的结果与实验值非常接近.对POL的计算表明,其在基态下的局部结构与过氧化氢分子类似.采用多体理论计算得到的吸收谱表明其最低吸收峰位于6.3 eV处.这一结果不支持实验认为的位于3.8 eV处的吸收峰是由POL缺陷导致的说法.对于NOV缺陷,计算表明其基态的Si—Si键长为2.51?而三重态下的值则为3.56?.相应的GW+BSE计算表明中性氧空位缺陷导致了位于7.4 eV处的吸收峰,与实验测量结果一致.     

First-Principles Calculations of the AS,, Electronic Structures

The changes in the GaAs electronic structure due to the arsenic-antisite defect have been calculated by using LDF-LMTO-ASA method to a D_2d-symmetry-supercell ( Ga₁₅As₁₇□₃₂)c,o ntains 15 gallium atoms, 17 arsenic atoms and 32 empty spheres. The results show that the central AsGa atom is antibonding with its nearest neighbor arsenic atoms and therefore induces gap states in GaAs. The gap states are composed of A₁-like state and T₂-like state. The bonding properties of gap states have been analyzed in detail by using density of states (DOS) and combined coefficients of wavefunctions. Our results of E_A1 =E_v+0.70 eV and= E_T2+1.07eV are in good agreement with experiments as well as previous calculation results obtained by other self-consistent methods.     

Simulations of the effects of density and temperature profile on SMBI penetration depth based on the HL-2A tokamak configuration

Using the trans-neut module of the BOUT++ code, we study how the fueling penetration depth of supersonic molecular beam injection(SMBI) is affected by plasma density and temperature profiles. The plasma densities and temperatures in L-mode are initialized to be a set of linear profiles with different core plasma densities and temperatures. The plasma profiles are relaxed to a set of steady states with different core plasma densities or temperatures. For a fixed gradient, the steady profiles are characterized by the core plasma density and temperature. The SMBI is investigated based on the final steady profiles with different core plasma densities or temperatures. The simulated results suggest that the SMB injection will be blocked by dense core plasma and high-temperature plasma. Once the core plasma density is set to be N_(i0)= 1.4N_0(N_0= 1 × 10~(19)m~(-3)) it produces a deeper penetration depth. When N_(i0) is increased from 1.4N_0 to 3.9N_0 at intervals of 0.8N_0, keeping a constant core temperature of T_(e0)= 725 eV at the radial position of ψ = 0.65, the penetration depth gradually decreases. Meanwhile, when the density is fixed at N_(i0)= 1.4N_0 and the core plasma temperature T_(e0) is set to 365 eV,the penetration depth increases. The penetration depth decreases as T_(e0) is increased from 365 eV to 2759 eV. Sufficiently large N_(i0) or T_(e0) causes most of the injected molecules to stay in the scrape-off-layer(SOL) region, lowering the fueling efficiency.     

空位缺陷和相变对冲击压缩下蓝宝石光学性质的影响

为了探究冲击压缩下蓝宝石光学性质的变化行为,本文采用第一性原理方法,在180 GPa的压力范围内计算了蓝宝石理想晶体和含空位点缺陷晶体的光学性质.吸收光谱数据表明,仅考虑压力和温度因素不能解释冲击消光实验的结果,而冲击诱导的氧离子空位点缺陷应该是导致该结果的一个重要原因.波长在532 nm处的折射率数据表明:1)蓝宝石的两个高压结构相变将导致其折射率明显上升;在Corundum和Rh_2O_3相区,其折射率将随冲击压力增大而降低;在CalrO_3相区,压力小于172 GPa时,其折射率随冲击压力增大而缓慢地降低,但172 GPa以上时折射率却随冲击压力增大而逐渐增大;2)空位点缺陷对折射率随冲击压力的变化规律有明显的影响.本文结果不仅有助于增强用空位点缺陷的物理机理来解释蓝宝石冲击透明性损伤现象的可靠性,而且对未来进一步的实验研究以及发展新型窗口材料有重要的参考作用.     

Direct and indirect optical transitions in Zn3P2

Absorption measurements were made on single crystals of Zn₃P₂ at temperatures of 300, 80 and 5 K, and photo-voltage spectral responses-were measured at 300 K for Au- and In---Zn₃P₂ contacts. Interband absorption was interpreted as a process involving three mechanisms: (1) indirect transitions from the valence band at the Γ point, (2) either excitations from acceptor level to the conduction band at the Γ point, or second indirect transitions associated with the creation of excitons, and (3) band-to-band direct transitions at the Γ point. The effect of the lighting configuration on spectral PV plots is also discussed, and the origin of two peaks of PV responses is interpreted as being in accordance with optical data. The indirect energy gap has been estimated as 1.315eV at 300 K and 1.335 eV at 80 and 5 K, and the direct one as 1.505, 1.645 and 1.685 eV at 300, 80 and 5 K, respectively.     

利用时域核磁共振研究脲醛树脂的固化过程

利用时域核磁共振(TD NMR)技术对无固化剂的脲醛树脂在不同温度下固化过程进行了研究,实验结果表明:脲醛树脂在120℃左右时开始固化,在180℃左右时固化反应基本结束.随着温度的升高,自旋-自旋弛豫时间(T2)的总峰面积逐渐减少,当温度高于140℃时,总峰面积减少的程度较大.在30℃~140℃温度范围内,自旋-晶格弛豫时间T1(1)=0.014ms时,随着温度的升高,峰面积先逐渐减少而后略微增加;T1(2)=327.455 ms时,随着温度的升高,峰面积逐渐减少.在30℃~180℃温度范围内,自旋-晶格弛豫时间(T1)分布的总峰面积随着温度的升高而逐渐减少.在固化过程中,随着温度的升高,脲醛树脂体系的质量逐渐减少,说明脲醛树脂胶黏剂体系发生了缩聚交联反应,同时生成了甲醛和水.     

正交相Fe2(MoO4)3的制备与表征及其负膨胀行为的第一性原理研究

采用水热合成法制备出Fe₂(MoO₄)₃样品, 并用高温X-射线衍射、热重和差示扫描量热同步热分析仪对其进行表征, 发现样品在510 ℃附近发生低温单斜相和高温正交相之间的可逆相变, 且正交相表现出负膨胀特征. 采用第一性原理计算了正交相Fe₂(MoO₄)₃ 的原子、电子结构以及声子谱、声子态密度, 并和可获得的实验结果进行了系统的比较. 结果显示正交相Fe₂(MoO₄)₃中MoO₄四面体较之FeO₆八面体具有更强的刚性. 发现最低频的光学支处具有最负的格林乃森(Grüneisen)系数, MoO₄四面体和FeO₆ 八面体相连的桥氧原子的横向振动、FeO₆八面体柔性扭曲转动以及MoO₄四面体的刚性翻转共同导致了Fe₂(MoO₄)₃负膨胀现象的发生.     

R型铁氧体BaFe4-xTi2+xO11的化学组态以及磁性行为的研究

本文通过传统的固相反应法制备了R型六角铁氧体BaFe_4-xTi_2+xO₁₁ (x= 0, 0.25, 0.5, 0.75, 1), 并且对它的原子价态以及磁性行为进行了研究. X 射线光电子能谱(XPS)结果显示了随着掺杂含量的增加, 体系中Fe³⁺离子逐渐减少而Fe²⁺离子逐渐增加. 由于具有非对称结构的阻挫晶格中存在各种关联作用的竞争, 使得BaFe_4-xTi_2+xO₁₁体系表现出了复杂的磁有序行为, 在T₁～250 K和T₂～83 K两处存在磁转变. 对这一系列掺杂样品, 在相变温度T₁之上表现顺磁行为, 而在相变温度T₂前后的磁化强度都表现出低场下随磁场的增加快速增加, 高场下则线性变化且在5×10⁴ Oe时还未达到饱和的行为, 表明这一系列掺杂样品是典型的倾斜反铁磁态(canted antiferromagnetic) 或者亚铁磁态.     

The Theoretical Calculation of Elastic Constants and Phonon Dispersion Curves of Ca0.86Sr0.14CuO2

The lattice dynamical properties of Ca_0.86Sr_0.14CuO₂ are studied with a shell model. Its elastic constants and phonon dispersion curves are predicted. The calculated optical mode frequencies in Γ point are in reasonable agreement with infrared spectra, except an Azu symmetry mode around 180 cm^-1. Our calculated results can serve as guide for further experimental investigation.     

Structural and Optical Properties of Point Defects in α-SiO2 Cluster

First-principles methods based on the density functional theory (DFT) are used in order to calculate the structural and optical properties of α-SiO₂ cluster with the non-bridging oxygen hole centers (NBOHC) and NBOHC-E' defects. We clarify the stable structure of the NBOHC-E' point defects for the first time using the functional B3LYP, which is also tested to investigate the influence of electronic properties. The calculation is carried out for cluster configurations extracted from supercell. The results of optical absorption peak for Si₂O₇H₆ and Si₅O₁₆H₁₂ with NBOHC-E' defects are found at 2.66 eV, which is higher than the often observed OA peak at 2.0 eV for the NBOHC defect in α-SiO₂. The overall absorption spectra are in qualitative agreement with the experiment.     

The interaction of Cu and S2 with aluminum and alumina surfaces: a comparative study

The bonding interactions between Cu and Al are much stronger than those between Cu and Al₂O₃. Cu atoms supported on alumina show a narrow 3d band with a centroid shifted 0.35 eV with respect to that of the 3d band in bulk metallic Cu. In contrast, Cu atoms deposited on aluminum exhibit shifts of 1.3–1.6 eV in the centroid of the 3d band. Similar differences are observed when comparing the behavior of Ag and Pt overlayers on alumina and aluminum. The d band shifts on the oxide substrate are in the order of 0.3–0.4 eV, whereas on the metal substrate they vary from 0.8 to 2.0 eV. These trends are explained in terms of a simple model that takes into account changes in the energy of the Al(3s, 3p) bands when going from metallic aluminum to alumina. The sticking coefficient of S₂ on alumina surfaces is at least one order of magnitude smaller than on aluminum, a difference that also reflects variations in the position of the Al(3s,3p) bands. Submonolayer coverages of Cu do not produce significant changes in the electronic properties of Al₂O₃. In contrast, the deposition of small amounts of sulfur (0.1 ML) induces a substantial reduction (0.4–0.5 eV) in the binding energies of the O KVV, O 1s and Al 2p features of alumina. This is consistent with a transfer of electrons from alumina into the S atoms that produces a transformation similar to a change from n-type to p-type semiconductors. The reactivity of Cu/Al₂O₃ surfaces toward sulfur is much larger than that of pure Al₂O₃ surfaces. Cu clusters supported on alumina react with S₂ to form CuS_x compounds that decompose at temperatures between 850 and 1100 K.     

氢化氧化铝的同位素效应研究

在B3LYP/6-311++G(d,p)水平上预测了Al2O3H3分子的较低能量构型.其基态构型具有C s对称性,电子态为1A′.通过研究Al2O3M3和M2(M=H,D,T)的能量E、定容热容C V和熵S,用电子振动近似讨论了Al2O3+3/2M2→Al2O3M3反应的氢同位素效应,得到了Al2O3氢化的热力学函数?H0,?S0,?G0,及平衡压力与温度的关系.研究表明,氧化物Al2O3吸附氢(氘,氚)反应的同位素排代效应顺序为氚排代氘,氘排代氢,与钛等金属的同位素排代顺序相反.但排代效应都非常弱,且随着温度的增加趋于消失.     

Bandgap narrowing in the layered oxysulfide semiconductor Ba_3Fe_2O_5Cu_2S_2: Role of FeO_2 layer

A new layered Cu-based oxychalcogenide Ba_3Fe_2O_5Cu_2S_2 has been synthesized and its magnetic and electronic properties were revealed. Ba_3Fe_2O_5Cu_2S_2 is built up by alternatively stacking [Cu_2S_2]~(2-) layers and iron perovskite oxide[(FeO_2)(BaO)(FeO_2)]~(2-)layers along the c axis that are separated by barium ions with Fe~(3+) fivefold coordinated by a square-pyramidal arrangement of oxygen. From the bond valence arguments, we inferred that in layered CuC h-based(Ch =S, Se, Te) compounds the +3 cation in perovskite oxide sheet prefers a square pyramidal site, while the lower valence cation prefers the square planar sites. The studies on susceptibility, transport, and optical reflectivity indicate that Ba_3Fe_2O_5Cu_2S_2 is an antiferromagnetic semiconductor with a Ne′el temperature of 121 K and an optical bandgap of 1.03 eV. The measurement of heat capacity from 10 K to room temperature shows no anomaly at 121 K. The Debye temperature is determined to be 113 K. Theoretical calculations indicate that the conduction band minimum is predominantly contributed by O 2p and 3 d states of Fe ions that antiferromagnetically arranged in FeO_2 layers. The Fe 3d states are located at lower energy and result in a narrow bandgap in comparison with that of the isostructural Sr_3Sc_2O_5Cu_2S_2.     

Numerical simulation of nonlinear ionization nonuniformities innonequilibrium disk MHD generator

The formation and evolution of the ionization nonuniformities from initial disturbances of finite amplitude in the nonequilibrium Ar-Cs plasma in a disk magnetohydrodynamic (MHD) generator is studied by the numerical simulation, The simulations are carried out in the wide interval of electron temperatures corresponding to the region at which the seed partially ionizes, the region of the linear plasma stability at the fully ionized seed, and the region of the instability corresponding to the partial ionization of Ar at high electron temperatures. Initial disturbances of finite amplitude in electron temperature and density are introduced at the time t=0 into the homogeneous plasma distribution, and the critical amplitudes determining the development of the instability are calculated. The initial disturbances are constructed using random functions with different spatial scales, The results are compared with the calculation of the critical amplitudes from the nonlinear theory of the plane ionization waves, It is found that at electron temperatures lower than 5500 K, the temperature dependence of the critical amplitudes and the structure of the nonlinear waves agree well with the nonlinear theory, In the electron temperature region corresponding to the partial ionization of the noble gas (T_e>5500 K), the finite ionization rate of argon atoms is essential for analysis of the instability, In this region the margin of the plasma stability is wider than it is predicted by the nonlinear theory, The nonuniformity in the argon ion number density plays the dominating role in the instability development at high electron temperatures (T_e>5500 K) in comparison with the nonuniformity in T_e in the initial disturbances,