关于Zn1-xBexO电子结构和光学性质的研究

采用基于密度泛函理论和平面波赝势技术的CASTEP程序对Zn_1-xBe_xO合金电子结构和光学性质进行了计算.当0≤x≤1,其带隙从0.963 eV变化到7.293 eV.分析了晶格畸变和能带间排斥效应对带隙的影响.当Be含量x=0.125,0.25,0.375,0.5,0.625,0.75时,a/b轴压应变控制着带隙变化;当x=0.875,1时,c轴压应变控制着带隙变化.能带间的p-d排斥影响价带顶变动,Γ_1v与Γ_1c之间排斥影响导带底变动.这些能带间的排斥效应被用来分析Zn_1-xBe_xO带隙变动.另外,也分析了Zn_1-xBe_xO介电函数虚部ε₂. 关键词： 带结构 光学性质 应变 排斥     

β型烧绿石结构氧化物超导体$lt;i$gt;A$lt;/i$gt;Os$lt;sub$gt;2$lt;/sub$gt;O$lt;sub$gt;6$lt;/sub$gt;($lt;i$gt;A$lt;/i$gt;=K,Rb,Cs)电子能带结构的第一性原理计算

用全势线性缀加平面波方法计算β型烧绿石结构氧化物超导体AOs₂O₆ （A=K,Rb,Cs）的电子能带结构及态密度.计算发现电子自旋轨道耦合和在位库仑势U的作用增大了费米面处态密度值.通过计算还得到这三种化合物电子关联常数λ_c分别为1.55,1.12和0.73.由实验测量与能带计算得到的电子比热容系数的比值得到电子质量提高参数.通过分析这三种化合物电子质量提高参数,推算出它们的电声子耦合常数λ_ep分别为1.56,0.78和1.08.由此提出KOs₂O₆为强电子关联和强电声子耦合系统,而RbOs₂O₆和CsOs₂O₆的电子关联性与电声子耦合为中等. 关键词： β型烧绿石超导体 能带结构 电子关联 电声子耦合     

Electronic structure and hot carrier relaxation in ⟨001⟩ anatase TiO2 nanowire

We present an electronic structure and non-adiabatic excited state dynamics study of ?001? anatase TiO₂ nanowire (NW) by combining density matrix formalism and ab initio electronic structure calculations. Our results show that quantum confinement increases the energy gap as the dimension of TiO₂ is reduced from the bulk to a NW with a diameter of several nanometres and that the probability of electronic transitions induced by lattice vibrations for the NW follows band gap law. The electron non-radiative relaxation to the bottom of the conduction band is involving Ti 3d orbitals, while the hole non-radiative relaxation of holes to the top of the valence band occurs by subsequent occupation of O 2p orbitals.     

Electronic structure of a superconducting Bi2212 crystal by angle resolved ultra violet photoemission

The electronic structure of a high quality superconducting Bi₂Sr₂CaCu₂O_8+δ (Bi2212) single crystal is studied by angle resolved ultra violet photoemission (ARUPS) using He I (21.2 eV). Our results appear to show two bands crossing the Fermi level in ΓX direction of the Brillouin zone as reported by Takahashiet al. The bands at higher binding energy do not show any appreciable dispersion. The nature of the states near the Fermi level is discussed and the observed band structure is compared with the band structure calculations.     

MBiO2Cl(M=Ca，Sr，Ba)材料的合成、能带计算及发光性能研究

采用固相合成方法制备出CaBiO₂Cl,SrBiO₂Cl和BaBiO₂Cl粉体,研究了该Sillen系列铋基氧卤化合物的光学带隙、电子结构及发光性能.基于密度泛函理论计算表明,SrBiO₂Cl和BaBiO₂Cl均为直接带隙半导体材料,与吸收光谱实验结果相符合.在X射线和紫外光激发下三者均具有宽的可见光发射带(400—550nm),尤其是BaBiO₂Cl粉体的光输出强度约为Bi 关键词： 光致发光 电子结构 Sillen化合物     

Tight-binding study of structural and electronic properties of silver clusters

Tight-binding model is developed to study the structural and electronic properties of silver clusters. The ground state structures of Ag clusters up to 21 atoms are optimized by molecular dynamics-based genetic algorithm. The results on small Ag_n clusters (n = 3-9) are comparable to ab initio calculations. The size dependence of electronic properties such as density of states, s-d band separation, HOMO-LUMO gap, and ionization potentials are discussed. Magic number behavior at Ag₂, Ag₈, Ag₁₄, Ag₁₈, Ag₂₀ is obtained, in agreement with the prediction of electronic ellipsoid shell model. We suggest that both the electronic and geometrical effect play significant role in the coinage metal clusters. Received 7 August 2000     

Half-metallic properties of Ti2FeSi full-Heusler compound

In this study, the electronic structure and magnetic properties of novel half-metallic Ti₂FeSi full-Heusler compound with CuHg₂Ti-type structure were examined by density functional theory (DFT) calculations. The electronic band structures and density of states of the Ti₂FeSi compound show the spin-up electrons are metallic, but the spin-down bands are semiconductor with a gap of 0.45 eV, and the spin-flip gap is of 0.43 eV. Fe atom shows only a small magnetic moment and its magnetic moment is antiparallel to that of Ti atoms, which is indicative of ferrimagnetism in Ti₂FeSi compound. The Ti₂FeSi Heusler compound has a magnetic moment of 2 μ_B at the equilibrium lattice constant a=5.997 Å.     

Optical band gap studies and estimation of two photon absorption coefficient in alkali bismuth borate glasses

The optical absorption spectra of the glasses with composition xBi₂O₃·(30???x)R₂O·70B₂O₃ (R?Li, Na, K) and xBi₂O₃·(70???x)B₂O₃·30Li₂O (0?≤?x?≤?20) have been recorded in the wavelength range 350–650?nm. The glass samples were prepared by the normal melt–quench technique. The fundamental absorption edge for all the series of glasses is analyzed using the theory of Davis and Mott. The position of absorption edges and the values of optical band gap are dependent on the mol% of Bi₂O₃. The absorption in these glasses is associated with indirect transitions. The values of Urbach's energy and band tailing parameters are reported. The two photon absorption coefficient, β, in these glasses has also been estimated from the optical band gap and its value ranges from 1.3 to 11.6?cm/GW. The relationship between β and glass composition has also been discussed in terms of the electronic structure of the glass system.     

Impact of iron composition on the calculated electronic and magnetic properties of Fe3−xNixSi

ABSTRACT

The structural, electronic and magnetic properties of Fe_3?xNi_xSi alloys with variable iron composition (0?≤?x?≤?1) have been investigated within by using Projector augmented wave (PAW) method. The exchange–correlation potential was treated with the generalised gradient approximation (GGA) for the calculation of the structural properties and for the computation of the electronic and magnetic properties in order to treat the d states. These alloys crystallize in cubic Heusler structures; The Fe₃Si and Fe₂NiSi have a regular structure DO₃ and L2₁ respectively. To describe the experimental proprieties we use the on-site Coulomb interactions of U_eff(Ni)?=?3.1?eV and U_eff(Fe)?=?3.4?eV. A good agreement between calculated and experimental magnetic moments is found for the cubic Heusler phases without the addition of Hubbard-model. The obtained results of the density of states and the spin-polarized band structure show that the Fe₂NiSi alloy has half-metallic property. Through the obtained values of the total spin magnetic moment, we conclude that in general, the Fe₂NiSi alloy is half-metallic ferromagnet material whereas the Fe₃Si alloy has a metallic nature.     

First-principles calculations on implanted TiO<Subscript>2</Subscript> by 3d transition metal ions

3d transition metal (V, Cr and Fe) ions are implanted into TiO₂ by the method of metal ion implantation. The electronic band structures of TiO₂ films doped 3d transition metal ions have been analyzed by ab initio band calculations based on a self-consistent full-potential linearized augmented plane-wave method within the first-principle formalism. Influence of implantation on TiO₂ films is examined by the method of UV-visible spectrometry. The results of experiment and calculation show that the optical band gap of TiO₂ films is narrowed by ion implantation. The calculation shows that the 3d state of V, Cr and Fe ions plays a significant role in red shift of UV-Vis absorbance spectrum.     

Al掺杂SnO2 材料电子结构和光学性质

采用基于第一性原理的线性缀加平面波方法(FP-LAPW),研究Al掺杂SnO₂材料Sn_1-xAl_xO₂ (x= 0,0.0625,0.125,0.1875,0.25)的电子结 构和光学性质,包括能带结构、电子态密度、介电函数和其他一些光学性质.计算结果表明,掺杂Al之后价带上部分折叠态增加,价带宽度发生收缩,对导带底起作用的Sn 5s态减少,使得带隙增宽,且态密度整体向高能方向发生移动.随着Al掺杂量的增加带隙越来越宽,Al杂质能级在导带部分与Sn 5p态电子相互作用逐渐增强,虚部谱中的第一介电峰的强度随掺杂Al浓度增大而减弱.同时,吸收谱及其他光学谱线与介电函数虚部谱线相对应,各谱线均发生蓝移现象,对应带隙增宽,从理论上指出了光学性质与电子结构之间的内在关系. 关键词： 能带结构 态密度 光学性质 介电函数     

LiGaX2(X=S,Se, Te)的电子结构,光学和弹性性质的第一性原理计算

采用基于第一性原理的密度泛函理论赝势平面波方法,对LiGaX₂(X=S, Se, Te)的能带结构、态密度、光学以及弹性性质进行了理论计算. 能带结构计算表明LiGaS₂ 的禁带宽度为4.146 eV, LiGaSe₂ 的禁带宽度为3.301 eV, LiGaTe₂ 的禁带宽度为2.306 eV; 其价带主要由Ga-4p 层电子和X- np 层电子的能态密度决定; 同时也对LiGaX< 关键词： 电子结构 光学性质 弹性性质 LGX     

第一性原理计算MgB2薄膜拉伸对超导电性的影响

应用全势线性响应线性糕模轨道方法计算MgB₂的电子能带结构、声子谱及电声子耦合常数,并讨论MgB₂的超导电性.通过比较MgB₂薄膜双轴拉伸前后超导电性的变化可以看出，随着a轴晶格常数增大和c轴晶格常数减小,声子谱中硼的E_2g声子频率显著下降，使得电声子耦合强度λ和声子对数平均频率ω_ln增强,提高了MgB₂薄 关键词： 超导电性 能带结构 声子频率 电声子耦合     

Cr和W掺杂的单层MoS2电子结构的第一性原理研究

采用密度泛函理论框架下的第一性原理平面波赝势方法, 研究了Cr和W掺杂对单层二硫化钼(MoS₂)晶体的电子结构性质的影响. 计算结果表明: 当掺杂浓度较高时, W对MoS₂的能带结构几乎没有影响, 而Cr的掺杂则影响很大, 表现为能带由直接带隙变为间接带隙, 且禁带宽度减小. 通过进一步分析, 得出应力的产生是导致Cr掺杂的MoS₂电子结构变化的最直接的原因.     

First-principles calculation of structural and electronic properties of pyrochlore Lu2Sn2O7

A Density functional theory method within generalized gradient approximation has been performed to obtain the static lattice parameters, oxygen positional parameter, bond length and bond angle and electronic properties of ideal Lu₂Sn₂O₇ pyrochlore. The results are in excellent agreement with available experimental measurements. Density of states (DOS) of this compound was presented and analysed. We also notice the presence of the hybridization between oxygen and Lu metal. The band structure calculations show that the compound has direct band gap of 2.67 eV at the Γ point in the Brillouin zone and this indicates that the material has a semi-conducting feature.     

Structural,electronic, optical and elastic properties of the complex K2PtCl6-structure hydrides ARuH6 (A = Mg,Ca, Sr and Ba): first-principles study

We report a systematic study of the structural, electronic, optical and elastic properties of the ternary ruthenium-based hydrides A₂RuH₆ (A = Mg, Ca, Sr and Ba) within two complementary first-principles approaches. We describe the properties of the A₂RuH₆ systems looking for trends on different properties as a function of the A sublattice. Our results are in agreement with experimental ones when the latter are available. In particular, our theoretical lattice parameters obtained using the GGA-PBEsol to include the exchange-correlation functional are in good agreement with experiment. Analysis of the calculated electronic band structure diagrams suggests that these hydrides are wide nearly direct band semiconductors, with a very slight deviation from the ideal direct-band gap behaviour and they are expected to have a poor hole-type electrical conductivity. The TB-mBJ potential has been used to correct the deficiency of the standard GGA for predicting the optoelectronic properties. The calculated TB-mBJ fundamental band gaps are about 3.53, 3.11, 2.99 and 2.68 eV for Mg₂RuH₆, Ca₂RuH₆, Sr₂RuH₆ and Ba₂RuH₆, respectively. Calculated density of states spectra demonstrates that the topmost valence bands consist of d orbitals of the Ru atoms, classifying these materials as d-type hydrides. Analysis of charge density maps tells that these systems can be classified as mixed ionic-covalent bonding materials. Optical spectra in a wide energy range from 0 to 30 eV have been provided and the origin of the observed peaks and structures has been assigned. Optical spectra in the visible range of solar spectrum suggest these hydrides for use as antireflection coatings. The single-crystal and polycrystalline elastic moduli and their related properties have been numerically estimated and analysed for the first time.     

The changes in electronic structure and optical properties of magnetic semiconductors at magnetic phase transitions

The influence of magnetic phase transitions on electronic structure and optical properties of magnetic semiconductors is discussed. Europium chalcogenides and chromium chalcogenide spinels are the main subjects of the investigation. It is shown, that many-body effects are responsible for the changes of optical properties and non-rigid band behavior of electronic structure. Magnetic phase transition leads to energy shift of wide bands and change in density of states of “magnetic” d(f)-electrons without any significant shift of their energies. The influence of fluctuations at T ～ T _c and antiferromagnetic semiconductors are also considered.     

GeI2 monolayer: a model thermoelectric material from 300 to 600 K

ABSTRACT

In this work, the electronic structure, optical properties and thermoelectric properties of the GeI₂ monolayer are calculated by the first principles with the Boltzmann transport equation. The monolayer is calculated as an indirect band gap semiconductor with an indirect band gap of a value 2.19?eV. This GeI₂ monolayer is good for absorbing low-energy photons, and it is insensitive to high-energy photons. The material is stable at temperatures up to 600?K, so we calculated the thermal conductivity (K_L), Seebeck coefficient (S), power factor (PF) and thermoelectric figure of merit (ZT) of the GeI₂ monolayer at various carrier concentrations from 300 to 600?K. Due to the lower group velocity, the GeI₂ monolayer has a lower thermal conductivity of 0.48?W/m?K at 300K. In P-type doping, the power factor can up to 0.11?mW/m?K², and its ZT value is 4.04 at 600?K of the GeI₂ monolayer, indicating that the GeI₂ monolayer is a potential thermoelectric material.     

Elastic,mechanical, electronic properties and hardness of Nb2AsC from first principles

The study aims at the elastic, mechanical, electronic properties and hardness of Nb₂AsC using first principles based on the density functional theory method within the generalised gradient approximation. The calculated lattice parameters of Nb₂AsC are in good agreement with the experimental data. The five independent elastic constants are firstly calculated as a function of pressure, and our results indicate that it is mechanically stable in the applied pressure. The elastic anisotropy is examined through the computation of the direction dependence of Young's modulus. The pressure dependences of the bulk modulus, shear modulus, average velocity of acoustic waves and Debye temperature of Nb₂AsC are systematically investigated. The band structure and density of states are discussed, and the results show that the strong hybridisations C p–Nb d and As p–Nb d would be beneficial to the structure stability of Nb₂AsC. Based on the Mulliken population analysis, the hardness of Nb₂AsC is predicted.