Sb掺杂闪锌矿GaAs电子结构和光学性质第一性原理研究

采用基于密度泛函理论(DFT)框架下广义梯度近似平面波超软赝势法,计算了不同浓度Sb掺杂闪锌矿GaAs体系GaAs1-xSbx(x=0,0.25,0.5,0.75,1)的电子结构和光学性质,包括能带、态密度、复介电函数和吸收系数。计算结果表明,Sb掺杂导致体系晶格常数线性增大,并使得体系导带和价带组成发生改变,禁带宽度呈二次多项式变化。随着掺杂浓度的增加,体系静态介电常数线性增大,吸收带边出现了明显的红移现象。分析了掺杂Sb诱发GaAs1-xSbx体系的电子和光学性质改变,为Sb掺杂闪锌矿GaAs在光电子学和微电子学方面的实际应用提供了一定的理论依据。     

Evolution of Superconducting Properties of LiFeAs Single Crystals Doped with Magnetic or Nonmagnetic Impurities

The effect of magnetic Co²⁺ and nonmagnetic Ga³⁺ impurities on the crystal structure and superconducting properties of LiFeAs single crystals has been investigated. A large T _c decrease of about 4.8 K/at% is observed in Ga-doped LiFeAs. This rate is higher than that of the material doped with magnetic Co impurities (～3.7 K/at%). The greater T _c suppression in the Ga case is likely due to the pair breaking associated with the significant changes in the crystal structure of the doped material. The increase of the critical current densities in intermediate magnetic fields (H⊥?ab) indicates that a very small amount of Ga (0.5 at%) acts as an effective pinning site for flux pinning enhancement in the material. The analysis of the temperature and field dependencies of the magnetic relaxation is consistent with the collective pinning model for the Co-doped material, while the magnetic relaxation measurements combined with the peak position of the critical current density in the B–T phase diagram of Ga-doped LiFeAs suggest an elastic–plastic transition of the vortex lattice at higher temperatures and fields.     

Effect of Grain Size on Magnetic Properties of ZnO Doped with Nickel Single Impurities

The purpose of our study is to find a suitable material to be used in spintronic applications and to find the relation between the parameters of deposition by spray pyrolysis technic (temperature, concentration of Ni doping) and the ferromagnetic properties (Curie temperature, magnetic moment). The nickel-doped zinc oxide, Zn.1?x.NixO (x = 0.01, 0.03, 0.05), and diluted magnetic semiconductors (DMSs) are synthesized by the spray pyrolysis technic. The results of The X-ray diffraction (XRD) of prepared substrates confirm the incorporation of the dopants into the ZnO lattice structure. The spin-polarized electronic properties has been found and was investigated in detail by using the density-functional theory (DFT), the local density approximation (LDA), and The Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA). As result, nickel doping brings up a half-metallic appearance due to the hybridization between the 3d state of Nickel impurities and the oxygen 2p state. The mechanism of the interatomic exchange has been explained as being a p-d double exchange.     

Effect of Impurities and Structural Defects on the Transport Properties of n-Type GaAs Crystals

The carrier mobility and concentration in n-type GaAs single crystals, undoped, doped during growth (Te, Sn, and Ge), and neutron-transmutation doped (NTD) with Se and Ge, were measured as a function of temperature. Although the donor concentration was the same in all of the doped crystals, the carrier mobility and concentration in the GaAs crystals were found to be lower than those in the GaAs, GaA, and NTD GaAs crystals by about a factor of three and one order of magnitude, respectively. These results are interpreted in terms of the compensation ratio and predominant scattering processes in the crystals. The large compensation ratio in the GaAs crystals is due to the high acceptor concentration. The low carrier mobility in GaAs is associated with the strong scattering of charge carriers by ionized defects, because of their high density, and also, in contrast to the undoped, GaAs, and GaAs, and NTD GaAs crystals, with additional scattering by nonuniformly distributed structural defects. In calculations of the compensation ratio and analysis of the scattering mechanisms, the Brooks–Herring or Cornell–Weisskopf approximation was used, depending on the compensation ratio in the material.     

Theoretical Study of TiO2 Doped with Single and Double Impurities

Using first principles calculations within the density-functional theory (DFT) and the local density approximation (LDA), the electronic structure and magnetic properties of rutile TiO₂ doped with single and double-impurities Ti(Cr, Mn) O₂ has been investigated. The Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) has been used. Chromium doping brings up a half-metallic appearance; it is due to the hybridization between the 3d state of chromium and the oxygen 2p state. The mechanism of the interatomic exchange in this case has been deducted as a double exchange. We also studied Mn doped TiO₂, which gave rise to ferromagnetism, due to the p–d exchange. We also study the advantage of codoping TiO₂ diluted magnetic oxide to find a suitable material to be used in spintronics.     

Magnetic and Electronic Properties of Point Defects in ZrO2

Using the Korringa–Kohn–Rostoker Coherent Potential Approximation (KKR-CPA) method in connection with the Generalized Gradient Approximation (GGA), we study the magnetic and electronic properties of different point defects in cubic ZrO₂. In particular, we discuss the zirconium interstitial (Zr_i), zirconium antisite (Zr_O), zirconium vacancy (V_Zr), oxygen interstitial (O_i), oxygen antisite (O_Zr), and oxygen vacancy (V_O) defects. It has been shown that oxygen vacancy and zirconium interstitial (V_O, Zr_i) are n-type, while the other point defects are p-type. The magnetic moments are observed only in the oxygen interstitial and antisite (O_i, O_Zr) cases. The corresponding ferromagnetic states are more stable than the spin–glass states. It has been found that the mechanism responsible of such stabilities is the double exchange. Based on Mean Field Approximation (MFA), the Curie temperature (T _C ) is estimated. Moreover, it has been found that the O_i and O_Zr defects provide half-metallic properties being the responsible for ferromagnetism.     

Ag-O-N共掺杂闪锌矿ZnS光催化性质的第一性原理研究

采用第一性原理方法计算比较了Ag、O和N单掺杂、Ag-O和Ag-N二元掺杂以及Ag-O-N三元共掺杂闪锌矿ZnS的晶型结构、电荷分布、能带和态密度以及光学特性.N、O对晶格结构的影响大于Ag,结构优化和电荷分布结果显示晶格畸变随着掺杂组分的增多不断增大.形成能计算表明,N最易掺杂,组分越多,掺杂越难实现.Ag、O和N各组分掺杂ZnS后能带间隙均有所减小,Ag、N掺杂ZnS更有利于价带顶上移,部分能带越过费米能级,形成电子跃迁的过渡能级,有利于光催化反应.Ag-O-N三元掺杂ZnS后在可见光区的吸收峰最大,有利于提高ZnS对可见光的利用率.     

Al掺杂纤锌矿ZnO的电子结构研究

采用基于密度泛函理论(DFT)的总体能量平面波超软赝势法,电子交换关联能结合广义梯度近似(GGA)PW91形式,对Al原子不同掺杂位置的ZnO超原胞进行了几何优化,计算分析了晶体结构参数及掺杂模型的电子结构.结果表明,几何优化后掺杂晶胞z轴方向出现收缩现象,两种模型在杂质掺入后半导体ZnO导电能力提高,形成置换杂质模型的可能性要大于间隙杂质模型,掺杂Al和基体ZnO的原子数配比直接影响导电性能,最后给出了电子轨道跃迁规律.     

Electronic Structure and Magnetic Properties of Zinc-Blende Co-Doped GaN with N Vacancies

The electronic structure and magnetic properties of zinc-blende structure of (Ga,Co)N phase with N vacancy defects are investigated using the Korringa–Kohn–Rostoker (KKR) method combined with coherent potential approximation (CPA). The results show that (Ga,Co)N phase is ferromagnetically polarized with an enhancement of the polarization and that the electronic structure can be modified simply by changing the concentration of N vacancies. Moreover, the (Ga,Co)N with high density of N vacancies shows a drastic increase of the magnetic moment of cobalt in the parent GaN compound, to reach a maximum value of 1.7 μ_B/Co at 8 at.%, which is in good agreement with the experimental values reported in the literature.     

掺铁与铜的硫系玻璃的电学性质与能带结构

用高温熔融法在 Sb_(12)Ge_(28)Se_(60)玻璃中掺入 Fe 和 Cu。对 Cu_x(Sb_(12)Ge_(28)(Se_(60))_(100-x)系统玻璃,当 Cu 含量为5 at%时,电导率提高3.4个数量级,电导活化能下降0.23eV。分析表明,Cu 引入后与母体玻璃中的阴离子形成了正常结构键合的共价键,各元素的价键均得到满足,电导活化能的下降是由于禁带宽度降低导致(E_f-E_v)值下降所致。对 Fe_x(Sb_(12)Ge_(28)Se_(60))_(100-x)系统玻璃,DSC 与 SEM 的测定表明,Fe 引入后玻璃产生了分相。当 Fe 含量为2 at%时,电导率增加了6.3个数量级,电导活化能则下降0.61eV,光声子谱的分析表明,E_(opt)仅下降了0.04eV。根据这些数据和 ESR 的测定结果得出了该系统玻璃的能带结构模型,导致电导率激增的原因是由于 Fe~(2+)(d~(?))施主缺陷态的形成。     

Realizing Zinc Blende GaAs/AlGaAs Axial and Radial Heterostructure Nanowires by Tuning the Growth Temperature

Vertical zinc blende GaAs/AlGaAs heterostructure nanowires were grown at different temperatures by metalorganic chemical vapor deposition via Au-assisted vapor-liquid-solid mechanism.It was found that radial growth can be enhanced by increasing the growth temperature.The growth of radial heterostructure can be realized at temperature higher than 500℃,while the growth temperature of axial heterostructure is lower than 440℃.The room temperature photoluminescence properties of the nanowires were investigated and the relevant growth mechanism was discussed.     

Effect of Intrinsic Defects on Electronic and Magnetic Properties in Tm-Doped GaN: First-Principles Calculations

Based on the density functional theory, we investigate the electronic and magnetic properties of various types of defect complexes formed by dopant Tm and Ga vacancies, N vacancies, or O interstitial in Tm-doped GaN. Formation energies are first calculated for all defect complexes to assess their stability. The single Tm dopant is found to introduce the local magnetic moment of about 2 μ_B/Tm in GaN. However, in the case of defect complexes, the magnetic moments of Tm can be suppressed by the existence of Ga vacancies around it, while the presence of N vacancies or O interstitial does not influence the magnetic moment of Tm. In addition, each Ga vacancy in the neutral charge state induces the local magnetic moment of about 2.1 μ_B and one octahedral O interstitial can lead to the local moment of about 1.6 μ_B.     

Magnetic Properties of SrO Doped with 3d Transition Metals

Based on the density functional theory and using the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) method, we study the (Sr, TM)O doped systems where TM = V, Cr, Mn, Fe, Co, and Ni atoms. In particular, we start first by relaxing the parameters of the corresponding structures. Then we discuss its electronic structures, magnetic stabilities, and half-metal properties using 3d transition metals. Among others, it has been shown that doping with Cr, Mn, Fe, and Co, the ferromagnetic phase can be stabilized using a double exchange mechanism. Moreover, we find that the half-metallic properties of these compounds are formed due to a large exchange splitting and the delocalized properties of the majority spin e _g state and the minority spin t _eg states.     

The Electronic and Magnetic Properties and the Topological Phase Graphene Sheet with Fe,Co, Si,and Ge Impurities

The structural, electronic, and magnetic properties of pure graphene sheet and graphene sheet with Fe, Co, Si, and Ge impurities are investigated. The calculated results are done within density functional theory in the presence of spin-orbit coupling using the generalized gradient approximation. Electron density of states, band order, electron charge distribution, magnetic moment of these sheets, and the effect of pressure on the band order of graphene sheet with Fe impurity are investigated.     

参杂尖晶石型镍基铁氧体的磁性能与介电性能

采用溶胶-凝胶法制备了立方晶系尖晶石型镍基铁氧体微粉Ni0.5M0.5Fe2O4（M=Zn、Mn、Cu）,采用X射线衍射仪、扫描电子显微镜、振动样品磁强计和矢量网络分析仪对粉末的结构、形貌、磁性以及电磁性能进行了表征,结果表明,三种粉末在室温下具有超顺磁性,其饱和磁化强度MS分别为76．0、59．4和54．4emu·g-1。在2—11GHz范围内,Ni0.5M0.5Fe2O4的电磁损耗角正切值tgδ随频率的增大而逐渐减小;Ni0.5M0.5Fe2O4和 Ni0.5M0.5Fe2O4的tgδ随频率的增大先增大后减小。     

Electronic Structure of Doped Insulators and High Temperature Superconductivity

The Fermi liquid theory of the normal state and the BCS-Eliashberg theory of the superconducting state were designed for good metals not for doped antiferromagnetic insulators, such as the high temperature superconductors. Consequently, it is necessary to understand the electronic structure of the doped insulator and to develop a new mechanism and many-body theory of superconductivity for these materials. It will be argued that, since the motion of a single hole in an antiferromagnet is frustrated, the driving force for the physics of a finite concentration of doped holes is the need to reduce their zero-point kinetic energy. This proceeds in three steps that are reflected in a sequence of crossovers and phase transitions. First of all, the system forms a charge-inhomogeneous state – an electronic liquid crystal phase, involving an array of metallic stripes, which lowers the kinetic energy along a stripe. In the direction perpendicular to the stripes, the kinetic energy is lowered by pair hopping, which proceeds in two steps. Local pair hopping induces spin pairing and then, at a lower temperature, pair hopping from stripe to stripe produces superconducting phase coherence. Some of the experimental support for the various aspects of this model will be described.     

双元素掺杂的a-Si∶H光电特性研究

本文研究由氮(N)、硼(B)以及卤素(X)元素掺杂的 a-Si∶H 膜在静电场下的光电特性.实验结果表明,(B+N)、(B+X)双元素掺杂比单元素更明显地提高 a-Si∶H 膜的表面电位 V_(?),降低残余电位V_R,并在 X/Si=10~(-2),B_2H_6/SiH_4=0.3×10~(-4)时,得到了 V_(?)=60V/μm,σ_D=10~(-4)((?)cm)~(-1)的静电复印用a-Si∶H 材料.     

Electronic and Magnetic Properties of MnAu Superlattices

Self-consistent ab initio calculations, based on the Density Functional Theory (DFT) approach and using full potential linear augmented plane wave (FLAPW) method, are performed to investigate both electronic and magnetic properties of the MnAu superlattices. Polarized spin and spin-orbit coupling are included in calculations within the framework of the antiferromagnetic state between two adjacent MnAu superlattices. Obtained data from ab initio calculations are used as input for the Monte Carlo simulations to compute other magnetic parameters. On the other hand, and within the framework of Monte Carlo simulations, we examine the magnetic properties in the binary MnAu superlattices modelized by the Mn ion with spin moment, S=5/2. The considered Hamiltonian takes into account the nearest neighbor and second nearest interactions, and an external magnetic field h. The magnetization of the MnAu superlattice is calculated versus temperature for a fixed size. The magnetic hysteresis cycle is established for T=120 K. The effect of magnetization versus the second exchange interaction (J _c ) are established with absence and presence of magnetic field (h).     

Electronic and Magnetic Properties of MnSb Compounds

The self-consistent ab initio calculations, based on density functional theory (DFT) approach and using full-potential linear augmented plane wave (FLAPW) method, are performed to investigate both electronic and magnetic properties of the MnSb compounds. Polarized spin and spin-orbit coupling are included in calculations within the framework of the ferromagnetic state between two adjacent Mn atoms. The ferromagnetic energy of MnSb systems is obtained. Magnetic moment considered to lie along the (001) axes are computed. The exchange interactions between the magnetic atoms Mn–Mn in MnSb are given using the mean field theory. Obtained data from ab initio calculations are used as input for the high-temperature series expansions (HTSEs) calculations to compute other magnetic parameters. The HTSEs of the magnetic susceptibility of the magnetic moments in MnSb (m _Mn) is given up to the tenth order series in (x = J ₁(Mn–Mn)/ k _B T). The critical temperature T _C is deduced by HTSEs of the magnetic susceptibility series combined with the Padé approximation method. The critical exponent γ associated with the magnetic susceptibility is deduced as well.