Mn掺杂对半金属铁磁体Fe_2Si电磁特性的影响

此文用基于密度泛函理论第一性原理的贋势平面波方法,计算了Fe_2Si及Mn掺杂Fe_2Si体系的能带结构、电子态密度和磁学特性,分析了不同位置Mn掺杂对Fe_2Si电磁特性的影响,获得了纯的和不同位置Mn掺杂的Fe_2Si体系是铁磁体,自旋向上的能带结构穿过费米面表现金属特性,纯Fe_2Si的半金属隙为0.164e V;Mn掺杂在Fe1位时,自旋向下部分转变为A-M间的间接带隙半导体,体系呈现半金属特性,此时磁矩为2.00μB,是真正的半金属性铁磁体;掺杂在Fe2位时,自旋向下部分的带隙值接近于0,体系呈现金属特性;掺杂在Fe3位时,自旋向下部分转变为L-L间的直接带隙半导体,体系呈现半金属特性等有益结果 .自旋电荷密度分布图表明Mn原子的3d电子比较局域,和周围原子成键时3d电子更倾向于形成共价键.体系的半金属性和磁性主要来源于Fe-3d电子与Mn-3d电子之间的d-d交换,Si-3p电子与Fe、Mn-3d电子之间的p-d杂化.这些结果为半金属铁磁体Fe_2Si的电磁调控提供了有效的理论指导.     

Structural, electronic and magnetic properties of the 3d transition metal-doped GaN nanotubes

We have performed first-principles calculations on the structural, electronic and magnetic properties of seven different 3d transition-metal (TM) impurity (V, Cr, Mn, Fe, Co, Ni and Cu) doped armchair (5,0) and zigzag (8,0) gallium nitride nanotubes (GaNNTs). The results show that there is distortion around 3d TM impurities with respect to the pristine GaNNTs for 3d TM-doped (5,5) and (8,0) GaNNTs. The change of total magnetic moment follows Hund’s rule for 3d TM-doped (5,5) and (8,0) GaNNTs, respectively. The total density of states (DOS) indicates that Cr-, Mn-, Fe- and Ni-doped (5,5) GaNNTs as well as Cr-, Mn-, Ni- and Cu-doped (8,0) GaNNTs are all half-metals with 100% spin polarization. The study suggests that such TM-doped nanotubes may be useful in spintronics and nanomagnets.     

Origins of ferromagnetism in transition metal doped diamond

Using first-principles calculations, we investigated the electronic and magnetic properties of Mn-doped, Fe-doped, and Co-doped diamond. It is found that the Mn-, Fe-, and Co-doped diamond are stabilized in ferromagnetic configurations. The origins of the magnetic ordering are explained successfully by the phenomenological band coupling model based on the p–d and d–d level repulsions between the dopant ions and host elements. According to Heisenberg model, high Curie temperature may be expected for Mn-, Fe-, and Co-doped diamond if there are no native defects or other impurities.     

Magnetic properties of diluted magnetic semiconductors: Quantum Monte Carlo approach

The magnetic correlation between magnetic impurities in semiconductors is investigated by performing the quantum Monte Carlo (QMC) simulation. The Anderson Hamiltonian with the realistic parameters obtained by the local density approximation (LDA) calculation is employed. The LDA calculation gives a dispersion of the host (GaAs) electron and the mixing energy between host and magnetic impurity (Mn). The mixing between host and impurity electrons generates the impurity bound state in the energy gap of semiconductors. The long range ferromagnetic coupling is observed when the Fermi energy locates between the band edge and the impurity bound state. The ferromagnetic coupling is enhanced by decreasing temperature.     

Mn掺杂ZnO纳米线磁性质的第一性原理研究

本文采用第一性原理方法系统研究了Mn原子单掺杂和双掺杂ZnO纳米线的稳定性和磁性质.所有掺杂纳米线的束缚能都为负值,表明掺杂增强了纳米线的稳定性.表面掺杂纳米线显示了直接带隙半导体特性,而中间掺杂纳米线显示了间接带隙半导体特性.纳米线的总磁矩主要来源于Mn原子3d轨道的贡献.由于杂化,相邻的O原子和Zn原子也产生了少量自旋.在超原胞内,Mn原子和O原子磁矩平行排列,表明它们之间是铁磁耦合.     

Electron correlation effects on Compton profiles of copper in the GW approximation

The Compton profiles (CPs) of copper are calculated by the GW approximation with FLAPW basis sets on the LDA. In the quasiparticle band structure in the GW approximation, the width of fully occupied 3d valence band which is overestimated in the LDA, is in good agreement with experimental observation. The dynamical screening effects are important for band width narrowing. The occupation number densities are evaluated from the spectral function calculation within the GW calculations. The CPs obtained using these GW calculations successfully reproduce experimental results.     

Magnetism and electronic structure of Mn- and V-doped zinc blende ZnTe from first-principles calculations

By means of the first-principles full potential linearized augmented plane-wave method within the local density approximation for the exchange-correlation functional, we have investigated the magnetism and electronic structure of Mn- and V-doped zinc blende ZnTe. Total energy calculations show that, for high doping concentration (12.5%), ZnTe:Mn has an antiferromagnetic ground state while the ferromagnetic state is more favorable than the antiferromagnetic state for ZnTe:V. Furthermore, ZnTe with a low doping of Mn (6.25%) has a stable ferromagnetic ground state, which is in agreement with the experimental results. The calculated magnetic moment of ZnTe doped with Mn (V) mainly originates from transition metal Mn (V) atom with a little contribution from Te atom due to the hybridization between Mn (V) 3d and Te 5p electrons. Electronic structure indicates that Mn-doped ZnTe is a semiconductor, but V-doped ZnTe shows a half-metallic characteristic. We also discuss the difference between electronic and magnetic properties for ZnTe doped with 12.5% and 6.25% Mn.     

Hyperfine Interactions and Magnetism of 3d Transition-Metal-Impurities in II–VI and III–V Compound-Based Diluted Magnetic Semiconductors

The electronic structure of II–VI and III–V compound-based diluted magnetic semiconductors is calculated based on the local density approximation (LDA) using the Korringa–Kohn–Rostoker method combined with the coherent potential approximation. The magnetism of 3d transition-metal-atom-doped ZnO, ZnS, ZnSe, ZnTe, GaN, GaAs is investigated from first-principles. It is suggested that the double exchange mechanism stabilizes the ferromagnetism in these DMSs. In order to obtain microscopic information on the electronic structure of transition-metal-impurities in semiconductors, the hyperfine field of respective impurities in each host material is calculated. It is found that the agreement with the experimental values is not good, probably because the LDA is not sufficient to describe the core states of transition metals. However, it is suggested that the hyperfine fields clearly reflect the local magnetic moments for 3d impurities. This revised version was published online in September 2006 with corrections to the Cover Date.     

Atomic displacements in dilute alloys of Cr, Nb and Mo

Kanzaki lattice static method is used to calculate the atomic displacements due to substitutional impurities in 3d (Cr) and 4d (Nb, Mo) metals. Wills and Harrison interatomic potential is used to calculate dynamical matrix and the impurity-induced forces up to second nearest neighbors. The calculated atomic displacements for 3d, 4d and 5d impurities in Cr (V, Mn, Fe, Ni, Nb, Mo, Ta and W), Nb (V, Cr, Mn, Fe, Zr, Mo, Ta and W) and Mo (V, Cr, Mn, Fe, Zr, Nb, Ta and W) are tabulated up to 10 NN’s. The strain field due to 3d impurities is least in Cr metal while it is larger in Nb and Mo metals. For 4d and 5d impurities the strain is larger in Cr metal than in Nb and Mo hosts. Similar trend is found for relaxation energies also.     

Electronic structure of LiGaS2

X-ray photoelectron spectroscopy (XPS) measurement has been performed to determine the valence band structure of LiGaS₂ crystals. The experimental measurement is compared with the electronic structure obtained from the density functional calculations. It is found that the Ga 3d states in the XPS spectrum are much higher than the calculated results. In order to eliminate this discrepancy, the LDA+U method is employed and reasonable agreement is achieved. Further calculations show that the difference of the linear and nonlinear optical coefficients between LDA and LDA+U calculations is negligibly small, indicating that the Ga 3d states are actually independent of the excited properties of LiGaS₂ crystals since they are located at a very deep position in the valence bands.     

Mn掺杂的ZnS(001)表面的电子态特性

利用第一性原理计算Mn在ZnS(001)表面上几种掺杂位置的形成能、局域分波态密度和磁矩.对Mn在ZnS(001)表面上的三种位置的形成能进行比较,得到两种填隙位置是非常稳定的掺杂位置.分析ZnS(001):Mn各种再构表面的电子态密度和电荷密度分布.结果表明,三种表面模型中,自旋向上的Mn原子的3d态和近邻S原子的3p态都有一定的杂化,并且替代掺杂的Mn和邻近S原子的p-d杂化最明显,形成的共价键最强.而自旋向下的Mn原子的3d态比较局域,受S原子的3p态影响较小.计算了三种掺杂表面的磁矩,并分析计算结果.     

Study of magnetic and optical properties of Zn_(1-x)TM_xTe(TM = Mn,Fe,Co,Ni) diluted magnetic semiconductors:First principle approach

We present structural,magnetic and optical characteristics of Zn_(1-x)TM_xTe(TM = Mn,Fe,Co,Ni and x = 6.25%),calculated through Wien2 k code,by using full potential linearized augmented plane wave(FP-LAPW) technique.The optimization of the crystal structures have been done to compare the ferromagnetic(FM) and antiferromagnetic(AFM) ground state energies,to elucidate the ferromagnetic phase stability,which further has been verified through the formation and cohesive energies.Moreover,the estimated Curie temperatures T_c have demonstrated above room temperature ferromagnetism(RTFM) in Zn_(1-x)TM_xTe(TM =Mn,Fe,Co,Ni and x= 6.25%).The calculated electronic properties have depicted that Mn- and Co-doped ZnTe behave as ferromagnetic semiconductors,while half-metallic ferromagnetic behaviors are observed in Fe- and Ni-doped ZnTe.The presence of ferromagnetism is also demonstrated to be due to both the p-d and s-d hybridizations between the host lattice cations and TM impurities.The calculated band gaps and static real dielectric constants have been observed to vary according to Penn's model.The evaluated band gaps lie in near visible and ultraviolet regions,which make these materials suitable for various important device applications in optoelectronic and spintronic.     

D-Type Anti-Ferromagnetic Ground State in Ca_2Mn_2O_5

We study the electronic and magnetic properties of an oxygen-deficient perovskite Ca_2 Mn_2 O_5 based on the first principle calculations. The calculations show that the ground state of Ca_2 Mn_2 O_5 is a D-type anti-ferromagnetic structure with the anti-ferromagnetic spin coupling along the c-direction. The corresponding electronic structure of the D-type state is investigated, and the results display that Ca_2 Mn_2 O_5 is an insulator with an indirect energy gap of ~2.08 eV. By the partial density-of-state analysis, the valence band maximum is mainly contributed to by the O-2 p orbitals and the conduction band minimum is contributed to by the O-2 p and Mn-3 d orbitals. Due to the Coulomb repulsion interaction between electrons, the density of state of Mn-3 d is pulled to-6--4.5 eV.     

非球对称势场与轨道有序化:NiO电子结构再研究

用含有非球对称修正的FPLMTO和LDA+UDFT,LDA+USIC研究了NiO的电子结构,发现由于非球对称势场的引入,导致了占据的3d轨道eg分量的轨道有序化.结果表明在这一类复杂的强关联电子中,原子球内非球对称性多极势场对电子结构的影响已经比较明显:除了上述的轨道有序化以外,eg分量的位置下移,宽度明显变窄,呈现局域化的趋势;两种不同LDA+U方法计算得出的电荷转移能隙分别为337eV(DFT)和25eV(SIC).同时发现在NiO中,上、下Hubbard带实际上是由占据的和非占据的3deg轨道构成,3dt2g轨道和O2p轨道的杂化带具有较大的带宽,因而也具有一定的巡游特性.结果表明在NiO中有特征明显的3d电子轨道有序化.通过研究得出结论:在NiO这一类体系的电子结构计算中,势场的非球对称性部分和球间区势场的作用,以及轨道极化等因素都应该恰当地处理 关键词： 关联电子系统 过渡金属氧化物 电子结构     

Hole-mediated stabilization of cubic GaN

We propose here a new approach to stabilize the cubic zinc-blende (ZB) phase by incorporation of impurities into a compound that has a hexagonal wurtzite (WZ) ground state. For GaN, we suggest that this can be achieved by adding 3d acceptors such as Zn, Mn, or Cu because the p-d repulsion between the 3d impurity levels and the valence band maximum is larger in the ZB phase than in the WZ phase. This makes the top of the valence states of the ZB structure higher than that of the WZ structure. As holes are created at the top of the valence states by the impurities, it will cost less energy for the holes to be created in the ZB structure, thus stabilizing this phase. Our first-principles total energy calculations confirm this novel idea.     

正交多铁性材料DyMnO3的磁性质研究

基于密度泛函理论结合投影缀加平面波方法, 通过VASP软件包执行计算, 在分别考虑电子自旋阻挫共线与非共线的磁性结构基础上, 研究了正交结构下多铁性DyMnO₃材料在不同磁性构型下的晶格参数、总能、磁性、电子态密度和能带结构. 计算过程中选取广义梯度近似赝势, 同时使用局域自旋密度近似+U方法处理强关联作用下3d电子的计算结果. 计算结果表明: Mn离子为A型反铁磁态磁性构型的情况下能量最低结构最为稳定, Dy稀土离子磁性甚微, 可忽略不计; 当考虑电子自旋为非共线排列时, 正交DyMnO₃的总能提高、磁矩增大; 从电子结构图分析可知, 材料为间接能隙绝缘体, 能隙宽度约为0.38 eV, 加U后为1.36 eV, 导致晶格畸变的主要原因为Mn-3d与O-2p电子之间强烈的杂化作用. 关键词： 多铁性 反铁磁 密度泛函理论 非共线     

First-principles investigation of impurity concentration influence on bonding behavior,electronic structure and visible light absorption for Mn-doped BiOCl photocatalyst

We performed first-principles calculation to investigate the bonding behavior, electronic structure and visible light absorption of Mn_xBi_1−xOCl (x=0, 0.0625, 0.09375 and 0.125) using density functional theory (DFT) within a plane-wave ultrasoft pseudopotential scheme. The relaxed structural parameters are consistent with the experimental results. The bonding behavior, bond orders, Mulliken charges and bond populations as well as formation energies are obtained. The calculated band structures and density of states show that Mn incorporation results in some impurity energy levels of Mn 3d states in forbidden band as well as valence band and conduction band, and that Mn 3d states, for the modest Mn doping concentration, not only can act as the capture center of excited electrons under longer wavelength light irradiation, but also may trap the photo-excited holes, improving the transfer of photo-excited carriers to the reactive sites. Our calculated optical absorption spectra exhibit that the spectral absorption edge is obviously red-shifted and extends to the visible, red and infrared light region due to the incorporation of Mn. Our calculated absorption spectra are in excellent agreement with the experimental results of Mn-doped BiOCl photocatalyst.     

First principle calculation of the electronic and magnetic properties of Mn-doped 6H-SiC

The electronic and magnetic properties of 6H-SiC with Mn impurities have been calculated using GGA formalism. Various configurations of Mn sites were considered. It was found that 6H-SiC doped with Mn atoms possess a moment for both types of substitution. The Mn atom at Si site possesses larger magnetic moment than Mn atom at C site. The energy levels appearing in the band gap due to vacancies and due to Mn impurities are determined and the calculated densities of states (DOSs) are used to analyse the different value of the magnetic moments for different types of substitution. A model that explains the magnetic moment at Mn site is proposed.     

铁磁性高锰硅化物Mn_4Si_7电子特性的第一性原理计算

本文采用基于第一性原理的密度泛函理论超软雁势平面波方法,对铁磁性半导体高锰硅化合物Mn_4Si_7进行了理论计算.结果表明块体Mn_4Si_7是准直接带隙半导体材料,其价带主要是由Mn的3d轨道电子构成,导带主要是由Mn的3d及Si的3p轨道电子构成.相同自旋轨道下,自旋向下态的电子更容易占据较高的能级.而自旋向上态的电子对Mn_4Si_7的禁带宽度起主导作用. Mn_4Si_7的费米能级附近各轨道未被电子占满,且自旋向上态与自旋向下态电子的不对称分布使其具有了磁性.为Mn_4Si_7磁学特性提供主要贡献的是Mn的3d轨道电子,而Si的3p和3s轨道电子提供了一个小的贡献.