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.     

Microscopic analysis of the valence band and impurity band theories of (Ga,Mn)As

We analyze microscopically the valence and impurity band models of ferromagnetic (Ga,Mn)As. We find that the tight-binding Anderson approach with conventional parametrization and the full potential local-density approximation+U calculations give a very similar band structure whose microscopic spectral character is consistent with the physical premise of the k·p kinetic-exchange model. On the other hand, the various models with a band structure comprising an impurity band detached from the valence band assume mutually incompatible microscopic spectral character. By adapting the tight-binding Anderson calculations individually to each of the impurity band pictures in the single Mn impurity limit and then by exploring the entire doping range, we find that a detached impurity band does not persist in any of these models in ferromagnetic (Ga,Mn)As.     

Electronic structures and magnetic couplings of B-, C-, and N-doped BeO

The electronic structures and magnetic properties of B-, C-, and N-doped BeO supercells are investigated by means of ab initio calculations using density functional theory. The magnetic exchange constants of C-doped BeO at different doping levels are also calculated. A phenomenological band structure model based on p-d exchange-like p-p level repulsion between the dopants is proposed to explain the magnetic ground states in B-, C-, and N-doped BeO systems. The evolution from the antiferromagnetic phase to the ferromagnetic phase of C-doped BeO supercell with C concentration decreasing can also be well explained using this model. The findings in this study provide a simple guide for the design of band structure for a magnetic sp-electron semiconductor.     

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.     

First-Principles Studies for the Electronic Structures of Diluted Magnetic Semiconductors （Ga, Fe）As

First-principles LMTO-ASA band calculations are performed for Ga1-xFezAs (x = 1, 1/4, 1/8) by assuming supercell structures. It is found that the antiferromagnetic (AFM) state is stable for x = 1/4. For x = 1/8, ferromagnetic (FM) state is more stable than AFM state, and no stable magnetic state exists for x = 1. In both the cases the magnetic moments of As and Ga atoms are parallel to those of the nearest Fe atoms due to the p-d hybridization. Fhrther, the band structure shows rather localized Fe 3d state in the gap, and the parallel polarization is confined rather in the vicinity of Fe site.     

Optical properties of the filled tetrahedral semiconductors LiZnX (X=N, P, and As)

We present first principles calculations of the electronic and the optical properties of the filled tetrahedral compounds LiZnN, LiZnP, and LiZnAs performed with the full potential linearized augmented plane wave method within the local density approximation. The origin of the small gap of LiZnN is attributed to the strong p-d coupling in this compound. The assignments of the structures in the optical spectra and band structure transitions are discussed in detail. The predicted values of the dielectric constants for LiZnP and LiZnAs are close to those of the binary compounds GaP and GaAs, respectively.     

X-ray photoemission of valence electrons in cuprous halides,and lead and cadmium iodides.

The XPS spectra of valence bands in CuCl, CuBr, and CuI show the existence of two separate bands after appropriate deconvolution. The evaluation of the average p-d mixing rates on the basis of the tight-binding approximation with a few simplifying assumptions indicates, in accordance with optical data, that the upper valence band arises mainly from the 3d state of copper. The XPS spectra coincide qualitatively but not quantitatively with the spectra obtained by excitation with monochromatized synchrotron radiation at 40, 61, and 76 eV. A major difference is that the ratio of integrated intensity of the upper band to that of the lower band is larger in the case of excitation at 40, 61, or 76 eV. The XPS spectra of PbI₂ and CdI₂ have also been measured. In both materials, the valence band spectra have composite structures and give two well-defined peaks after deconvolution. The profile of the spectrum of PbI₂ appears to have some deviation from the reported energy band structure.     

Zn1-xMnxS(001)稀磁半导体薄膜的能量稳定性、磁性和电子结构

通过基于广义梯度近似的总能密度泛函理论研究不同Mn掺杂浓度的ZnS(001)薄膜的电学和磁学特性. 计算单个Mn原子和两个Mn原子处于各种掺杂位置及不同的磁耦合状态时的能量稳定性.计算了单个Mn原子掺杂和两个Mn原子掺杂的ZnS(001)薄膜的态密度. 不同掺杂组态的p-d杂化的程度不同. 不同掺杂组态,Mn原子所处的晶场环境不同,所以不同掺杂组态的Mn的3d分波态密度峰的劈裂有很大的不同. 掺杂两个Mn原子时,得到三种稳定组态的基态都是反铁磁态. 分析了以上三种能量稳定的组态中,两个Mn原子在不同磁耦合状态下的3d态密度图. 当两原子为铁磁耦合时,由于d-d电子相互作用,使反键态的态密度峰明显加宽. 随着Mn掺杂浓度的增加,Mn原子有相互靠近,并围绕S原子形成団簇的趋势. 对于这样的组态,Mn原子之间为反铁磁耦合能量更低.     

Fe_9Si的电子结构及铁磁性质的第一性原理研究

采用第一性原理的密度泛函理论(Density Functional Theory)赝势平面波方法,对Fe_9Si的电子结构和铁磁性质进行理论计算.计算结果表明:(1)Fe_9Si具有负的形成热-0.1094 eV/atom,结合能5.124eV/atom,表明Fe_9Si合金具有强结合力和结构稳定性;(2)Fe_9Si具有典型的金属能带特征,穿过Fermi能级的能带最主要是Fe的3d态电子的贡献,其次是来自Si的3p态电子的贡献.结合键不是单一金属键,而是金属键和共价键组成的混合键;(3)Fe_9Si的铁磁性主要来自Fe原子的未满层壳的3d态电子的自旋.计算结果为Fe_9Si铁磁性材料的设计与应用提供了理论依据.     

Fe9Si的电子结构及铁磁性质的第一性原理研究

采用第一性原理的密度泛函理论（Density Functional Theory）赝势平面波方法, 对Fe9Si的电子结构和铁磁性质进行理论计算。 计算结果表明： (1) Fe9Si具有负的形成热-0.1094 eV/atom, 结合能5.124 eV/atom, 表明Fe9Si合金具有强结合力和结构稳定性; (2) Fe9Si具有典型的金属能带特征, 穿过Fermi能级的能带最主要是Fe的3d态电子的贡献, 其次是来自Si的3p态电子的贡献。 结合键不是单一金属键, 而是金属键和共价键组成的混合键; (3) Fe9Si的铁磁性主要来自Fe原子的未满层壳的3d态电子的自旋。 计算结果为Fe9Si铁磁性材料的设计与应用提供了理论依据。     

First-Principles Studies for the Electronic Structures of Diluted Magnetic Semiconductors (Ga, Fe)As

First-principles LMTO-ASA band calculations are performed for Ga1-xFexAs (x = 1, 1/4, 1/8) by assuming supercell structures. It is found that the antiferromagnetic (AFM) state is stable for x = 1/4. For x = 1/8, ferromagnetic(FM) state is more stable than AFM state, and no stable magnetic state exists for x = 1. In both the cases the magneticmoments of As and Ga atoms are parallel to those of the nearest Fe atoms due to the p-d hybridization. Further, theband structure shows rather localized Fe 3d state in the gap, and the parallel polarization is confined rather in thevicinity of Fe site.     

Optoelectronic and thermoelectric properties of Zintl YLi3X2(X=Sb,Bi) compounds through modified Becke—Johnson potential

In the present work, we investigate the structural, optoelectronic and thermoelectric properties of the YLi₃X₂(X=Sb, Bi) compounds using the full potential augmented plane wave plus local orbital (FP-APW+lo) method. The exchange-correlation potential is treated with the generalized gradient approximation/local density approximation (GGA/LDA) and with the modified Becke-Johnson potential (TB-mBJ) in order to improve the electronic band structure calculations. In addition, the estimated ground state properties such as the lattice constants, external parameters, and bulk moduli agree well with the available experimental data. Our band structure calculations with GGA and LDA predict that both compounds have semimetallic behaviors. However, the band structure calculations with the GGA/TB-mBJ approximation indicate that the ground state of the YLi₃Sb₂ compound is semiconducting and has an estimated indirect band gap (Γ-L) of about 0.036 eV while the ground state of YLi₃Bi₂ compound is semimetallic. Conversely the LDA/TB-mBJ calculations indicate that both compounds exhibit semiconducting characters and have an indirect band gap (Γ-L) of about 0.15 eV and 0.081 eV for YLi₃Sb and YLi₃Bi₂ respectively. Additionally, the optical properties reveal strong responses of the herein materials in the energy range between the IR and extreme UV regions. Thermoelectric properties such as thermal conductivity, electrical conductivity, Seebeck coefficient, and thermo power factors are also calculated.     

First-principles study of structural, electronic and magnetic properties in Cd1−xFexS diluted magnetic semiconductors

In this paper, we report theoretical investigations of structural, electronic and magnetic properties of ordered dilute ferromagnetic semiconductors Cd_1−xFe_xS with x=0.25, 0.5 and 0.75 in zinc blende (B3) phase using all-electron full-potential linear muffin tin orbital (FP-LMTO) calculations within the density functional theory and the generalized gradient approximation. The analysis of band structures, density of states, total energy, exchange interactions and magnetic moments reveals that both the alloys may exhibit a half-metallic ferromagnetism character. The value of calculated magnetic moment per Fe impurity atom is found to be 4 μ_B. Moreover, we found that p-d hybridization reduces the local magnetic moment of Fe from its free space charge value of 4 μ_B and produces small local magnetic moments on Cd and S sites.     

First-principles prediction of half-metallic ferromagnetism in Cd(TM)O2 (TM=Cr, Mn, Fe, Co, Ni) compounds

We report the electronic structure of Cd(TM)O₂ (TM=Cr, Mn, Fe, Co, Ni) in the chalcopyrite structures. From this study we find that Cd(TM)O₂ is a half-metallic ferromagnetic compound. From the energy consideration we find that Cd(TM)O₂ is more stable in chalcopyrite structure rather than in rock salt structure. A careful analysis of the spin density reveals the ferromagnetic coupling between the p-d states and the cation dangling-bond p states, which is believed to be responsible for the stabilization of the ferromagnetic phase. The calculated heat of formation, bulk modulus and cohesive energy are reported.     

Theoretical investigations of half-metallic ferromagnetism in new Half-Heusler YCrSb and YMnSb alloys using first-principle calculations

Structural,electronic,and magnetic properties of new predicted half-Heusler YCrSb and YMnSb compounds within the ordered MgAgAs Clb-type structure are investigated by employing first-principal calculations based on density functional theory.Through the calculated total energies of three possible atomic placements,we find the most stable structures regarding YCrSb and YMnSb materials,where Y,Cr(Mn),and Sb atoms occupy the(0.5,0.5,0.5),(0.25,0.25,0.25),and(0,0,0) positions,respectively.Furthermore,structural properties are explored for the non-magnetic and ferromagnetic and anti-ferromagnetic states and it is found that both materials prefer ferromagnetic states.The electronic band structure shows that YCrSb has a direct band gap of 0.78 eV while YMnSb has an indirect band gap of 0.40 eV in the majority spin channel.Our findings show that YCrSb and YMnSb materials exhibit half-metallic characteristics at their optimized lattice constants of 6.67  and 6.56 ,respectively.The half-metallicities associated with YCrSb and YMnSb are found to be robust under large in-plane strains which make them potential contenders for spintronic applications.     

Ferromagnetism in GaN:Gd: a density functional theory study

First-principle calculations of the electronic structure and magnetic interaction of GaN:Gd have been performed within the generalized gradient approximation (GGA) of the density functional theory with the on-site Coulomb energy U taken into account (also referred to as GGA+U). The ferromagnetic p-d coupling is found to be over 2 orders of magnitude larger than the s-d exchange coupling. The experimental colossal magnetic moments and room-temperature ferromagnetism in GaN:Gd reported recently are explained by the interaction of Gd 4f spins via p-d coupling involving holes introduced by intrinsic defects such as Ga vacancies.     

Energy band structure of spin-1 condensates in optical lattices

The energy band structure of spin-1 condensates with repulsive spin-independent and either ferromagnetic or antiferromagnetic spin-dependent interactions in one-dimensional (1D) periodic optical lattices is discussed. Within the two-mode approximation, Bloch bands of spin-1 condensates are presented. The results show that the Bloch bands exhibit a complex structure as the atom density of m F=0 hyperfine state increases: bands splitting, reversion, intersection and loop structure are excited subsequently. The complex band structure should be related to the tunneling and spin-mixing dynamics.     

Magnetic properties and electronic structure of GdNi4Si compound

Electronic structure of the ternary GdNi₄Si compound, crystallizing in hexagonal CaCu₅ structure (P6/mmm space group) was studied by magnetic measurements, X-ray photoelectron spectroscopy (XPS) and ab initio calculations. Core levels and valence band were investigated. The valence band of the XPS spectra is determined mainly by the Ni(3d) and Gd(4f) bands. The peaks’ positions are in good agreement with binding energies of a metallic gadolinium and nickel. The experimental valence band spectrum as well as the calculated density of states exhibit the domination of the Ni(3d) states in region from −4 eV to the Fermi level.     

Effects of disorder on the Curie temperature of GaMnN, GaCrN, InCrN, and InMnN diluted magnetic semiconductors

The critical Curie temperatures of ordered and disordered diluted magnetic semiconductors based on GaN, InN, CrN, and MnN compounds are investigated using the classical Heisenberg model within the mean field approximation. The equilibrium structural lattice parameters of all the structures investigated are obtained from first principles. We show that the Curie T_c temperatures of disordered GaN and InN doped with small concentrations of Mn and Cr depends, to a great extent, on the Mn and Cr concentrations. Our calculations on these systems show that a T_c above room-temperature can be observed in these systems and it is affected greatly by the degree of disorder of Mn and Cr randomly distributed on the Ga and In sites. In addition, the ferromagnetic stability in these diluted magnetic semiconductors is studied systematically. Our results indicate that 3d Mn and Cr impurity states in GaN and InN favor the ferromagnetic state rather than the spin-glass phase.