Heusler合金Co2MnAl(100)表面电子结构、磁性和自旋极化的第一性原理研究

基于第一性原理计算方法系统地研究了L2₁和B2结构下的Heusler合金Co₂MnAl(100)表面原子的原子弛豫、电子结构、磁性和自旋极化行为. L2₁和B2结构的Co₂MnAl(100)表面由于Co–Mn和Co–Al的成键差异, 使得不同原子分别发生不同程度的伸缩. 与块体相比, Co和Mn原子的自旋磁矩由于表面效应而明显增大, 电子结构计算显示L2₁结构块体中的带隙被表面态破坏, 表面效应使得两种结构的CoCo端面自旋极化率降低, 但MnAl端面并未受到显著影响, 呈现了较大的自旋极化, 预测其在隧道结中可能具有很好的应用潜力.     

Inverse-Heusler合金Ti_2NiAl/GaAs隧道异质结的自旋极化和电磁特性

基于密度泛函理论的第一性原理计算,研究了Hg_2CoTi型Inverse-Heusler合金Ti_2NiAl和GaAs异质界面上的原子相互作用、磁性、态密度和自旋极化.结果表明,在所研究的12种异质结构(包括顶位和桥位连接)中,界面态的存在不同程度地破坏了合金的半金属性,导致不超过75%的自旋极化率发生;仅当Heusler合金TiAl端面的Al原子或者TiNi端面的Ti原子位于As原子的顶位时,合金异质结可以保留着约72%的自旋极化率,根据Julliere模型,能最大实现68.9%的自旋电子迁移率,有望在隧穿磁阻中实际应用.     

单层GaSe表面Fe原子吸附体系电子自旋性质调控

Ⅲ族金属单硫化物因其优越的光电和自旋电子特性而备受关注,实现对其自旋性质的有效调控是发展器件应用的关键.本文采用密度泛函理论系统地研究了GaSe表面Fe原子吸附体系的几何构型及自旋电子特性.Fe/GaSe体系中Fe吸附原子与最近邻Ga,Se原子存在较强的轨道耦合效应,使体系呈现100%自旋极化的半金属性.其自旋极化贡献主要来源于Fe-3d电子的转移及Fe-3d,Se-4p和Ga-4p轨道杂化效应.对于Fe双原子吸附体系,两Fe原子之间的自旋局域导致原本从Fe转移至GaSe的自旋极化电荷量减少,从而费米能级附近的单自旋通道转变为双自旋通道,费米能级处的自旋极化率转变为0.研究结果揭示了Fe_n/GaSe吸附体系自旋极化特性的形成和转变机制,可为未来二维自旋纳米器件的设计与构建提供参考.     

CrO_2块体及其(001)和(100)表面的电子结构和磁性质

基于密度泛函理论的第一性原理计算,我们研究了CrO_2块体以及其(100)和(001)表面的磁性和电子结构.CrO_2(100)表面中表面层Cr原子向内收缩,而(001)表面中的表面层Cr原子却向外伸展;越往内层,原子驰豫幅度越小.由于表面效应.表面层原子的磁矩有了大幅的提升,其中(100)表面中表面层Cr原子的磁矩最高,为3.03μB.电子结构计算表明,CrO_2(100)和(001)表面均保持了块体中良好的半金属性,并且半金属带隙在(100)表面中受到了进一步的拓宽,因此具有更稳定的半金属性     

LiMgPbSb型四元Heusler合金CoFeTiSb的半金属性及其无序效应

基于密度泛函理论的GGA计算,我们研究了LiMgPbSb型Heusler合金CoFeTiSb电子结构,发现CoFeTiSb在费米面处存在100%的自旋极化率,并且具有2μB的原胞总磁矩.此外,我们考虑了Co-Fe、Co-Ti和Fe-Ti交换无序对CoFeTiSb合金电子结构的影响,发现这三种交换无序均使得CoFeTiSb完全丧失了半金属性,在Co-Fe交换无序下,CoFeTiSb合金具有85%的自旋极化率,而在Co-Ti无序下,CoFeTiSb合金的自旋极化率只有5%.     

Mn掺杂LiZnN新型稀磁半导体磁电性质的第一性原理计算

采用基于密度泛函理论的第一性原理平面波超软赝势方法,对纯LiZnN、Mn掺杂LiZnN及Li不足和过量时Mn掺杂LiZnN体系进行几何结构优化,分析体系的电子结构、半金属性和磁电性质.结果表明,Mn的掺入使体系产生自旋极化杂质带,自旋极化率为100%,表现出半金属铁磁性,且形成较强的Mn-N共价键.当Li不足时,Mn-N键的共价性最强,键长变短,体系半金属性明显增强,形成能最低,结构最稳定.Li过量时,体系半金属性消失,表现为金属性,杂质带宽度增大,体系导电能力增强.表明Mn掺杂LiZnN新型稀磁半导体可以通过Mn的掺入和改变Li的含量来实现磁性和电性的分离调控.掺杂体系的基态均为铁磁性,其净磁矩主要由Mn原子贡献,通过海森堡模型计算发现,Li空位可以有效提高体系的居里温度.     

Cr掺杂BaTiO3中的高自旋极化半金属态

具有高自旋极化率的半金属材料是一类极具应用前景的自旋电子材料.立方BaCrO3具有非常稳定的半金属性,而四方BaTiO3则是一种多功能绝缘体.因此,本研究采用基于密度泛函的第一性原理方法,探究BaTi1-xCrxO3(x=0,0.125,0.25,0.5,0.75,1)体系中Cr、Ti互掺导致的晶体结构和电子结构的变化.研究表明,Cr离子的掺杂会使体系发生四方相到立方相结构相变;同时,受Cr-Ti轨道杂化的影响,体系存在绝缘态-半金属态转变.此外,Cr掺杂调控了体系的磁性,掺入的Cr离子表现为+4价态,提供2μB的局域磁矩.最后,本文构建了d-p杂化分子轨道模型,基本解释了体系半金属性的产生机制.本研究预测了BaTi1-xCrxO3体系中丰富的物理性质,为其在自旋电子器件方面的应用提供了理论指导.     

铁电层厚度对(BiCoO_3)_n/(La_(2/3)Sr_(1/3)MnO_3)_1超晶格磁电性质的影响

本文利用第一性原理,对由四方相BiCoO_3(BCO)和La_(2/3)Sr_(1/3)MnO_3(LSMO)所构造的超晶格的形成能、电子结构与磁电性质进行了系统研究.研究表明,超晶格中BCO层厚度变化可对超晶格的电子结构,及其极化位移与自旋极化率等磁电性质产生重要影响.特别的,在BiO/Mn_BO_2为界面构型的超晶格中,当BCO侧为一个单胞层时,超晶格表现出半金属性,可获得100%自旋极化率.上述研究结果对于新型自旋电子器件研究具有一定科学意义.     

四元Heusler合金VFeScP/Ag(100)异质结的结构、磁性以及电子性质

基于密度泛函理论的第一性原理计算,研究了四元Heusler合金VFeScP/Ag(100)异质结的ScP-Ag、VFe-Ag、FeFe-Ag、VV-Ag、ScSc-Ag和PP-Ag这6种原子端面的结构、原子磁性、态密度和自旋极化。结果表明,由于界面原子复杂的相互作用,界面原子层呈现不同程度不平整,从而可能加剧界面层的电子散射。与块体相比,界面原子的配位数变化引起的d电子局域性和磁直接交互的共同作用,导致了的界面原子复杂的磁行为。电子态密度研究发现,原来块体中的高自旋极化率已经被破坏。最大的自旋极化率出现在ScSc-Ag异质结构中,约为53%,预测在自旋阀中有一定的应用潜力。     

应变和C掺杂对单层BN纳米片的电子结构和磁学性质的影响

利用基于密度泛函理论的第一性原理计算方法, 研究了应变和C原子掺杂对单层BN纳米片的电子结构和磁学性质的影响. 计算结果表明未掺杂的单层BN纳米片具有宽的直接带隙, 在压缩和拉伸应变的作用下, 带隙会分别增大和减小, 但应变对带隙的调制整体效果不太明显. 单个C原子掺入BN纳米片的态密度揭示体系呈现出半金属性(Half-metallicity), 磁矩主要源于C 2p态, 而B 2p和N 2p态在极化作用下也能提供部分磁矩. 两个C原子掺入BN纳米片时, 磁性基态会随着C原子的间距发生变化: 当两C原子为最近邻(nn)和次近邻(nnn)时, 反铁磁态为磁性基态; 而当两C原子为次次近邻(nnnn)时, 铁磁态为基态, 并且其态密度也显示出半金属性.     

Electronic structure of the NiMnSb-semiconductor (110) interface

The electronic structure and magnetic properties of the (110) interface between the half-metallic Heusler alloy NiMnSb and semiconductors with different configurations of contact atoms have been calculated in terms of the density functional theory. It has been shown that spin polarization depends substantially on the configuration of atoms at the contacts. The nature of the interface states at the contacts under consideration has been analyzed. A nearly 100% spin polarization is obtained for the configuration with nickel and antimony, which in the alloy occupy the corresponding sites of the anion and the cation in the semiconductor. The evaluation of the adhesion energy at the interfaces has demonstrated that contacts with a maximum spin polarization also have the highest energy and are energetically favorable and stable.     

Scanning tunneling microscopy chemical signature of point defects on the MoS2(0001) surface

Using ab initio calculations, we have studied the modification of the electronic structure of the MoS2(0001) surface by several point defects: a surface S vacancy and different transition metal atoms substituting a S atom (Pd, Au, Fe, and V). With a S vacancy, a gap state appears with weight mostly on the Mo and S atoms surrounding the vacancy. The substitutional atoms of complete d band (Pd and Au) do not present magnetic polarization and slightly modify the DOS near the Fermi energy. On the other hand, the incomplete d band atoms (Fe and V) present spin polarization and modify significantly the states near the band edges. From calculated STM images and STS curves, we show that this chemical signature can be measured and used to characterize the surface defects of the substrate which are suitable nucleation centers for nanocluster growth.     

Spin-polarized surface states on Fe-deposited Au(111) surface: A theoretical study

We have studied electronic structure of Fe-deposited Au(111) by performing ab initio density functional theory calculations. We find that the magnetic moment on the deposited Fe layer is enhanced as compared to that in bulk iron. We observe a large number of new states on the Fe-deposited surface — one of which is in the majority spin channel having similar dispersion to that on the clean surface, and others in the minority spin channel. The effective mass of electrons in surface states near the Fermi level increases on Fe deposition. The electronic properties are found to be insensitive to the stacking of near-surface layers. We need to use very thick slabs in our calculations to avoid splitting of surface states due to spurious interactions between the two surfaces of the slab. Using the local density of states profiles for different surface states, we conclude that in scanning tunneling microscope experiments one can detect two of the surface states — one in the majority channel below the Fermi level, and another in the minority channel appearing just above the Fermi energy. We compare our results to those from scanning tunneling spectroscopy experiments.     

Local spin polarization at surfaces probed by hollow atoms

The relaxation of hollow atoms produced by slow multiply charged ions impinging on surfaces produces characteristic Auger electron spectra. These spectra, which serve as fingerprints of the interaction, can be used to probe local spin ordering at surfaces by relating changes in the intensities of different spin states to local spin polarization at the surface. The area from which the electrons are captured is of the order of a few Angstrom(2), only. The potential of the method is illustrated by He(2+) and N(6+) ions interacting with a ferromagnetic Ni(110) crystal. From the Auger spectra we determine a spin polarization of approximately 90% at room temperature.     

Heusler合金Co2CrGa(1 0 0)的表面结构、磁性和自旋极化

基于第一性原理计算,我们系统地呈现了三元合金Co2CrGa (100)表面的原子弛豫、磁性、电子结构以及表面原子极化行为。结果显示,由于Co-Ga和Co-Cr成键的差异,表面的Co和Cr原子分别向内层收缩和向外部真空层伸展。与块体相比较,表面Co和Cr原子的自旋磁矩由于局域性的提升而明显增大。在研究的Co2CrGa(100)不同原子端面中,可以观察到块体中的半金属带隙在CoCo和GaGa原子端面被大量的表面态所破坏,仅仅在CrGa和CrCr原子覆盖的端面,检测到100%的理想极化,预测其在隧道结中可能具有较佳的应用潜力。     

Spin-resolved photoemission of surface states of W(110)-(1 x 1)H

The surface electronic states of W(110)-(1 x 1)H have been measured using spin- and angle-resolved photoemission. We directly demonstrate that the surface bands are both split and spin-polarized by the spin-orbit interaction in association with the loss of inversion symmetry near a surface. We observe 100% spin polarization of the surface states, with the spins aligned in the plane of the surface and oriented in a circular fashion relative to the Smacr; symmetry point. In contrast, no measurable polarization of nearby bulk states is observed.     

Magnetic dichroism of potassium atoms on the surface of helium nanodroplets

The population ratio of Zeeman sublevels of atoms on the surface of superfluid helium droplets (T=0.37 K) has been measured. Laser induced fluorescence spectra of K atoms are measured in the presence of a moderately strong magnetic field (2.9 kG). The relative difference between the two states of circular polarization of the exciting laser is used to determine the electron spin polarization of the ensemble. Equal fluorescence levels indicate that the two spin sublevels of the ground-state K atom are equipopulated, within 1%. Thermalization to 0.37 K would give a population ratio of 0.35. We deduce that the rate of spin relaxation induced by the droplet must be <520/s. For the K2 triplet dimer we find instead full thermalization of the spin.     

Co_(2~-)基Heusler合金Co_2FeAl_(1–x)Si_x(x=0.25,x=0.5,x=0.75)的结构、电子结构及热电特性的第一性原理研究

运用基于密度泛函理论的第一性原理方法,对Co_2FeAl_(1–x)Si_x(x=0.25, 0.5, 0.75)系列Heusler合金的电子结构、四方畸变、弹性常数,声子谱以及热电特性进行了计算研究.结果显示, Co_2FeAl_(1–x)Si_x系列合金的电子结构均为半金属特性,向下自旋态(半导体性)均呈现良好的热电特性,并且随着硅原子浓度的增加功率因子随之增加.计算的声子谱不存在虚频,均满足动力学稳定性条件,弹性常数均满足玻恩稳定性条件,机械稳定性均良好.随着晶格常数c/a的比值变化,体系的能量最低点均出现在c/a=1处,即结构稳定性不随畸变度c/a的变化而变化,说明不存在马氏体相变.此系列合金薄膜的电子结构呈现较高的自旋极化率,在替代浓度x=0.75时自旋极化率达到100%,且当x=0.75时薄膜在畸变度c/a=1.2时存在马氏体相变.随着晶格畸变度的改变,总磁矩也发生变化,且主要由Fe和Co两种过渡金属原子的磁矩变化所决定.     

The effect of surface channeling of neutral atoms

Special features of reflection of neutral atoms from crystal surfaces are considered in the case of grazing incidence. It is shown that polarization effects and the long-range part of the potential of interaction of ions with atomic chains and planes can play an important role in describing processes of reflection of atoms from the surface. The analysis in this paper shows that the polarization of accelerated neutral atoms in the case of grazing incidence on the surface can lead to the formation of bound surface states. The threshold condition for the angle of arrival of atoms at the plane at which the capture by such states is possible is obtained.