BN链掺杂的石墨烯纳米带的电学及磁学特性

基于密度泛函理论第一性原理系统研究了BN链掺杂石墨烯纳米带(GNRs)的电学及磁学特性, 对锯齿型石墨烯纳米带(ZGNRs)分非磁态(NM)、反铁磁态(AFM)及铁磁性(FM)三种情况分别进行考虑. 重点研究了单个BN链掺杂的位置效应. 计算发现: BN链掺杂扶手椅型石墨烯纳米带(AGNRs) 能使带隙增加, 不同位置的掺杂, 能使其成为带隙丰富的半导体. BN链掺杂非磁态ZGNR的不同位置, 其金属性均降低, 并能出现准金属的情况; BN链掺杂反铁磁态ZGNR, 能使其从半导体变为金属或半金属(half-metal), 这取决于掺杂的位置; BN链掺杂铁磁态ZGNR, 其金属性保持不变, 与掺杂位置无关. 这些结果表明: BN链掺杂能有效调控石墨烯纳米带的电子结构, 并形成丰富的电学及磁学特性, 这对于发展各种类型的石墨烯基纳米电子器件有重要意义. 关键词： 石墨烯纳米带 BN链掺杂 输运性质 自旋极化     

硅团簇自旋电子器件的理论研究

利用过渡金属掺杂的硅基团簇, 构建了一种自旋分子结; 并利用第一性原理方法, 对其电子自旋极化输运性质进行了研究. 计算表明, 通过过渡金属掺杂可以有效地产生自旋极化电流, 磁性金属Fe和非磁性金属Cr和Mn掺杂的体系呈现出较明显的自旋极化透射现象, 但分子结的自旋极化输运能力与团簇孤立状态下的磁矩无一致性.从Sc到Ni的掺杂, 体系的自旋极化透射能力先增大后迅速减小, 在Fe掺杂的Si₁₂团簇中出现最大值. 关键词： 硅团簇 自旋极化输运 密度泛函理论 非平衡格林函数     

双空位掺杂氟化石墨烯的电子性质和磁性

基于密度泛函理论,计算了外来原子X(Al,P,Ga,As,Si)双空位替代掺杂氟化石墨烯的电子特性和磁性.通过对计算结果分析发现,与石墨烯的双空位掺杂类似,氟化石墨烯的双空位掺杂也是一种较为理想的掺杂方式.通过不同原子掺杂,氟化石墨烯的电子性质与磁性均发生很大变化:Al和Ga掺杂使氟化石墨烯由半导体变为金属,并且具有磁性;P和A8掺杂使氟化石墨烯变为自旋半导体;Si掺杂氟化石墨烯仍是半导体,只改变带隙且没有磁性.进一步讨论磁性产生机制获得了掺杂原子浓度与磁性的关系,并且发现不同掺杂情况的磁性是由不同原子的不同轨道电子引起的.双空位掺杂不仅丰富了氟化石墨烯的掺杂方式,其不同电磁特性也使此类掺杂结构在未来的电子器件中具有潜在应用.     

石墨烯纳米片磁有序和自旋逻辑器件第一原理研究

尺寸效应和拓扑阻挫能够在有限石墨烯纳米片段中形成磁有序,本文对能够产生大自旋或电子自旋反铁磁耦合的石墨烯有限片段进行合理分类,提出几种能够作为基本逻辑门的特殊结构并对其进行第一原理电子结构计算,为设计高密度超快自旋器件提供了有效方案和理论依据.计算结果证明：基于有限石墨烯片段的逻辑门结构能够在室温下进行错误率较低的可纠错运算.     

功能化扶手椅型石墨烯纳米带异质结的磁器件特性

石墨烯在未来纳米电子器件领域具有广泛的应用前景, 但是基于扶手椅型石墨烯纳米带(AGNR)的磁输运性质的研究还比较少. 本文理论上提出AGNR边缘桥接过渡金属Mn原子, 再用双F 原子(或双H原子)饱和形成特殊化学修饰的纳米带(AGNR-Mn-F₂或AGNR-Mn-H₂), 并运用基于第一性原理和非平衡态格林函数相结合的方法对其磁输运性质进行理论计算. 结果表明: 这两种纳米带所构成的异质结(F₂-AGNR-Mn-H₂)具有优良的磁器件特性, 即在很宽的偏压范围内, 能实现100%的自旋极化, 且在P(在左右电极垂直加上相同方向的磁场)和AP构型(在左右电极垂直加上相反方向的磁场)时, 分别具有单自旋和双自旋过滤效应; 同时发现, 这种异质结也具有双自旋二极管效应, 它的最大整流比可达到10⁸. 此外, 改变开关磁场的方向, 即从一种磁构型变换为另一种磁构型时, 能产生明显的自旋阀效应, 其巨磁阻高达10⁸%. 这意味着这种特殊的异质结能同时实现优良的自旋过滤、双自旋二极管及巨磁阻效应, 这对于发展自旋磁器件有重要意义.     

Rashba spin-splitting control at the surface of the topological insulator Bi2Se3

The electronic structure of Bi(2)Se(3) is studied by angle-resolved photoemission and density functional theory. We show that the instability of the surface electronic properties, observed even in ultrahigh-vacuum conditions, can be overcome via in situ potassium deposition. In addition to accurately setting the carrier concentration, new Rashba-like spin-polarized states are induced, with a tunable, reversible, and highly stable spin splitting. Ab initio slab calculations reveal that these Rashba states are derived from 5-quintuple-layer quantum-well states. While the K-induced potential gradient enhances the spin splitting, this may be present on pristine surfaces due to the symmetry breaking of the vacuum-solid interface.     

Spin-dependent transport properties of debrominated tetrabromopolyaromatic with Cu or Co doping embedded between zigzag graphene nanoribbon electrodes

Using density functional theory combined with non-equilibrium Green's function method, we investigate the spin-dependent transport properties of debrominated tetrabromopolyaromatic (D-TBPA) molecules embedded between zigzag graphene nanoribbon electrodes, and the effects of copper and cobalt side doping have also been considered. Our results show that the copper doping can insert new energy levels around the Fermi Level and keep spin degeneration of band structure, the cobalt doping can also induce spin splitting. The results on spin transport properties of D-TBPAs embedded into zigzag graphene nanoribbon electrodes show that these systems exist spin filtering and negative differential resistance behaviors. Corresponding physical mechanism on the spin-dependent transport property has been revealed according to the frontier molecular orbital characteristics.     

Effect of single magnetic atom on spin-polarized transport of armchair graphene nanoribbons

Using a perturbative method, the influence of a single magnetic impurity on the spin-polarized current flowing through a metallic armchair graphene nanoribbon is investigated theoretically. It is shown that the nonlinear correction depends strongly on the relative spin orientations in the two spin-polarized reservoirs. Also, the effects of magnetic impurity position and width of nanoribbon on the nonlinear conductance are discussed.     

Dependences of spin polarization on the control parameters in the spin-polarized injection through the magnetic p-n junction

Effective spin-polarized injection from magnetic semiconductor (MS) to nonmagnetic semiconductor (NMS) has been highlighted in recent years. In this paper we study theoretically the dependence of nonequilibrium spin polarization (NESP) in NMS during spin-polarized injection through the magnetic p-n junction. Based on the theory in semiconductor physics, a model is established and the boundary conditions are determined in the case of no external spin-polarized injection and low bias. The control parameters that may influence the NESP in NMS are indicated by calculating the distribution of spin polarization. They are the doping concentrations, the equilibrium spin polarization in MS and the bias. The effective spin-polarized injection can be realized more easily by optimizing the above parameters.     

Electron and spin transport in adiabatic quantum pumps based on graphene nanoribbons

The adiabatic quantum pump effect in double-barrier structures based on graphene nanoribbons has been analyzed numerically. The charge transmitted through the system in one cycle of variation of pumping potentials (barrier heights) has been calculated. This charge is shown to be quantized under certain conditions. The possibility of the generation of not only electron currents but also spin and purely spin ones is considered. In devices based on zigzag nanoribbons, the spin current emerges when taking into account their magnetic structure and applying a transverse electric field breaking the symmetry between the up and down spins. In devices based on armchair nanoribbons without any magnetic structure, this symmetry breaks when using a ferromagnetic insulator (deposited, for example, on the region between gates) through the proximity effect.     

Spin-polarized tunneling in a ferromagnetic graphene junction: Interplay between the exchange interaction and the orbital effect of the magnetic field

The influence of magnetic vector potential barrier (MVPB) on the spin-polarized transport of massless Dirac particles in ferromagnetic graphene is studied theoretically. The phenomenon of Klein tunneling of relativistic particles across a rectangular potential barrier prevents any of the massless fermions from being confined but they can be electrically confined by quantum dots with integrable dynamics (Bardarson et al., 2009) [36]. Utilization of only the in-plane exchange splitting in the ferromagnetic graphene cannot produce 100% spin polarization. This tunneling can be confined using the magnetic vector potential barrier, which leads to high degree of spin polarization. By combining the orbital effect and the Zeeman interaction in graphene junction, it is found that the junction mimics behavior of half-metallic tunneling junction, in which it acts as a metal to particles of one spin orientation but as an insulator or a semiconductor to those of the opposite orientation. The idea of the half-metallic tunneling junction can provide a source of ∼100% spin-polarized current, which is potentially very useful. Adjustment of the position of the Fermi level in ferromagnetic layer by placing a gate voltage on top of the ferromagnetic layer shows that reverse of the orientation of the completely spin-polarized current passing through the junction is controlled by adjusting the gate voltage. These interesting characteristics should lead to a practical gate voltage controlled spin filtering and spin-polarized switching devices as a perfect spin-polarized electron source for graphene-based spintronics.     

Magnetic ground state of coupled edge-sharing CuO2 spin chains

By means of density functional theory, we investigate the magnetic ground state of edge-sharing CuO2 spin chains, as found in the (La,Ca,Sr)14Cu24O41 system, for instance. Our data rely on spin-polarized electronic structure calculations including on-site interaction [local density approximation plus the multiorbital mean-field Hubbard model (LDA+U)] and an effective model for the interchain coupling. Strong doping dependence of the magnetic order is characteristic for edge-sharing CuO2 spin chains. We determine the ground state magnetic structure as function of the spin-chain filling and quantify the competing exchange interactions.     

First-principle study of magnetism induced by vacancies in graphene

Spin-polarized density functional theory has been used to study the effects of vacancy defects on the magnetic properties of graphene. Structural optimization shows that introducing a carbon vacancy cluster into a graphene sheet changes the spatial distribution of the neighbor atoms, particularly those located around the vacancy. From spin-polarized DOS and LPDOS calculations, we find that only vacancies containing unpaired electrons show magnetism. These results lead us to formulate a relation between the vacancy-induced magnetic moment and the size and shape of the vacancy clusters in graphene sheet.     

Magnetism in quantum dots on graphene-graphane nanoribbons

The magnetic properties of quantum dots on a graphene nanoribbon are investigated in the context of the method of the generalized gradient approximation of spin-polarized density functional theory. It is shown that the antiferromagnetic ordering of magnetic moments is most stable for ribbons with periodic alternation of graphene-graphane nanosegments. When the graphene nanoribbon is separated by the rows of C atoms with hydrogen atoms adsorbed on them in the form of a two-periodic superlattice, the ferrimagnetic ordering in the coupled unequal quantum dots turns out to be most favorable.     

Spin-polarization and x-ray magnetic circular dichroism in GaAs

The combination of angular spin momentum with electronics is a promising successor to charge-based electronics. The conduction bands in GaAs may become spin-polarized via optical spin pumping, doping with magnetic ions, or induction of a moment with an external magnetic field. We investigated the spin populations in GaAs with x-ray magnetic circular dichroism for each of these three cases. We find strong anti-symmetric lineshapes at the Ga L₃ edge indicating conduction band spin splitting, with differences in line width and amplitude depending on the source of spin polarization.     

Structural stability and electronic properties of Ni-doped armchair graphene nanoribbons

The size dependent electronic properties of armchair graphene nanoribbons (AGNR) with Ni doped atoms have been investigated using spin-unrestricted density functional theory. We predict antiferromagnetic (AFM) ground states for Ni-termination and one edge Ni-doping. The computed formation energy reveals that one edge Ni-terminated AGNR are energetically more favourable as compared to pristine ribbons. One edge substitutional doping is energetically more favourable as compared to centre doping by ∼1 eV whereas both edge doping is unfavourable. The bond length of substitutional Ni atoms is shorter than that of Ni adsorption in GNR, implying a stronger binding for substitutional Ni atoms. It is evident that binding energy is also affected by the coordination number of the foreign atom. The results show that Ni-interaction perturbs the electronic structure of the ribbons significantly, causing enhanced metallicity for all configurations irrespective of doping site. The band structures reveal the separation of spin up and down electronic states indicating towards the existence of spin polarized current in Ni-terminated and one edge doped ribbons. Our calculation predicts that AGNR containing Ni impurities can play an important role for the fabrication of spin filters and spintronic devices.     

The spin-dependent transport properties of endohedral transition-metal-fullerene X@C66H4 (X=Fe,Co, Mn,Ni)

Inspired by recent experiments on the successful synthesis of hydrofullerene C₆₆H₄ in Tian et al. (2019) [12] with two negatively curved heptagons. Based on the density functional theory and nonequilibrium Green's function method, we report the spin-dependent transport through transition-metal-atom-encapsulated C₆₆H₄ hydrofullerene, i.e., X@C₆₆H₄(X=Fe, Co, Mn, Ni), contacted by single gold atoms via semi-infinite non-magnetic Au electrodes. It is found that, Mn- and Fe-doped systems show highly spin-polarized transmission as well as considerable magnetic moments whereas Ni-doped systems show completely spin-unpolarized transmission and nonmagnetic. Interestingly, Co-doped systems show two spin states, i.e., spin-polarized and spin-unpolarized ones. Further analysis shows that, for Mn-, Fe- and Ni-doped systems, the spin-polarized/unpolarized state is caused by the finite/(nearly-)zero magnetism of the encapsulated metal atom. While the magnetism in Co-doped systems is quenched for the top hexagonal doping case, but not for the side heptagonal doping one, which induces the spin-unpolarized/spin-polarized state. And the screening effect of electrodes on the magnetism of Co is the underlying physical mechanism. Our findings would be beneficial to the design of spintronics devices.     

Spin Transport in a Magnetic Superlattice with Broken Two-Fold Symmetry

We theoretically investigate the spin-dependent electron transport properties in a magnetic superlattice （MSL） with broken two-fold symmetry. An abnormal barrier in the MSL can break the two-fold symmetry of the system when it is not located at the two-fold symmetry center. A two-fold symmetry breaking factor is introduced to describe the two-fold symmetry breaking degree. Our numerical calculations show that the transmission, the conductance and the spin polarization are non-trivially dependent on the two-fold symmetry breaking factor. When the factor is large enough, the polarization almost approaches 100% in a proper Fermi energy range. However, for two mutually mirror-symmetric MSLs with the same factor, their polarizations may be either similar or distinct. These features provide some clues to the design and applications of MSL-based spin filters or spin-dependent tunneling electron devices.     

Fully spin-polarized transport induced by B doping in graphene nanoribbons

B-doping induced spin polarization in zigzag-edged graphene nanoribbons is studied by density functional calculations by two kinds of doping: (1) doping only one B atom in the central scattering region; (2) periodically doping in the whole system. It is found that even a single B dopant may cause large spin polarization in the current, which can be understood by the breaking of spin-degeneracy due to the impurity atoms and the Fermi level shift resulting from the hole-donating of the B atoms. More interestingly, 100% spin polarized current under finite bias is obtained through periodical doping although the transmission function around the Fermi level is not 100% spin polarized. This can be interpreted by a rigid shift model of the special band structures of the left and right leads in this case. It demonstrates that only transmission function at equilibrium conditions is not sufficient in the study of electron transport, but current should be considered in certain situations.     

Defective graphene and nanoribbons: electronic,magnetic and structural properties

We make use of first-principles calculations, based on the density functional theory(DFT), to investigate the alterations at the structural, energetic, electronic andmagnetic properties of graphene and zigzag graphene nanoribbons (ZGNRs) due to theinclusion of different types of line and punctual defects. For the graphene it is foundthat the inclusion of defects breaks the translational symmetry of the crystal withdrastic changes at its electronic structure, going from semimetallic to semiconductor andmetallic. Regarding the magnetic properties, no magnetization is observed for thedefective graphene. We also show that the inclusion of defects at ZGNRs is a good way tocreate and control pronounced peaks at the Fermi level. Furthermore, defective ZGNRsstructures show magnetic moment by supercell up to 2.0μ_B. For the non defectiveZGNRs is observed a switch of the magnetic coupling between opposite ribbon edges from theantiferromagnetic to the ferrimagnetic and ferromagnetic configurations.