Spin-polarized transport in a normal/ferromagnetic/normal zigzag graphene nanoribbon junction

We investigate the spin-dependent electron transport in single and double normal/ferromagnetic/normal zigzag graphene nanoribbon (NG/FG/NG) junctions. The ferromagnetism in the FG region originates from the spontaneous magnetization of the zigzag graphene nanoribbon. It is shown that when the zigzag-chain number of the ribbon is even and only a single transverse mode is actived, the single NG/FG/NG junction can act as a spin polarizer and/or a spin analyzer because of the valley selection rule and the spin-exchange field in the FG, while the double NG/FG/NG/FG/NG junction exhibits a quantum switching effect, in which the on and the off states switch rapidly by varying the cross angle between two FG magnetizations. Our findings may shed light on the application of magnetized graphene nanoribbons to spintronics devices.     

Valley selection rule in a Y-shaped zigzag graphene nanoribbon junction

The valley valve effect was predicted in a straight zigzag graphene nanoribbon （ZGR） p/n junction. In this work, we address a possible valley selection rule in a Y-shaped ZGR junction. By modeling the system as a three-terminal device and calculating the conductance spectrum, we found that the valley valve effect could be preserved in the system and the Y-shaped connection does not mix the valley index or the pseudoparities of quasiparticles. It is also shown that the Y-shaped ZGR device can be used to separate spins in real space according to the unchanged valley valve effect. Our finding might pave a way to manipulate and detect spins in a multi-terminal graphene-based spin device.     

锯齿型石墨烯纳米窄带中量子霍尔体系的电场调控

应用含自洽格点在位库仑作用的Kane-Mele模型,研究锯齿型石墨烯纳米窄带平面内横向电场对边界带能带结构和量子自旋霍尔(QSH)体系的影响.研究结果显示,当电场强度较弱时,外加电场的方向可以调控自旋向下的两个边界带一起朝不同方向移动,导致波矢q=0.5处自旋向下的两个纯边界态的能量简并劈裂方向可由电场调控;当电场强度进一步增强到超过0.69 V/nm,自旋向下的两个边界带出现较大带隙,能带反转,而自旋向上的电子结构无能隙,系统呈现半金属性,同时QSH体系不再是B类.特别当电场强度为1.17 V/nm时,在自旋向下能带的能隙中,q=0.5处存在自旋向上的纯边界态,意味着在8格点边界处可以产生自旋向上的纯边界电流.当电场强度持续增加时,QSH系统从B类到C类经历3个阶段的变化.当电场强度超过1.42 V/nm后,自旋向上的两个边界带也出现能带反转,分别成为导带和价带,系统成为C类的普通量子霍尔体系.     

Spin-polarized transport in graphene nanoribbon superlattices

By the Green’s function method,we investigate spin transport properties of a zigzag graphene nanoribbon superlattice(ZGNS) under a ferromagnetic insulator and edge effect.The exchange splitting induced by the ferromagnetic insulator eliminates the spin degeneracy,which leads to spin-polarized transport in structure.Spin-dependent minibands and minigaps are exhibited in the conductance profile near the Fermi energy.The location and width of the miniband are associated with the geometry of the ZGNS.In the optimal structure,the spin-up and spin-down minibands can be separated completely near the Fermi energy.Therefore,a wide,perfect spin polarization with clear stepwise pattern is observed,i.e.,the perfect spin-polarized transport can be tuned from spin up to spin down by varying the electron energy.     

Edge magnetization in Bernal-stacked trilayer zigzag graphene nanoribbons

We have used a tight-binding Hamiltonian of an ABA-stacked trilayer zigzag graphene nanoribbon with β-alignment edges to study the edge magnetizations. Our model includes the effect of the intralayer next-nearest-neighbor hopping, the interlayer hopping responsible for the trigonal warping and the interaction between electrons, which is considered by a single band Hubbard model in the mean field approximation. Firstly, in the neutral system we analyzed the two magnetic states in which both edge magnetizations reach their maximum value; the first one is characterized by an intralayer ferromagnetic coupling between the magnetizations at opposite edges, whereas in the second state that coupling is antiferromagnetic. The band structure, the location of the edge-state bands and the local density of states resolved in spin are calculated in order to understand the origins of the edge magnetizations. We have also introduced an electron doping so that the number of electrons in the ribbon unit cell is higher than in neutral case. As a consequence, we have obtained magnetization steps and charge accumulation at the edges of the sample, which are caused by the edge-state flat bands.     

基于石墨烯电极的Co-Salophene分子器件的自旋输运

采用基于非平衡格林函数结合第一性原理的密度泛函理论的计算方法,研究了基于锯齿型石墨纳米带电极的Co-Salophene分子器件的自旋极化输运性质.计算结果表明,当左右电极为平行自旋结构时,自旋向上的电流明显大于自旋向下的电流,自旋向下的电流在[-1V,1V]偏压下接近零,分子器件表现出优异的自旋过滤效应.与此同时,在自旋向上电流中发现负微分电阻效应.当左右电极为反平行自旋结构时,器件表现出双自旋过滤和双自旋分子整流效应.除此之外,整个分子器件还表现出较高的巨磁阻效应.通过分析器件的自旋极化透射谱、局域态密度、电极的能带结构和分子自洽投影哈密顿量,详细解释该分子器件表现出众多特性的内在机理.研究结果对设计多功能分子器件具有重要的借鉴意义.     

Band gap opening in zigzag graphene nanoribbon modulated with magnetic atoms

The effects of magnetic atom on the band structure of zigzag-edged graphene nanoribbons are investigated by the density functional theory. The results show that for narrow zigzag-edged graphene nanoribbons, the band gap can be opened duo to the spin-up/spin-down charges being re-enriched on the edge sites. However, for the wide zigzag-edged graphene nanoribbons, a spin-up/spin-down half-metallic property can be observed. Moreover, it is found that the Seebeck coefficients in the narrow zigzag-edged graphene nanoribbons are reversed and enlarged, which provides a way to design novel thermoelectric device.     

Rectifying performance in zigzag graphene nanoribbon heterojunctions with different edge hydrogenations

Using nonequilibrium Green?s functions in combination with the density functional theory, we investigated the electronic transport behaviors of zigzag graphene nanoribbon (ZGNR) heterojunctions with different edge hydrogenations. The results show that electronic transport properties of ZGNR heterojunctions can be modulated by hydrogenations, and prominent rectification effects can be observed. We propose that the edge dihydrogenation leads to a blocking of electronic transfer, as well as the changes of the distribution of the frontier orbital at negative/positive bias might be responsible for the rectification effects. These results may be helpful for designing practical devices based on graphene nanoribbons.     

硼或氮掺杂的锯齿型石墨烯纳米带的非共线磁序与电子输运性质

基于非共线磁序密度泛函/非平衡格林函数方法,研究了硼或氮掺杂的锯齿型石墨烯纳米带的非共线磁序与电子透射系数.未掺杂的石墨烯纳米带的计算结果表明磁化分布主要遵循类似于Neel磁畴壁的螺旋式磁化分布.相比于未掺杂的情况,硼/氮掺杂的石墨烯纳米带的磁化分布出现了双区域的特征,即杂质原子附近的磁化较小,杂质原子左(右)侧区域的磁化分布更接近于左(右)电极的磁化方向,这为通过掺杂手段在石墨烯纳米带边缘上构建不同磁畴壁提供了可能性.与未掺杂的透射系数不同的是,硼/氮掺杂的石墨烯纳米带的透射系数在费米面附近随着磁化偏转角增大而减小,表明非共线磁序引起的自旋翻转散射占据主导地位.而在E=±0.65 eV处,出现了一个较宽的dip结构,投影电子态密度的分析表明其来源于杂质原子形成的束缚态所引起的背散射.我们的研究结果对于理解石墨烯纳米带中的非共线磁序与杂质散射以及器件设计具有一定的意义.     

Band structure and edge states of star-like zigzag graphene nanoribbons

Connecting three zigzag graphene nanoribbons(ZGNRs) together through the sp~3 hybrid bonds forms a star-like ZGNR(S-ZGNR). Its band structure shows that there are four edge states at k = 0.5, in which the three electrons distribute at three outside edge sites, and the last electron is shared equally(50%) by two sites near the central site. The lowest conductance step in the valley is 2, two times higher than that of monolayer ZGNR(M-ZGNR). Furthermore, in one quasithree-dimensional hexagonal lattice built, both of the Dirac points and the zero-energy states appear in the band structure along the z-axis for the fixed zero k-point in the x-y plane. In addition, it is an insulator in the x-y plane due to band gap 4 eV, however, for any k-point in the x-y plane the zero-energy states always exist at k_z = 0.5.     

Effects of vertical strain on zigzag graphene nanoribbon with a topological line defect

We report the elastic, electronic and magnetic properties of naked ZGNRs with topological line defects (LD-ZGNRs) lying symmetrically on the ribbon's middle under vertical strains at four types of line defect atoms by using a first-principles approach. By changing the position and size of the local deformation of the ribbon, the optimal position is obtained. Moreover, an apparent spin-splitting of the energy band is obtained when a local deformation is created by the vertically applied strain.     

含Stone-Wales缺陷zigzag型石墨烯纳米带的电学和光学性能研究

本文采用基于密度泛函理论的第一性原理对zigzag型石墨烯纳米带中含有不同Stone-Wales缺陷的电子结构特性和光学性能进行研究. 考虑了两种模型:不计电子自旋和考虑电子自旋的情况.研究发现:不计电子自旋情况下,含对称Stone-Wales缺陷的石墨烯纳米带在缺陷区域出现了凹凸不平的折皱构型,两种不同的Stone-Wales缺陷都引起了电荷的重新分布.考虑电子自旋时,Stone-Wales缺陷的引入对石墨烯纳米带自旋密度有显著影响,也引起了不同自旋的电子态密度的变化.进一步研究了纳米带的光学性能,发现 关键词： 石墨烯纳米带 Stone-Wales缺陷 电子结构 光学性能     

金掺杂锯齿型石墨烯纳米带的电磁学特性研究

本文采用基于密度泛函理论的第一性原理计算了金原子填充锯齿型石墨烯纳米带 (ZGNRs)中双空位结构的电磁学特性. 计算结果表明: 边缘位置是金原子的最稳定掺杂位置, 杂质原子的引入导致掺杂边缘的磁性被抑制, 不过掺杂率足够大时, 掺杂边缘的磁性反而恢复了. 金掺杂纳米带的能带结构对掺杂率敏感: 随着掺杂率的增大, 掺杂纳米带分别表现半导体特性、半金属特性以及金属特性. 本文的计算表明金原子掺杂可以调制ZGNR的磁性以及能带特性, 为后续实验起指导作用, 有利于推动石墨烯材料在自旋电子学方面的应用.     

掺杂三角形硼氮片的锯齿型石墨烯纳米带的磁电子学性质

利用基于密度泛函理论的第一性原理方法,研究了三角形BN片掺杂的锯齿型石墨烯纳米带(ZGNR)的磁电子学特性.研究表明:当处于无磁态时,不同位置掺杂的ZGNR都为金属;当处于铁磁态时,随着杂质位置由纳米带的一边移向另一边时,依次可以实现自旋金属-自旋半金属-自旋半导体的变化过程,且只要不在纳米带的边缘掺杂,掺杂的ZGNR就为自旋半金属;当处于反铁磁态时,在中间区域掺杂的ZGNR都为自旋金属,而在两边缘掺杂的ZGNR没有反铁磁态.掺杂ZGNR的结构稳定,在中间区域掺杂时反铁磁态是基态,而在边缘掺杂时铁磁态为基态.研究结果对于发展基于石墨烯的纳米电子器件具有重要意义.     

氢化与非氢化石墨烯纳米条带的密度泛函研究

基于密度泛函理论的第一性原理计算方法,研究了宽度N=8的边缘氢化和非氢化条带的结构和电子性质. 研究表明,扶手形无氢化石墨纳米条带的边缘碳原子是以三重键相互结合,它在边缘的成键强度比氢化时要高,具有更强的化学活性,可作为纳米化学传感器的基础材料. 能带结构计算表明,无论是扶手形条带还是锯齿形条带,它们都是具有带隙的半导体,且无氢化条带的带隙要比氢化的条带带隙宽度大,氢化对于条带的电子性质具有显著修饰作用. 通过锯齿形石墨纳米条带顺磁性、铁磁性和反铁磁性的计算,发现反铁磁的状态最稳定,并且边缘磁性最强,这有利于条带在自旋电子器件中的应用. 关键词： 石墨纳米条带 成键机理 电子结构 自旋分布     

含单排线缺陷锯齿型石墨烯纳米带的电磁性质

基于密度泛函理论的第一性原理方法,研究了含单排线缺陷锯齿型石墨烯纳米带(ZGNR)的电磁性质,主要计算了该缺陷处于不同位置时的能带结构、透射谱、自旋极化电荷密度、总能以及布洛赫态.研究表明,含单排线缺陷的ZGNR和无缺陷的ZGNR在非磁性态和铁磁态下都为金属.虽然都为金属,但其呈金属性的成因有差异.在反铁磁态下,单排线缺陷越靠近ZGNR的边缘,对ZGNR电磁性质的影响越明显,缺陷由ZGNR对称轴线向边缘移动过程中,含单排线缺陷的ZGNR有一个半导体-半金属-金属的相变过程.虽然线缺陷靠近中线的ZGNR为半导体,但由于缺陷引入新的能带,导致含单排线缺陷的ZGNR的带隙小于无缺陷ZGNR的带隙.单排线缺陷紧邻边界时,含缺陷ZGNR最稳定;单排线缺陷位于次近邻边界位置时,含缺陷ZGNR最不稳定.在反铁磁态下,对单排线缺陷位于对称轴线的ZGNR施加适当的横向电场,可以实现半导体到半金属的转变.这些研究结果对于发展基于石墨烯的纳米电子器件有重要的意义.     

Tailoring atomic structure to control the electronic transport in zigzag graphene nanoribbon

We have performed ab initio   density functional theory calculation to study the electronic transport properties of the tailored zigzag-edged graphene nanoribbon (ZGNR) with particular electronic transport channels. Our results demonstrated that tailoring the atomic structure had significantly influenced the electronic transport of the defective nanostructures, and could lead to the metal-semiconducting transition when sufficient atoms are tailored. The asymmetric I–V$I\text{–}V$ characteristics as a result of symmetry breaking have been exhibited, which indicates the route to utilize GNR as a basic component for novel nanoelectronics.     

含带隙石墨烯纳米带的自旋筛选输运

利用Landauer-Büttiker公式和非平衡格林函数方法,研究了在电荷和自旋偏压共同作用下的扶手椅型石墨烯纳米带的自旋相关的电子输运性质. 当系统存在两种偏压时,不用自旋的电子具有不同的偏压窗口. 同时,含带隙石墨烯纳米带具有与自旋无关的导电电压阈值. 通过设置适当的两种偏压值,系统可以产生易于调节的单一自旋的电流.     

Structural defects influence on the conductance of strained zigzag graphene nanoribbon

In this paper, we investigate the influence of point structural defects on the transport properties of zigzag graphene nanoribbons (ZGNRs) under uniaxial strain field, using the numerical studies based on the ab-initio calculation, the standard tight-binding model and Green's functions. The calculation results show that the direction of applied strain and defect type significantly affect the conductance properties of ZGNRs. The conductance of the defective nanoribbons generally decreases and some dips corresponding to complete electron backscattering is appeared. This behavior is originated from the different coupling between the conducting electronic states influenced by the wave function modification around the Fermi energy which depends on the defect type. We show that the presence of defects leads to a significant increase in local current. Furthermore, we have investigated the strain-tunable spin transport of defective ZGNRs in the presence of the exchange magnetic field and Rashba spin-orbit coupling (RSOC).     

Spin transport in a Zigzag normal/ferromagnetic graphene junction

We study magnetic proximity effect induced low-energy spin transport in the normal/ferromagnetic junction of a semi-infinite zigzag graphene nanoribbon. Due to the absence of a spin flip in a single interface, the spin transfer in this model can be described by the two-spin channel model. We identify each spin channel as either a perfect conducting or a non-conducting channel. This feature leads to spin filter in symmetric zigzag graphene nanoribbon and spin precession in antisymmetric zigzag graphene nanoribbon, and helps to directly determine the exchange-splitting intensity directly, even without an external auxiliary bias.