石墨烯量子磁输运性质的电场效应

最近研究发现石墨烯在一维周期性电学或磁学调制势下, 其扩散电导率会出现Weiss振荡. 本文进一步探索了面外加垂直磁场和面内加横向电场以及一维周期性弱调制电学势的多场耦合作用下, 石墨烯的量子磁输运性质, 结果表明: Weiss振荡振幅和电导率数值都随着静电场的增加而增加. 有趣的是, 当电场与磁场的比值达到某一临界值, 即时, 输运电导率的Weiss振荡突然消失. 这一奇特现象在传统的二维电子气体中是不存在的, 因此可以归因于石墨烯载流子外加电磁场的反常相对论性能谱. 关键词：石墨烯; 静电场; Weiss振荡; 磁输运性质     

石墨烯量子磁输运性质的电场效应

最近研究发现石墨烯在一维周期性电学或磁学调制势下, 其扩散电导率会出现Weiss振荡. 本文进一步探索了面外加垂直磁场和面内加横向电场以及一维周期性弱调制电学势的多场耦合作用下, 石墨烯的量子磁输运性质, 结果表明: Weiss振荡振幅和电导率数值都随着静电场的增加而增加. 有趣的是, 当电场与磁场的比值达到某一临界值, 即时, 输运电导率的Weiss振荡突然消失. 这一奇特现象在传统的二维电子气体中是不存在的, 因此可以归因于石墨烯载流子外加电磁场的反常相对论性能谱. 关键词：石墨烯; 静电场; Weiss振荡; 磁输运性质     

锯齿形石墨烯反点网络加工与输运性质研究

具有特定边界的石墨烯纳米结构在纳电子学、自旋电子学等研究领域表现出良好的应用前景.然而石墨烯加工成纳米结构时,无序的边界不可避免地会降低其载流子迁移率.氢等离子体各向异性刻蚀技术是加工具备完美边界石墨烯微纳结构的一项关键技术,刻蚀后的石墨烯呈现出规则的近原子级平整的锯齿形边界.本文研究了氮化硼上锯齿形边界石墨烯反点网络的磁输运性质,低磁场下可以观测到载流子围绕着一个空位缺陷运动时的公度振荡磁阻峰.随着磁场的增大,朗道能级简并度逐渐增大,载流子的磁输运行为从Shubnikov-de Haas振荡逐渐向量子霍尔效应转变.在零磁场附近可以观测到反点网络周期性空位缺陷的边界散射所导致的弱局域效应.研究结果表明,在氮化硼衬底上利用氢等离子体刻蚀技术加工锯齿形边界石墨烯反点网络,其样品质量会明显提高,这种简单易行的方法为后续高质量石墨烯反点网络的输运研究提供了新思路.     

电光调制对外部光注入垂直腔表面发射激光器的偏振转换及其非线性动力学行为的操控性研究

针对主和副垂直腔表面发射激光器构成的外部注入激光器系统的偏振转换及其非线性动力学行为, 利用周期性极化铌酸锂晶体中准相位匹配线性电光调制, 本文提出了一种新的操控方案并且探索了其控制规律. 研究结果发现, 受到平行光注入或正交光注入的副激光器输出偏振度随外加电场成周期性振荡变化, 其振荡波峰轨迹包络曲线为正弦曲线, 而振荡波谷轨迹包络曲线为余弦曲线; 选取一定的主激光器偏置电流, 通过对来自主激光器的光进行电光调制, 受到两种方式注入的副激光器可以输出任意偏振模, 并且其非线性动力行为经历不同的演变. 另外, 副激光器的偏振度仅依赖于外加电场, 与副激光器的偏置电流无关.     

变形及电场作用对石墨烯电学特性影响的第一性原理计算

利用基于密度泛函理论的第一性原理方法,系统研究了变形、电场及共同作用对石墨烯电学特性影响的电子机理.研究表明,本征石墨烯的能隙及态密度值在费米能级处均为0,呈现出半金属特性;在一定的变形量下对石墨烯施加剪切、拉伸、扭转及弯曲变形作用,发现剪切和扭转变形对打开石墨烯能隙的作用明显;对本征石墨烯施加不同方向的电场,可知010电场方向对打开石墨烯能隙的作用效果最强.这是因为该电场方向下石墨烯C-C原子间的布居数正值较大,成键键能较高,而负值数值较小,反键键能较低;线性增加电场强度,石墨烯的能隙呈线性增长势;变形及电场共同作用下,外加电场提高了变形对打开石墨烯能隙的作用效果,但不及两种外场叠加的作用效果.     

光子辅助Fano共振隧穿周期双阱势特性（英文）

周期双阱势的光学性质是激光物理和量子光学的前沿研究领域之一。该文研究了具有时间周期双阱势的石墨烯系统中光子辅助狄拉克电子的Fano型共振隧穿。利用双量子阱结构,电子通过两量子阱之间的薄势垒的共振隧穿将导致束缚态能级的分裂,Fano型共振谱将分裂为两个不对称共振峰。通过改变相位、频率和振幅来调制Fano峰的形状,可以用来调制Dirac在石墨烯中的电子输运性质。数值分析表明,两个振荡场的相对相位可以调节非对称Fano型共振峰的形状。当相对相位从0增加到π时,共振峰谷从峰的一侧移到另一侧;在临界相位3π/11处,不对称共振峰变得对称。此外,还可以通过改变振荡场的频率和振幅以及静态势阱的结构来调制Fano峰的分布。这些有趣的物理性质可以用来调节石墨烯中Dirac的电子输运性质。     

外加轴向磁场下碳纳米管场效应晶体管的电学性质

在紧束缚理论的基础上推导出轴向磁场下碳纳米管的能带公式,研究外加磁场下碳纳米管场效应晶体管的电学特性.说明磁场可使碳管的导电性质在金属型和半导体型之间转变,转变的磁场周期为0.5₀.进一步应用场效应晶体管Natori理论模拟计算了外加磁场对碳纳米管场效应晶体管的电流-电压特性的影响,研究结果显示zigzag管和armchair管的电流随外电压和磁场都有振荡行为,而且两类管的振荡行为有明显差别. 关键词： 碳纳米管 紧束缚理论 费米能 能带结构     

石墨烯莫尔超晶格

石墨烯莫尔超晶格来源于六方氮化硼衬底对石墨烯的二维周期势调控. 由于这种外加的周期势对石墨烯能带具有显著的调制作用, 近年来引发了人们广泛的关注. 利用氮化硼衬底上外延的单晶石墨烯薄膜, 我们系统研究了基底调制下的莫尔超晶格以及相关的物理特性. 首先, 我们在电子端和空穴端都观测到了超晶格狄拉克点, 并且超晶格狄拉克点同本征狄拉克点类似, 都表现出绝缘体的特性. 在低温强磁场下, 可以观测到到单层石墨烯和双层石墨烯的量子霍尔效应. 并且, 从朗道扇形图中, 可以清晰的看到磁场下形成的超晶格朗道能级. 此外, 利用红外光谱的方法研究了强磁场下石墨烯超晶格体系不同朗道能级之间的跃迁, 发现这种跃迁满足有质量狄拉克费米子的行为, 对应38 meV的本征能隙. 在此基础上, 我们在380 meV位置发现一个同超晶格能量对应的光电导峰. 通过利用旋量势中三个不同的势分量对光电导峰进行拟合, 发现赝自旋杂化势起主导作用. 进一步研究表明赝自旋杂化势强度随载流子浓度的增大显著降低, 表明电子-电子相互作用引起的旋量势的重构.     

锯齿型石墨纳米带叠层复合结的电子输运

基于紧束缚格林函数方法,研究了两半无限长锯齿型石墨纳米带叠层复合结的电子输运性质.结果表明,层间次近邻相互作用、叠层区长度及门电压对复合结的电子透射谱有重要调制作用.层间次近邻相互作用导致复合结的透射谱关于费米能呈现非对称性,与实验结果很好相符.低于费米能第一子能区内周期性出现透射系数为0和1的台阶,呈现全反射与透射现象.随散射结长度增加,透射系数在1内周期性振荡,呈现明显的量子干涉效应.在门电压调控下,低于费米能的透射系数出现了从1到0的转变,类似于开关效应.相关结果对基于石墨烯器件的设计与应用有指导意义.     

He离子辐照对石墨烯微观结构及电学性能的影响

本文采用5.4 ke V不同剂量的He离子辐照单层石墨烯,利用X射线光电子能谱(XPS)、拉曼光谱(Raman)和半导体参数分析仪表征辐照前后石墨烯微观结构和电学性能变化.研究结果表明:随着辐照剂量增大,单层石墨烯的缺陷密度逐渐增加,当辐照剂量增至1.6×10~(13) He~+/cm~2,石墨烯开始由纳米晶结构向无定形碳结构转变,不断增多的缺陷致使石墨烯电导率持续降低,其电子输运机制也由玻尔兹曼扩散输运转变为跃迁输运;狄拉克电压(V_(dirac))向正电压方向的偏移量随辐照剂量增大而增大,其主因是辐照缺陷和吸附杂质导致石墨烯P型掺杂效应增强.     

Temperature effects on the magnetoplasmon spectrum of a weakly modulated graphene monolayer

In this work, we determine the effects of temperature on the magnetoplasmon spectrum of an electrically modulated graphene monolayer as well as a two-dimensional electron gas (2DEG). The intra-Landau band magnetoplasmon spectrum within the self-consistent field approach is investigated for both the aforementioned systems. Results obtained not only exhibit Shubnikov-de Haas (SdH) oscillations but also commensurability oscillations (Weiss oscillations). These oscillations are periodic as a function of inverse magnetic field. We find that both the magnetic oscillations, SdH and Weiss, have a greater amplitude and are more robust against temperature in graphene compared to a conventional 2DEG. Furthermore, there is a π phase shift between the magnetoplasmon oscillations in the two systems which can be attributed to Dirac electrons in graphene acquiring a Berry's phase as they traverse a closed path in a magnetic field.     

Magnetotransport through ac driven ferromagnetic graphene nanoribbons

We present a theoretical study on the spin-dependent transport through the ferromagnetic graphene nanoribbons in the presence of a magnetic and an in-plane ac electric field, and find that when the ac field is applied, in the two-terminal ferromagnetic graphene device, for the parallel configurations of the electrodes' magnetizations, the width of the even-number conductance plateaus decrease, the new conductance plateaus appear at the odd-number positions, and the even-number conductance plateaus at the high energy are quenched under the sufficiently strong ac field. In contrast, for the antiparallel configuration of the electrodes' magnetizations, the odd-plateaus of the conductance shrink, and the new plateaus developed at the even-number positions. The magnetic resistance exhibits a successive rectangular-like oscillation structure close to the band edge, whereas experiences an alternative transition between the sharp peak and dip near the zero energy with increasing the ac field strength. In the six-terminal ferromagnetic graphene device, the variations of the longitudinal and Hall resistances' plateaus as well as the addition of the new quantized plateaus with the rise of the ac field strength are also revealed.     

Thermodynamic properties of a magnetically modulated graphene monolayer

The effect of magnetic modulation on thermodynamic properties of a graphene monolayer in the presence of a constant perpendicular magnetic field is reported here. One-dimensional spatial electric or magnetic modulation lifts the degeneracy of the Landau levels and converts into bands and their bandwidth oscillates with magnetic field, leading to Weiss-type oscillations in the thermodynamic properties. The effect of magnetic modulation on the thermodynamic properties of a graphene sheet is studied and then compared with electrically modulated graphene and magnetically modulated conventional two-dimensional electron gas (2DEG). We observe Weiss-type and de Haas-van Alphen oscillations at low and high magnetic fields, respectively. There is a definite phase difference in Weiss-type oscillations in thermodynamic quantities of magnetically modulated graphene compared to electrically modulated graphene. On the other hand, the phase remains the same and the amplitude of the oscillation is large when compared with the magnetically modulated two-dimensional electron gas (2DEG). Explicit asymptotic expressions of the density of states and the Helmholtz free energy are provided to understand the phase and amplitude of the Weiss-type oscillations qualitatively. We also study thermodynamic properties when both electric and magnetic modulations are present. The Weiss-type oscillations still exist when the modulations are out-of-phase.     

Electronic Bloch oscillation in a pristine monolayer graphene

In a pristine monolayer graphene subjected to a constant electric field along the layer, the Bloch oscillation of an electron is studied in a simple and efficient way. By using the electronic dispersion relation, the formula of a semi-classical velocity is derived analytically, and then many aspects of Bloch oscillation, such as its frequency, amplitude, as well as the direction of the oscillation, are investigated. It is interesting to find that the electric field affects the component of motion, which is non-collinear with electric field, and leads the particle to be accelerated or oscillated in another component.     

Magnetothermoelectric transport in modulated and unmodulated graphene

We draw motivation from recent experimental studies and present a comprehensive study of magnetothermoelectric transport in a graphene monolayer within the linear response regime. We employ the modified Kubo formalism developed for thermal transport in a magnetic field. Thermopower as well as thermal conductivity as a function of the gate voltage of a graphene monolayer in the presence of a magnetic field perpendicular to the graphene plane is determined for low magnetic fields (～1 T) as well as high fields (～8 T). We include the effects of screened charged impurities on thermal transport. We find good qualitative and quantitative agreement with recent experimental work on the subject. In addition, in order to analyze the effects of modulation, which can be induced by various means, on the thermal transport in graphene, we evaluate the thermal transport coefficients for a graphene monolayer subjected to a periodic electric modulation in a magnetic field. The results are presented as a function of the magnetic field and the gate voltage.     

Corrugated graphene: effects of in-plane and tilted out-of-plane magnetic fields

The electronic energy level structure of the corrugated graphene electron subjected to a magnetic field tilted with respect to the graphene plane and an in-plane homogeneous magnetic field is investigated theoretically within the perturbation framework. It is shown that the anisotropy induced by the tilted magnetic field strongly modifies the Fermi velocity of the corrugated graphene electron, and the corrugated structure yields intrinsic Weiss oscillations in both Fermi velocity and the graphene Landau levels.     

Novel electric field effects on Landau levels in graphene

A new effect in graphene in the presence of crossed uniform electric and magnetic fields is predicted. Landau levels are shown to be modified in an unexpected fashion by the electric field, leading to a collapse of the spectrum, when the value of electric to magnetic field ratio exceeds a certain critical value. Our theoretical results, strikingly different from the standard 2D electron gas, are explained using a "Lorentz boost," and as an "instability of a relativistic quantum field vacuum." It is a remarkable case of emergent relativistic type phenomena in nonrelativistic graphene. We also discuss few possible experimental consequence.