A comparison of the transport properties of bilayer graphene,monolayer graphene,and two-dimensional electron gas

We studied and compared the transport properties of charge carriers in bilayer graphene, monolayer graphene, and the conventional semiconductors (the two-dimensional electron gas (2DEG)). It is elucidated that the normal incidence transmission in the bilayer graphene is identical to that in the 2DEG but totally different from that in the monolayer graphene. However, resonant peaks appear in the non-normal incidence transmission profile for a high barrier in the bilayer graphene, which do not occur in the 2DEG. Furthermore, there are tunneling and forbidden regions in the transmission spectrum for each material, and the division of the two regions has been given in the work. The tunneling region covers a wide range of the incident energy for the two graphene systems, but only exists under specific conditions for the 2DEG. The counterparts of the transmission in the conductance profile are also given for the three materials, which may be used as high-performance devices based on the bilayer graphene.     

Electronic transport of bilayer graphene with asymmetry line defects

In this paper,we study the quantum properties of a bilayer graphene with(asymmetry) line defects.The localized states are found around the line defects.Thus,the line defects on one certain layer of the bilayer graphene can lead to an electric transport channel.By adding a bias potential along the direction of the line defects,we calculate the electric conductivity of bilayer graphene with line defects using the Landauer-Biittiker theory,and show that the channel affects the electric conductivity remarkably by comparing the results with those in a perfect bilayer graphene.This one-dimensional line electric channel has the potential to be applied in nanotechnology engineering.     

Effect of a nonuniform magnetic field on the Landau states in a biased AA-stacked graphene bilayer

We study the Landau states in the biased AA-stacked graphene bilayer under an exponentially decaying magnetic field along one spatial dimension. The results show that the energy eigenvalues of the system are strongly dependent on the inhomogeneity of the magnetic field and the bias voltage between the graphene layers, and in particular the reordering and mixing of finite Landau states could occur. Moreover, we also demonstrate that the current carrying states induced by the decaying magnetic field propagate vertically to the magnetic-field gradient within the graphene sample and can be further modulated by the bias voltage between the layers.     

Dissimilar stability of proteins in graphene bilayer: a molecular dynamics study

In order to study protein stability on graphene surface and in a confined space simultaneously, two parallel graphene single layers were built and two structurally dissimilar protein molecules were placed in between. Molecular dynamics simulation results showed a significant denaturing effect of graphene layers on GA module, a 3-α helices bundle protein, while another α/β structure protein, protein G, kept its NMR structure intact throughout all simulations. Such extremely different denaturation behaviours of the proteins offer a good chance to investigate the mechanism of graphene toxicity. Further analysis showed Van der Waals interaction could be the main cause of the denaturing effect. Although the solvation effect can contribute, its contribution is not comparable with the Van der Waals interaction.     

Theoretical investigation of structural and optical properties of semi-fluorinated bilayer graphene

We have studied the structural and optical properties of semi-fluorinated bilayer graphene using density functional theory. When the interlayer distance is 1.62 , the two graphene layers in AA stacking can form strong chemical bonds.Under an in-plane stress of 6.8 GPa, this semi-fluorinated bilayer graphene becomes the energy minimum. Our calculations indicate that the semi-fluorinated bilayer graphene with the AA stacking sequence and rectangular fluorinated configuration is a nonmagnetic semiconductor(direct gap of 3.46 e V). The electronic behavior at the vicinity of the Fermi level is mainly contributed by the p electrons of carbon atoms forming C=C double bonds. We compare the optical properties of the semifluorinated bilayer graphene with those of bilayer graphene stacked in the AA sequence and find that the semi-fluorinated bilayer graphene is anisotropic for the polarization vector on the basal plane of graphene and a red shift occurs in the [010]polarization, which makes the peak at the low-frequency region located within visible light. This investigation is useful to design polarization-dependence optoelectronic devices.     

Transport properties in monolayer–bilayer–monolayer graphene planar junctions

The transport study of graphene based junctions has become one of the focuses in graphene research. There are two stacking configurations for monolayer–bilayer–monolayer graphene planar junctions. One is the two monolayer graphene contacting the same side of the bilayer graphene, and the other is the two-monolayer graphene contacting the different layers of the bilayer graphene. In this paper, according to the Landauer–Büttiker formula, we study the transport properties of these two configurations. The influences of the local gate potential in each part, the bias potential in bilayer graphene,the disorder and external magnetic field on conductance are obtained. We find the conductances of the two configurations can be manipulated by all of these effects. Especially, one can distinguish the two stacking configurations by introducing the bias potential into the bilayer graphene. The strong disorder and the external magnetic field will make the two stacking configurations indistinguishable in the transport experiment.     

Electrically controllable spin transport in bilayer graphene with Rashba spin-orbit interaction

We study the spin transport in bilayer graphene nanoribbons (BGNs) in the presence of Rashba spin-orbit interaction (SOI) and external gate voltages. It is found that the spin polarization can be significantly enhanced by the interlayer asymmetry or longitudinal mirror asymmetry produced by external gate voltages. Rashba SOI alone in BGNs can only generate current with spin polarization along the in-plane y direction, but the polarization components can be found along the x, y and z directions when a gate voltage is applied. High spin polarization with flexible orientation is obtained in the proposed device. Our findings shed new light on the generation of highly spin-polarized current in BGNs without external magnetic fields, which could have useful applications in spintronics device design.     

Projective representation of D_6 group in twisted bilayer graphene

Within the framework of continuum model, we study the projective representation of emergent D_6 point group in twisted bilayer graphene. We then construct tight-binding models of the lowest bands without and with external electromagnetic fields, based on the projective representation.     

Interlayer transport of electron in bilayer graphene with phonon-induced lattice distortion in the presence of biased potential

The interlayer transport of electron in bilayer graphene influenced by phonon in the presence of biased potential is investigated using the tight-binding approach. The in-plane optical mode E_2g and out-of-plane optical mode B_1g associated with the applied biased potential are considered to compute and discuss the interlayer transport probability of an electron initially localized on the bottom layer at the Dirac point in the Brillouin zone. Without the biased potential, the interlayer transport probability is equal to 0.5 regardless of the phonon displacement except for a few special cases. Applying a biased potential to the layers, we find that in different phonon mode the function of the transport probability with respect to applied biased potential and phonon displacement is complex and various, but on the whole the transport probability decreases with the increase in the absolute value of the applied biased potential. These phenomena are discussed in detail in this paper.     

Electric-field-induced plasmon in AA-stacked bilayer graphene

The collective excitations in AA-stacked bilayer graphene for a perpendicular electric field are investigated analytically within the tight-binding model and the random-phase approximation. Such a field destroys the uniform probability distribution of the four sublattices. This drives a symmetry breaking between the intralayer and interlayer polarization intensities from the intrapair band excitations. A field-induced acoustic plasmon thus emerges in addition to the strongly field-tunable intrinsic acoustic and optical plasmons. At long wavelengths, the three modes show different dispersions and field dependence. The definite physical mechanism of the electrically inducible and tunable mode can be expected to also be present in other AA-stacked few-layer graphenes.     

Observation of quadratic magnetoresistance in twisted double bilayer graphene

Magnetoresistance ({MR}) provides rich information about Fermi surface, carrier scatterings, and exotic phases for a given electronic system. Here, we report a study of the magnetoresistance for the metallic states in twisted double bilayer graphene (TDBG). We observe quadratic magnetoresistance in both Moiré valence band (VB) and Moiré conduction band (CB). The scaling analysis shows validity of Kohler's rule in the Moiré valence band. On the other hand, the quadratic magnetoresistance appears near the halo structure in the Moiré conduction band, and it violates Kohler's rule, demonstrating the {MR} scaling related to band structure in TDBG. We also propose an alternative scaling near the halo structure. Further analysis implies that the observed quadratic magnetoresistance and alternative scaling in conduction band are related to the halo boundary. Our results may inspire investigation on {MR} in twisted 2D materials and provide new knowledge for {MR} study in condensed matter physics.     

Strain induced topological transitions in twisted double bilayer graphene

We theoretically study the band structures and the valley Chern numbers of the AB–AB and AB–BA stacked twisted double bilayer graphene under heterostrain effect. In the absence of heterostrain, due to the constrains by the spatial symmetries, the central two flat bands of the AB–AB are topological trivial bands, while in the AB–BA they have a finite Chern number. The heterostrain breaks all the point group symmetries and the constrains are lifted, hence the topological properties of the two arrangements can be tuned by different strain magnitudes ϵ and directions ϕ. The heterostrain has dissimilar impacts on the Chern numbers of the AB–AB and AB–BA, owing to their different band gaps, and these gaps can be modified by a vertical electric field. Our results show that the topological transitions for both arrangements occur in the ϵ range of 0.1%–0.4%, which can be realized in the graphene-based sample.     

Interlayer transport of an electron in bilayer graphene with phonon-induced lattice distortion in the presence of biased potential

The interlayer transport of an electron in bilayer graphene influenced by a phonon in the presence of a biased potential is investigated using the tight-binding approach. The in-plane optical mode E2g and out-of-plane optical mode B1g associated with the applied biased potential are considered to compute and discuss the interlayer transport probability of an electron initially localized on the bottom layer at the Dirac point in the Brillouin zone. Without the biased potential, the interlayer transport probability is equal to 0.5 regardless of the phonon displacement except for a few special cases. Applying a biased potential to the layers, we find that in different phonon modes the function of the transport probability with respect to the applied biased potential and phonon displacement is complex and various, but on the whole the transport probability decreases with the increase in the absolute value of the applied biased potential. These phenomena are discussed in detail in this paper.     

The electronic transport properties of defected bilayer sliding armchair graphene nanoribbons

By applying non-equilibrium Green's functions (NEGF) in combination with tight-binding (TB) model, we investigate and compare the electronic transport properties of perfect and defected bilayer armchair graphene nanoribbons (BAGNRs) under finite bias. Two typical defects which are placed in the middle of top layer (i.e. single vacancy (SV) and stone wale (SW) defects) are examined. The results reveal that in both perfect and defected bilayers, the maximum current refers to β-AB, AA and α-AB stacking orders, respectively, since the intermolecular interactions are stronger in them. Moreover it is observed that a SV decreases the current in all stacking orders, but the effects of a SW defect is nearly unpredictable. Besides, we introduced a sequential switching behavior and the effects of defects on the switching performance is studied as well. We found that a SW defect can significantly improve the switching behavior of a bilayer system. Transmission spectrum, band structure, molecular energy spectrum and molecular projected self-consistent Hamiltonian (MPSH) are analyzed subsequently to understand the electronic transport properties of these bilayer devices which can be used in developing nano-scale bilayer systems.     

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.     

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.     

Electric- and magnetic-field-tuned Landau levels and Hall conductivity in AA-stacked bilayer graphene

We theoretically study the combined effect of magnetic and electric fields on the Landau levels and Hall conductivity in AA-stacked bilayer graphene. From the analytic expressions derived, we obtain explicit criterions for determining the zero-energy Landau level and different level crossings in the graphene bilayer. For providing a scheme of experimental verification, we further explore the quantum Hall effect in such a biased bilayer. It is found that the zero-conductance Hall plateau in this system can vanish at certain specific combinations of magnetic and electric fields, accompanying with the occurrence of resonance Hall conductivity steps.     

L型石墨纳米结的热输运

利用非平衡格林函数方法研究了由半无限长扶手椅型和锯齿型边界石墨纳米带连接而成的L型石墨纳米结的热输运性质.结果表明,L型石墨纳米结的热导依赖于L型石墨纳米结的夹角和石墨纳米带的宽度.在L型石墨纳米结的夹角从30°增加到90°再增加到150°过程中,其热导显著增大.夹角为90°的L型石墨纳米结的热导随着扶手椅型纳米带宽度增加时,在低温区热导随着宽度的增大而降低,在高温区热导随宽度的增大而升高.对于夹角为150°的L型石墨纳米结,其热导无论是在低温区还是在高温区都随着锯齿型纳米带宽度的增加而降低.利用声子透射谱对这些热输运现象进行了合理的解释.研究结果阐明了不同L型石墨纳米结中的热输运机理,为设计基于石墨纳米结的热输运器件提供了重要的物理模型和理论依据. 关键词： 石墨纳米结 热输运 热导     

垂直电场下扭转双层石墨烯光学吸收性质的理论研究

层间扭转角度是对石墨烯物理性质宽波段可调谐的一个新参量.本文采用2°<θ<15°扭转角度下的连续近似模型,获得了不同扭转角度双层石墨烯分别在有、无电场下的能带结构,通过电子-光子相互作用跃迁速率,计算模拟了范霍夫奇点附近电子带内跃迁和带间跃迁所引起的光学吸收谱.结果表明,在无外加电场时,带间跃迁吸收峰的位置随着扭转角度的增大而发生从红外到可见光波段的蓝移,且吸收系数增大,带内跃迁的光学吸收系数相对于带间跃迁高出2个数量级;而存在外加电场时,两个范霍夫奇点在波矢空间的位置发生偏移,带间跃迁吸收峰发生分裂,且两个分裂的吸收峰位置随着电场强度的不断增大而反向行进.上述研究结果对石墨烯材料在光电器件方面的应用有一定指导作用.