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β_1=E/(ν_F·B)=1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.     

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     

三角形石墨烯量子点阵列的磁电子学特性和磁输运性质

基于密度泛函理论的第一性原理计算方法,研究了三角形石墨烯纳米片用不同连接方式拼接而成的四种一维量子点阵列(1D QDAs)的磁电子学性质和磁输运性质.结合能计算表明所有1D QDAs是非常稳定的.特别是研究发现1D QDAs的电子和磁性质不仅依赖于磁性态,也明显依赖于连接方式,如在无磁态时,不同量子点阵列(QDAs)可为金属或窄带隙半导体.在铁磁态时,不同QDAs能为半金属(half-metal)或带隙不同的双极化磁性半导体.而在反铁磁态时,不同QDAs为带隙不等的半导体.这些结果意味着连接方式对有效调控纳米结构电子和磁性质扮演重要的角色.1D QDAs呈现的半金属或双极化磁性半导体性质对于发展磁器件是非常重要的,而这些性质未曾在本征石墨烯纳米带中出现.同时,我们也研究了一种阵列的磁器件特性,发现其拥有完美的(100%)单或双自旋过滤效应,尤其是呈现超过109%的巨磁阻效应.     

Fe位Al掺杂对Sr2FeMoO6磁结构和磁输运性质的影响

研究了Sr2Fe1-xAlxMoO6(0≤x≤0.30)系列多晶样品的磁学和输运性质.室温X射线衍射谱图的精修结果显示Al3+掺杂没有改变样品的晶格结构,但提高了Sr2FeMoO6晶格的阳离子有序度.5K时样品的磁化曲线说明平均单位分子饱和磁矩随着Al含量的增加而下降,但平均单位Fe离子磁矩却逐渐提高.磁化曲线的拟合结果显示样品内反铁磁相互作用对饱和磁矩的贡献随着Al含量的增加而下降,说明一定量的Fe离子被Al替代后,抑制了样品内Fe-O-Fe反相边界的形成,从而提高了Sr2FeMoO6晶格的阳离子有序度和平均单位Fe离子磁矩.对饱和磁矩的分析表明非磁性Al3+离子掺杂会形成无磁相互作用的Mo-O-Al-O-Mo区,可以将原来较大的Mo-O-Fe亚铁磁区分割成许多小的区域,并且使这些亚铁磁区间的磁耦合作用变弱,从而提高了低场磁电阻效应.阳离子有序度的提高使来源于自旋相关电子在反相边界处散射的高场磁电阻明显降低,导致了样品的磁电阻在x=0.15时达到了最大值.     

耦合电磁场对石墨烯量子磁振荡的影响

我们研究了包含自旋轨道耦合与杂质散射在内的石墨烯量子磁振荡对外加电磁场的响应.我们发现,石墨烯中自旋轨道耦合、电磁场以及边界共同修正了朗道能谱,且当电场与磁场比值超过某一临界值时,量子磁振荡会突然消失,这与非相对论二维电子气的情况显著不同.这种现象可以通过朗道量子化轨道由封闭转化为开放的半经典理论来解释.此外,我们还发现杂质散射和温度的共同作用会使得磁振荡振幅衰减.我们的结果可用于分析石墨烯及其类似结构(硅烯、锗烯、锡烯等)的费米能级与朗道能谱的相互作用,进而探测自旋轨道耦合引起的能隙.     

缺陷对准周期磁超晶格输运性质的影响

研究了缺陷结构的Fibonacci磁超晶格的输运性质.结果表明,缺陷结构对纳米准周期链的电子输运性质有重要影响,特别是低能区电子输运 关键词： 缺陷 Fibonacci链 磁纳米结构 输运性质     

半磁半导体的输运性质和杂质态

半磁半导体是一类新型的半导体材料．本文主要阐述ｓｐ－ｄ交换相互作用对半磁半导体（ＳＭＳｃ）中输运特性和杂质行为的影响，讨论了量子输运特性，巨大负磁阻效应，磁场感生的金属─绝缘体转变等一系列新的物理效应以及束缚磁极化子的形成．     

N掺杂对zigzag型石墨烯纳米带的能带结构和输运性质的影响

用第一性原理研究了N掺杂zigzag型石墨烯纳米带(z-GNRs)的能带结构、透射谱和电流电压特性,研究结果表明N掺杂将使得z-GNRs的能带结构中出现能隙,材料从金属转变为半导体;随着杂质浓度的增大,相同偏压下电流明显减小,同时体系费米面附近的透射率逐渐减小;z-GNRs的长度、宽度以及N原子的替代掺杂位置均会对输运性质产生影响,在宽度较小的情况下,掺杂浓度和掺杂位置两种因素共同影响体系的输运性质. 关键词： 石墨烯纳米带 N掺杂 能带结构 输运性质     

串型耦合双量子点处于自旋阻塞区时磁输运性质的研究

使用双杂质安德森模型的哈密顿量,从理论上研究了串型耦合双量子点系统处于自旋阻塞区时的磁输运性质,并用主方程近似方法求解了哈密顿量.结果表明,自旋轨道耦合作用导致的双量子点间的自旋反转隧穿能够解除系统的自旋阻塞.同时也研究了超精细相互作用导致的在量子点内自旋反转和双量子点之间的自旋关联对系统的磁输运性质的影响,取得了一些有价值的结果,并对相关的物理问题进行了讨论.     

Strain induced novel quantum magnetotransport properties of topological insulators

Recent theoretical and experimental researches have revealed that the strained bulk HgTe can be regarded as a three-dimensional topological insulator (TI). Motivated by this, we explore the strain effects on the transport properties of the HgTe surface states, which are modulated by a weak 1D in-plane electrostatic periodic potential in the presence of a perpendicular magnetic field. We analytically derive the zero frequency (dc) diffusion conductivity for the case of quasielastic scattering in the Kubo formalism, and find that, in strong magnetic field regime, the Shubnikov–de Haas oscillations are superimposed on top of the Weiss oscillations due to the electric modulation for null and finite strain. Furthermore, the strain is shown to remove the degeneracy in inversion symmetric Dirac cones on the top and bottom surfaces. This accordingly gives rise to the splitting and mixture of Landau levels, and the asymmetric spectrum of the dc conductivity. These phenomena, not known in a conventional 2D electron gas and even in a strainless TI and graphene, are a consequence of the anomalous spectrum of surface states in a fully stained TI. These results should be valuable for electronic and spintronic applications of TIs, and thus we fully expect to see them in the further experiment.     

Electrostatic field effect on molecular structures at metal surfaces

We report on the structural transitions of molecules on metal surfaces by external electrostatic field. An electrode–molecule–electrode model is considered to quantify the effect of electrostatic forces at the molecule–electrode interface. Within a quasi-parallel-plate capacitor approach, this model reveals how external electrostatic fields change the delicate balance between molecule–substrate and molecule–molecule interactions, leading to substantial changes in the molecular conformation. The predictions are validated by scanning tunneling microscopy (STM) observations of four different molecules and electrode facets. In addition, first-principles simulations verify the results of our model calculations.     

Electrostatic field penetration through a slot on a conducting half-plane

In this study, electrostatic field penetration through a slot on a conducting half-plane is investigated based on the Mellin transform and mode matching. To formulate the field behavior by the slotted half-plane, a slotted conducting wedge with a line charge is considered as an analysis model. A fast convergent series solution is obtained through the application of eigenfunction expansion and residue calculus. Numerical computations of the electrostatic field are conducted to demonstrate the penetration characteristics for the slotted conducting half-plane in terms of the slot dimensions and the position of the line charge. The computed results are also compared with simulated results.     

Electrostatic discharge effect on safe distance determination for 500 kV ac power line's helicopter inspection

The effect of electrostatic discharge on safe distance determination for 500 kV ac transmission line's helicopter inspection is investigated. Potential difference exists while helicopter is near the power transmission line, which is harmful to the inspection persons. The electric field in the helicopter cabin is calculated when it is near the power line. The result indicates that the potential difference becomes higher with the decrease of the distance between the helicopter and transmission line. Considering the discharge energy and the guarantee of the persons' safety, the safe distance for 500 kV ac transmission lines is determined as d ≥ 15 m.     

Magnetoresistance oscillations of two-dimensional electron systems in lateral superlattices with structured unit cells

Model calculations for commensurability oscillations of the low-field magnetoresistance of two-dimensional electron systems (2DES) in lateral superlattices, consisting of unit cells with an internal structure, are compared with recent experiments. The relevant harmonics of the effective modulation potential depend not only on the geometrical structure of the modulated unit cell, but also strongly on the nature of the modulation. While higher harmonics of an electrostatically generated surface modulation are exponentially damped at the position of the 2DES about 90 nm below the surface, no such damping appears for strain-induced modulation generated, e.g., by the deposition of stripes of calixarene resist on the surface before cooling down the sample.     

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.     

Low temperature magnetotransport properties of La0.55Ho0.15Sr0.3MnO3

Magnetotransport data measured in thin films of La_0.55Ho_0.15Sr_0.3MnO₃ down to very low temperatures (0.25 K) are reported. The samples presented colossal magnetoresistance with a T_C close to 200 K. A minimum in the resistance vs. T curve and a drop in the ZFC magnetization were also observed. It was also found a T-dependent relaxation effect after the magnetic field was either applied or removed. These results can be understood within the framework where electronic scattering occurs across magnetic domain walls in a reentrant spin-glassy-like phase.