Modulation of the magnetic properties in zigzag-edge graphene nanoribbons by connection sites

Based on the first-principles method, the magnetic properties for zigzag–edge graphene nanoribbon (ZGNR) junctions are investigated. The results show the system had the ferromagnetic or antiferromagnetic ground state depending on the connection sites between ZGNR electrodes and the central ZGNR. The junction displays a metallic behavior when the central ZGNR is connected at the middle site of electrodes with a ferromagnetic state, but shows obvious spin semiconductor feature when the connection site is shifted to the edge of the ZGNR. For the antiferromagnetic states, all models show a semiconductor behavior, which originates from the spin-degenerate edge states. While for the antiparallel spin configuration, the spin density of the central ribbon is affected by connection sites, and it shows weaker little by little with the central ribbon moving from bottom to the middle site, which is different obviously from ferromagnetic or antiferromagnetic state. When one edge state of the central ZGNR is broken, bipolar spin semiconductor features can be obtained with different band gaps at suitable connection sites.     

Hot electron energy relaxation in quantizing magnetic fields

A systematic study of the radiative recombination of hot electrons between Landau levels in several n-InSb samples has been carried out as a function of the free electron concentration and the applied electric and magnetic fields (E ～ H). The magnetic field range investigated was 0.5 to 3.0 T. Electron temperatures are derived in dependence of the total electric and the magnetic field applied. The carrier heating is impeded with increasing electron concentration and also with increasing magnetic field. From the direct observation of the recombination radiation intensity on electric field response the electron lifetime in the first Landau level is found to be less than 10^?8. The results obtained are discussed in light of a new theoretical approach. We give evidence that electron-electron scattering plays an important role both for the carrier distribution established and for the electron lifetime in the first Landau level.     

激光诱导碳纳米管制备石墨烯纳米带及其电学性能分析

通过将纳米管解压缩可以很容易地生产石墨烯纳米带,因为碳纳米管结构可以被认为是卷起的石墨烯筒。这是一种特殊的2D石墨结构,具有出色的性能。应用领域广泛,包括晶体管、光学和微波通信设备、生物传感器、化学传感器、电子存储和处理设备以及纳米机电系统和复合材料。通过扫描电子显微镜（SEM）观察薄膜的形貌,通过拉曼光谱法表征石墨烯的性质,并通过半导体参数测量系统测量薄膜的电导率。拉曼光谱表明,通过优化工艺可以增强石墨烯的拉曼特性。碳纳米管制备石墨烯带的两个重要参数是激光能量密度和辐照时间。在这项研究中,通过准分子激光辐照碳纳米管薄膜来生产石墨烯纳米带。实验结果表明,在150 mJ的激光能量下,观察到连接时碳纳米管没有打开。在450 mJ的能量下,可以有效地破坏碳纳米管,并且使其部分地形成石墨烯带。此时,膜的电导率达到最大值。由于蓄热作用,在碳纳米管壁上出现大量的多孔结构。     

Magneto-electronics properties of fluorine-passivated “Christmas Tree” graphene nanoribbons

Magneto-electronics properties of fluorine-passivated “Christmas Tree” graphene nanoribbons (F-CTGNRs) are investigated theoretically. It is found that F-CTGNRs have a ferromagnetic ground state and can be a spin metal, a spin semiconductor, a spin half-metal, or a spin gapless semiconductor, completely depending on their geometrical sizes. More importantly, for the F-CTGNR based device, the dual spin-filtering effect with the perfect (100%) spin polarization and high-performance dual spin diode effect with a rectification ratio about 10¹⁰ can be achieved. It means that the F-CTGNR can act as an excellent dual spin diode. The magnetoresistance (MR) value larger than 10⁴% can be reached in the calculated bias region, which is comparable to the previously reported experimental values for the MgO tunnel junction. These distinctive features can be attributed to their unique band overlap pattern for two electrodes and particular sensitivity to a switching magnetic field. Our findings suggest that F-CTGNRs have desirable natures for developing magnetic devices.     

Electronic structures and transport properties of BN nanodot superlattices of armchair graphene nanoribbons

The electronic and transport properties of embedded boron nitride(BN) nanodot superlattices of armchair graphene nanoribbons are studied by first-principles calculations.The band structure of the graphene superlattice strongly depends on the geometric shape and size of the BN nanodot,as well as the concentration of nanodots.The conduction bands and valence bands near the Fermi level are nearly symmetric,which is induced by electron-hole symmetry.When B and N atoms in the graphene superlattices with a triangular BN nanodot are exchanged,the valance bands and conduction bands are inverted with respect to the Fermi level due to electron-hole symmetry.In addition,the hybridization ofπorbitals from C and redundant B atoms or N atoms leads to a localized band appearing near the Fermi level.Our results also show a series of resonant peaks appearing in the conductance.This strongly depends on the distance of the two BN nanodots and on the shape of the BN nanodot. Controlling these parameters might allow the modulation of the electronic response of the systems.     

掺有BN纳米点石墨烯纳米带的电子结构和输运性质的第一性原理研究

利用第一性原理研究了含有BN纳米点的扶手椅型石墨烯纳米带的电子结构和输运性质。研究发现,能带结构与BN纳米点的几何形状、尺寸以及纳米点浓度密切相关。由于电子-空穴对的对称性,使得费米能级附近的价带和导带几乎关于费米能级是对称的。对于含有三角形BN纳米点的石墨烯纳米带,当B和N交换时,价带和导带正好关于费米能级相互倒置。而且,由于C原子和多余B原子(或多余N原子)之间的轨道杂化,使得在费米能级附近形成一条局域的能带。另外,当电子在含有BN纳米点的石墨烯纳米带中输运时,在电导谱中会出现一系列的共振峰,其依赖于BN纳米点之间的距离和纳米点的形状。由此通过控制这些参数可以调制系统的电子响应。     

锯齿-扶手椅-锯齿型石墨烯纳米带电子输运特性研究

基于密度泛函理论和非平衡格林函数,研究了中心散射区长度对于锯齿-扶手椅-锯齿型石墨烯纳米带(Z-A-ZGNRs)的电子输运特性的影响.结果表明:中心散射区长度对导电性能有很大的影响.散射区长度较小时,在一定区间内具有明显的负微分电阻现象,长度增加时,这种效应减弱.Z-A-ZGNRs在-2～2 V偏压下存在整流现象,散射...     

The cyclotron resonance of holes in InGaAs/GaAs heterostructures with quantum wells in quantizing magnetic fields

The spectra of the cyclotron resonance of holes in the InGaAs/GaAs selectively doped heterostructures with quantum wells are studied in pulsed magnetic fields as high as 50 T at 4.2 K. The previously observed effect of the inverted (compared with the results of the single-particle calculation of the Landau levels) ratio of the spectral weight of two split components of the line of the cyclotron resonance, which is attributed to the effects of the exchange interaction of holes, is confirmed. It is found that the ratios of intensities of the components of the line of the cyclotron resonance profoundly differ on the ascending and descending branches of the magnetic field pulse, which may be associated with a long time of the spin relaxation of holes between the two lowest Landau levels, which constitute tens of milliseconds.     

Conductivity of charge-ordered layered crystals in quantizing magnetic fields in the inversion region of the nonoscillating magnetoresistance component

The conductivity of charge-ordered layered crystals in electric and quantizing magnetic fields orthogonal to the layers is calculated. Charge ordering is considered as an alternation of layers with different charge-carrier densities. When scattering by acoustic phonons, conductivity is calculated in the approximation of profound quantization, so that the intersubband transitions can be assumed to be suppressed. It is shown that, in the transition of the layered crystal from the disordered to almost completely ordered state, the conductivity oscillations exhibit double periodicity. In this case, the high “carrier” frequency is controlled by the effective attractive electron-electron interaction responsible for charge ordering, whereas the low “modulation” frequency is defined by the width of the narrow miniband orthogonal to the layers. It is also shown that, in the transition from a disordered to an almost completely ordered state, the longitudinal conductivity of layered crystals decreases by two or three orders of magnitude. At the same time, the relative contribution of oscillations sharply increases, with satisfactory agreement to the experimental data.     

Tunneling current of the contact between impurity-containing graphene nanoribbons

The current-voltage characteristics of a tunneling contact between two graphene nanoribbons containing impurity atoms are obtained based on the previously calculated density of states. The dependences on the nanoribbon geometrical and energy characteristics are calculated.     

Negative differential resistance and rectification effects in step-like graphene nanoribbons

Step-like zigzag graphene nanoribbons (ZGNRs) with different step widths are designed, and their electronic transport properties are investigated by using the non-equilibrium Green’s function method combined with the density functional theory. The results reveal that one with a small step structure can exhibit better conductive capability and interesting negative differential resistance (NDR) behavior under negative applied biases. More importantly, with the increase of step width, these step-like ZGNR nanojunctions present valuable rectification effects, and show a rule that the rectification ratio increases with increasing the step width. It is also shown that the rectification effect can be further inversed and enhanced through introducing a defect around the step. Transmission spectra, densities of states, energy barriers, transmission eigenstates, and transmission pathways are analyzed subsequently to understand the electronic transport properties of these step-like ZGNR devices, which can be used in developing nanoscale NDR devices and rectifiers.     

Structural,electronic, and magnetic properties of zigzag-edged BN nanoribbons with 558-type line defects

The 558-type line defect is introduced into zigzag-edged BN nanoribbons (ZBNNRs), similar to that be found in graphene, and the structural, electronic, and magnetic properties for such defective ZBNNRs are investigated systematically. It shows that they are highly stable. In the nonmagnetic state, the metallic property of ZBNNRs remains unchanged regardless of the defect positions, but different defect positions give rise to different influences on the total DOS and PDOS at the Fermi level. Interestingly, in the magnetic state, the thermal stability of magnetism is enhanced greatly when the line defect appears at most positions, even comparable with room temperature. When the line defect position shifts from the B-edge to the N-edge, the transition from the half-metal to the magnetic semiconductor or spin unpolarized semiconductor is induced. And also, the response of defect ribbons to an in-plane transverse electric field is essentially different from defect-free ribbons, and the half-metallic feature can be enhanced by an electric field for the line defect introduced into certain positions in a ribbon.     

Resonance modulation of electron-electron relaxation by a quantizing magnetic field

It is found that a magnetic field turns on intersubband electron relaxation as the Landau levels of the spatial quantization subbands of a single heterojunction scan the Fermi level. The observed steady decrease in intersubband relaxation time as the magnetic field increases is attributed to the addition of electron-phonon interaction to electron-electron interaction.     

Electronic properties of BN nanocones under electric fields

The electronic properties of BN nanocones with 240° disclination under electric fields are investigated using first-principles calculations based on the density-functional theory. The cones are studied under the influence of electric fields, up to 1.13 V/Å, applied along the cone axis. The densities of states (DOS) of these BN nanocones show different patterns depending on the termination two atoms (BN, BB or NN) and the electric field strength. A decreasing of the gap is observed with increasing field. The field emission properties are very sensitive to the DOS and we show that the termination atoms, as well as the electric field, contribute to enhance the electron field emission.     

High-quality single-crystal CdSe nanoribbons and their optical properties

Large-scale synthesis of single-crystal CdSe nanoribbons is achieved by a modified thermal evaporation method, in which two-step-thermal-evaporation is used to control CdSe sources＇ evaporation. The synthesized CdSe nanoribbons are usually several micrometers in width, 50 nm in thickness, and tens to several hundred micrometers in length. Studies have shown that high-quality CdSe nanoribbons with regular shapes can be obtained by this method. Room-temperature photolumines-cence indicates that the lasing emission at 710 nm has been observed under optical pumping （266 nm） at power densities of 25-153 kW/cm^2. The full width half maximum （FWHM） of the lasing mode is 0.67 nm     

Electric and magnetic fields

The content of the first of the two courses in electro-magnetic fields offered by the electrical engineering department at MIT to its juniors is summarized. The course deals with electroquasi-statics and magnetoquasistatics throughout, treating electrostatic and magnetostatic phenomena as special cases. This method of presentation allows the introduction of many more engineering examples into lecture demonstrations and homework than is possible in a treatment of statics followed by a treatment of the full dynamic equations, or by an overemphasis on static phenomena with only a belated discussion of quasistatic ideas. Details on several lecture demonstrations and examples are given, and it is shown how they are integrated into the "flow" of ideas. Further, our treatment of magnetization differs from the conventional one and the advantages of this treatment are discussed.