Electronic transport properties of hafnium/zirconium multilayers

We have investigated the electrical resistivity, superconducting transition temperature, and upper critical field as a function of layer thickness in hafnium/zirconium ( ) metallic superlattices. These films have equal Hf and Zr layer thicknesses (d_h and d_z, respectively). We have studied a series of samples with modulation wavelength λ = d_h + d_z ranging from 20 to 250 Å. All films show a metallic type of resistivity, with remarkably little difference in both room temperature and liquid helium resistivities. All structures undergo a transition to a superconducting state, with the transition temperature remaining nearly constant across the series. This indicates that the interface region of these structures is of rather high quality. Upper critical fields both parallel and perpendicular to the sample plane were determined. Because of the relatively large superconducting coherence length, these films behave essentially three-dimenstionally throughout the range of λ studied. However, we also observe a somewhat anomalous behavior in the ratio of the parallel to perpendicular critical fields near the transition temperature, the origin of which is not yet known.     

掺杂六角形石墨烯电子输运特性的研究

锯齿型和扶手椅型六角形石墨烯分别跨接在两Au电极上, 构成分子纳器件, 同时考虑对六角形石墨烯分别进行B, N和BN局部规则掺杂. 利用第一性原理方法, 系统地研究了这些器件的电子输运特性. 计算结果表明: B及BN掺杂到扶手椅型六角形石墨烯, 对其电流有较好的调控效应, 同时发现本征及掺杂后的锯齿型六角形石墨烯均表现为半导体性质, 且N及BN掺杂时, 表现出明显的负微分电阻现象, 特别是N掺杂的情况, 能呈现显著的负微分电阻效应, 这也许对于发展分子开关有重要应用. 通过其透射特性及掺杂诱发的六角形石墨烯电子结构的变化, 对这些结果的内在原因进行了说明.     

多孔石墨烯纳米片电学性质的研究

采用紧束缚近似方法,研究了三角形锯齿型石墨烯纳米片(Triangular zigzag graphene nanosheets, TZGN)的电子结构.研究表明单孔TZGN结构的零能级都是外边缘态,跟孔的大小没有关系.多孔TZGN结构受孔间结构的影响,零能级会随着孔数目的增加逐渐出现内外边缘耦合态,导带和价带能级个数也会随着孔的大小和孔的数量的增加而减少.研究结果拓宽了石墨烯纳米结构在纳机电器件方面的应用.     

Electronic properties of bearded graphene nanoribbons

We investigate the electronic properties of graphene nanoribbons with attachment of bearded bonds as a model of edge modification. The main effect of the addition of the beards is the appearance of additional energy subbands. The originally gapless armchair graphene nanoribbons become semiconducting. On the other hand, the originally semiconducting armchair graphene nanoribbons may or may not change to gapless systems depending on the width. With the inclusion of a transverse electric field, the band structures of bearded graphene nanoribbons are further altered. An electric field creates additional band-edge states, and changes the subband curvatures and spacings. Furthermore, the energy band symmetry about the chemical potential is lifted by the field. With varying width, the bandgap demonstrates a declining zigzag behavior, and touches the zero value regularly. Modifications in the electronic structure are reflected in the density of states. The numbers and energies of the density of state divergent peaks are found to be strongly dependent on the geometry and the electric field strength. The beard also causes electron transfer among different atoms, and alters the probability distributions. In addition, the electron transfers are modified by the electric field. Finally, the field introduces more zero values in the probability distributions, and removes their left–right symmetry.     

Electronic properties of Li-doped zigzag graphene nanoribbons

Zigzag graphene nanoribbons (ZGNRs) are known to exhibit metallic behavior. Depending on structural properties such as edge status, doping and width of nanoribbons, the electronic properties of these structures may vary. In this study, changes in electronic properties of crystal by doping Lithium (Li) atom to ZGNR structure are analyzed. In spin polarized calculations are made using Density Functional Theory (DFT) with generalized gradient approximation (GGA) as exchange correlation. As a result of calculations, it has been determined that Li atom affects electronic properties of ZGNR structure significantly. It is observed that ZGNR structure exhibiting metallic behavior in pure state shows half-metal and semiconductor behavior with Li atom.     

Electronic and magnetic properties of graphene nanoribbons

Molecular orbital calculations of the electronic properties of graphene nanoribbons as a function of length in the nanometre range show a pronounced decrease in the band gap and ionization potential with increasing length. It is shown that length can be used to design the materials to be insulating, semiconducting or metallic. A low ionization potential (work function), less than single walled carbon nanotubes, is obtained at the longest length of the calculation (2.3?nm). This latter result suggests the possibility of using graphene nanoribbons as electric field induced electron emitters. Calculations on boron and nitrogen doped carbon nanoribbons indicate that the triplet state is more stable than the singlet state.     

Electronic transport properties of Co cluster-decorated graphene

Interactions of magnetic elements with graphene may lead to various electronic states that have potential applications.We report an in-situ experiment in which the quantum transport properties of graphene are measured with increasing cobalt coverage in continuous ultra-high vacuum environment. The results show that e-beam deposited cobalt forms clusters on the surface of graphene, even at low sample temperatures. Scattering of charge carriers by the absorbed cobalt clusters results in the disappearance of the Shubnikov–de Haas(Sd H) oscillations and the appearance of negative magnetoresistance(MR)which shows no sign of saturation up to an applied magnetic field of 9 T. We propose that these observations could originate from quantum interference driven by cobalt disorder and can be explained by the weak localization theory.     

Electronic properties of graphene with a topological defect

Various types of topological defects in graphene are considered in the framework of the continuum model for long-wavelength electronic excitations, which is based on the Dirac–Weyl equation. The condition for the electronic wave function is specified, and we show that a topological defect can be presented as a pseudomagnetic vortex at the apex of a graphitic nanocone; the flux of the vortex is related to the deficit angle of the cone. The cases of all possible types of pentagonal defects, as well as several types of heptagonal defects (with the numbers of heptagons up to three, and six) are analyzed. The density of states and the ground state charge are determined.     

Electronic and magnetic properties of hydrogenated graphene nanoflakes

We investigated the energetic stability, electronic, and magnetic properties of hydrogenated graphene nanoflakes (GNFs) by using density-functional theory (DFT). Hydrogenated GNFs were found to be the stable heterojunction structures. As the increase of H coverage, a transition of a small-gap semiconductor to wide-gap semiconductor occurs, accompanied with a nonmagnetic (with the coverage χ=0$\chi =0$) → magnetic (with the coverage 0<χ<1$0<\chi <1$) → nonmagnetic (with the coverage χ=1$\chi =1$) transfer for hexagonal nanoflakes and magnetic (with the coverage 0?χ<1$0?\chi <1$) → nonmagnetic (with the coverage χ=1$\chi =1$) transfer for triangular nanoflakes. The efficacious tune of band gaps and the magnetic moments on these nanoflakes by hydrogenation offers an effectual avenue for the applications of C-based nanomagnets.     

Electronic and transport properties of boron-doped graphene nanoribbons

We report a spin polarized density functional theory study of the electronic and transport properties of graphene nanoribbons doped with boron atoms. We considered hydrogen terminated graphene (nano)ribbons with width up to 3.2 nm. The substitutional boron atoms at the nanoribbon edges (sites of lower energy) suppress the metallic bands near the Fermi level, giving rise to a semiconducting system. These substitutional boron atoms act as scattering centers for the electronic transport along the nanoribbons. We find that the electronic scattering process is spin-anisotropic; namely, the spin-down (up) transmittance channels are weakly (strongly) reduced by the presence of boron atoms. Such anisotropic character can be controlled by the width of the nanoribbon; thus, the spin-up and spin-down transmittance can be tuned along the boron-doped nanoribbons.     

Electronic properties of Z-shaped graphene nanoribbon under uniaxial strain

Based on tight-binding approximation and a generalized Green's function method, the effect of uniaxial strain on the electron transport properties of Z-shaped graphene nanoribbon (GNR) composed of an armchair GNR sandwiched between two semi-infinite metallic armchair GNR electrodes is numerically investigated. Our results show that the increase of uniaxial strain enhances the band gap and leads to a metal-to-semiconductor transition for Z-shaped GNR. Furthermore, in the Landauer–Büttiker formalism, the current–voltage characteristics, the noise power resulting from the current fluctuations and Fano factor of strained Z-shaped GNR are explored. It is found the threshold voltage for the current and the noise power increased so that with reinforcement of the uniaxial strain parameter strength, the noise power goes from the Poisson limit to sub-Poisson region at higher bias voltages.     

Electronic and magnetic properties of silicon adsorption on graphene

Graphene has proved to be extremely sensitive to its surrounding environment, such as the supporting substrate and guest adatoms. In this work, the structural stabilities, and electronic and magnetic properties of graphene with low-coverage adsorption of Si atoms and dimers are studied using a first-principles method. Our results show that graphene with Si adatoms is metallic and magnetic with a tiny structural change in the graphene, while graphene with Si addimers is semi-metallic and nonmagnetic with a visible deformation of the graphene. The spin-polarized density of states is calculated in order to identify the electronic origin of the magnetic and nonmagnetic states. The present results suggest that the electronic and magnetic behaviors of graphene can be tuned simply via Si adsorptions.     

Electronic and magnetic properties of quasifreestanding graphene on Ni

For the purpose of recovering the intriguing electronic properties of freestanding graphene at a solid surface, graphene self-organized on a Au monolayer on Ni(111) is prepared and characterized by scanning tunneling microscopy. Angle-resolved photoemission reveals a gapless linear pi-band dispersion near K[over] as a fingerprint of strictly monolayer graphene and a Dirac crossing energy equal to the Fermi energy (EF) within 25 meV meaning charge neutrality. Spin resolution shows a Rashba effect on the pi states with a large (approximately 13 meV) spin-orbit splitting up to EF which is independent of k.     

Electronic transport properties on V-shaped-notched zigzag graphene nanoribbons junctions

Using nonequilibrium Green?s functions in combination with the density functional theory, the spin-dependent electronic transport properties on V-shaped notched zigzag-edged graphene nanoribbons junctions have been calculated. The results show that the electronic transport properties are strongly depending on the type of notch and the symmetry of ribbon. The spin-filter phenomenon and negative differential resistance behaviors can be observed. A physical analysis of these results is given.     

自旋轨道耦合作用下石墨烯pn结的电子输运性质

本文研究了自旋轨道耦合作用下石墨烯纳米带pn结的电子输运性质. 当粒子的入射能量处于pn结两端势能之间时, 粒子将会以隧穿的形式通过石墨烯pn结, 同时伴随着电子空穴转换. 电导随费米能的变化曲线呈不等高阶梯状, 并在费米能位于pn结两端能量中点时取得最大值. 随着石墨烯pn结长度的增加, 电导以指数形式衰减. 自旋轨道耦合作用导致的能隙会使电导显著减小, 而边缘态的粒子则可以几乎毫无阻碍地通过pn结. 本文用一个简单的子带隧穿模型解释了上述特征. 最后还研究了在pn转换区中掺入替位杂质的情况. 在弱杂质下, 电导随费米能变化的曲线将不再对称; 当杂质较强时, 仅边界态的形成的电导台阶能够保持.