Nonlinear response of a clean one-dimensional wire

We study nonlinear transport in a clean one-dimensional wire fabricated by cleaved edge overgrowth in molecular beam epitaxy. At low electron densities, and with a large applied bias, we observe a feature in the differential conductance similar to the so-called "0.7 structure," found in quantum point contact devices. Using a simple model we suggest a link between this phenomenon, charge neutrality, and unidirectional dynamics in the wire.     

Energy partitioning of tunneling currents into Luttinger liquids

Tunneling of electrons of definite chirality into a quantum wire creates counterpropagating excitations, carrying both charge and energy. We find that the partitioning of energy is qualitatively different from that of charge. The partition ratio of energy depends on the excess energy of the tunneling electrons (controlled by the applied bias) and on the interaction strength within the wire (characterized by the Luttinger-liquid parameter κ), while the partitioning of charge is fully determined by κ. Moreover, unlike for charge currents, the partitioning of energy current should manifest itself in dc experiments on wires contacted by conventional (Fermi-liquid) leads.     

Laser action of C,N, O,F, Cl,and Br in hollow cathode discharges

C, N, O, F, Cl, and Br red and infrared laser emission was investigated using hollow cathode discharges in gas mixtures of helium and molecules containing the corresponding atoms. A total of 33 laser lines with wavelengths from 0.7 to 2.0m was observed. Three laser lines of atomic C, five laser lines of atomic N, two laser lines of atomic Cl and five laser lines of atomic Br were observed for the first time. Dissociation charge transfer and dissociative excitation transfer are suggested as being responsible for populating the upper laser levels.     

Spin-dependent transport through magnetic nanojunctions

Coherent electronic transport through a molecular device is studied using non-equilibrium Green's function (NEGF) formalism. Such device is made of atomic nanowire which is connected to ferromagnetic electrodes. The molecule itself is described with the help of Hubbard model (Coulomb interactions are treated by means of the Hartree-Fock approximation), while the coupling to the electrodes is modeled through the use of a broad-band theory. It was shown that magnetoresistance varies periodically with increasing length of the atomic wire (in the linear response regime) and oscillates with increasing bias voltage (in the nonlinear response regime). Since the TMR effect for analyzed structures is predicted to be large (tens of percent), these junctions seem to be suitable for application as magnetoresistive elements in future electronic circuits.     

Controllable double-well magneto-optic atom trap with a circular current-carrying wire

We propose a controllable double-well magneto-optic trap (MOT) for neutral atoms that uses a circular current-carrying wire and a biased magnetic field. One can cause the proposed double-well trap to evolve continually into a single-well trap by reducing the current in the wire or increasing the bias field. We estimate that in a weak intensity approximation a cold atomic sample with a temperature of approximately 300 K and a number of approximately 10(6) atoms can be stored in each MOT. One can control the number of trapped atoms by changing the current in the wire.     

Transport properties of carbon atomic wire in the environment of H2O molecules: An ab initio study

The transport properties of carbon atomic wire in the environment of H₂O molecules are studied by the non-equilibrium Green function method based on density functional theory. In particular, the carbon wire with seven atoms sandwiched between the Al(1 0 0) electrodes is considered. It is found that the transport properties are sensitive to the variation of the number and the position of the H₂O molecule adsorbed on the carbon wire. To our surprise, with different positions of a single H₂O molecule on the carbon wire, the equilibrium conductance shows an evident odd–even oscillatory behavior. For example, the equilibrium conductance of the carbon wire becomes bigger when the H₂O is adsorbed on the odd-numbered carbon atoms; an opposite conclusion is obtained for the H₂O adsorbed on the even-numbered carbon atoms. For the cases of two H₂O molecules, the equilibrium conductance varies largely and the contribution of the third eigenchannel becomes larger in some special configurations. The calculated current–voltage curves show different behavior with the variation of the positions of the H₂O molecules. In certain cases, large negative differential resistance (NDR) is shown, while in other cases, it only slightly deviates from the linear behavior. The above behavior is analyzed via the charge transfer and the density of states (DOS) and reasonable explanations are presented.     

Nature of electron states and magneto-transport in a graphene geometry with a fractal distribution of holes

We investigate theoretically the spin-dependent electron transport in a Rashba quantum wire with rough edges. The charge and spin conductances are calculated as function of the electron energy and wire length by adopting the spin-resolved lattice Green function method. For a single disordered Rashba wire, it is found that the charge conductance quantization is destroyed by the edge disorder. However, a nonzero spin conductance can be generated and its amplitude can be manipulated by varying the wire length, which is attributed to the broken structure symmetries and the spin-dependent quantum interference induced by the rough boundaries. For a large ensemble of disordered Rashba wires, the average charge conductance decreases monotonically, however, the average spin conductance increases to a maximum value and then decreases, with increasing wire length. Further study shows that the influence of the rough edges on the charge and spin conductances can be eliminated by applying a perpendicular magnetic field to the wire. In addition, a very large magnitude of the spin conductance can be achieved when the electron energy lies between the two thresholds of each pair of subbands. These findings may not only benefit to further apprehend the transport properties of the Rashba low-dimensional systems but also provide some theoretical instructions to the application of spintronics devices.     

STM针尖和外电场在Si(111)-7×7表面单原子操纵中的作用

利用第一性原理的离散变分局域密度泛函方法,采用团簇模型（Si₃₄H₃₆-W₁₁)来模拟STM操纵Si(111)-7×7表面顶角吸附原子的过程．通过分析在进行原子操纵过程中体系的能量与电子云密度分布来研究针尖和外电场的作用．结果表明,当针尖与样品间距离较近时,利用两者间有较强的相互作用,能有效地降低脱出能的能垒高度．外电场对体系脱出能的影响与其大小及极性有关,当样品上所加正偏压增强时,脱出能曲线高度单调下降,而外电场极性为负时,反而稍有增高．仅考虑针尖和样品之间的静态电子相互作用及静电场的作用,尚不能使被操纵原子脱离样品表面．最后讨论了在Si(111)-7×7表面上进行原子操纵的其他机理.     

Spin transfer in magnetic tunnel junctions with hot electrons

Recent data on the bias dependence of the spin transfer effect in magnetic tunnel junctions have shown that torque remains intact at bias voltages for which the tunneling magnetoresistance has been strongly reduced. We show that the current induced excitations due to hot electrons, while reducing the magnetoresistance, enhance both the charge current and the spin transfer in magnetic tunnel junctions in such a manner that the ratio of the torque to the charge current does not significantly change.     

True atomic resolution imaging of surface structure and surface charge on the GaAs(110)

We demonstrated a novel method to detect the van der Waals and the electrostatic force interactions simultaneously on an atomic scale, which is based on frequency modulation detection method. For the first time, the surface structure and the surface charge at atomic-scale point defects on the GaAs(110) surface have been simultaneously resolved with true atomic resolution under ultra-high vacuum condition. From the bias voltage dependence of the image contrast, we can verify that the sign of the atomically resolved surface charge at the point defect was positive.     

Generation of spin current by Coulomb drag

Coulomb drag between two quantum wires is exponentially sensitive to the mismatch of their electronic densities. The application of a magnetic field can compensate this mismatch for electrons of opposite spin directions in different wires. The resulting enhanced momentum transfer leads to the conversion of the charge current in the active wire to the spin current in the passive wire.     

Even-odd behavior of conductance in monatomic sodium wires

With the aid of the Friedel sum rule, we perform first-principles calculations of conductances through monatomic Na wires, taking into account the sharp tip geometry and discrete atomic structure of electrodes. We find that conductances (G) depend on the number (L) of atoms in the wires; G is G(0)( = 2e(2)/h) for odd L, independent of the wire geometry, while G is generally smaller than G(0) and sensitive to the wire structure for even L. This even-odd behavior is attributed to the charge neutrality and resonant character due to the sharp tip structure. We suggest that similar even-odd behavior may appear in other monovalent atomic wires.     

分子线电子输运特性的第一性原理研究

从第一性原理出发 ,利用密度泛函理论研究了SH -C8H16-SH分子和金表面的相互作用 ,并利用分子前线轨道理论和微扰理论定量地确定了该相互作用能常数 ,然后 ,利用弹性散射格林函数方法研究了该分子与金表面形成的分子线的伏 安特性 .研究结果表明 ,当含有硫氢官能团的有机分子化学吸附于金表面时 ,硫原子将与金原子形成以共价键为主的混和键 ,此时 ,扩展的分子轨道使分子线的电导呈现出欧姆特性 ,而对于局域的分子轨道 ,电子的输运只能通过隧道效应来实现 .对分子线伏 安特性的计算结果显示 ,在零偏压附近 ,存在一个电流禁区 ,随着偏压的增加 ,分子线的电导呈现出平台特征 .     

Detecting Spin Bias with Circularly Polarized Light

We theoretically study the spin transport through a two-terminal quantum dot device under the influence of a symmetric spin bias and circularly polarized light.It is found that the combination of the circularly polarized light and the applied spin bias can result in a net charge current.The resultant charge current is large enough to be measured when properly choosing the system parameters.The resultant charge current can be used to deduce the spin bias due to the fact that there exists a simple linear relation between them.When the external circuit is open,a charge bias instead of a charge current can be induced,which is also measurable by present technologies.These findings indicate a new approach to detect the spin bias by using circularly polarized light.     

Excitation mechanism in polymer layers doped with tetra-methylester of perylene tetracarboxylicacid: influences of doping concentration and external bias

A spin-coated layer of poly(N-vinylcarbazole) (PVK) doped with tetra-methyl ester of perylene-3,4,9,10-tetracarboxylicacid (TMEP) is prepared, and the effects of dopant concentration and bias voltages on excitation mechanisms are studied through analyzing its emission characteristics. The investigation indicates that in photoluminescence (PL) cases energy and charge transfers between PVK and TMEP are cooperated for low dopant concentration, whereas the electroluminescence (EL) spectra imply that energy transfer is more important than charge trapping and transfer in these devices. For high dopant concentration, charge transfer is dominated in PL spectra but has a minor effect on EL spectra, and the external bias has less influence on the EL characteristics. A kinetic analysis is given to rationalize the experiment results.     

Strong isotope effects on the charge transfer in slow collisions of He2+ with atomic hydrogen, deuterium, and tritium

Probabilities and cross sections for charge transfer by He2+ impact on atomic hydrogen (H), deuterium (D), and tritium (T) at low collision energies are calculated. The results are obtained using an ab initio theory, which solves the time-dependent Schr?dinger equation. For the H target, excellent agreement is achieved between the present and previous results. Differences by orders of magnitude are observed between the cross sections for H, D, and T. A method is introduced to separate the contributions of charge-transfer mechanisms due to radial and rotational coupling. The large differences observed for H, D, and T are attributed to isotope effects in the rotational coupling mechanism.     

Charge transport in one‐dimensional chains of nanoparticles

Gold electrodes were bridged by a one‐dimensional chain of citrate stabilized gold particles. The resulting device exhibits current fluctuations at constant bias voltage due to conformational changes in the citrate molecules induced by charge transfer across the molecules. Moreover, fluctuations of the differential conductance as a function of the bias voltage became evident. The effect is attributed to the interplay of two origins: (a) dislocation of particles in the chain by action of the electric field; (b) interaction between mechanical motion of citrate molecules located on particles and charge transfer. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Charge transfer and stability of NFE random alloys

The stability of random alloys is determined primarily by a balance between the energy of the volume change upon alloy formation (elastic energy), and the charge transfer between atomic cells. The charge transfer in homovalent, nearly free electron alloys was determined by taking the wave function to be that of the free atoms inside the ion cores, and to be plane waves in the interstitial regions. For a given valence, it was possible to establish an electronegativity series based on charge transfer, to determine charge transfer as a function of composition, and to compute the energy of alloy formation. The theory properly accounts for the formation of alloys of the alkali and alkaline earth metals, and shows why so few such alloys form.     

Electric Charging in Dielectric Barrier Discharges with Asymmetric Gamma‐Coefficients

A dielectric barrier discharge with different electrode surfaces is investigated and the dynamic evolution of surface charge on the dielectric surface is measured optically. It is found that the amount of surface charge after the positive and the negative half‐cycle are not equal, i.e. a bias charge emerges. To understand this phenomenon, the transfered charge per half‐cycle is estimated from the Paschen curve and the shape of the driving voltage waveform. It turns out that the charge bias is necessary to compensate for the asymmetry of the discharge conditions due to the different surfaces and hence is required for a balanced charge transfer in the steady state (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Multiple-charge-quanta shot noise in superconducting atomic contacts

We have measured shot noise in aluminum atomic point contacts containing a small number of conduction channels of known transmissions. In the normal state, we find that the noise power is reduced from its Poissonian value and reaches the partition limit, as calculated from the transmissions. In the superconducting state, the noise reveals the large effective charge associated with each elementary transfer process, in excellent agreement with the predictions of the quantum theory of multiple Andreev reflections.