Out-of-equilibrium Kondo effect in a mesoscopic device

We study the nonequilibrium regime of the Kondo effect in a quantum dot laterally coupled to a narrow wire. We observe a split Kondo resonance when a finite bias voltage is imposed across the wire. The splitting is attributed to the creation of a double-step Fermi distribution function in the wire. Kondo correlations are strongly suppressed when the voltage across the wire exceeds the Kondo temperature. A perpendicular magnetic field enables us to selectively control the coupling between the dot and the two Fermi seas in the wire. Already at fields of order 0.1 T only the Kondo resonance associated with the strongly coupled reservoir survives.     

Electronic conductance via atomic wires: a phase field matching theory approach

A model is presented for the quantum transport of electrons, across finite atomic wire nanojunctions between electric leads, at zero bias limit. In order to derive the appropriate transmission and reflection spectra, familiar in the Landauer-Büttiker formalism, we develop the algebraic phase field matching theory (PFMT). In particular, we apply our model calculations to determine the electronic conductance for freely suspended monatomic linear sodium wires (MLNaW) between leads of the same element, and for the diatomic copper-cobalt wires (DLCuCoW) between copper leads on a Cu(111) substrate. Calculations for the MLNaW system confirm the correctness and functionality of our PFMT approach. We present novel transmission spectra for this system, and show that its transport properties exhibit the conductance oscillations for the odd- and even-number wires in agreement with previously reported first-principle results. The numerical calculations for the DLCuCoW wire nanojunctions are motivated by the stability of these systems at low temperatures. Our results for the transmission spectra yield for this system, at its Fermi energy, a monotonic exponential decay of the conductance with increasing wire length of the Cu-Co pairs. This is a cumulative effect which is discussed in detail in the present work, and may prove useful for applications in nanocircuits. Furthermore, our PFMT formalism can be considered as a compact and efficient tool for the study of the electronic quantum transport for a wide range of nanomaterial wire systems. It provides a trade-off in computational efficiency and predictive capability as compared to slower first-principle based methods, and has the potential to treat the conductance properties of more complex molecular nanojunctions.     

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.     

Bias drop and phonon emission in molecular wires

Using the recursion-transfer-matrix (RTM) method combined with the non-equilibrium Green's function (NEGF) method and density-functional theory, we perform ab initio calculations for the electron transport of molecular wires bridged between electrodes. We present an effective potential of molecular wire under a finite bias voltage and discuss the phonon emission and local heating due to inelastic electron-phonon coupling effects. We find that it is strongly dependent on contact conditions. When the contacts to electrodes are bad, excitation phonon modes at contacts become dominant for the energy dissipation.     

Carbon-atom wires: charge-transfer doping, voltage drop, and the effect of distortions

We present first-principles calculations on electrical conduction through carbon atomic wires. The changes in charge distribution induced by a large bias exhibit the primary involvement of the wire's pi states. A significant fraction ( approximately 40%) of the voltage drops across the atomic wire itself. At zero bias, there is a large transfer of charge from the electrodes to the wire, effectively providing doping without introducing scattering centers. This transfer leads, however, to potential barriers at the wire-electrode junctions. Bending the wire reduces its conductance.     

Organometallic benzene-vanadium wire: A one-dimensional half-metallic ferromagnet

Using density functional theory we perform theoretical investigations of the electronic properties of a freestanding one-dimensional organometallic vanadium-benzene wire. This system represents the limiting case of multidecker Vn(C6H6)(n+1) clusters which can be synthesized with established methods. We predict that the ground state of the wire is a 100% spin-polarized ferromagnet (half-metal). Its density of states is metallic at the Fermi energy for the minority electrons and shows a semiconductor gap for the majority electrons. We find that the half-metallic behavior is conserved up to 12% longitudinal elongation of the wire. Ab initio electron transport calculations reveal that finite size vanadium-benzene clusters coupled to ferromagnetic Ni or Co electrodes will work as nearly perfect spin filters.     

Ab initio calculations of transport properties of atomic bridges by the recursion-transfer-matrix method

We present an efficient self-consistent method for approaching quantum transport through atomic-scale structures. Using the recursion-transfer-matrix (RTM) method with a separable form of nonlocal pseudopotentials, scattering waves propagating between metallic electrodes through nano-bridged structures are efficiently calculated on the basis of the density-functional formalism. We performed calculations with this method of the conductance of Al atomic wires with various kinds of single atoms mixed at the contact to one electrode. We found that the transport properties are considerably affected by the bonding nature of the atom at the contact. The conductance is largely determined by the atomic species at the contact and does not change much as the length of the atomic wire increases.     

DC voltage profile of a 1D pumped wire with two dynamical and one static impurities

In this work we study the behavior of the voltage profile of a 1D quantum wire with an impurity when transport is induced by two ac voltages that oscillating with a phase lag define a quantum pump. The voltage profile sensed along the wire by the voltage probe, that we assume weakly coupled to the system, exhibits a Friedel's oscillations structure inside the region delimited by the position of the two ac voltages that induce transport. On the other hand, outside this region the oscillations are suppressed. Using perturbation theory in the coupling constant of the voltage probe we derived analytical expressions for the DC current valid for the adiabatic regime. We also compare our analytical results with the exact numerical calculations using Keldysh non-equilibrium Green's functions formalism.     

真空及空气中金属丝电爆炸特性研究

开展了铝丝在真空和空气环境中的电爆炸特性研究.从金属丝电爆炸的电压、电流波形得到了金属丝内的沉积能量,并基于以上电参数特征分析了电爆炸产物的状态,获得了空气中铝丝电爆炸电流暂停时间随初级储能电容充电电压的变化规律.真空和空气中铝丝电爆炸在电压击穿时刻的沉积能量分别为2.8和6 eV/atom.采用波长为532 nm、亚纳秒激光探针对金属丝电爆炸物理过程开展了高时空分辨率的阴影和纹影诊断.阴影图像清晰地展示了不同气氛环境中高密度电爆炸产物的膨胀过程,根据光学诊断图像分析了高密度丝核沉积能量的结构和空气中铝丝电爆炸产生的激波的膨胀轨迹.真空和空气环境中高密度电爆炸产物的平均膨胀速度分别为1.9和3 km/s.基于实验数据和输运参数模型,估算了金属丝在电压击穿时刻的温度.     

Coulomb blockade and transport in a chain of one-dimensional quantum dots

A long one-dimensional wire with a finite density of strong random impurities is modeled as a chain of weakly coupled quantum dots. At low temperature T and applied voltage V its resistance is limited by breaks: randomly occurring clusters of quantum dots with a special length distribution pattern that inhibit the transport. Because of the interplay of interaction and disorder effects the resistance can exhibit T and V dependences that can be approximated by power laws. The corresponding two exponents differ greatly from each other and depend not only on the intrinsic electronic parameters but also on the impurity distribution statistics.     

Effect of intermolecular distance and contact hollow-type on the transport properties of parallel atomic wires

We use non-equilibrium Green's function combined with density functional theory to investigate the electronic transport properties of two parallel molecular wires made of carbon atomic chains (triynes) capped with thiol. The results show that the transport behaviors clearly depend on the intermolecular distance when the two wires are separated by a relatively small distance. However, with increasing the wire spacing, the transport properties are dramatically affected by the molecule-electrode contact hollow-type and insensitive to the intermolecular distance. A quantum interference mechanism is proposed to interpret the contact hollow-type dependence of transport properties at large intermolecular distance.     

Charging effects in biased molecular devices

The influence of charging effects on the transport characteristics of a molecular wire bridging two metallic electrodes in the limit of weak contacts is studied by the generalized Breit–Wigner formula. Molecule is modeled as a quantum dot with discrete energy levels, while the coupling to the electrodes is treated within a broad-band theory. Owing to this model we find self-consistent occupation of particular energy levels and orbital energies of the wire in the presence of transport. The nonlinear conductance and current–voltage characteristics are investigated as a function of bias voltage in the case of symmetric and asymmetric coupling to the electrodes. It is shown that the shape of those curves are determined by the combined effect of the electronic structure of the molecule and by electron–electron repulsion.     

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.     

Lattice controlled transport in quantum wires at low temperatures

Theory of acoustic scattering rate of the carriers in a quantum wire has been developed under the condition of low temperature when the approximations of the traditional theory are hardly valid. The scattering rates thus obtained are then used to estimate the zero-field mobility characteristics in a narrow channel GaAs–GaAlAs quantum wire. On comparison with other available results it is revealed that the finite energy of the acoustic phonons and the complete phonon distribution without any truncation lead to significantly different transport characteristics at low temperatures.     

Molecular wires acting as coherent quantum ratchets

The effect of laser fields on electron transport through a molecular wire weakly coupled to two leads is investigated. The molecular wire acts as a coherent quantum ratchet if the molecule is composed of periodically arranged, asymmetric chemical groups. This setup presents a quantum rectifier with a finite dc response in the absence of a static bias. The nonlinear current is evaluated in closed form within the Floquet basis of the isolated, driven wire. The current response reveals multiple current reversals together with a nonlinear dependence on the amplitude and the frequency of the laser field. The current saturates for long wires at a nonzero value, while it may change sign upon decreasing its length.     

Magnetization transport and quantized spin conductance

We analyze transport of magnetization in insulating systems described by a spin Hamiltonian. The magnetization current through a quasi-one-dimensional magnetic wire of finite length suspended between two bulk magnets is determined by the spin conductance which remains finite in the ballistic limit due to contact resistance. For ferromagnetic systems, magnetization transport can be viewed as transmission of magnons, and the spin conductance depends on the temperature T. For antiferromagnetic isotropic spin-1/2 chains, the spin conductance is quantized in units of order (gmu(B))(2)/h at T=0. Magnetization currents produce an electric field and, hence, can be measured directly. For magnetization transport in electric fields, phenomena analogous to the Hall effect emerge.     

Tunable quantum dot resonator embedded in a quantum wire

Combined quantum wire and quantum dot system is theoretically predicted to show unique conductance properties associated with Coulomb interactions. We use a split gate technique to fabricate a quantum wire containing a quantum dot with two tunable potential barriers in a two-dimensional electron gas. We observe the effects of the quantum dot cavity on the electron transport through the quantum wire, such as Coulomb oscillations near the pinch-off voltage and periodic conductance oscillations on the first conductance plateau.     

Quantum transport andI–Vcharacteristics of quantum size field effect transistor

Quantum electron transport is expected to occur in nanometer-size field effect transistors. We show that the amplitude of the transmitted wave equals 1 only when the electric field in the conducting channel is zero. By reducing the dimension of the quantum transport from bulk to a two-dimensional electron gas system, and further to a one-dimensional quantum wire, the current-bias relation is not affected while the gate control over the drain current weakens. Starting from the Poisson and Schrödinger equations, we have studied numerically the quantum wave transport through the conduction channel where scattering processes are neglected, theI–Vcharacteristic of a typical heterojunction high electron mobility transistor shows a linear relationship between drain current and voltage at low drain bias, but the drain current decreases with increasing drain voltage at a high bias.     

Pioneering experiments on atomic-beam-assisted generation of drag currents in the Globus-M spherical tokamak

Research data for drag currents in the Globus-M spherical tokamak are presented. The currents are generated by injecting atomic beams of hydrogen and deuterium. Experiments were carried out in the hydrogen and deuterium plasma of the tokamak. It has a divertor configuration with a lower X-point, a displacement along the larger radius from–1.0 to–2.5 cm, and a toroidal field of 0.4 T at a plasma current of 0.17–0.23 MA. The beam is injected into the tokamak in the equatorial plane tangentially to the magnetic axis of the plasma filament with an impact diameter of 32 cm. To provide a 28-keV 0.5-MW atomic beam with geometrical sizes of 4 × 20 cm (at a power level of 1/e), an IPM-2 ion source is used. The generation of noninductive currents is detected from a rise in the loop current and a simultaneous dip of the loop voltage. The injection of the hydrogen and deuterium atomic beams into the deuterium plasma results in a noticeable and reproducible dip of the loop voltage (up to 0.5 V). Using the ASTRA transport code, a model is constructed that allows rapid calculation of noninductive currents. Calculations performed for a specific discharge confirm that the model adequately describes the effect of drag current generation.     

Self-Aligned SiGe MOS-Gate FET with Modulation-Doped Quantum Wire Channel for Millimeter Wave Application

This paper describes a self-aligned SiGe MOS-gate field-effect transistor (FET) having a modulation-doped (MOD) quantum wire channel. An analytical model based on modified charge control equations accounting for the quantum wire channel, is presented predicting the transport characteristics of the MOS-gate MODFET structure. In particular, transport characteristics of devices having strained SiGe layers, realized on Si or Ge substrates, are computed. The transconductance g_m and unity-current gain cutoff frequency (f_T) are also computed as a function of the gate voltage V_G. The calculated values of f_T suggest the operation of one-dimensional SiGe MODFETs to be around 200 GHz range at 77°K, and 120 GHz at 300°K.