Quintuple-period Si atomic wires with alternative double and triple modulations by metal: Mg/Si(557)

The formation of Mg-induced quasi-one-dimensional atomic wires on a Si(557) surface was studied by low energy electron diffraction (LEED), scanning tunneling microscopy (STM), and first-principles calculations. The atomic wires were produced on the Si(557) surface without faceting when heated to 330 ?C. The atomic wires had a × 5 period along the wires, as observed by LEED. STM images showed the existence of three kinds of atomic wires in a unit cell: an atomic wire located at the step edge and the others on the terrace. Interestingly, alternative double and triple modulations resulting in the × 5 period was observed at the atomic wire located at the step edge. Among the variety of atomic structure models available, the one based on a honeycomb-chain-channel model, which is that of a metal/Si(111)-(3 × 1) surface, reproduced the STM images well and was relatively stable energetically.     

The effect of semi-infinite crystalline electrodes on transmission of gold atomic wires using DFT

First principle calculations of the conductance of gold atomic wires containing chain of 3–8 atoms each with 2.39 Å bond lengths are presented using density functional theory. Three different configurations of wire/electrodes were used. For zigzag wire with semi-infinite crystalline electrodes, even–odd oscillation is observed which is consistent with the previously reported results. A lower conductance is observed for the chain in semi-infinite crystalline electrodes compared to the chains suspended in wire-like electrode. The calculated transmission spectrum for the straight and zig-zag wires suspended between semi-infinite crystalline electrodes showed suppression of transmission channels due to electron scattering occurring at the electrode-wire interface.     

Conductance of short quantum wires with sharp boundaries

The conductance of short ballistic wires with boundaries, whose curvature radius is comparable to the electron wavelength, is investigated. It is found that, in such wires, no conductance quantization takes place. Instead, pronounced interference effects are observed. These effects are related to the Fabry-Perot interference by the wire edges and with the mesoscopic interference caused by the coherent scattering of electrons by the wire edges and by the impurities located in the near-wire regions of the 2D electron gas.     

Solderability study of RABiTS-based YBCO coated conductors

The solderability of commercially available YBa₂Cu₃O_7?x (YBCO) coated conductors that were made from Rolling Assisted Biaxially Textured Substrates (RABiTS)-based templates was studied. The coated conductors, also known as second-generation (2G) high temperature superconductor (HTS) wires (in the geometry of flat tapes about 4 mm wide), were laminated with copper, brass, or stainless steel strips as stabilizers. To understand the factors that influence their solderability, surface profilometry and scanning electron microscopy were used to characterize the wire surfaces. The solderability of three solders, 52In48Sn, 67Bi33In, and 100In (wt.%), was evaluated using a standard test (IPC/ECA J-STD-002) and with two different commercial fluxes. It was found that the solderability varied with the solder and flux but the three different wires showed similar solderability for a fixed combination of solder and flux. Solder joints of the 2G wires were fabricated using the tools and the procedures recommended by the HTS wire manufacturer. The solder joints were made in a lap-joint geometry and with the superconducting sides of the two wires face-to-face. The electrical resistances of the solder joints were measured at 77 K, and the results were analyzed to qualify the soldering materials and evaluate the soldering process. It was concluded that although the selection of soldering materials affected the resistance of a solder joint, the resistivity of the stabilizer was the dominant factor.     

A peculiarity of silver-based corona wire heating on ozone generation

The effect of corona wire temperature on the ozone generation in the positive dc corona electrostatic precipitator is studied experimentally. The external heating of the corona wire can suppress the ozone generation. In this study, nichrome and two kinds of silver-based wires 0.1 mm diameter were tested as discharges electrodes. The nichrome corona wire heating shows a well-known monotonic decreasing the rate of ozone production. In the case of the tested silver-based wires the rate of ozone production decreases nonlinearly and passes through a local minimum in the range from 35 to 55 °C with increasing the wire temperature. At the wire temperature about 46 °C ozone generation by positive dc corona discharge is decreased by 53% with Ag:Mn = 0.85:0.15 wire and by 25% with Ag:Ni = 0.7:0.3 wire as compared to the same wire at 26 °C. Under these conditions the corona wire heating increases slightly the corona current and speed of airflow.     

Microscopic theory of Auger recombination in quantum wires

An analysis is made of mechanisms for Auger recombination of nonequilibrium carriers in cylindrical quantum wires. It is shown that two different Auger recombination mechanisms take place in these wires: a quasi-threshold and a nonthreshold mechanism. Both mechanisms are associated with the presence of heterobarriers but are of a different nature. The quasi-threshold mechanism is attributed to the spatial confinement of the carrier wave functions to the region of the quantum wire and in this case the quasi-momentum conservation law is violated and the Auger recombination process is intensified. As the radius of the wire increases, the quasi-threshold Auger recombination process goes over to a threshold process. The nonthreshold mechanism is caused by the scattering of an electron (hole) at the heterojunction; the rate of this nonthreshold Auger recombination tends to zero in the limit of an infinite-radius wire.     

Electrons in a curvilinear quantum wire

The adiabatic motion of electrons in curvilinear quantum wires was studied. It was assumed that the cross section of a wire was constant along its length. The potential that limited electron motion across a wire and the shape of the cross section of the wire were considered arbitrary, while the curvature and the torsion (defined as the derivative of the cross section rotation angle with respect to the length) were assumed to be small. An effective nonrelativistic Hamiltonian for the motion of electrons along a wire with the conservation of transverse quantum numbers was obtained. The spin-orbit coupling Hamiltonian related to the curvature and torsion of a wire was found. Particular cases of a rectilinear twisted quantum wire with a noncircular cross section and a curvilinear quantum wire on a plane were studied. Various transverse potential models limiting the motion of electrons were considered. In particular, the coefficients of the effective Hamiltonian for quantum wires with rectangular and circular cross sections and hard walls and for wires with a parabolic potential were found.     

Optimization of two parallel wires of laser plasma filaments for channeling electromagnetic energy

We theoretically investigated the electromagnetic wave (EMW) transmission along two parallel wires of laser plasma filaments produced by the filamentation of ultrafast laser pulses in air. Many factors, such as wire diameter and separation, electron density, and operating frequency are shown to influence the propagation loss. By taking into consideration the radiation and transmission effects of the wires, the calculations of the two parallel filament wires reasonably agrees with that of the standard commercial twin-lead wire. Specifically, the optimum separation of the two wires is determined for a given frequency and an effective electron density of the wires. When compared with free-space propagation, transmission enhancement of tens dB is obtained using optimized wire configurations. Thus, the two plasma wires may be a potential channel for point to point directed delivery of EM energy or communication of pulsed-modulated EM radiation.     

Negative Coulomb drag in coupled wire and quantum dot system

We measure the Coulomb drag between parallel split-gate quantum wires with a quantum dot embedded in one of the two wires (drive wire). We observe negative Coulomb drag when a Coulomb oscillation peak appears in the drive wire and the conductance of the other wire (drag wire) is slightly below the first plateau. This indicates that correlation holes are dragged in the drag wire by single electron tunneling through the quantum dot in the drive wire. The drag is only promoted in the drag wire near the barrier regions of the dot, and low compressibility of the drag wire is necessary for the negative drag to occur.     

Magnetic characterization of FeCo nanowire arrays by first-order reversal curves

FeCo nanowire arrays have been obtained by current pulse electrodeposition into nanoporous alumina templates. First-order reversal curve (FORC) diagrams have been used to investigate magnetostatic interaction and average coercivity of individual FeCo nanowires embedded in porous alumina templates. The FeCo nanowires with a wires length up to 3 μm and wires diameter ranging from 25 to 50 nm showed interacting single-domain behavior. Using FORC diagrams, the spread of coercivity distribution was seen to be almost independent of the wires diameter, but with increase in diameter the inter-wire magnetostatic interaction was increased. It was found that for arrays with higher diameter, the coercivity of the arrays is lower than the average coercivity of the individual wires. It was detected that an increase in wire diameter results in a considerable increase in the spread of the distribution in the H_u direction of FORC distribution. Curve fitting on the experimental data proved a relatively linear relation between interaction field and square diameter of the nanowires.     

Phonon effect on binding energies of impurity states in cylindrical quantum wires of polar semiconductors under an electric field

Phonon effect on hydrogenic impurity states in cylindrical quantum wires of polar semiconductors under an applied electric field is studied theoretically by a variational approach. The binding energies are calculated as functions of the transverse dimension of the quantum wire, and the donor-impurity position under different fields. The electron–phonon interaction is considered in the calculations by taking both the confined bulk longitudinal optical phonons and interface optical phonons as well as the impurity-ion–phonon coupling. The numerical results for the CdTe and GaAs quantum wires are given and discussed as examples. It is confirmed that the electron–phonon interaction obviously reduces both the binding energy and the Stark energy-shift of the bound polarons in quantum wires.     

Optical emission from the one-dimensional electron gas in narrow modulation-doped GaAs/(InGa)As/(AlGa)As quantum wires fabricated by lateral top barrier modulation

We report on the fabrication and photoluminescence characterisation of n-type doped quantum wires, which are based on a modulation-doped GaAs/(InGa)As/(AlGa)As quantum well structure, as used in inverted high electron mobility transistors. Lateral patterning was performed by electron beam lithography followed by a selective wet etch process to remove the n-type doped GaAs top barrier between the wire regions. The removal of the top barrier was verified by micro-Raman spectroscopy. Spatially indirect emission from the one-dimensional (ID) electron gas formed in the quantum wires is observed in low-temperature photoluminescence, even for the narrowest geometrical wire width of 23 nm. The emission shows a blue-shift for wire widths below 100 nm, which amounts to up to 60 meV for the narrowest wires.PACS: 78.66.Fd, 73.20.Dx, 78.55.Cr     

Exciton trapping, binding and diamagnetic shift in T-shaped quantum wires

We determine the exciton states of T-shaped quantum wires. We use anisotropic effective-mass models to describe the electron and hole states. Pair correlation along the wire axis and in the lateral directions is included. We accurately model the measured redshifts between exciton photoluminescence in quantum wells and T-shaped wires. This redshift arises from enhanced exciton binding and the difference between well and wire confinement energy. We predict a large enhancement of binding energy only when lateral correlation is included, indicating that T-shaped wires arequasirather thanquantum1D wires. We calculate exciton shapes and diamagnetic shifts to determine how the exciton is distorted when confined in a T-wire.     

Self polarization in GaAs–(Ga, Al)As quantum well wires: electric field and geometrical effects

The response of an electron to an external electric field in different shapes of infinite quantum well wires has been investigated. The self-polarization effect which can be defined as the influence of the barrier potential on the impurity electron is studied for the quantum well wire of square, rectangular and cylindrical cross-sections. An external electric field vanishes due to the self-polarization effect has been calculated. It is shown that the self-polarization effect outside of the center depends on both the geometrical form of the wire and the impurity position in the same structure.     

Small-span bending test for determination of elastic-plastic properties of ultrathin Pt wires

A mechanical testing methodology for determination of elastic-plastic properties of very thin metallic wires using small-span bending under lateral load is described. Sufficient strain is locally developed in the tested section of wire by unsymmetrical bending with two opposite probes where one end of the wire is fixed on a substrate with rigid joint by Joule heat welding. From the load-displacement relationships obtained experimentally, Young’s modulus is determined by analytical formulation. Moreover, yield stress and hardening modulus of the wire are identified using an optimization strategy with finite-element analyses. The thin Pt wires with a nominal diameter of about 625 nm are examined by the testing scheme and the wires are found to have higher yield stress compared to bulk Pt.     

Conductance of short quantum wires with sharp boundaries

The conductance of short ballistic wires with boundaries, whose curvature radius is comparable to the electron wavelength, is investigated. It is found that, in such wires, no conductance quantization takes place. Instead, pronounced interference effects are observed. These effects are related to the Fabry-Perot interference by the wire edges and with the mesoscopic interference caused by the coherent scattering of electrons by the wire edges and by the impurities located in the near-wire regions of the 2D electron gas. Original Russian Text ? D.A. Kozlov, Z.D. Kvon, A.E. Plotnikov, D.V. Shcheglov, A.V. Latyshev, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 86, No. 10, pp. 752–756.     

Atom chips in the real world: the effects of wire corrugation

We present a detailed model describing the effects of wire corrugation on the trapping potential experienced by a cloud of atoms above a current carrying micro wire. We calculate the distortion of the current distribution due to corrugation and then derive the corresponding roughness in the magnetic field above the wire. Scaling laws are derived for the roughness as a function of height above a ribbon shaped wire. We also present experimental data on micro wire traps using cold atoms which complement some previously published measurements [CITE] and which demonstrate that wire corrugation can satisfactorily explain our observations of atom cloud fragmentation above electroplated gold wires. Finally, we present measurements of the corrugation of new wires fabricated by electron beam lithography and evaporation of gold. These wires appear to be substantially smoother than electroplated wires.     

Ground state description of bound polarons in parabolic quantum wires

The Feynman-Haken variational path integral theory is, for the first time, generalized to calculate the ground-state energy of an electron coupled simultaneously to a Coulomb potential and to a longitudinal-optical (LO) phonon field in parabolic quantum wires. It is shown that the polaronic correction to the ground-state energy is more sensitive to the electron-phonon coupling constant than the Coulomb binding parameter and monotonically stronger as the effective wire radius decreases. We apply our calculations to several semiconductor quantum wires and find that the polaronic correction can be considerably large. Received 16 November 1998     

抛物量子线中强耦合极化子的有效质量

采用改进的线性组合算符法、Lagrange乘子和变分法,在考虑电子与LO声子相互作用情况下,研究了抛物量子线中强耦合极化子的有效质量和光学声子平均数。通过数值计算,讨论了约束强度ω₀和拉格朗日乘子u对极化子的有效质量m^*和光学声子平均数N及极化子振动频率λ的影响。计算结果表明:有效质量m^*和光学声子平均数N及极化子振动频率λ都随着约束强度ω₀和拉格朗日乘子u的增加而增大。     

Magnetic anisotropy and anisotropic ballistic conductance of thin magnetic wires

The magnetocrystalline anisotropy of thin magnetic wires of iron and cobalt is quite different from the bulk phases. The spin moment of monatomic Fe wire may be as high as 3.4 μ_B, while the orbital moment as high as 0.5 μ_B. The magnetocrystalline anisotropy energy (MAE) was calculated for wires up to 0.6 nm in diameter starting from monatomic wire and adding consecutive shells for thicker wires. I observe that Fe wires exhibit the change sign with the stress applied along the wire. It means that easy axis may change from the direction along the wire to perpendicular to the wire. We find that ballistic conductance of the wire depends on the direction of the applied magnetic field, i.e. shows anisotropic ballistic magnetoresistance. This effect occurs due to the symmetry dependence of the splitting of degenerate bands in the applied field which changes the number of bands crossing the Fermi level. We find that the ballistic conductance changes with applied stress. Even for thicker wires the ballistic conductance changes by factor 2 on moderate tensile stain in our 5×4 model wire. Thus, the ballistic conductance of magnetic wires changes in the applied field due to the magnetostriction. This effect can be observed as large anisotropic BMR in the experiment.