Transport In Wires Research Articles

Superdiffusive transport in two-dimensional fermionic wires

Superdiffusive transport in two-dimensional fermionic wires

Improving the superconducting properties of 100 m class MgB2 wire with 18+1 filaments produced via an internal Mg diffusion process

Internal Mg diffusion (IMD) technology reveals the expansive prospects of MgB2 superconducting wires. To facilitate the practical application of IMD, a series of 100 m class MgB2 wires with 18 + 1 filaments were prepared in this study. Based on the optical micrographs and performance analyses, the annealing temperature, B powder type and diameter of wires were refined. The results indicated that introducing C-coated B powder into wires would accelerate the diffusion of Mg melt and enhance the J e and layer J c significantly, while shrinking the wire diameter would further improve the wire transport capability. The optimal J e and layer J c of wire with an 0.8 mm diameter reached 1.2 × 104 and 1.1 × 105 A cm−2, respectively at 4.2 K and 8 T, which contained C-coated B powder and was annealed at 650 °C. These improved wires were eligible for magnet use in terms of performances, integrity, length and uniformity.

Topologically protected subdiffusive transport in two-dimensional fermionic wires

Topologically protected subdiffusive transport in two-dimensional fermionic wires

Charge Puddles Driven Complex Crossover of Magnetoresistance in Non-Topological Sulfur Doped Antimony Selenide Nanowires.

A race to achieve a crossover from positive to negative magnetoresistance is intense in the field of nanostructured materials to reduce the size of memory devices. Here, the unusual complex magnetoresistance in nonmagnetic sulfur-doped Sb2Se3nanowires is demonstrated. Intentionally, sulfur is doped in such a way to nearly achieve the charge neutrality point that is evident from switching of carrier type from p-type to n-type at 13 K as inferred from the low-temperature thermoelectric power measurements. A change from 3D variable range hopping (VRH) to power law transport with α = 0.18 in resistivity measurement signifies a Luttinger liquid transport with weak links through the nanowires. Interestingly, high magnetic field induced negative magnetoresistance (NMR) occurring in hole dominated temperature regimes can only be explained by invoking the concept of charge puddles. Spot energy dispersive spectroscopy (EDS), magnetic force microscopy (MFM) measurements, Tmottand Regel plot indicate an enhanced disorder in these sulfurized nanowires that are found to be the precursor for the formation of these charge puddles. Tunability of conducting states in these nanowires is investigated in the light of interplay of carrier type, magnetic field, temperature, and intricate intra-inter wire transport that makes this nanowires potential for large scale spintronic devices.

Tuning multichannel conductance via through-space conjugated naphthalene

The single-molecular conductance results and theoretical calculations reveal that through-bond and through-space conjugation are integrated into one single molecule, allowing for multichannel transport in Naph-based molecular wires.

Controlling Charge Transport in Molecular Wires through Transannular π-π Interaction.

This paper describes the influence of the transannular π-π interaction in controlling the carrier transport in molecular wires by employing the STM break junction technique. Five pentaphenylene-based molecular wires that contained [2.2]paracyclophane-1,9-dienes (PCD) as the building block were prepared as model compounds. Functional substituents with different electronic properties, ranging from strong acceptors to strong donors, were attached to the top parallel aromatic ring and used as a gate. It was found that the carrier transport features of these molecular wires, such as single-molecule conductance and a charge-tunneling barrier, can be systematically controlled through the transannular π-π interaction.

Electron Transport in AlGaAs Cylindrical Quantum Wire Sandwiched between Two GaAs Cylindrical Quantum Well Wires

In this work, we study theoretically and analytically the electronic transport through a nanowire structure composed of a finite cylindrical quantum wire (CQWR) based on barrier AlGaAs semiconductor, sandwiched between two semi-infinite cylindrical GaAs quantum well wires (CQWWRs). Using the Green function approach to determine the electronic structure of this artificial nanostructure, which is analyzed as a function of the geometrical and physical parameters of nanowires structure. The results show the eigen states (confined states), when they interact with the incoming electronic waves from the first semi-infinite cylindrical GaAs quantum well wire. The decrease of the radius of the system leads to the energy quantization of the electrons and the electronic states move towards high energies until a critical radius Rc=20Å below which no electronic state can exist. In addition, we found that the electronic energy levels of the finite cylindrical quantum wire depend on the mole fraction of aluminum and the ratio between the radius of the cylindrical nanowires and the thickness of the barrier, which are the most important parameters in the optimization of the cylindrical quantum wires nanostructure.Keywords: Cylindrical Quantum Wire, Nanowire, Electronic States, Green Function

Nonequilibrium Electrical, Thermal and Spin Transport in Open Quantum Systems of Topological Superconductors, Semiconductors and Metals

We study nonequilibrium transport in various open quantum systems whose systems and leads/baths are made of topological superconductors (TSs), semiconductors, and metals. Using quantum Langevin equations and Green’s function method, we derive exact expressions for steady-state electrical, thermal, and spin current at the junctions between a system and leads. We validate these current expressions by comparing them with the results from direct time-evolution simulations. We then show how an electrical current injected in TS wires divides into two parts carried by single electronic excitations and Cooper pairs. We further show ballistic thermal transport in an open TS wire in the topological phase under temperature or voltage bias. The thermal current values grow significantly near the topological phase transition, where thermal conductance displays a sharp quantized peak as predicted earlier. We relate the quantized thermal conductance to the zero-frequency thermoelectric transmission coefficient of the open TS wire. We also observe a large thermoelectric current near the topological transition of the TS wires. We introduce a differential spin conductance which displays a quantized zero-bias peak at zero temperature for a spinful TS wire in the topological phase. The role of superconducting baths in transport is demonstrated by thoroughly examining the features of zero-temperature differential electrical conductance and thermal conductance in open systems with TS baths. Our new thermoelectric and spin transport findings in various two-terminal geometries are beneficial to the present challenges in probing the emergence of Majorana quasi-particles in experiments.

Electronic Transport in Molecular Wires of Precisely Controlled Length Built from Modular Proteins.

DNA molecular wires have been studied extensively because of the ease with which molecules of controlled length and composition can be synthesized. The same has not been true for proteins. Here, we have synthesized and studied a series of consensus tetratricopeptide repeat (CTPR) proteins, spanning 4 to 20 nm in length, in increments of 4 nm. For lengths in excess of 6 nm, their conductance exceeds that of the canonical molecular wire, oligo(phenylene-ethylenene), because of the more gradual decay of conductance with length in the protein. We show that, while the conductance decay fits an exponential (characteristic of quantum tunneling) and not a linear increase of resistance with length (characteristic of hopping transport), it is also accounted for by a square-law dependence on length (characteristic of weakly driven hopping). Measurements of the energy dependence of the decay length rule out the quantum tunneling case. A resonance in the carrier injection energy shows that allowed states in the protein align with the Fermi energy of the electrodes. Both the energy of these states and the long-range of hopping suggest that the reorganization induced by hole formation is greatly reduced inside the protein. We outline a model for calculating the molecular-electronic properties of proteins.

Electron transport probing the electrically tunable topological phase transition in a Dirac semimetal

Electron transport in a Dirac semimetal (DSM) quantum wire without/with an external electric field is investigated by using the Green-function-based Landauer-B\"uttiker formula. Normal-topological phase transition is certificated by the different characteristics between the DSM quantum wire without and with the electric field, including the orbital decomposition for the energy bands, transversally local charge and spin polarization density distributions, topological Chern number, transport behaviors, and disorder effects. Topologically trivial surface states are found in the former system and their transport can be localized by disorders. However, topologically nontrivial surface states are generated in the latter one and their transport is robust against strong disorders when the electric field strength is larger than a critical value. In addition, switching effect of the electron transport in the DSM quantum wire can be achieved by varying the electric field strength. Further studies indicate that the switching effect shows a strong robustness against disorder, displaying the feasibility of designing a topological field effect transistor based on the considered system.

Promotion and suppression of single-molecule conductance by quantum interference in macrocyclic circuits

Promotion and suppression of single-molecule conductance by quantum interference in macrocyclic circuits

Root formation and metallurgical challenges in laser beam and laser-arc hybrid welding of thick structural steel

Single-pass laser beam welding (LBW) of steel components with wall thickness of > 10 mm is of high interest due to enhanced productivity. Deep penetration LBW provides excessive hardness and certain quality issues such as root humping in flat position, which is associated with disability of surface tension to sustain melt dropout. High hardness is associated with fast cooling rates and shortage of filler wire transportation to the root of the fusion zone. Use of laser-arc hybrid welding (LAHW) can promote acicular ferrite by adding filler metal and additional heat input from the arc. However, LAHW may promote humping and adjustment of many parameters is required hindering its application. In this work, a 16 kW disk laser was used in butt welding of 12 mm and 15 mm thick plates with different bevelling geometries. Root humping occurred within a wide range of process parameters providing narrow process window. Twelve millimeter thick plates were successfully welded with a single-pass technique providing good quality of root by using zero air gap regardless bevelling geometry. Welding of 15 mm plates was more challenging, and the process was sensitive even with a slight parameter change. Improved results were achieved with application of small air gap. Acceptable hardness in both weld metal and heat affected zone (< 290 HV) was achieved for both plate thicknesses providing good toughness of minimum 27 J at −50°C.

Testing of an MgB2 coil for a wind turbine generator pole

We report on the testing of a 1 m long and 0.5 m wide racetrack coil wound from 4.5 km of MgB2 tape-shaped superconductors with a copper strip soldered to the nickel matrix. The cross-section of the coil corresponds to what is needed for the field windings of a 10 MW direct drive wind turbine generator pole, but where the straight section has been shortened. Ten double pancake sub-coils were wet-wound, with epoxy as the only electrical insulation, and assembled. The entire coil was cooled down to 14 K, and the direct current was ramped up. Voltage measurements were made over the double pancake coils. For eight of the ten double pancake coils, excessive voltages appeared at currents much lower than the design current. These voltages may either be attributed to the wire quality or to the winding process. A quench was observed at 149 A, which is in reasonable agreement with the predicted load line of the coil obtained by scaling wire transport measurements at 1 T and 20 K with hysteretic magnetization measurements in the range 1-5 T and 5-20 K.

Hidden duality and accidental degeneracy in cycloacene and Möbius cycloacene

The accidental degeneracy appearing in cycloacenes as triplets and quadruplets is explained with the concept of segmentation, introduced here with the aim of describing the effective disconnection of π orbitals on these organic compounds. For periodic systems with time reversal symmetry, the emergent nodal domains are shown to divide the atomic chains into simpler carbon structures analog to benzene rings, diallyl chains, anthracene (triacene) chains, and tetramethyl-naphthalene skeletal forms. The common electronic levels of these segments are identified as members of degenerate multiplets of the global system. The peculiar degeneracy of Möbius cycloacene is also explained by segmentation. In the last part, it is shown that the multiplicity of energies for cycloacene can be foreseen by studying the continuous limit of the tight-binding model; the degeneracy conditions are put in terms of Chebyshev polynomials. The results obtained in this work have important consequences on the physics of electronic transport in organic wires, together with their artificial realizations.

Engineering electron wavefunctions in asymmetrically confined quasi one-dimensional structures

We present results on electron transport in quasi-one dimensional quantum wires in GaAs/AlGaAs heterostructures obtained using an asymmetric confinement potential. The variation of the energy levels of the spatially quantized states is followed from strong confinement through weak confinement to the onset of two dimensionality. An anticrossing of the initial ground and first excited states is found as the asymmetry of the potential is varied, giving rise to two anticrossing events, which occur on either side of symmetric confinement. We present results analyzing this behavior and showing how it can be affected by the inhomogeneity in background potential. The use of an enhanced source-drain voltage to alter the energy levels is shown to be a significant validation of the analysis by showing the formation of double rows of electrons, which correlate with the anticrossing.

Machining of cylindrical parts by wire electrical discharge grinding using thin plate guide

In order to meet the processing requirements of precise cylindrical parts on hard and difficult-to-machine materials, this paper introduces wire electrical discharge grinding (WEDG) using thin plate wire guide for cylindrical parts machining. The spindle system and the wire transport system to realize this processing method was added to a low speed wire electrical discharge machining (LS-WEDM) machine tool. In order to realize the online measurement of the machining accuracy of the cylindrical parts, an online measurement system of two-degree-of-freedom based on laser displacement sensor was built. The reasons for the incomplete removal of the machined surface were analyzed. Through the application of a new tool setting strategy and reasonable selection of processing parameters, the phenomenon of incomplete processing can be avoided. The experimental research on the effects of part rotational speed, wire feed rate, wire running speed and wire tension on roundness, surface roughness Sa, and surface morphology were carried out. Finally, example of the cylindrical parts is given to verify the machining accuracy.

Nano-sized amorphous/semi-crystalline boron ratio and external Mg effects on transport and structural properties in MgB2 IMD wires

In the present study, Mg diffusion process has been investigated by transport measurements and surface morphology analysis in round MgB2 wires made with internal Mg diffusion (IMD) method. These wires were produced using semi-amorphous boron, nano-sized amorphous boron or mixtures of them via cold drawing process. In addition to the conventional IMD wire production technique, a thin layer of magnesium was initially coated on the interior of the iron sheath and diffused into the boron powder during sintering. It has been found that Mg coating is effective on improving the engineering current density of IMD wires having relatively thick MgB2 cores. Surprisingly, Mg coating is also effective in non-stoichiometric hybrid IMD wires containing highly excessive amount of Mg.

Coherent Effects in Charge Transport in Molecular Wires: Toward a Unifying Picture of Long-Range Hole Transfer in DNA.

In the framework of a multistep mechanism in which environmental motion triggers comparatively faster elementary electron-transfer steps and stabilizes hole-transfer products, microscopic coherence is crucial for rationalizing the observed yield ratios of oxidative damage to DNA. Interference among probability amplitudes of indistinguishable electron-transfer paths is able to drastically change the final outcome of charge transport, even in DNA oligomers constituted by similar building blocks, and allows for reconciling apparently discordant experimental observations. Properly tailored DNA oligomers appear to be a promising workbench for studying tunneling in the presence of dissipation at the macroscopic level.

Anisotropic Charge Transport in Nanoscale DNA Wire

A new computational framework based on multiscale modeling approach is developed to calculate the current–voltage (V–I) characteristics of a double stranded DNA (dsDNA) attached between two gold el...

Transport in magnetically doped one-dimensional wires: can the helical protection emerge without the global helicity?

We study the phase diagram and transport properties of arbitrarily doped quantum wires functionalized by magnetic adatoms. The appropriate theoretical model for these systems is a dense one-dimensional Kondo lattice (KL) which consists of itinerant electrons interacting with localized quantum magnetic moments. We discover the novel phase of the locally helical metal where transport is protected from a destructive influence of material imperfections. Paradoxically, such a protection emerges without a need of the global helicity, which is inherent in all previously studied helical systems and requires breaking the spin-rotation symmetry. We explain the physics of this protection of the new type, find conditions, under which it emerges, and discuss possible experimental tests. Our results pave the way to the straightforward realization of the protected ballistic transport in quantum wires made of various materials.