Wire Junctions Research Articles

Behavior of 1 Ω resistors at frequencies below 1 Hz and the problem of assigning a dc value

This paper presents a systematic study of the effects of reversals of current polarity on 1 Ω standard resistors. A selection of 1 Ω standards were measured on a dc cryogenic current comparator bridge and also on an ac current comparator bridge operating at 1 Hz. We find that the apparent 4 terminal resistance depends strongly on the dc reversal rate, in one case showing a variation of more than 0.1 μΩ/Ω for cycle times between 10 s and 1000 s. Even in the best cases, effects of at least 0.01 μΩ/Ω are present. Furthermore, the assumption that the apparent resistance value converges to a limiting value for long waiting times after reversal is not always upheld, making it difficult to quote a simple dc value. Existing models of Peltier heating at wire junctions do not completely explain our observations. The problems presented have been the limiting factor for 1 Ω measurements as part of a recent on-site comparison of quantum Hall effect resistance standards.

Effect on Electrical Characteristics of Contact Condition among Wire Junctions of Mesh Surfaces for Reflector Antennas

Effect on Electrical Characteristics of Contact Condition among Wire Junctions of Mesh Surfaces for Reflector Antennas

Triazatriangulene as binding group for molecular electronics.

The triazatriangulene (TATA) ring system was investigated as a binding group for tunnel junctions of molecular wires on gold surfaces. Self-assembled monolayers (SAMs) of TATA platforms with three different lengths of phenylene wires were fabricated, and their electrical conductance was recorded by both conducting probe-atomic force microscopy (CP-AFM) and scanning tunneling microscopy (STM). Similar measurements were performed for phenylene SAMs with thiol anchoring groups as references. It was found that, despite the presence of a sp(3) hybridized carbon atom in the conduction path, the TATA platform displays a contact resistance only slightly larger than the thiols. This surprising finding has not been reported before and was analyzed by theoretical computations of the transmission functions of the TATA anchored molecular wires. The relatively low contact resistance of the TATA platform along with its high stability and directionality make this binding group very attractive for molecular electronic measurements and devices.

Silver Nanowire Electrodes: Conductivity Improvement Without Post-treatment and Application in Capacitive Pressure Sensors.

Transparent electrode based on silver nanowires (AgNWs) emerges as an outstanding alternative of indium tin oxide film especially for flexible electronics. However, the conductivity of AgNWs transparent electrode is still dramatically limited by the contact resistance between nanowires at high transmittance. Polyvinylpyrrolidone (PVP) layer adsorbed on the nanowire surface acts as an electrically insulating barrier at wire–wire junctions, and some devastating post-treatment methods are proposed to reduce or eliminate PVP layer, which usually limit the application of the substrates susceptible to heat or pressure and burden the fabrication with high-cost, time-consuming, or inefficient processes. In this work, a simple and rapid pre-treatment washing method was proposed to reduce the thickness of PVP layer from 13.19 to 0.96 nm and improve the contact between wires. AgNW electrodes with sheet resistances of 15.6 and 204 Ω sq−1 have been achieved at transmittances of 90 and 97.5 %, respectively. This method avoided any post-treatments and popularized the application of high-performance AgNW transparent electrode on more substrates. The improved AgNWs were successfully employed in a capacitive pressure sensor with high transparency, sensitivity, and reproducibility.Electronic supplementary materialThe online version of this article (doi:10.1007/s40820-014-0018-0) contains supplementary material, which is available to authorized users.

GaAs nanowire array solar cells with axial p-i-n junctions.

Because of unique structural, optical, and electrical properties, solar cells based on semiconductor nanowires are a rapidly evolving scientific enterprise. Various approaches employing III-V nanowires have emerged, among which GaAs, especially, is under intense research and development. Most reported GaAs nanowire solar cells form p-n junctions in the radial direction; however, nanowires using axial junction may enable the attainment of high open circuit voltage (Voc) and integration into multijunction solar cells. Here, we report GaAs nanowire solar cells with axial p-i-n junctions that achieve 7.58% efficiency. Simulations show that axial junctions are more tolerant to doping variation than radial junctions and lead to higher Voc under certain conditions. We further study the effect of wire diameter and junction depth using electrical characterization and cathodoluminescence. The results show that large diameter and shallow junctions are essential for a high extraction efficiency. Our approach opens up great opportunity for future low-cost, high-efficiency photovoltaics.

Anomalous length-independent frontier resonant transmission peaks in armchair graphene nanoribbon molecular wires

Molecular wires constitute the building blocks for nanoscale interconnects. However, the exponential decrease in conductance with wire length severely limits their applications. We predict, using first principles calculations, that armchair graphene nanoribbon (AGNR) wires, connected by transverse zigzag edges to wider AGNR electrodes, can exhibit anomalous resonant transmission peaks that are nearly independent of the wire length. We propose a new model to explain the unusual length independence of peak energies from the locally repeating property of the wavefunction in the middle-AGNR. We further uncover that this locally repeating pattern originates from states of a perfect AGNR with infinite length. The pattern can be well preserved when the AGNR is connected to wider AGNR leads because of the zigzag edges serving as electron sources and drains. The length independence of peak widths results from the zigzag edges absorbing most of the wavefunction renormalization as the length increases, so that the coupling strength to the electrodes does not change significantly. These anomalous properties arising from intrinsic wavefunction properties of the AGNRs are in sharp contrast to typical transmission properties of traditional molecular wire junctions, which suggests promising potential application as “molecular wire” interconnects in nano-electronics.

Single Molecular Resistive Switch Obtained via Sliding Multiple Anchoring Points and Varying Effective Wire Length

A single molecular resistive (conductance) switch via control of anchoring positions was examined by using a molecule consisting of more than two same anchors. For this purpose, we adopted the covered quaterthiophene (QT)-based molecular wire junction. The QT-based wire consisted of two thiophene ring anchors on each side; thus, shift of anchors was potentially possible without a change in the binding modes and distortion of the intramolecular structure. We observed three distinct conductance states by using scanning tunneling microscope-based break junction technique. A detailed analysis of the experimental data and first-principles calculations revealed that the mechanism of the resistive switch could be explained by standard length dependence (exponential decay) of conductance. Here, the length is the distance between the anchoring points, i.e., length of the bridged π-conjugated backbone. Most importantly, this effective tunneling length was variable via only controlling the anchoring positions in the same molecule. Furthermore, we experimentally showed the possibility of a dynamic switch of anchoring positions by mechanical control. The results suggested a distinct strategy to design functional devices via contact engineering.

Electroluminescence of a Polythiophene Molecular Wire Suspended between a Metallic Surface and the Tip of a Scanning Tunneling Microscope

The electroluminescence of a polythiophene wire suspended between a metallic surface and the tip of a scanning tunneling microscope is reported. Under positive sample voltage, the spectral and voltage dependencies of the emitted light are consistent with the fluorescence of the wire junction mediated by localized plasmons. This emission is strongly attenuated for the opposite polarity. Both emission mechanism and polarity dependence are similar to what occurs in organic light emitting diodes (OLED) but at the level of a single molecular wire.

Early detachment of the Solitaire stent during thrombectomy retrieval: an in vitro investigation

ObjectiveTo elucidate the early detachment of the Solitaire stent during mechanical thrombectomy, we tested the stent in vitro under various conditions and evaluated with microscopy.MethodsA total of five stents were...

Tunable channel width of a UV-gate field effect transistor based on ZnO micro-nano wire

A n-channel field effect transistor (FET) based on ZnO microwire has been fabricated for ultraviolet detection, where a PEDOT:PSS/ZnO wire junction serves as the gate. The sensitivity of the junction FET was enhanced by two orders of magnitude with a fast response time <1 s at 3 V compared with a Ag–ZnO–Ag detector under the illumination of UV light (325 nm). Such a great improvement in photoresponse is attributed to the introduction of a depletion layer, resulting in a lower dark current. The change of the junction FET channel width with various UV light intensities was calculated and discussed in terms of the carrier diffusion theory and the ideal I–V characteristics of the depletion mode JFET.

Unfrozen spin effect in a composite 1D topological superconductor

The spin of Majorana fermions (MF) is nearly frozen by the applied magnetic field in a composite 1D topological superconductor due to the weak spin-orbit coupling (SOC). In this work, we address theoretically the strong SOC limit and weak-magnetic-field effect, under which the spin degree of freedom of MFs is reactivated and dependent on the wire direction. The quantized zero-energy conductance peak in a bent N/S wire junction can be severely suppressed. The MF phase is significantly modified due to the spin noncollinearity and the fusion of MFs is independent of the wire direction, which is opposite to the strong-magnetic-field case. It is also found that the spatial distribution of an MF could be heavily affected by the wire direction. The findings are helpful to identify and braid MFs in the 1D wire network.

Soldering a 3D Wire Lattice Structure

Soldering the junctions in a novel woven structure constructed from metallic wires is expected to significantly enhance the mechanical properties of that structure. The ideal bond geometry has solder localized at wire junctions and a uniform amount of solder at each junction throughout the structure. By examining the effects of solvent variations, liquid flux concentration, and solder powder concentration on the soldered joint geometry and distribution, we have developed a new procedure to bond these wire junctions using a very fine solder powder suspended in a flux solution.

Frequency-dependent admittance of a short superconducting weak link

We consider the linear and nonlinear electromagnetic responses of a nanowire connecting two bulk superconductors. The Andreev states appearing at a finite phase bias substantially affect the finite-frequency admittance of such a wire junction. Electron transitions involving the Andreev levels are easily saturated, leading to the nonlinear effects in photon absorption for the subgap photon energies. We evaluate the complex admittance analytically at an arbitrary frequency and arbitrary, possibly nonequilibrium, occupation of the Andreev levels. Special care is given to the limits of a single-channel contact and a disordered metallic weak link. We also evaluate the quasistatic fluctuations of admittance induced by fluctuations of the occupation factors of the Andreev levels. In view of possible qubit applications, we compare properties of a weak link with those of a tunnel Josephson junction. Compared to the latter, a weak link has smaller low-frequency dissipation. However, because of the deeper Andreev levels, the low-temperature quasistatic fluctuations of the inductance of a weak link are exponentially larger than of a tunnel junction. These fluctuations limit the applicability of nanowire junctions in superconducting qubits.

Majorana fermions and odd-frequency Cooper pairs in a normal-metal nanowire proximity-coupled to a topological superconductor

We discuss a strong relationship between Majorana fermions and odd-frequency Cooper pairs which appear at a disordered normal (N) nano wire attached to a topologically nontrivial superconducting (S) one. The transport properties in superconducting nano wire junctions show universal behaviors irrespective of the degree of disorder: the quantized zero-bias differential conductance at 2e^{2}/h in NS junction and the fractional current-phase (J-\varphi) relationship of the Josephson effect in SNS junction J\propto \sin(\varphi/2). Such behaviors are exactly the same as those in the anomalous proximity effect of odd-parity spin-triplet superconductors. We show that odd-frequency Cooper pairs support the universal transport properties. The odd-frequency pairs exist wherever the Majorana fermions stay.

Luttinger liquid in a non-equilibrium steady state

We propose and investigate an exactly solvable model of non-equilibrium Luttinger liquid on a star graph, modeling a multi-terminal quantum wire junction. The boundary condition at the junction is fixed by an orthogonal matrix , which describes the splitting of the electric current among the leads. The system is driven away from equilibrium by connecting the leads to heat baths at different temperatures and chemical potentials. The associated non-equilibrium steady state depends on and is explicitly constructed. In this context, we develop a non-equilibrium bosonization procedure and compute some basic correlation functions. Luttinger liquids with general anyon statistics are considered. The relative momentum distribution away from equilibrium turns out to be the convolution of equilibrium anyon distributions at different temperatures. Both the charge and heat transport are studied. The exact current–current correlation function is derived and the zero-frequency noise power is determined.

Networks of quantum wire junctions: A system with quantized integer Hall resistance without vanishing longitudinal resistivity

We consider a honeycomb network built of quantum wires, with each node of the network having a Y junction of three wires with a ring through which flux can be inserted. The junctions are the basic circuit elements for the network, and they are characterized by $3\ifmmode\times\else\texttimes\fi{}3$ conductance tensors. The low energy stable fixed point tensor conductances result from quantum effects, and are determined by the strength of the interactions in each wire and the magnetic flux through the ring. We consider the limit where there is decoherence in the wires between any two nodes, and study the array as a network of classical three-lead circuit elements whose characteristic conductance tensors are determined by the quantum fixed point. We show that this network has some remarkable transport properties in a range of interaction parameters: It has a Hall resistance quantized at ${R}_{xy}=h/{e}^{2}$, although the longitudinal resistivity is nonvanishing. We show that these results are robust against disorder, in this case nonhomogeneous interaction parameters $g$ for the different wires in the network.

Zero-bias conductance quantization in a normal / superconducting junction of nano wire

We discuss a strong relationship between Majorana fermions and odd-frequency Cooper pairs which appear at a disordered normal nano wire attached to a topologically nontrivial superconducting one. The zero-bias differential conductance in a normal / superconducting nano wire junctions is quantized at 2e2/h irrespective of degree of disorder, length of disordered segment, and random realization of disordered potential. Such behaviors are exactly the same as those in the anomalous proximity effect of px-wave spin-triplet superconductors. We show that odd-frequency Cooper pairs assist the unusual transport properties.

Solution-processed flexible transparent conductors composed of silver nanowire networks embedded in indium tin oxide nanoparticle matrices

Although silver nanowire meshes have already demonstrated sheet resistance and optical transmittance comparable to those of sputter-deposited indium tin oxide thin films, other critical issues including surface morphology, mechanical adhesion and flexibility have to be addressed before widely employing silver nanowire networks as transparent conductors in optoelectronic devices. Here, we demonstrate the efficacy of low temperature solution-processed flexible metal nanowire networks embedded in a conductive metal oxide nanoparticle matrix as transparent conductors, and investigate their microstructural, optoelectronic, and mechanical properties in attempting to resolve nearly all of the technological issues imposed on silver nanowire networks. Surrounding silver nanowires by conductive indium tin oxide nanoparticles offers low wire to wire junction resistance, smooth surface morphology, and excellent mechanical adhesion and flexibility while maintaining the high transmittance and the low sheet resistance. In addition, we discuss the relationship between sheet resistance and transmittance in the silver nanowire composite transparent conductors and their maximum achievable transmittance. Although we have selected silver nanowires and indium tin oxide nanoparticle matrix as demonstration materials, we anticipate that various metal nanowire meshes embedded in various conductive metal oxide nanoparticle matrices can effectively serve as transparent conductors for a wide variety of optoelectronic devices owing to their superior performance, simple, cost-effective, and gentle processing.

Electric current in star junctions of molecular wires

There is an enormous amount of literature on molecular-size circuits. However, no common regularities have ever been reported regarding the embedding of Y-like or, more generally, starlike molecular junctions into circuits. At the same time, connections of several wires are unavoidable components of any circuit. We show that in star junctions of $\mathcal{N}$ identical molecular wires, which are equally coupled to the feeding leads, the branched current is inversely proportional to ${\mathcal{N}}^{2}$. It is also proved that, independently of molecular structure, the minimal resistance that is associated with the terminal-to-terminal current through such junctions is equal to $h{\mathcal{N}}^{2}/8{e}^{2}$. In words, it is divided by eight von Klitzing resistance times ${\mathcal{N}}^{2}$. These predictions rule the performance of quantum wire junctions and provide useful references for studies of complex multiterminal molecular devices. The current-voltage dependence predicted for the circuits that contain a star junction of molecular wires is compared with I-V characteristics of constructively similar junctions of conventional resistors, where the Ohm law prescribes the proportionality of branched current to $1/\mathcal{N}$. The newly derived basic formulas include the exact solution of Lippman-Schwinger equation for a general model of multiterminal, rigid (coherent) scatterer, and also, a new and general trace formula for multiterminal transmission.

Entanglement entropy of quantum wire junctions

We consider a fermion gas on a star graph modeling a quantum wire junction and derive the entanglement entropy of one edge with respect to the rest of the junction. The gas is free in the bulk of the graph, the interaction being localized in its vertex and described by a non-trivial scattering matrix. We discuss all point-like interactions, which lead to a unitary time evolution of the system. We show that for a finite number of particles N, the Rényi entanglement entropies of one edge grow as ln N with a calculable prefactor, which depends not only on the central charge, but also on the total transmission probability from the considered edge to the rest of the graph. This result is extended to the case with a harmonic potential in the bulk.