Inter-wire Coupling Research Articles

Complex Three-Dimensional Magnetic Ordering in Segmented Nanowire Arrays.

A comprehensive three-dimensional picture of magnetic ordering in high-density arrays of segmented FeGa/Cu nanowires is experimentally realized through the application of polarized small-angle neutron scattering. The competing energetics of dipolar interactions, shape anisotropy, and Zeeman energy in concert stabilize a highly tunable spin structure that depends heavily on the applied field and sample geometry. Consequently, we observe ferromagnetic and antiferromagnetic interactions both among wires and between segments within individual wires. The resulting magnetic structure for our nanowire sample in a low field is a fan with magnetization perpendicular to the wire axis that aligns nearly antiparallel from one segment to the next along the wire axis. Additionally, while the low-field interwire coupling is ferromagnetic, application of a field tips the moments toward the nanowire axis, resulting in highly frustrated antiferromagnetic stripe patterns in the hexagonal nanowire lattice. Theoretical calculations confirm these observations, providing insight into the competing interactions and resulting stability windows for a variety of ordered magnetic structures. These results provide a roadmap for designing high-density magnetic nanowire arrays for spintronic device applications.

On designing an efficient numerical-based forbidden pattern free crosstalk avoidance codec for reliable data transfer of NoCs

Inter-wire coupling capacitances can lead to crosstalk fault that is strongly dependent on the transition patterns appearing on the wires. These transition patterns can cause mutual influences between adjacent wires of NoCs and as a result threaten the reliability of data transfer seriously. To increase the reliability of NoCs against the crosstalk fault, Forbidden Pattern Free (FPFs) codes are used. To generate FPF codes, numerical systems are among the overhead-efficient mechanisms. The algorithms of numerical systems have direct effect on the amounts of the codec overheads including power consumption, area occupation and performance of NoCs. To find an overhead-efficient numerical system, this paper proposes an Algorithm for Generating FPF Numerical systems (AGFN). AGFN can provide a tradeoff for designers in selecting overhead-efficient FPF numerical systems. Using this algorithm, collection of Fibonacci-based and non-Fibonacci-based numerical systems can be generated. Then, using AGFN, non-Fibonacci-based FPF numerical system called Summation-based-Subtracted-Added-Penultimate (S2AP) is generated. Experimental results indicate that S2AP not only reduces the worst crosstalk effects by completely removing the Triplet Opposite Direction (TOD) transitions, but also it can significantly improve power consumption, area occupation and critical path overheads of codec with respect to the other state-of-the-art FPF codes.

Forbidden Free Pattern Crosstalk Avoidance

In this work, we present the crosstalk is a noise caused by inter-wire coupling capacitance between adjacent wires, and causes logic malfunctions and delay faults. Since the degree of crosstalk interference is determined by various factors, such as physical inter-wire coupling capacitance, length of wires running in parallel, switching speed of signals, and transition timing of adjacent signals, it is very difficult for VLSI designers to estimate the crosstalk influence in sufficient accuracy. In this paper, a technique for reduction of maximum bus delay caused by crosstalk is proposed. By approximated equation of bus delay, it becomes clear that our technique is effective for repeater-inserted bus. The result of simulation shows the total bus delay can be reduced.

Micromagnetic Structure of Co Stripe Arrays With Tuned Anisotropy

An interwire magnetostatic coupling is one of the most important effects in the micro- and nanowire arrays, because of it strongly influences on the magnetization switching, spin-wave dynamics and magneto-transport properties. Manipulation of the magnetostatic coupling is possible not only by variation of a dimension of nanowires and geometry of arrays, but also by changing the magnetic anisotropy. In this paper, we report on an investigation of the magnetic behavior of arrays consisting of finite number of Co microstripes with two mutually perpendicular induced anisotropies. We have studied an effect of the interwire coupling on the switching field of magnetization and analyzed the domain structure of the arrays under an impact of the induced anisotropies.

Interwire coupling forIn(4×1)/Si(111) probed by surface transport

The In/Si(111) system reveals an anisotropy in the electrical conductivity and is a prototype system for atomic wires on surfaces. We use this system to study and tune the interwire interaction by adsorption of oxygen. Through rotational square four-tip transport measurements, both the parallel $({\ensuremath{\sigma}}_{||})$ and perpendicular $({\ensuremath{\sigma}}_{\ensuremath{\perp}})$ components are measured separately. The analysis of the I(V) curves reveals that ${\ensuremath{\sigma}}_{\ensuremath{\perp}}$ is also affected by adsorption of oxygen, showing clearly an effective interwire coupling, in agreement with density-functional-theory-based calculations of the transmittance. In addition to these surface-state mediated transport channels, we confirm the existence of conducting parasitic space-charge layer channels and address the importance of substrate steps by performing the transport measurements of In phases grown on Si(111) mesa structures.

Bus encoder design for reduced crosstalk, power and area in coupled VLSI interconnects

This research work presents a novel circuit for simultaneous reduction of power, crosstalk and area using bus encoding technique in RC modeled VLSI interconnect. Bus-invert method is used to reduce inter-wire coupling, which is actually responsible for crosstalk, delay and power dissipation in coupled interconnects. The proposed method focuses on simplified and improved encoder circuit for 4, 8 and 16 coupled lines. In past, the researchers developed encoders that usually focused on minimizing power dissipation and/or crosstalk, thereby paying heavy penalty in terms of chip area. However, the proposed encoder and decoder while significantly reducing crosstalk demonstrates an overall reduction of power dissipation by 68.76% through drastically limiting switching activity. Furthermore, while reducing the complexity, chip area and transistor count of the circuit is reduced by more than 57%.

Electron Transport at the Nanometer‐Scale Spatially Revealed by Four‐Probe Scanning Tunneling Microscopy

AbstractElectron transport at the nanometer‐scale is the key to novel applications of nanomaterials in electronic and energy technologies. Due to the restricted dimensionality, one of the distinctive characteristics of nano‐systems is their transport properties critically depend on structural details. Therefore, an important requirement for transport research of a specific nanomaterial system is to examine structures and properties in a coherent manner. In this regard, four‐probe scanning tunneling microscopy (STM), which combines four independently controllable STMs with a scanning electron microscope (SEM) in the same cryogenic environment, is uniquely useful for probing electron transport on multiple length‐scales and revealing how transport is coupled to the electronic and structural properties down to the atomic scale for individual nanomaterials. By utilizing this unique tool, extensive research has been undertaken to explore aspects of nanotransport, which include (a) intertwined electronic and structural phase transitions in surface supported two‐dimensional structures, (b) effects of atomic defects and interwire coupling on the electronic and transport properties of ultra thin quantum wire systems, (c) grain boundary resistances in copper nanowires with one‐to‐one correspondence to the grain boundary structure, (d) defect scattering effects in two‐dimensional electron gas systems, and (e) evaluation of transport behaviors of individual semiconductor nano‐junctions and nanodevices. In this paper, transport measurement techniques are first introduced with a four‐probe STM and then the recent progress on its applications is reviewed with a focus on the spatially resolved electron transport at the nanometer‐scale. The goal is to stimulate further advancement and utilization of techniques capable of characterizing materials properties at the nanometer‐scale to facilitate the exploration of the great promise of nanoscience and nanotechnology.

Finite-Element Supported Transmission-Line Models for Calculating High-Frequency Effects in Machine Windings

Wave-propagation effects in machine windings are commonly simulated on the basis of a transmission-line model (TLM) where the parameters are extracted from cross-sectional electroquasistatic and magnetoquasistatic finite-element (FE) models of a stator slot. In this paper, three improvements are proposed. First, an additional radially-axially oriented FE model deals with the complicated return path of the common-mode current through the laminated core and leads to an additional impedance to be inserted into the TLM. Secondly, an additional TLM section is constructed using parameters extracted from an axisymmetric FE model of the winding overhang. Thereby, the impedance mismatches caused by the end-winding overhangs along the wave propagation direction are incorporated in the model. Thirdly, the capacitive and inductive inter-wire couplings are taken into account. The simulation results indicate the need of all three additions.

Correlating Electronic Transport to Atomic Structures in Self-Assembled Quantum Wires

Quantum wires, as a smallest electronic conductor, are expected to be a fundamental component in all quantum architectures. The electronic conductance in quantum wires, however, is often dictated by structural instabilities and electron localization at the atomic scale. Here we report on the evolutions of electronic transport as a function of temperature and interwire coupling as the quantum wires of GdSi(2) are self-assembled on Si(100) wire-by-wire. The correlation between structure, electronic properties, and electronic transport are examined by combining nanotransport measurements, scanning tunneling microscopy, and density functional theory calculations. A metal-insulator transition is revealed in isolated nanowires, while a robust metallic state is obtained in wire bundles at low temperature. The atomic defects lead to electron localizations in isolated nanowire, and interwire coupling stabilizes the structure and promotes the metallic states in wire bundles. This illustrates how the conductance nature of a one-dimensional system can be dramatically modified by the environmental change on the atomic scale.

Molecular Conduction through Adlayers: Cooperative Effects Can Help or Hamper Electron Transport

We use a one-electron, tight-binding model of a molecular adlayer sandwiched between two metal electrodes to explore how cooperative effects between molecular wires influence electron transport through the adlayer. When compared to an isolated molecular wire, an adlayer exhibits cooperative effects that generally enhance conduction away from an isolated wire's resonance and diminish conductance near such a resonance. We also find that the interwire distance (related to the adlayer density) is a key quantity. Substrate-mediated coupling induces most of the cooperative effects in dense adlayers, whereas direct, interwire coupling (if present) dominates in sparser adlayers. In this manner, cooperative effects through dense adlayers cannot be removed, suggesting an optimal adlayer density for maximizing conduction.

A computationally efficient method for calculating the maximum conductance of disordered networks: Application to one-dimensional conductors

Random networks of carbon nanotubes and metallic nanowires have shown to be very useful in the production of transparent, conducting films. The electronic transport on the film depends considerably on the network properties, and on the interwire coupling. Here we present a simple, computationally efficient method for the calculation of conductance on random nanostructured networks. The method is implemented on metallic nanowire networks, which are described within a single-orbital tight binding Hamiltonian, and the conductance is calculated with the Kubo formula. We show how the network conductance depends on the average number of connections per wire, and on the number of wires connected to the electrodes. We also show the effect of the inter/intrawire hopping ratio on the conductance through the network. Furthermore, we argue that this type of calculation is easily extendable to account for the upper conductivity of realistic films spanned by nanowire networks. When compared to experimental measurements, this quantity provides a clear indication of how much room is available for improving the film conductivity.

Steady-state current transfer and scattering theory

The correspondence between the steady-state theory of current transfer and scattering theory in a system of coupled tight-binding models of one-dimensional wires is explored. For weak interwire coupling both calculations give nearly identical results, except at singular points associated with band edges. The effect of decoherence in each of these models is studied using a generalization of the Liouville-von Neuman equation suitable for steady-state situations. An example of a single impurity model is studied in detail, leading to a lattice model of scattering off target that affects both potential scattering and decoherence. For an impurity level lying inside the energy band, the transmission coefficient diminishes with increasing dephasing rate, while the opposite holds for impurity energy outside the band. The efficiency of current transfer in the coupled wire system decreases with increasing dephasing.

A semianalytical finite element method for elastic guided waves propagating in helical structures

Steel multiwire cables are widely used in civil engineering as load-carrying members. The basic element of these cables is usually a simple straight strand made of a straight core and one layer of helical wires. Several difficulties arise in the understanding of guided ultrasonic waves in such structures, partly due to the helical geometry and the interwire coupling effects. In the context of nondestructive evaluation, this paper aims at theoretically investigating the propagation of elastic waves in helical waveguides. A numerical method is chosen based on a semianalytical finite element technique that relies on a specific nonorthogonal curvilinear coordinate system. This system is shown to be translationally invariant along the helix centerline so that a spatial Fourier transform can be explicitly performed along the axis and the problem is reduced to two dimensions. A single helical wire is first considered. The convergence and accuracy of the proposed method are assessed by comparing finite element results with reference solutions. The method is then extended to study a seven wire cable with simplified contact conditions. Dispersion curves are presented for wave numbers as well as energy velocities. Significant differences with the infinite straight cylinder are observed.

Testing Network-on-Chip Communication Fabrics

Network-on-chip (NoC) communication fabrics will be increasingly used in many large multicore system-on-chip designs in the near future. A relevant challenge that arises from this trend is that the test costs associated with NoC infrastructures may account for a significant part of the total test budget. In this paper, we present a novel methodology for testing such NoC architectures. The proposed methodology offers a tradeoff between test time and on-chip self-test resources. The fault models used are specific to deep submicrometer technologies and account for crosstalk effects due to interwire coupling. The novelty of our approach lies in the progressive reuse of the NoC infrastructure to transport test data to the components under test in a recursive manner. It exploits the inherent parallelism of the data transport mechanism to reduce the test time and, implicitly, the test cost. We also describe a suitable test-scheduling approach. In this manner, the test methodology developed in this paper is able to reduce the test time significantly as compared to previously proposed solutions, offering speedup factors ranging from 2x to 34x for the NoCs considered for experimental evaluation.

Quantum ballistic transport in nanowire junctions

The quantum ballistic transport of electrons through nanowire junctions formed by putting one nanowire (with square, rectangular, and circular cross sections) on top of another wire (the two wires are not lying in the same plane) is studied using tight-binding models and the Green's function approach. Different tight-binding models are considered to find the optimum number of tight-binding sites in the transverse plan for approximating the continuum model in a finite range of energy and also to find the minimal site number for qualitative description of the transport characteristics. Resonant dips and peaks found in the interwire and intrawire conductances can be explained by the formation of bound and quasibound states at the cross junction. How the conduction channels of the wires are coupled together in the formation of the bound and quasibound states are analyzed using the projected Green's functions. Quasibound states unbound in both wires give rise to resonant peaks in the interwire conductance as well as resonant dips in the intrawire conductance. Quasibound states unbound in only one wire give rise to only resonant dips in the intrawire conductance without corresponding resonant peaks in the interwire conductance. Some exception in the latter case is discussed. Reduction of interwire coupling strength is shown to suppress the conductance at energy far from the subband edges. In the weak interwire coupling regime, larger conductances are found at energies close to the subband edges. A comparison of the square and rectangular wire junctions studies is made. Increase in the dimension perpendicular to a square wire junction reduces the interwire conductance and enhances the intrawire conductance. On the other hand, degeneracy of subbands due to the higher symmetry of a square cross section enhances the interwire conductance. Comparison of circular wire junctions with square wire junctions show that some conductance features found in square wire junctions disappear in circular wire junctions owing to the weaker interwire coupling in circular wire junctions, which is the result of a smaller contact area between wires. The energies and probability densities of some bound and quasibound states are also determined and analyzed in the present study.

Surface Plasmon Structures for Surface-Enhanced Raman Scattering

ABSTRACTSurface-enhanced Raman scattering (SERS) can enhance the intensity of Raman radiation by many orders of magnitude for molecules bound to metallic nanostructures. SERS is generally thought to have been a result of excitation of surface plasmons in nanostructured metal, which greatly enhances local electric field experienced by the molecule. Common SERS substrates fabricated using noble metal colloidal or electrochemically roughened thin films suffer from lack of homogeneity where only few hot spots yield high enhancement. We explore semiconducting nanowires and metallic nanorods as an economical, stable, and uniform SERS substrate for the detection of trace amount of chemicals and bio-species. We utilize bulk synthesized semiconducting nanowires as nano-scale structures that can be coated with noble metals or their colloidal forms to allow for excitation of surface plasmons over broad frequency range. The use of nanowires as SERS substrates has several advantages. The surface properties of these nanowires are highly reproducible and well defined as compared to other systems like colloidal aggregates, electrochemical roughening etc. The synthesized nanowires offer many unique features (sharp vertices, noncircular cross-sections, inter-wire coupling) that may lead to larger field enhancement factors. High density of nanowires means close interaction between adjacent nanowires, which enables SERS to manifest for a broad selection of excitation sources. We have synthesized germanium dioxide and zinc oxide nanowires using the vapor-liquid-solid growth mechanism utilizing a simple quartz tube furnace set up. The nanowires are grown either using thin gold film (5-15 nm) or colloidal gold (20 nm or 60 nm) as catalyst on substrates of silicon, quartz, and alumina. The resulting nanowires are dense (100-300 nm diameter, 10-40 μm long) and randomly distributed on the substrate. The nanowires are subsequently coated with thin films (10-15 nm) of gold that provide plasmons active surface. We have also investigated silver nanorods on glass formed by grazing angle deposition using e-beam evaporation. These nanorods have a diameter of ∼ 50-70 nm with lengths averaging 300-400 nm. These well aligned high aspect ratio and dense structures are ideal for excitation of surface plasmons. The synthesized structures have been characterized using SEM, TEM, and EDS. The SERS studies were conducted using EzRaman-L system from Enwave Optronics, Inc. Silver nanorods and gold-coated nanowires have been found to exhibit significant Raman enhancement for micro-molar concentrations of Rhodamine 6G and Nile blue, and are promising candidates for SERS applications.

Switching characteristics of coupled quantum wires with tunable coupling strength

We explore the switching characteristics of coupled quantum wires with a controllable interwire coupling. We perform extensive experimental and theoretical characterization of the coupling region between the wires, evaluating the local barrier that is formed in this region. With strong coupling between the wires, a nonmonotonic switching of their currents is obtained in a magnetic field, suggestive of quantum interference of electrons in their coupling region.

Observation of Plasmon Propagation, Redirection, and Fan-Out in Silver Nanowires

We report the coupling of free-space photons (vacuum wavelength of 830 nm) to surface plasmon modes of a silver nanowire. The launch of propagating plasmons, and the subsequent emission of photons, is selective and occurs only at ends and other discontinuities of the nanowire. In addition, we observe that the nanowires redirect the plasmons through turns of radii as small as 4 microm. We exploit the radiating nature of discontinuities to find a plasmon propagation length >3 +/- 1 microm. Finally, we observe that interwire plasmon coupling occurs for overlapping wires, demonstrating plasmon fan-out at subwavelength scales.

Delocalization in Weakly Coupled Disordered Wires: Application to Conjugated Polymers

It is well known that even for minimal disorder one-dimensional wires are insulators: all 1D electron states are localized. Here, the influence of interwire coupling on delocalization of 1D states is examined. Based on perturbation theoretic arguments for the formation of 3D states in coupled wires and subsequent scaling analysis, practical expressions for the microscopic conditions of electronic delocalization and coherent conductivity of coupled 1D wires are obtained. The model quantitatively explains the temperature dependent dc conductivity in conducting polymers at both sides of the metal-insulator transition and links the experimental data to microscopic material parameters.

Low power system on chip bus encoding scheme with crosstalk noise reduction capability

Inter-wire coupling is a major source of wire load and delay faults for on-chip buses implemented in ultra-deep submicron system on chip (SoC) systems. Elimination or minimisation of such faults is crucial to the performance and reliability of SoC designs. A novel on-chip bus encoding scheme targeting high-performance generic SoC systems is presented. In addition to its efficiency in terms of power, the scheme reduces delay faults by completely eliminating the most critical type of crosstalk coupled switched capacitance. The authors describe the technique and its implementation (using the widely adopted AMBA-AHB SoC bus standard) and provide experimental results indicating 22–36% energy saving for systems implemented in 0.18 µm CMOS technology.