Diffusive Wire Research Articles

Multi-terminal scattering approach to conductance and noise at scanning tunnelling microscope tips

Experiments that measure the average current and current fluctuations at one or two local tunnelling contacts on a mesoscopic multiprobe conductor are proposed and theoretically investigated. The average current and the current fluctuations at a single tunnelling tip are determined by an effective local distribution function , which is given as the product of local partial densities of states (injectivities) and the Fermi distribution functions in the electron reservoirs. The current correlations at two tips connected to a phase-coherent conductor are determined by spatially non-diagonal density of states elements and can give information about the phase and correlations of wavefunctions. All results are illustrated for measurements on a ballistic wire and on a metallic diffusive wire. Copyright © 1999 John Wiley & Sons, Ltd.

Evidence for Saturation of Channel Transmission from Conductance Fluctuations in Atomic-Size Point Contacts

The conductance of atomic size contacts has a small, random, voltage dependent component analogous to conductance fluctuations observed in diffusive wires (UCF). A new effect is observed in gold contacts, consisting of a marked suppression of these fluctuations when the conductance of the contact is close to integer multiples of the conductance quantum. Using a model based on the Landauer-Buettiker formalism we interpret this effect as evidence that the conductance tends to be built up from fully transmitted (i.e., saturated) channels plus a single, which is partially transmitted.

1/3-shot-noise suppression in diffusive nanowires

We report low-temperature shot-noise measurements of short diffusive Au wires attached to electron reservoirs of varying sizes. The measured noise suppression factor compared to the classical noise value $2e|I|$ strongly depends on the electric heat conductance of the reservoirs. For small reservoirs injection of hot electrons increases the measured noise, and hence the suppression factor. The universal $1/3$ suppression factor can only asymptotically be reached for macroscopically large and thick electron reservoirs. A heating model based on the Wiedemann-Franz law is used to explain this effect.

Energy Distribution Function of Quasiparticles in Mesoscopic Wires

We have measured with a tunnel probe the energy distribution function of Landau quasiparticles in metallic diffusive wires connected to two reservoir electrodes, with an applied bias voltage. The distribution function in the middle of a 1.5-\ensuremath{\mu}m-long wire resembles the half sum of the Fermi distributions of the reservoirs. The distribution functions in 5-\ensuremath{\mu}m-long wires are more rounded, due to interactions between quasiparticles during the longer diffusion time across the wire. From the scaling of the data with the bias voltage, we find that the scattering rate between two quasiparticles varies as ${\ensuremath{\varepsilon}}^{\ensuremath{-}2}$, where $\ensuremath{\varepsilon}$ is the energy transferred.

Electron heating effects in diffusive metal wires

We have investigated the electron heating in metallic diffusive wires of varying length at liquid-helium temperature by measuring the electric noise. The local increase of the electron temperature can be essential already for small currents and is well described by a heat-diffusion equation for the electrons. Depending on the electron thermal conductance and the electron–phonon coupling in the wire, different length regimes are identified. The quantitative knowledge of the electron temperature is important for analysis of nonequilibrium effects involving current heating in mesoscopic wires.

Semiclassical analysis of the quantum interference corrections to the conductance of mesoscopic systems.

The Kubo formula for the conductance of a mesoscopic system is analyzed semiclassically, yielding simple expressions for both weak localization and universal conductance fluctuations. In contrast to earlier work which dealt with times shorter than $O(\log \hbar^{-1})$, here longer times are taken to give the dominant contributions. For such long times, many distinct classical orbits may obey essentially the same initial and final conditions on positions and momenta, and the interference between pairs of such orbits is analyzed. Application to a chain of $k$ classically ergodic scatterers connected in series gives the following results: $-{1 \over 3} [ 1 - (k+1)^{-2} ]$ for the weak localization correction to the zero--temperature dimensionless conductance, and ${2 \over 15} [ 1 - (k+1)^{-4} ]$ for the variance of its fluctuations. These results interpolate between the well known ones of random scattering matrices for $k=1$, and those of the one--dimensional diffusive wire for $k \rightarrow \infty$.

Mesoscopic transport in Si metal-oxide-semiconductor field-effect transistors with a dual-gate structure

This article studies the mesoscopic transport of electrically controllable inversion layers of Si metal-oxide-semiconductor field-effect transistors that use a dual-gate structure. We have developed two kinds of devices: a quasi-one-dimensional device (1D-FET) and a Coulomb blockade device (CB-FET). In both devices, the field effect is used to change the channel structure by introducing potential barriers in the narrow inversion channel. The 1D-FET changes a long diffusive quantum wire into a short ballistic one. Strong oscillations in differential conductance, even negative differential conductance, have been observed at 4.2 K, indicating enhanced modulation of electron mobility by intersubband scattering suppression. The CB-FET, on the other hand, transforms a simple quantum wire into a coupled quantum-dot array. A clear change in transport properties is observed with changes in the barrier height at low temperatures. The experimental results are consistent with the theory of one-dimensional subbands and the Coulomb blockade of single-electron tunneling.

Experimental study of reduced shot noise in a diffusive mesoscopic conductor.

We demonstrate that a diffusive narrow wire of a length much longer than the elastic scattering length and the phase-coherence length, but of the order of the energy relaxation length, exhibits shot noise with an intensity lower than the full shot-noise level. At 4.2 K, the reduction factor is shown to vary between 0.2 and 0.45, depending on the width of the wire. The reduction is consistent with recent theoretical predictions.

Experimental study of reduced shot noise in a diffusive mesoscopic conductor

We show that a diffusive narrow wire of a length much longer than the elastic scattering length and the phase-coherence length, but of the order of the energy relaxation length, exhibits shot noise with an intensity lower than the full shot-noise level. At 4.2 K, the reduction factor is shown to vary between 0.2 and 0.45, depending on the width of the wire. The reduction is consistent with recent theoretical predictions.

Nonlocal magnetoresistance of diffusive wires in high magnetic fields

We have used a nonlocal configuration of measurement probes to study the magnetoresistance of diffusive n +-GaAs wires at high magnetic fields where ω cτ>1. At low temperatures we observe universal conductance fluctuations (UCF). For ω cτ>1 there is a large increase in the Lee-Stone correlation field but the amplitude is unchanged in strong disagreement with the theoretical prediction. We conclude that the UCF in this regime cannot be scaled in terms of a single parameter, the phase coherence length. At higher temperatures, where the UCF are quenched, we observe qualitatively new magnetoresistance oscillations which have the same periodicity as Shubnikov-de Haas oscillations but differ from them in many important respects. In particular, the new oscillations disappear at low temperatures. This indicates the importance of dissipation in the measurement of nonlocal resistance. There is excellent agreement between the temperature dependence of the new oscillations and a simple model.

Quantum transport in diffusive microstructures in high magnetic fields

We have studied magnetotransport in diffusive submicron wires (n+-GaAs) at high magnetic fields where ωcτ>1. The system allowed us to follow a transition from orthodox mesoscopic behaviour to the regime of edge state transport.We found that for ωcτ>1 the universal conductance fluctuations (UCF) can no longer be scaled in terms of only one parameter, the phase coherence length. This breakdown of universal scaling is seen as a large increase of the Lee-Stone correlation field while the magnitude of UCF is unchanged in strong disagreement with the theoretical prediction.A qualitatively new magnetoresistance oscillatory effect has been observed at intermediate temperatures (10K<T<50K) when the quantum transport along the edges coexists with classical (diffusive and dissipative) conduction in the bulk. The effect arises due to Landau quantization but entirely disappears at low temperatures. The latter distinguishes it from the other quantum transport phenomena and indicates the importance of dissipation in resistance measurement.

Symbolic generation of constrained random logic cells

A symbolic cell generator (SYC) that can generate the symbolic layout of a generic CMOS logic cell is presented. It accepts as input a SPICE-like netlist describing circuit components, connectivity, and the list of the I/O pins. Using this generator, the user can specify topological constraints on pin and transistor positions, the maximum lengths of polysilicon and diffusion wires, and a preferred layer for each electrical node. Cells are generated according to optimization criteria that take into account not only geometric factors, such as cell area, aspect ratio, and wirelength, but also electrical features, namely capacitance to the substrate and contact and via minimization. The generator's placement strategy includes transistor clustering into regions, global region placement by linear ordering, and two-dimensional local transistor placement. The routing combines Steiner trees and Lee algorithms. Object-oriented programming paradigms were used in the implementation of the program, written in C++ language. Experimental results for small and medium-sized cells are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Real-space and magnetic-field correlation of quantum-resistance fluctuations in the ballistic regime in narrow GaAs-AlxGa

We report the first experimental study, in the quasiballistic regime, of magnetic field correlation properties of quantum-resistance fluctuations, between phase coherent spatial regions. Our devices are GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As quasi-one-dimensional wires with an elastic-scattering length comparable to the voltage probe spacing and at most 10 transverse channels occupied. We find the correlation coefficients to be substantially different than recent results in diffusive wires (\ensuremath{\sim}0 compared to -0.5 for the latter), indicating that the universality, which characterizes fluctuations in diffusive quantum transport, is beginning to break down in this few-channel, ballistic limit.

Critical data of type II superconductors under hydrostatic pressure

Thin films of Nb/sub 3/Sn, Nb/sub 3/Ga, and NbN deposited on sapphire substrates by different preparation techniques and diffusion wires were measured under hydrostatic pressures up to 15 kbar. The results on critical current and upper critical induction under pressure can be described by scaling laws similar to those already known for temperature as parameter. The anisotropies of the critical currents and fields depend on the microstructure of the samples. The pinning mechanisms are not changed under hydrostatic pressure, so that the pinning forces in these superconductors can be compared to existing pinning theories.

Inhomogeneity of sample structure and peak effect in superconducting NbN and Nb(C,N) diffusion wires

Wires of NbN and Nb(C,N) prepared by a diffusion technique were found to exhibit a strong radial gradient in the superconducting transition temperature and in the lattice parameter. The peak effect observed on these samples is discussed with respect to this inhomogeneity of sample structure, which is found to be responsible for the disagreement between theoretical models predicting a peak effect and these experimental results.

Effect of low temperature neutron irradiation on superconducting properties of Nb 3Sn

Nb 3Sn diffusion wires were irradiated with fast neutrons at 4.6 K. Exposure to 3.9 × 10 18 n/cm 2 caused an increase in the critical current density and a decrease in the transition temperature.

Kritisches Verhalten von V3Si und Nb3Sn unter uniaxialem Zug im transversalen Magnetfeld

The critical data of superconductivity, i.e. temperature, magnetic field and current, were investigated on V3Si-single cristals and V3Si- and Nb3Sn-diffusion wires in transverse magnetic fields under uniaxial tension. The values of all the critical parameters were reduced under the influence of the tension. A comparison of the results, obtained for the single crystals and the diffusion wires, lead to an understanding of some anomalies in the behaviour of the diffusion wire samples.

Präparation und Supraleitungseigenschaften von Niobnitrid sowie Niobnitrid mit Titan-, Zirkon- und Tantalzusatz

A description of preparation for samples of niobium nitride without and with admixtures of titanium, zirconium and tantalum by diffusion of nitrogen under pressure into niobium wires without and with admixtures of titanium, zirconium and tantalum respectively, is given. For samples of pure niobium nitride with different compositions the influence of the nitrogen content on the transition temperature to superconductivity has been investigated with the result of a transition temperature not exceeding 16.5 °K even for the stoichiometric composition NbN. This is in contrary to other authors. For diffusion wires of the systems (Nb-Ti)N x , (Nb-Zr)N x and (Nb-Ta)N x the critical temperature, lattice parameter and nitrogen content as a function of the admixture of titanium, zirconium and tantalum respectively, are given. For the first time the magnetisation curve of niobium nitride has been measured showing the behaviour of type II superconductors. Concerning the kinetics of formation of niobium nitride by diffusion the nitrogen content is increasing for constant nitrogen pressure with raising temperature of diffusion, but is decreasing in the high temperature region.