Electrical conductance of double layers composed of two metal thin films with different disorder

Thin films of Al, Cu, Ag and Au are coated at low temperatures with a highly disordered film of the same or one of the other metals, respectively. The total conductance of the thin film sandwich first decreases, then passes through a minimum, and finally increases with increasing thickness of the disordered overlayer. These total conductance versus overlayer thickness curves are interpreted within the Fuchs-Sondheimer-model of the geometrical size effect. Using this model the calculated fraction of specularly scattered conduction electrons at the surface of the base film is very much reduced by lowering the condensation temperature of the base film. Moreover the decrease in conductance is dependent on the choice of coating metal. This observation is an indication of fissures in the surface of the base film.     

Electronic contribution to the heat transfer across metal single crystal-He II boundaries

Measurements of the Kapitza conductance by the ac technique are reported for oriented gallium and copper single crystal discs. The data show convincing evidence that conduction electrons participate in the transmission of heat across the metal-He II interfaces for both metals. Measurements of the Kapitza conductance as a function of transverse and longitudinal fields and the influence of temperature, sample thickness and impurities allow a qualitative understanding of the total heat transfer across the sample.     

Diffuse LEED intensities of disordered crystal surfaces: II. Multiple scattering on disordered overlayers

The diffraction of low energy electrons from disordered overlayers adsorbed on ordered substrates is treated theoretically by an extension of Beeby's multiple scattering method. A lattice gas model is assumed for the disordered adsorbate layer. Multiple scattering within a certain area around each atom — each atom of the overlayer and within the ordered substrate — is treated self-consistently, the remaining contributions to the total scattering amplitude being averaged. The theory can be used in the limiting cases of random distribution and of long range order within the adsorbate layer.     

The mystery of the alkali metals; the induced anomalous Hall effect in thin Cs films

Sandwiches made from Fe and Cs films are investigated as a function of the magnetic field and the Cs thickness. Conduction electrons which cross from the Fe to the Cs are marked by a drift velocity component perpendicular to the electric field. The anomalous Hall effect in the Fe provides this “non-diagonal” kick to the electrons that cross from the Fe into the Cs. The ballistic propagation of the conduction electrons can be monitored as a function of the Cs film thickness. The free propagation into the Cs is measured in terms of the non-diagonal conductance L_xy which we denote as the “induced anomalous Hall conductance”L _xy ⁰. For a normal (non-magnetic) metal in contact with Fe, L_xy increases with the thickness of the normal metal until the film thickness exceeds (half) the mean free path of the conduction electrons. For Cs on top of Fe the induced anomalous Hall conductance increases up to a Cs coverage of about 100 A, then, in contrast to other non-magnetic metals, L _xy ⁰ decreases for larger Cs coverage and approaches zero. This behavior cannot be explained with the free electron model. The strange behavior of the induced AHC in Cs films adds an even more challenging mystery to the already poorly understood properties of thin Cs films. These results defy explanation in the free electron model. Received 29 April 1999 and Received in final form 10 July 1999     

Dispersion of the conductance of quantum nanowires and Joule heating

The high-frequency ballistic conductance G(ω) of a quantum wire connecting two classical reservoirs is discussed. It is supposed that the transverse size of the wire is on the order of the de Broglie wavelength of the conduction electrons. An expression for G(ω) in a wide range of frequencies ω is given. The behavior of both active ReG(ω) and reactive ImG(ω) parts of the conductance is investigated. The frequency range is determined where the so-called kinetic inductance is dominant, i.e., ImG(ω) is positive and larger than ReG(ω). This range is defined by the condition that the time of flight of the conduction electrons along the wire length L exceeds the period of oscillation 2π/ω of the electric potential. The Joule heat generation that accompanies the current flow through the quantum wire takes place in the reservoirs over a distance on the order of the mean free path of conduction electrons. The total rates of Joule heat generation are the same in both reservoirs.     

Tuning the effective fine structure constant in graphene: opposing effects of dielectric screening on short- and long-range potential scattering

We reduce the dimensionless interaction strength alpha in graphene by adding a water overlayer in ultrahigh vacuum, thereby increasing dielectric screening. The mobility limited by long-range impurity scattering is increased over 30%, due to the background dielectric constant enhancement leading to a reduced interaction of electrons with charged impurities. However, the carrier-density-independent conductivity due to short-range impurities is decreased by almost 40%, due to reduced screening of the impurity potential by conduction electrons. The minimum conductivity is nearly unchanged, due to canceling contributions from the electron-hole puddle density and long-range impurity mobility. Experimental data are compared with theoretical predictions with excellent agreement.     

Photoelectron spectroscopy of Ar/Cu(100) interface states

Buried interface states in Ar/Cu(100) were studied by means of one- and two-photon photoemission experiments. With increasing Ar overlayer thickness, a transition from broad electron scattering resonances in the Ar conduction band into a hydrogen-like series of quasi-bound states at the Ar/Cu interface was observed. The thickness dependence of energies and lifetimes is compared to theoretical resonance positions and linewidths derived from a parameterized one-dimensional potential. PACS 73.20.-r; 73.40.Ns; 79.60.-i; 78.47.+p     

Application of Crystal Elements for Charged Particle Beam Steering and Radiation Beam Generation on the U-70 Synchrotron

Dependences of the tunnel magnetoresistance and in-plane component of the spin transfer torque on the applied voltage in a magnetic tunnel junction have been calculated in the approximation of ballistic transport of conduction electrons through an insulating layer with embedded magnetic or nonmagnetic nanoparticles. A single-barrier magnetic tunnel junction with a nanoparticle embedded in an insulator forms a double-barrier magnetic tunnel junction. It has been shown that the in-plane component of the spin transfer torque in the double-barrier magnetic tunnel junction can be higher than that in the single-barrier one at the same thickness of the insulating layer. The calculations show that nanoparticles embedded in the tunnel junction increase the probability of tunneling of electrons, create resonance conditions, and ensure the quantization of the conductance in contrast to the tunnel junction without nanoparticles. The calculated dependences of the tunnel magnetoresistance correspond to experimental data demonstrating peak anomalies and suppression of the maximum magnetoresistances at low voltages.     

Non-Fermi-liquid behavior and double-exchange physics in orbital-selective Mott systems

We study a multiband Hubbard model in its orbital-selective Mott phase, in which localized electrons in a narrow band coexist with itinerant electrons in a wide band. The low-energy physics of this phase is shown to be given by a generalized double-exchange model. The high-temperature disordered phase thus differs from a Fermi liquid, and displays a finite scattering rate of the conduction electrons at the Fermi level, which depends continuously on the spin anisotropy.     

Effects of epitaxial films of nanometric thickness on the X-ray photoelectron diffraction intensities from substrates

The effects on the X-ray photoelectron diffraction intensities from the substrate produced by epitaxial NiO(0 0 1) films of various thickness deposited on Ag(0 0 1) were investigated. The variations in the Ag XPD curves induced by the NiO films can be explained in terms of multiple scattering of the electrons emitted by the substrate atoms along the close-packed rows of the overlayer. Intensity minima in the XPD curves from the substrate in correspondence to intensity maxima in the XPD curves from the overlayer are observed when the thin film is commensurate with the substrate. For films of suitable thickness, the analysis of XPD curves from the substrate allows one to get information about the structure of the film and of the film–substrate interface.     

Generation of low energy resonant electrons during relaxation of57Fe

Collection of low energy electrons (<15 eV) duringConversionElectronMossbauerSpectroscopy (CEMS) provides enhanced surface sensitivity. Spectra collected from a 0.928⁵⁷Fe foil using retarding field energy analyzers in conjunction with spiraltron electron multipliers demonstrates both resonant and nonresonant count rates which decrease by as much as 50% at 10 eV bias potential. Spectra from samples with the topmost 1.0 nm chemically labeled had total spectral areas of 99.0%mm/sec. The area ratio of the resonant 1.0 nm overlayer to the resonant substrate was 1.43 at 0 eV bias potential while at 15 eV the ratio decreased to 0.72. By vacuum evaporating a 5.0 nm copper coating on the sample, near complete attenuation of the low energy electrons from the 1.0 nm overlayer was achieved. These results suggest that some low energy electrons below 15 eV are formed as primary products of electronic relaxation following nuclear decay and that they are not the result of straggling or other scattering phenomena.     

Conduction of Strongly Coupled Electrons Through Narrow Channels on Liquid Helium Surface: Simulation

The conduction of electrons through narrow channels formed on the surface of liquid helium is analyzed by numerical simulations. It is shown that, when electrons are strongly coupled, we have nonlinear and even negative dependence of conductance on the width of the constriction which is controlled by the gate voltage (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of topology on the transport properties of two interacting dots

The transport properties of a system of two interacting dots, one of them directly connected to the leads constituting a side-coupled configuration (SCD), are studied in the weak and strong tunnel-coupling limits. The conductance behavior of the SCD structure has new and richer physics than the better-studied system of two dots aligned with the leads (ACD). In the weak coupling regime and in the case of one electron per dot, the ACD configuration gives rise to two mostly independent Kondo states. In the SCD topology, the inserted dot is in a Kondo state while the side-connected one presents Coulomb blockade properties. Moreover, the dot spins change their behavior, from an antiferromagnetic coupling to a ferromagnetic correlation, as a consequence of the interaction with the conduction electrons. The system is governed by the Kondo effect related to the dot that is embedded into the leads. The role of the side-connected dot is to introduce, when at resonance, a new path for the electrons to go through giving rise to the interferences responsible for the suppression of the conductance. These results depend on the values of the intra-dot Coulomb interactions. In the case where the many-body interaction is restricted to the side-connected dot, its Kondo correlation is responsible for the scattering of the conduction electrons giving rise to the conductance suppression.Received: 7 February 2004, Published online: 24 September 2004PACS: 73.63.-b Electronic transport in nanoscale materials and structures - 73.63.Kv Quantum dots     

Analysis of the anomalous Hall effect in a double layer of a ferromagnetic and a normal metal

A new method is suggested to investigate the mechanism of the anomalous Hall effect (AHE) in ferromagnetic metals. Using a double layer of a ferromagnet and a normal metal of increasing thickness one can manipulate the AHE in the ferromagnet without changing the ferromagnet's structure and electronic properties. The conduction electrons from the normal metal carry their drift velocity across the interface into the ferromagnetic film and induce an additional AHE conductance ΔG_xy. Its dependence on the mean free path in the normal metal distinguishes between the side jump and the skew scattering mechanisms for the AHE in the ferromagnet.     

Localization under the effect of randomly distributed decoherence

Electron transport through disordered quasi one-dimensional quantum systems is studied. Decoherence is taken into account by a spatial distribution of virtual reservoirs, which represent local interactions of the conduction electrons with their environment. We show that the decoherence distribution has observable effects on the transport. If the decoherence reservoirs are distributed randomly without spatial correlations, a minimal degree of decoherence is necessary to obtain Ohmic conduction. Below this threshold the system is localized and thus, a decoherence driven metal-insulator transition is found. In contrast, for homogenously distributed decoherence, any finite degree of decoherence is sufficient to destroy localization. Thus, the presence or absence of localization in a disordered one-dimensional system may give important insight about how the electron phase is randomized.     

Influence de l'absorption d'atomes de plomb sur la résistance électrique de films d'or

Precise electrical resistance and thickness measurements are associated to study lead adsorption on gold films in ultra-high vacuum. Fuchs' theory is used to interpret the large size effects observed when the lead deposit is less than one monolayer. At low coverages the ratio of electrical resistivity variation to lead concentration is independent of temperature and equal to 1.8 ± 0.1 microOhms cm/at%. With temperatures ranging from ? 150 to + 150°C, the evolution of size effects is related to structural changes in the lead overlayer. A saturation in the diffuse scattering of conduction electrons by adsorbed adatoms is shown to occur when the lead deposit is about half a monolayer; this is used for comparing surface roughness of annealed and unannealed gold films. For lead coverage greater than a monolayer, an alloy identified by X-ray diffraction as AuPb₂ is formed. The electrical resistance variation is consistent with the thinning of the gold film by removing atoms which diffuse towards the free surface through the alloy layer.     

Tunneling between ferromagnetic films

FeGeCo junctions conductance G(V) is studied when mean magnetizations of the two ferromagnetic film are parrallel or antiparallel. Conductance measurement, in these two cases, is related to the spin polarizations of the conduction electrons.     

An approach based on the free-electron theory to calculate electron conductance of perfect metallic nanowires

For the first time, the quantum free-electron theory is utilized for determining the quantized electrical conductance in a perfect metallic multi-atom nanowire. In this formulation, both spin-up and spin-down conduction electrons are considered so that our final result comes from a series combination of the contributions to the conductance from the two possible types of spin-oriented electron, assuming a symmetric distribution of the involved counter-ions. In addition, several aspects related to the atom-lead coupling are discussed.     

Persistence of magnetic moments at high density in disordered systems near the metal insulator transition

Abstract

Spontaneous moment formation has been observed in disordered narrow band systems like doped semiconductors, or the liquid alloy Cs-Au. A recent model has provided a natural origin for this behaviour in a disordered system whenever the conduction band is almost empty. The intimate interplay between correlation, localization and magnetic properties characterizes this regime, and makes it different from the usual half filled band correlated system.

In this work we investigate the behaviour of the model at high density. Precisely correlation effects are strongly enhanced in the limit of low filling of the band, giving rise to persistent local moments even at high density. Conversely in the well studied case of a half filled band local nioments vanish at high density, and correlation effects become negligible in this limit.

At the light of the model we argue that the observed magnetic properties should persist at higher densities provided that only a few per cent electrons populate the conduction band.     

On ballistic transport in carbon nanotubes

We investigate theoretically the ballistic regime exhibited by conduction electrons in multiwalled carbon nanotubes in relation to the conductance quantization in these tubes. Starting from the fact that electron drift mobility is quantized in multiwall tubes, essential aspects related to both ballistic and diffusive regimes are discussed.