Effects of finite electron mean free path on the attenuation,electromagnetic generation,and detection of ultrasonic shear waves in superconductors

Use of the general many-body formalism allows us to derive a system of equations describing the propagation of electromagnetic shear waves coupled to ultrasonic shear waves in normal and superconducting metals with arbitrary electron mean free paths. From this system of equations we derive general expressions in terms of correlation functions for the attenuation coefficient as well as for the generation and detection (radiation) efficiency of ultrasonic shear waves, assuming specular reflection. It is shown that, for arbitrary mean free path, generation and radiation efficiency are equal. Furthermore, they are related in a simple way to the residual surface resistance caused by ultrasound generation. In order to incorporate the frictional force between electrons and lattice, a reactive part of which remains finite even at T = 0, one has to introduce the current-stress tensor correlation function. The calculation of this correlation function for an impure BCS superconductor is more complicated than the calculation of the conductivity from the current-current correlation function. If suitably normalized, the electromagnetic contribution to the attenuation coefficient depends on mean free path only through the conductivity. For sufficiently short mean free path the generation and radiation efficiency in a superconductor can become equal to and even larger than the efficiency in the normal metal because the frictional force, which opposes the electromagnetic field driving the ions, is less effective in the superconducting than in the normal state. It is shown that the interpretation of experiments on the radiation efficiency in superconducting tin has to be modified when the finite electron mean free path is taken into account. Agreement between theory and experiment can still be achieved by a suitable choice of the Fermi velocity and the mean free path. The London penetration depth calculated from this Fermi velocity turns out to be smaller than generally accepted values.     

Low-energy electron mean free path in thin films of copper phthalocyanine

The formation of thin organic films of copper phthalocyanine (CuPc) deposited onto the surface of gold-coated quartz crystal resonator was studied in situ under ultrahigh vacuum conditions by means of total electron-beam-induced current spectroscopy in combination with deposit thickness determination by piezocrystal microbalance technique. Variations in the fine structure of the total current spectra of CuPc layers of var-ious thicknesses in the 0–8 nm interval have been analyzed and the electron mean free path in thin CuPc films was determined as a function of the electron energy. For electron energies of 5.0, 7.2, 14.4, and 18.0 eV above the Fermi level, the mean free path is 6.4, 3.9, 2.6, and 2.3 nm.     

Epitaxial Ni films for ballistic ferromagnetic nanostructures

Nickel films of 170 nm thick were grown on sapphire substrates of the A, R and C orientations. Their electric characteristics and surface morphology were studied. Peculiarities of the film growth and optimum growth conditions for films of high electron mobility were determined. Nickel films grown on the sapphire A-plane with a high residual electron mean free path were found suitable for the fabrication of ballistic planar ferromagnetic nanostructures. Ballistic electron transport was observed in cross-type epitaxial Ni (111) nanostructures.     

Ballistic conduction in multiwalled carbon nanotubes

The electrical transport in multiwalled carbon nanotubes is shown to be ballistic at room temperature with mean free paths on the order of tens of microns. The measurements are performed both in air and in the transmission electron microscope by contacting the free end of a nanotube pointing out of a fiber to a liquid metal and measuring the dependence of the nanotube resistance between the contacts. For a specific representative nanotube the resistance per unit length is found to be Rt = 31 +/- 61 omega/micron and the contact resistance with the liquid metal, Rc = 165 +/- 55 omega microns, corresponding to a mean free path l = 200 microns. Current-to-voltage characteristics are in accord with the electronic structure. The nanotubes survive high currents (up to 1 mA, i.e., current density on the order of 10(9) A/cm2). In situ electron microscopy shows that a relatively large fraction of the nanotubes do not conduct (even at high bias), consistent with the existence of semiconducting nanotubes. Discrepancies with other measurements are most likely due to damage caused to the outer layer(s) of the nanotubes during processing. The measured mean free path of clean, undamaged arc-produced multiwalled carbon nanotubes is several orders of magnitude greater than that for metals, making this perhaps the most significant property of carbon nanotubes.     

The electrical conductivity of inhomogeneous thin metallic films

This paper reports the following results: (i) an explicit for the electrical conductivity of a thin metal film where the electron mean free path is not constant through its thickness, and (ii) calculations showing how in this case an illusory fit to the Fuchs-Sondheimer result can be obtained with a dependence of the specularity parameter on thickness or angle. A survey of experimental results is also presented.     

High thermoelectric figure-of-merit in kondo insulator nanowires at low temperatures

We predict a large thermoelectric figure-of-merit in Kondo insulator nanowires at low temperatures. The high ZT values are due to the Kondo effect for electrons and boundary scattering on phonons. We simulated the electron properties of the bulk Kondo insulators within the framework of dynamical mean field theory and found that electrons have short mean free path. In nanowire structures, electron transport is hardly affected by the boundary scattering due to their small intrinsic mean free paths while phonons are strongly scattered due to classical size effect. The results suggest that the nanostructures of Kondo insulators can be designed for high performance thermoelectric cooling devices at low temperatures.     

Structural and electrical properties of epitaxial Pb/Ag superlattices

Pb---Ag superlattices, grown epitaxially on Si(111) surfaces, are studied by reflection high energy electron diffraction, X-ray diffraction and resistivity measurements. The electron mean free path and superconducting transition temperature are determined as a function of period length. It is found that well-ordered single-crystal multilayers are formed and stable only in a limited region of period lengths.     

The impurity dependence of the electrical resistivity in the intermediate state

Measurements have been made of the electrical resistivity in the intermediate state of a superconducting cylinder of tin showing the resistivity to have the expected temperature and field dependence, but indicating that the dependence on the electron mean free path appears to be inadequately characterized by present theory.     

用蒙特卡罗方法计算俄歇电子谱中的背散射因子

俄歇电子谱（AES）要成为样品定量分析的手段，必须考虑背散射电子与俄歇电子平均逃逸深度对俄歇电子产额的影响。本文提出一种对固体中低能电子背散射过程的简化模型，并进行蒙特卡罗计算机模拟研究，以确定背散射因子的方法，不仅可减少计算时间又能保持必要的精度，而且具有不依赖于实验测量误差的优点。以铜、银材料为实例，所得的背散射因子与文献报道相符，表明所采用的简化模型是合理的。     

Oxidation of thin titamium films in ambient conditions

The room temperature oxidation of vapor-deposited titanium films was investigated as a function of film thickness by resistivity and ellipsometric measurements. The thickness of the films ranged from 3.0 to 120 nm. The electron mean free path in the films varied from 11.3 to 26.0 nm, and the product of the bulk resistivity and the electron mean free path was 1.5 × 10^-10Ω cm². The values of the surface electron scattering parameter in the films were between 0.2 and 0.6 and decreased to 0.18 at the onset of oxidation. The best value for the complex refractive index of titanium was found to be 3.61-i4.06. The refractive index of the oxide film was 2.75. After a 1 d exposure of the films to room air at atmospheric pressure both the resistance and ellipsometric measurements indicated a residual oxide thickness of 2.6±0.3 nm, regardless of the original metal film thickness. These results disagree with the theoretical and experimental results of Mindel and Pollack, which implied a rapid decrease in oxide thickness with decreasing titanium film thickness.     

俄歇定量分析基体效应校正因子的获得

本文探试了一种俄歇定量分析基体效应校正因子获取的新方法。通过测量淀积过程中的俄歇强度——时间曲线,并对此进行拟合,得到定量分析所需的相对背散射因子和相对非弹性碰撞平均自由程。将它们用于合金样品的定量分析,得到了较好的结果。实验表明,非弹性碰撞平均自由程效应的校正和背散射因子的校正对定量分析是同样重要的。     

Direct imaging of single Au atoms within GaAs nanowires

Incorporation of catalyst atoms during the growth process of semiconductor nanowires reduces the electron mean free path and degrades their electronic properties. Aberration-corrected scanning transmission electron microscopy (STEM) is now capable of directly imaging single Au atoms within the dense matrix of a GaAs crystal, by slightly tilting the GaAs lattice planes with respect to the incident electron beam. Au doping values in the order of 10(17-18) cm(3) were measured, making ballistic transport through the nanowires practically inaccessible.     

Properties of the conduction electrons in the metals of group VIII

The behaviour of the conduction electrons in the eighth group elements was deduced from the results of experiments on vacuum-deposited films, by combining measurements of the electrical resistivity with thickness determinations and optical investigations. The mean free path of the electrons, their concentration, their effective mass, and the proportion of them specularly reflected at the film surfaces (known as the scattering parameter) were calculated by solving a system consisting of the Sondheimer equation for the size-dependence of the film conductivity, the known formulae giving the mean free path at the Fermi level and the plasma wavelength of the electron gas, and the Dingle relation for the scattering parameter in the spectral region of the anomalous skin effect.The data thus obtained are in reasonable agreement with the generally accepted band structure of the transition metals.     

Monte Carlo calculation of backscattered light intensity by suspension: comparison with experimental data

An optical device for measuring high particle concentrations is presented. The sensor consists of two optical fibers used for the emission and reception of the light scattered by particles in suspension. To describe the light path in this medium, we developed a Monte Carlo calculation including a mean free path, a phase function for particle scattering, and an absorption rate. A similarity law links concentration variations to a homothetic space transformation. A comparison between our calculation and experimental data is given for well-stirred, dense suspensions of silica particles with different sizes. A good fit is found when the model parameters (mean free path and phase function) are chosen according to the data given by a particle sizer.     

Hardening and softening mechanisms of pearlitic steel wire under torsion

The mechanical behaviors and microstructure evolution of pearlitic steel wires under monotonic shear deformation have been investigated by a torsion test and a number of electron microscopy techniques including scanning electron microscopy (SEM) and transmission electron microscopy (TEM), with an aim to reveal the softening and hardening mechanisms of a randomly oriented pearlitic structure during a monotonic stain path. Significantly different from the remarkable strain hardening in cold wire drawing, the strain hardening rate during torsion drops to zero quickly after a short hardening stage. The microstructure observations indicate that the inter-lamellar spacing (ILS) decreases and the dislocations accumulate with strain, which leads to hardening of the material. Meanwhile, when the strain is larger than 0.154, the enhancement of dynamic recovery, shear bands (SBs) and cementite fragmentations results in the softening and balances the strain hardening. A microstructure based analytical flow stress model with considering the influence of ILS on the mean free path of dislocations and the softening caused by SBs and cementite fragmentations, has been established and the predicted flow shear curve meets well with the measured curve in the torsion test.     

Rendement d'émission électronique secondarie déduit d'un modéle analytique: Comparaison avec les valeurs expérimentales pour l'or et l'aluminium

Conditions for comparison between experimental data and the theoretical results obtained from Amelio's analytical model of secondary electron emission are defined. We show that the quantity numerically comparable is the yield δ₀ due to the contribution to secondary electron formation by the primary electrons during their penetration into the target. The results show that the model presented leads to a choice of the mean free path which is unrealistic and inconsistent with the theory in terms of the Boltzmann transport equation.     

Magneto-optic properties of particulate layers

Restriction of electron mean free path in small metallic particles leads to a modification of the bulk optical and magneto-optic constants. The effect of this restriction and particle shape on the effective macroscopic values ofbar{n}andbar{Q}for a particulate layer are considered. Values of saturation polar Kerr rotation and Faraday effect figure of merit are calculated for different volume concentrations of nickel particles in a quartz matrix.     

Penetration of an electric field into a superconductor. II

An expression has been obtained for an extra potential at the superconductor-normal metal interface which is valid for an aribtrary electron mean free path. The effect of anisotropy and paramagnetic impurities on the penetration length of an electric field into the superconductor has been investigated. In sufficiently pure superconductors with long energy relaxation time the electric field attenuates, oscillating at distances less than the correlation length.     

Effect of particle size on the mechanical properties of poly(vinylchloride)-copper particulate composite

The effect of particle size ratios on tensile strength, tensile modulus, impact strength, Vickers hardness and transverse rupture strength of particulate PVC-Cu composite has been studied. The strength values reach a maximum at a particular metal loading where the formation of a segregated network is believed to occur. This critical metal loading shifts towards lower values with increasing polymer-to-metal size ratios. The mean free path calculated from the proposed model is found to be same in all metal loadings, corresponding to strength maxima irrespective of the particle size ratios. A correlation between strength properties and the mean free path has been suggested.     

Plasmon resonance shift during grazing incidence ion sputtering on Ag(001)

Grazing incidence ion sputtering was used to create shallow ripple patterns on a Ag(001) surface. The anisotropic plasmon resonance associated with this ripple pattern can be sensitively measured with Reflection Anisotropy Spectroscopy. A slight red shift of the resonance energy is observed with increasing ion fluence. The observed resonance feature is described well with a skewed Lorentzian line shape. This line shape is the small roughness length scale limit of the Rayleigh Rice perturbation approach. The width of this line shape is directly related to imaginary part of the dielectric function, which shows a roughness induced reduction of the electron mean free path. The observed change in resonance energy and strength with ion fluence is discussed.