No abstract

The electrical properties of microfabricated nanobridges of copper, silver, and gold with contact diameters in the range 4 -32 nm have been studied. High-quality point-contact spectra are evidence that electron transport is ballistic in these nanobridges. A comparison of our spectra with spectra from mechanical point contacts shows that microfabricated nanobridges are at least as good as mechanical point contacts for study of the electron-phonon interaction. Further, in Au nanobridges we have observed defect motion induced two-level resistance fluctuations (TLFs). An expression is derived for the voltage dependence of the temperature Tz of a defect in a nanobridge at low lattice temperatures. Using this expression for Tz, the experimental voltage dependence of the TLF's is successfully described by a thermal-activation model for the fluctuation rates, in which the voltage dependence of the activation energy and defect temperature is included. The values for the attempt time, activation energy, and electromigration parameter are as expected for defects in metals. An analysis of the two TLF's studied, showing a striking difference in both voltage dependence and magnitude of the duty cycle, suggests that rearrangement of complex defects is the mechanism behind the TLF behavior.

We perform detailed temperature dependent measurements of the magnetoresistance ͑MR͒ and its angular dependence of epitaxial Fe ͑110͒ films. The angular dependence of the MR at Hϭ10 kOe is found to change strongly when going from Tϭ4.2 K to Tϭ230 K. We analyze the data on the basis of Döring's equation. Second-and fourth-order angular dependent terms are found to be of equal importance, indicating strong deviations of the MR from a simple cos 2 dependence. One of the MR components is the ordinary or Lorentz magnetoresistance, which is strong at low temperatures and becomes smaller at higher temperatures, due to the reduction of the mean free path. By subtracting the ordinary magnetoresistance from the MR data we obtain the anisotropic magnetoresistance. We decompose the temperature dependent anisotropic magnetoresistance in the temperature dependent k constants of Döring's equation. These constants show a reduction between Tϭ20 K and Tϭ100 K, which reflects the observed decrease of the anisotropic magnetoresistance. We present arguments that the temperature dependence of the anisotropic magnetoresistance is most likely due to the change from defect-dominated scattering to phonon-dominated scattering, each of which has its own anisotropic magnetoresistance.

We report an investigation of the low-temperature electrical transport properties of bismuth films under applied hydrostatic pressure. Films with their trigonal axis perpendicular to the film plane and thicknesses of 30, 50, and 500 nm were grown by molecular-beam epitaxy on BaF 2 substrates. At 500 nm thickness the behavior resembles that of bulk Bi. From the observed Shubnikov-de Haas oscillations we find a pressureinduced decrease in extremal Fermi cross section. For the 30-nm film, we obtain the low-temperature carrier densities for electrons and holes together with the corresponding mobilities from magnetoconductance data at pressures up to 20 kbar. We find that pressure strongly reduces the surface-induced excess hole concentration, clearly revealing a finite electron concentration at high pressures. We discuss our results within the context of a possible semimetal-semiconductor transition in thin Bi films.

An experimental study on the ion impact energy distribution and the total ion flux at the driven electrode of a parallel plate reactive ion etcher is presented. Results are shown for 13.56 MHz discharges in Ar, Ar/H2, N2, O2, Cl2, and SF6/He over a pressure range of 0.3–40 Pa. The ion impact energy distribution consists of a collision-free part and a collision-induced part. It is observed that in Ar, N2, O2, and Cl2 the collision-induced part contains single and double peaks at regular energy intervals. This peaked structure is attributed to charge exchange processes in the sheath. Both the collision-free part and the collision-induced part of the ion impact energy distribution are well described by a model based on a constant sheath width, a sinusoidal sheath voltage, and a power law for the electric field in the sheath. The only adjustable parameter in the model is the sheath thickness. The sheath thickness has also been determined independently from the total ion current density using the Child–Langmuir law. The obtained values are in good quantitative agreement with each other and with the observed optical sheath thickness, demonstrating the overall consistency of the present approach.