Jerome G. Beery

Growth Mechanism of Sputtered Films of YBa2Cu3O7 Studied by Scanning Tunneling Microscopy

The surface microstructures of c-axis-oriented films of YBa2Cu3O7, deposited by off-axis magnetron sputtering on MgO and SrTiO3 single crystal (100) substrates, have been investigated with scanning tunneling microscopy and atomic force microscopy. There is strong evidence that the films nucleate as islands and grow by adding material to the edge of a spirally rising step. This results in columnar grains, each of which contains a screw dislocation at its center. This microstructure may be of significance in determining superconducting properties such as critical current, and represents a significant difference between thin films (especially those grown in situ) and bulk materials.

Elastic scattering of 10.5- and 14.5-MeV polarized protons from nuclei and the optical model potential at intermediate energies

Abstract The results of extensive measurements of the angular distributions and azimuthal asymmetries for elastic scattering of 10.5 and 14.5 MeV polarized protons, from a wide range of nuclei, are presented. Attention is focused on the general features of these angular distributions and certain deviations from the general trends. The interpretations of both these aspects of the data are discussed. In addition, results are presented from an extensive search program designed to obtain fits to the data with a six-parameter local “optical” potential. From these individual fits to each nuclide have been obtained a set of average parameters at 10.5 MeV and another set at 14.5 MeV. The predictions of the optical model with the average parameters are compared not only to the polarization data but also to elastic differential cross sections and reaction cross sections for protons. With a modification of the magnitudes of the refractive and absorptive central potentials, as required to fit 14.5 MeV neutron elastic scattering cross sections, the potential has been used to predict differential elastic cross sections, reaction cross sections, total cross sections, and polarizations for neutrons, and the results compared with available experimental data. A study has been made of the dependence of the refractive central potential depth on proton energy and on the nuclear symmetry parameter, (N−Z) A , obtaining reasonable agreement with the results of other workers.

Conducting polymer-colloidal silica composites

Abstract We describe the preparation of conducting polymer-colloidal silica composites by the in situ deposition of a thin coating of chemically synthesized polyaniline or polypyrrole onto monodisperse silica particles (∼1 μm diameter). These composite materials have been characterized by thermogravimetric analysis, scanning electron microscopy, four-point probe conductivity measurements, Fourier transform infra-red microscopy and Rutherford back-scattering spectrometry.

An electrochemical, ellipsometric, and surface science investigation of the PtRu bulk alloy surface

We describe results of in situ and ex situ characterizations of the surface of a bulk PtRu alloy in sulfuric acid solutions at room temperature. Ellipsometric measurements taken during potential cycling show that a chemisorbed oxygen species forms on the PtRu alloy at potentials as low as +0.25 V vs. RHE. This is found also for Ru electrodes in the same solution. However, the surface oxygen species seems to form more reversibly on the alloy surface. The surface composition of the PtRu alloy appears to be stable up to +1.0 V vs. RHE. Both the in situ and ex situ techniques employed showed partial Ru loss from the surface of the alloy for potential excursions above +1.0 V. Implications of these results for the use of such PtRu alloys as carbon monoxide tolerant anode electrocatalysts in acid fuel cells are discussed.

Microstructure and growth mechanism of thin sputtered films of YBa2Cu3O7 on MgO substrates

The microstructures of very thin sputtered films of YBa2Cu3O7 deposited on (100) faces of single‐crystal MgO, have been investigated using scanning tunneling and atomic force microscopies. By a thickness of 10 nm, the substrate is completely covered by a fine‐grained layer of the superconductor. The average grain size is about 100 nm. Many of these grains show evidence of a spiral growth mechanism. In somewhat thicker films (20 nm), the grain size has increased considerably (to about 200 nm), and this trend continues as the thickness is increased. The small grain size close to the interface suggests a possible origin for widely observed ion‐beam dechanneling in this region of the film.

Surface resistance of YBa2Cu3O7 films on SrTiO3 and LaGaO3 substrates

Surface resistance measurements of films of YBa2Cu3O7 deposited onto single‐crystal substrates of LaGaO3 and SrTiO3 have been made at a frequency of 22 GHz. The measurements were made in either a copper or niobium cavity by replacing the end wall with the superconducting film. Typical surface resistance at 20 K are 1–2 mΩ for films on LaGaO3 and 6–8 mΩ for films on SrTiO3, as measured in the copper cavity. The LaGaO3 values lie within the sensitivity range of the Cu cavity (∼2 mΩ) and can only be considered upper limits. Similar measurements in a Nb superconducting cavity resulted in a surface resistance value of 0.2±0.1 mΩ at 4 K for the best LaGaO3‐based film. This value is more than an order of magnitude lower than Cu, and suggests that LaGaO3‐based films may offer immediate advantages in a number of applications.

Characterization of Y‐Ba‐Cu‐O superconducting thin films prepared by coevaporation of Y, Cu, and BaF2

We have studied the effect of varying the postdeposition anneal conditions on the properties of thin films of YBa2Cu3O7 prepared by coevaporation from Y, Cu, and BaF2. The temperature of this processing step determines both the microstructure of the films and the constituent phases. At low temperatures, BaF2 is incompletely reacted and the predominant superconducting phase is Y2Ba4Cu8O20−x. At higher temperatures, the equilibrium distribution of phases, which includes YBa2Cu3O7, is formed. As the temperature is raised, the texture of the films also changes. The thermodynamics of the formation reaction and of the reaction between YBa2Cu3O7 and various substrate materials is discussed.

Thermal conductivity of optical coatings

A model capable of explaining recent experimental results showing a considerable decrease in the thermal diffusivity of optical coatings is presented. The model assumes that the coatings contain microcracks. It predicts that such imperfections can cause reduction factors of 2 or more orders of magnitude in the thermal conductivity of the films, when compared to the corresponding bulk values. If the cracks enclose a region where light is absorbed the thermal isolation can lead to damage of the coating. A similar model consisting of a random distribution of voids only predicts a modest reduction of the net thermal conductivity.

Surface resistance of YBa sub 2 Cu sub 3 O sub 7 films on SrTiO sub 3 and LaGaO sub 3 substrates

Surface resistance measurements of films of YBa2Cu3O7 deposited onto single‐crystal substrates of LaGaO3 and SrTiO3 have been made at a frequency of 22 GHz. The measurements were made in either a copper or niobium cavity by replacing the end wall with the superconducting film. Typical surface resistance at 20 K are 1–2 mΩ for films on LaGaO3 and 6–8 mΩ for films on SrTiO3, as measured in the copper cavity. The LaGaO3 values lie within the sensitivity range of the Cu cavity (∼2 mΩ) and can only be considered upper limits. Similar measurements in a Nb superconducting cavity resulted in a surface resistance value of 0.2±0.1 mΩ at 4 K for the best LaGaO3‐based film. This value is more than an order of magnitude lower than Cu, and suggests that LaGaO3‐based films may offer immediate advantages in a number of applications.

Growth and characterization of (111) oriented GaInAs/GaAs strained‐layer superlattices

We describe the growth, ion beam, and photoluminescence characterization of Ga1−xInxAs/GaAs strained‐layer superlattices grown along the [111] axis. The layer thicknesses and composition are determined by Rutherford backscattering. Normal incidence channeling gives a minimum channeling yield of 5.7%. Strain conditions are found by off‐normal incidence channeling using the angular scan method. Comparison of the photoluminescence spectrum of the superlattice with theoretical calculations provides strong evidence for the existence of strain‐generated electric fields in [111] growth axis strained‐layer superlattices.