Showing papers by "John B Ketterson published in 1998"
TL;DR: In this article, the effect of a regular array of magnetic dipoles embedded in a superconducting film was investigated and a large asymmetry in critical currents was found between when the magnetic dipole are aligned and antialigned with respect to an externally applied magnetic field.
Abstract: The effect of a regular array of magnetic dipoles embedded in a superconducting film was investigated. A large asymmetry in critical currents was found between when the magnetic dipoles are aligned and antialigned with respect to an externally applied magnetic field. Enhanced pinning effects were observed when the flux lattice and the dipole lattice were commensurate. The data are used to infer pinning mechanisms, strengths, and sites.
218 citations
TL;DR: In this paper, a study of optical second-harmonic (SH) generation in AlN films deposited on borosilicate glass by reactive direct current magnetron sputtering is reported.
Abstract: Results of a study of optical second-harmonic (SH) generation in AlN films deposited on borosilicate glass by reactive direct current magnetron sputtering are reported. As determined by x-ray diffraction (XRD), the films were highly c-axis oriented. The transmitted SH signal for film thickness ranging from 340 to 7800 A was measured. The signal was consistent with the existence of a dead layer (in which no SH was generated) on the film surface with an effective thickness of ∼300 A. A set of 7800-A-thick films were deposited at substrate temperatures from ambient to Ts=400 °C. For films deposited at 100 °C or higher, minimal variation in the SH signal with Ts was found. Values of |d33|=4 pm/V and |d31|=0.04 pm/V, with d33 and d31 having the same sign, were obtained for Ts=400 °C. The approximate tilt angle of the optic axis of the films with respect to the substrate surface normal was determined. XRD results showed that the tilt was due to a tilt of the crystallographic texture axis.
30 citations
TL;DR: In this paper, a Te monolayer (ML) was added to Bi/CdTe superlattices to reduce the interfacial energy and change the growth mode from 3D island-like to layer-by-layer growth.
Abstract: Bi deposited on the $\mathrm{CdTe}(111)A$ (Cd-terminated) surface grows by three-dimensional (3D) islanding, while Bi deposited on the $\mathrm{CdTe}(111)B$ (Te-terminated) grows layer-by-layer. However, introducing a Te monolayer (ML) on the $\mathrm{CdTe}(111)A$ surface reduces the interfacial energy, thereby changing the growth mode of Bi from 3D islandlike to layer-by-layer growth. The Te ML remains where it is deposited, which differs from the growth mode in which the surface-active agent floats on the growing surface. By incorporating appropriate Te ML's, Bi/CdTe superlattices with sharper interfaces were observed. These superlattices were characterized by x-ray diffraction and transmission electron microscopy.
26 citations
TL;DR: In this paper, the authors investigated the planarizing effects of multilayered superconducting thin films and found that coating a standard niobium base electrode with alternating layers of aluminum and Niobium significantly reduced the film's overall surface roughness.
Abstract: We have investigated the planarizing effects of multilayered superconducting thin films. Atomic force microscopy measurements indicate that coating a standard niobium base electrode with alternating layers of aluminum and niobium significantly reduces the film’s overall surface roughness. Planarized films such as these were used as the base electrodes of superconducting tunnel junctions that show vastly improved leakage characteristics over conventional junctions fabricated under the same conditions.
18 citations
TL;DR: A simple model of the multiple-scattering images is made of the conical radiation from the scanning tip and compared with the data, deriving excellent results.
Abstract: In two previous papers we presented experimental results of measurements with a scanning plasmon optical microscope constructed in the Kretschmann configuration. The angular distribution of the conical radiation from the scanning tip was measured, and the multiple-scattering images of simple surface irregularities were detected. We make a simple model of the multiple-scattering images and compare it with our data, deriving excellent results.
4 citations
TL;DR: In this article, a variable path acoustic interferometer for use at cryogenic temperatures is described, which is enabled without mechanical coupling via two piezoelectric bimorphs wired and mounted in a manner that preserves the parallelism of two ultrasonic transducers that define the acoustic path.
Abstract: We describe a variable path acoustic interferometer for use at cryogenic temperatures. Movement is enabled without mechanical coupling via two piezoelectric bimorphs wired and mounted in a manner that preserves the parallelism of two ultrasonic transducers that define the acoustic path. A certain degree of in situ alignment can also be accomplished. Path length sweeps from 0 to 180 μm have been made at cryogenic temperatures and preliminary sound velocity measurements in liquid 4He and gaseous 3He near 4 K are presented which agree well with past measurements.
3 citations
TL;DR: In this article, the transfer of a surfactant covered film between two Langmuir troughs over a bridge-shaped glass slide was studied under the Landau-Levich theory.
Abstract: We have studied the transfer of a surfactant covered film between two Langmuir troughs over a bridge-shaped glass slide when the surfactant concentrations in the two troughs were maintained at different values. We applied the Landau-Levich theory to this problem, assuming the Deryagin-Landau disjoining pressure for water films on glass. The theoretical relationship between the rate of transfer and the surface pressure differential in the two troughs is in good agreement with the experiment.
2 citations
TL;DR: In this paper, X-ray diffraction patterns (θ-2θ scans and rocking curves) of Bi1-x Te1+x have been analyzed, showing that their crystallinity depends upon the compositional deviation from stoichiometric BiTe.
Abstract: Thin films of the hexagonal phase of Bi1-x Te1+x have been grown on CdTe(111) substrates using molecular beam epitaxy (MBE). Analysis of X-ray diffraction patterns (θ-2θ scans and rocking curves) of the films shows that their crystallinity depends upon the compositional deviation from stoichiometric BiTe. Measurements of the temperature-dependent thermoelectric power (TEP) of the films reveals that compositional changes cause the TEP to vary from electron dominant (n-type) to hole dominant (p-type), implying their possible application as a thermoelectric cooler or power generator. Measurements of the temperature-dependent resistivity of the films were conducted, and the analysis shows semimetallic behavior. These results demonstrate that Bi1-x Te1+x is an appropriate model system to study the dependencies of thermoelectric and structural properties on binary composition.
1 citations
TL;DR: In this paper, the authors have grown Bi1-xSbx alloy thin films on CdTe(111)B over a wide range of Sb concentrations (0≤x≤0.183) using MBE.
Abstract: We have grown Bi1-xSbx alloy thin films on CdTe(111)B over a wide range of Sb concentrations (0≤x≤0.183) using MBE. We have observed several differences with the bulk system. The 3.5 and 5.1% Sb alloys show semiconducting behavior, and the Sb concentration with the maximum bandgap is shifted to a lower Sb concentration, from 15% in bulk to 9%. The power factor S2/ρ (where S is thermoelectric power(TEP) and ρ electrical resistivity) peaks at a significantly higher temperature (250K) than previously reported for the bulk alloy (80K). The magnetotransport properties of Bi1-x,Sbx thin films (x = 0, 0.09, and 0.16) and Bi/CdTe superlattices have been determined by applying the Quantitative Mobility Spectrum Analysis (QMSA) and multicarrier fitting to the magneticfield- dependent resistivities and Hall coefficients, using algorithms which account for the strong anisotropy of the mobilities. The calculated S values are in good agreement with experimental results. The structural stability of bulk Bi is studied using the local density linear muffin-tin orbital method. It is shown that the internal displacement changes the Bi electronic structure from a metal to a semimetal, in qualitative agreement with a Jones-Peierls-type transition. The total energy is calculated to have a double well dependence on the internal displacement, and to provide a stabilization of the trigonal phase. We show that an increase of the trigonal shear angle leads to a semimetal-semiconductor transition in Bi.
1 citations
TL;DR: In this article, high quality Bi2 Te3 thin films on CdTe(111)B substrates using MBE have been grown using in-situ reflection high-energy electron diffraction (RHEED) and θ-2θ X-ray diffraction analysis.
Abstract: We have grown high quality Bi2 Te3 thin films on CdTe(111)B substrates using MBE. Structural properties have been investigated using in-situ reflection high-energy electron diffraction (RHEED) and θ-2θ X-ray diffraction analysis. They show that Bi2 Te3 films on CdTe(111) grow along the (00. l) in the hexagonal cell with a layer-by-layer growth mode, resulting in a smooth surface, and an X-ray Bragg peak FWHM of 0.2°. The thermopower and electrical conductivity of the stoichiometric Bi2 Te3 films were ∼200 μV/K and 103(Ωcm)−1, respectively, comparable to the single crystal bulk values. We have observed the antisite defect effect in Te-rich Bi2 Te3 films: excess Te occupies Bi lattice sites and behaves as an n-type dopant. Crystallinity and transport properties are strongly affected by non-stoichiometry.
24 May 1998
TL;DR: In this paper, the power factor (S/sup 2/spl sigma) for MBE-grown 1 /spl mu/m thick BiSb thin films was observed to peak at a significantly higher temperature than previous results for the bulk alloy (80 K), possibly due to an enhanced bandgap.
Abstract: We have successfully grown BiSb alloy thin films on CdTe(111)B over a wide range of Sb compositions using molecular beam epitaxy. It is well known that small bandgap (/spl sim/18 meV) bulk BiSb alloys are good n-type thermoelements at liquid nitrogen temperature. We have observed that the power factor (S/sup 2//spl sigma/) for MBE-grown 1 /spl mu/m thick BiSb thin films grown on CdTe(111) peak at a significantly higher temperature (250 K) than previous results for the bulk alloy (80 K), possibly due to an enhanced bandgap. For doping experiments we used the group IV(VI) element Sn(Te) as an acceptor(donor). Thermoelectric Power (TEP) and electrical resistivity were studied in the range of temperatures 2-300 K. Doping Sn into the BiSb system causes the TEP to change sign (from negative to positive), and the maximum value of the TEP can be controlled with the Sn dopant concentration.