Showing papers by "John B Ketterson published in 1993"

Buckytubes and Derivatives: Their Growth and Implications for Buckyball Formation

Abstract: Transmission electron microscopy (TEM) observations of graphite tubules (buckytubes) and their derivatives have revealed not only the previously reported buckytube geometries but also additional shapes of the buckytube derivatives. Detailed cross-sectional TEM images reveal the cylindrical cross section of buckytubes and the growth pattern of buckytubes as well as their derivatives. These observations of frozen growth stages of buckytubes and derivatives suggest a helical growth mechanism analogous to that of crystal growth via screw dislocations. The helicacy of buckytubes is analyzed by electron diffraction whereas the anisotropy of electronic structure is revealed by momentum transfer resolved electron energy loss spectrometry. Based on the TEM observations, it is proposed that buckytubes act as precursors to closed-shell fullerene (buckyball) formation and the possible steps in buckyball formation are outlined. In arc evaporation experiments in which residue rods (containing various amounts of buckytubes) were used as the starting anode for fullerene production, the amount of buckytubes in the rod was correlated with fullerene yield.

Semimetal-to-semiconductor transition in bismuth thin films.

Abstract: Field- and temperature-dependent magnetotransport measurements on Bi layers grown by molecular-beam epitaxy have been analyzed by mixed-conduction techniques. In the thin-film limit, the net hole density scales inversely with layer thickness while the mobility scales linearly. By studying the minority electron concentration as a function of temperature in the range 100--300 K, we have unambiguously confirmed the long-standing theoretical prediction that quantum confinement should convert Bi from a semimetal to a semiconductor at a critical thickness on the order of 300 \AA{}.

Semimetal-to-Semiconductor Transition in Bismuth Thin Films

Abstract: Field- and temperature-dependent magnetotransport measurements on Bi layers grown by molecular-beam epitaxy have been analyzed by mixed-conduction techniques. In the thin-film limit, the net hole density scales inversely with layer thickness while the mobility scales linearly. By studying the minority electron concentration as a function of temperature in the range 100-300 K, we have unambiguously confirmed the long-standing theoretical prediction that quantum confinement should convert Bi from a semimetal to a semiconductor at a critical thickness on the order of 300 A

Growth and characterization of buckybundles

Abstract: Buckybundles (bundles of buckytubes), approximately 50 μm in diameter and 1 cm in length, have been observed in a deposited carbon rod on the cathode electrode of a dc arc. Scanning electron microscopy images clearly show an evenly spaced array of parallel buckybundles. High resolution electron microscopy reveals that each buckybundle consists of closely packed buckytubes with their axes parallel to the bundle axis. Within each bundle it is estimated that there are about 107 buckytubes with diameters in the range (20–300 A). We have measured the deposited rod growth rate as a function of the He gas pressure and have evaluated the influence of the graphite feed rod diameter on the yield of buckybundles. The magnetic susceptibilities parallel and perpendicular to the bundle axes were measured. The results show that the bundles have anisotropic diamagnetic properties.

Atomic-force and fluorescence microscopy of Langmuir-Blodgett monolayers of L -α-dimyristoylphosphatidic acid

Abstract: We have obtained fluorescence and atomic-force-microscopy images of the liquid and solid phases of Langmuir-Blodgett monolayers of [ital L]-[alpha]-dimyristoylphosphatidic acid, some containing [similar to]1 mol % of [ital L]-[alpha]-phosphatidylethanolamine-[ital N]-4-nitrobenzo-2-oxa-1,3-diazole The fluorescence microscopy images are similar to those of monolayers at the air-water interface, but slight differences indicate that changes occur in the monolayers as a result of the deposition process The atomic-force-microscopy images show the clean glass substrates to be very much rougher than when the monolayers are deposited on them We propose that a water layer provides a smooth surface on which the Langmuir-Blodgett monolayers rest We measured a [similar to]25-A height difference between the liquid and solid phase region, one explanation being that the alkyl chains of the molecule are standing up in the solid phase and lying down in the liquid phase This result is consistent with the fluorescence and atomic-force-microscopy images which show an increase in the amount of solid phase domains as a result of the deposition process Further, the atomic-force-microscopy images of the monolayers show features which have not been described before and which could not be attributed to the presence of the dye Finally, we showed that if the Langmuir-Blodgett monolayers aremore » not stored in a humid environment, they begin to collapse into multilayers« less

Second order optical nonlinearities of radio frequency sputter‐deposited AlN thin films

Abstract: Polarized second harmonic generation measurements were performed on AlN films deposited on (100) sapphire substrates by the reactive rf sputtering technique. The bulk effective second order nonlinearity observed in these films is typically about 6×10−9 esu at 1.06 μm, several times larger than that of quartz or KTP. The tensorial properties of the nonlinearity are consistent with the crystal symmetry of AlN and the microcrystallinity of these films.

Transport properties in n‐type InSb films grown by metalorganic chemical vapor deposition

Abstract: We have measured the temperature and magnetic field dependence of the Hall mobility and transverse magnetoresistance in n‐type InSb films epitaxially grown on GaAs substrates by metalorganic chemical vapor deposition. The films show a giant magnetoresistance: e.g., at 240 K the resistivity increases over 20 times at a magnetic field of 5 T; the low field coefficient of resistivity at 77 K is as high as 47.5 μΩ cm/G. The Hall mobility decreases with magnetic field and saturates at higher fields. By taking the interface carrier transport into account, the observed field dependence of the Hall mobility and magnetoresistance may be understood based on a two‐layer model.

Observation of an evolving standing-wave pattern involving a transverse disturbance in superfluid 3He-B.

Abstract: We have observed a series of oscillations in the transverse acoustic response of superfluid 3 He-B, which, in analogy with earlier observations involving longitudinal sound, suggest an evolving standing-wave pattern. These oscillations were observed with 60.8 MHz transverse sound at pressures between 1 and 3 bars. The change in the reciprocal of the phase velocity can be obtained from these data, and it is compared with a recent theoretical prediction [G. F. Moores and J. A. Sauls, J. Low Temp. Phys. 91, 13 (1993)]

Superlattices of YBa2Cu3O7−δ/PrBa2Cu3O7−δ grown by the pulsed organometallic beam epitaxy method

Abstract: We report on the growth of YBa2Cu3O7−δ/PrBa2Cu3O7−δ superlattices by pulsed organometallic beam epitaxy technique. X‐ray diffraction and analytical high resolution transmission electron microscopy measurements show high‐quality superlattice crystalline structures are being grown. Electrical measurements of these layered oxides have also shown very promising results. This work demonstrates the capabilities of pulsed organometallic chemical vapor deposition as an alternative method for growing high temperature superconducting oxides in a controlled manner.

Magneto-optical determination of the T-point energy gap in bismuth.

Abstract: We have carried out infrared magnetotransmission measurements on a 5000-A-thick bismuth film grown by molecular beam epitaxy onto a CdTe substrate. The data for photon energies below 300 meV display at least ten orders of minima due to interband transitions at the L points. In contrast to previous results from magnetoreflectivity experiments on bulk Bi, two concurrent series of resonances separated by a nearly constant energy of ≃5 meV are observed. An additional series of strong oscillations emerges at somewhat higher photon energies, due to interband transitions at the T point

Measurements of the transverse acoustic impedance of superfluid 3He-B.

Abstract: The transverse acoustic impedance in the [ital B] phase of superfluid [sup 3]He has been measured using a cw technique. These measurements were performed over a pressure range of 4.4--20.9 bar and at a frequency of 61 MHz. A simultaneous measurement of the longitudinal response at 61 MHz was made on a separate sound path. The availability of both probes allows a direct comparison of the transverse response with known features in the longitudinal response.

Multilayered Josephson transmission line based photon counting detector with ultra-high temporal and high spatial resolution

Abstract: A superconducting Josephson transmission line (JTL) fabricated with multilayered tunnel junctions with thin (∼100 A) superconducting layers may be used as an ionizing radiation detector. The suppression of the superconducting energy gap in the layers of such a JTL, due to the local deposition of energy by incident radiation, will initiate the propagation of one or more fluxons in the device. These fluxons represent digital information in the form processable by single flux quantum (SFQ) superconducting digital circuitry. Designs for JTL based detectors with temporal resolutions on the order of picoseconds and spatial resolution on the order of microns, along with numerical simulation results, are presented.

Ferroelectric properties of a-axis textured BaTiO3 thin films

Abstract: Ferroelectric and dielectric properties were measured for BaTiO3 thin films prepared by metalorganic chemical vapor deposition which were highly a-axis textured. No ferroelectric hysteresis was observed from the as-deposited BaTiO3 films on Pt coated MgO. Upon applying an electric field exceeding a threshold electric field, E1 ~ 50 - 100 kV/cm, a ferroelectric hysteresis was observed. A spontaneous polarization Ps > 15 μC/cm2 was measured for the textured films.

Fabrication of Potassium Titanyl Phosphate Films by Pulsed Excimer Laser Ablation for Nonlinear Optical Application

Abstract: We have fabricated highly-orientated crystalline potassium titanyl phosphate (KTP) films on a variety of substrates by pulsed excimer laser ablation. The resulting films have been extensively characterized in terms of their compositional, crystalline, and optical properties. Nonlinear optical property of secondary harmonic generation of the films has been evaluated with a high d value comparable to that of KTP bulk crystals. The results are encouraging that the KTP films deposited are very promising for nonlinear optical waveguide device applications.