J.C. Barbour

Strongly enhanced flux pinning by high‐energy proton irradiation of a Tl2Ca2Ba2Cu3Oy single crystal

Flux creep data are reported which clearly demonstrate a dramatic enhancement in the pinning potential for magnetic vortices (fluxoids) in a Tl2Ca2Ba2Cu3Oy single crystal upon irradiation with 4.5 MeV protons. The creep rate is markedly reduced between 5 and 80 K, and the effective pinning potential is quadrupled from ≊100 to ≊400 meV at 20 K in a field of 50 mT. This stronger pinning enhances the magnetization hysteresis and hence the inferred low‐field critical current density by a factor of 5 at 40 K.

Oxygen ion irradiation of Tl2Ca2Ba2Cu3O10 superconductors

The superconducting transport properties of polycrystalline Tl2Ca2Ba2Cu3O10 thin films irradiated with 740 keV oxygen ions were monitored as a function of fluence. Both the transition temperature (Tc ) and the critical current density (Jc ) decreased rapidly with fluence; however, the transition temperature onset remained constant. A fluence of 2×1014 O/cm2 (0.028 dpa) was sufficient to eliminate superconductivity in the films. Jc at 76 K decreased from 25u2009000 A/cm2 in the unirradiated sample to 2000 A/cm2 after a fluence of 2.1×1013 O/cm2. A room‐temperature anneal caused both Tc and the normal‐state resistivity to recover slightly after low‐fluence irradiations.

Stoichiometry and irradiation effects in melt grown Tl-Ca-Ba-Cu-O single crystals

Melt-grown crystals in the Tl-Ca-Ba-Cu-O system with the same structure type can have substantial differences in the superconducting transition, both in width and onset temperature. These differences are attributed to stoichiometry variations arising from cation site substitution. Magnetization and electrical resistivity data are presented which emphasize the extreme sensitivity of the superconductivity to the exact stoichiometry in this system. High quality single crystals exhibit large flux creep due to a weak pinning potential for magnetic flux lines. Flux pinning and thus the critical current density are shown to be significantly enhanced by irradiation with high-energy protons or neutrons. 25 refs., 3 figs., 1 tab.

Temperature dependent microstructural modification in ion-irradiated Tl-type high temperature superconductors

Abstract Ion irradiation damage creation and recovery were examined in Tl-based high temperature superconductors, HTSC, using TEM, resistivity, and magnetic measurements for irradiation temperatures of 20 to 650 K. During 1.5 MeV Kr + and Xe + ion irradiations of single-crystal Tl-1212 and Tl-1212 Tlue5f8Baue5f8Caue5f8Cuue5f8O HTSC, microstructural modification was observed in situ by electron diffraction and shows a remarkable temperature dependence. At selected sample temperatures, irradiations continued until a critical fluence, D c , was reached where the original structure disappeared. The temperature dependence of D c shows a minimum near the superconducting transition temperature, T c , and is correlated with the temperature dependence of the thermal conductivity, which has a maximum near T c . At an irradiation temperature near this maximum in thermal conductivity, a minimum amount of damage recovery occurs because heat can be dissipated away from the displacement cascade. Ion irradiation suppresses the T c . The rate of decrease in the T c as a function of damage (measured in displacements per atom, dpa) was found to be the same for various incident ions (He + , O 2+ , Au 5+ which shows that the damage accumulation is a result of atomic collisions. Further, the rate of decrease in T c was found to be the same for both transport and magnetization measurements, indicating that the displacements effect the bulk of the samples through point defect creation. An activation energy of 0.4 eV for ion irradiation damage recovery over the temperature range from 100 to 650 K was determined from normal state resistance versus time immediately after irradiation.

Reciprocal space analysis of the microstructure of luminescent and nonluminescent porous silicon films

The microstructure of anodically prepared porous silicon films was determined using a novel X-ray diffraction technique. This technique uses double-crystal diffractometry combined with position-sensitive X- ray detection to efficiently and quantitatively image the reciprocal space structure of crystalline materials. Reciprocal space analysis of newly prepared, as well as aged, p{sup {minus}} porous silicon films showed that these films exhibit a very broad range of crystallinity. This material appears to range in structure from a strained, single-crystal, sponge-like material exhibiting long-range coherency to isolated, dilated nanocrystals embedded in an amorphous matrix. Reciprocal space analysis of n{sup +} and p{sup +} porous silicon showed these materials are strained single-crystals with a spatially-correlated array of vertical pores. The vertical pores in these crystals may be surrounded by nanoporous or nanocrystalline domains as small as a few nm in size which produce diffuse diffraction indicating their presence. The photoluminescence of these films was examined using 488 nm Ar laser excitation in order to search for possible correlations between photoluminescent intensity and crystalline microstructure.

Characterization of Carbon Nitride Films Produced by Pulsed Laser Deposition

Carbon Nitride (CN{sub x}) films have been grown by ion-assisted pulsed-laser deposition (IAPLD). Graphite targets were laser ablated while bombarding the substrate with ions from a broad-beam Kaufman-type ion source. Ion voltage, current density, substrate temperature, and feed gas composition (N{sub 2} in Ar) were varied. Resultant films were characterized by Raman. Fourier transform infrared (FTIR), and Rutherford back scattering (RBS) spectroscopy. Samples with {approximately} 30% N/C ratio have been fabricated. The corresponding Raman and FTIR spectra indicate that nitrogen is incorporated into the samples by insertion into sp{sup 2}-bonded structures. A low level of C{identical_to}N triple bonds is also found. As the ion current and voltage are increased with a pure Ar ion beam, Raman peaks associated with nanocrystalline graphite appear in the spectra. Adding low levels of nitrogen to the ion beam first reduces the Raman intensity in the vicinity of the graphite disorder peak without adding detectable amounts of nitrogen to the films (as measured by RBS). At higher nitrogen levels in the ion beam, significant amounts of nitrogen are incorporated into the samples, and the magnitude of the ``disorder`` peak increases. By increasing the temperature of the substrate during deposition, the broad peak due mainly to sp{sup 2}-bonded C-N in the FTIR spectra is shifted to lower wavenumber. This could be interpreted as evidence of single-bonded C-N; however, it is more likely that the character of the sp{sup 2} bonding is changing.

Transport and pinning in high quality TlCaBaCuO films

Abstract Tlue5f8Caue5f8Baue5f8Cuue5f8O High temperature superconductors, especially Tl 2 Ca 2 Ba 2 Cu 3 O 10 with a T c of 120K, are of considerable interest for practical applications. Thin films in this system have demonstrated high T c ′s, compositional tolerance, and a lack of weak links at the grain boundaries. Flux pinning in the system is appreciably weaker than the YBa 2 Cu 3 O 7 materials as evidenced by large magnetic relaxation and the decrease in critical current density at high magnetic field. Recent work has evaluated the effect of improving film quality on the pinning and explored methods of increasing the pinning. We have made substantial improvements in the film quality that resulted in “decreased” pinning; ion irradiation slightly enhances the pinning.

Flux Motion Effects in Tl-Ca-Ba-Cu-O Thin Films

Flux motion effects, flow and creep, have been identified as one of the major problem areas in high temperature superconductors. In this paper, we examine these effects in polycrystalline thin films of the Tl-Ca-Ba-Cu-O high temperature superconductors. Films with transport critical current densities to 9×105 A/cm2 at 77 K and no weak-link behavior have been obtained. The activation energy for flux motion as a function of temperature has been determined by static magnetization and magnetotransport measurements. Estimated activation energies at 77 K from these techniques are 200 meV at 50 mT and 41 meV at 1 T, respectively. We discuss a new superconducting device, the superconducting flux flow transistor, fabricated from photolithographically patterned films, that utilizes switching between the superconducting and flux flow states as the active device mode. These devices show the potential for operation to 60 GHz utilizing the current films

ECR plasma synthesis of silicon nitride films on GaAs and InSb

Growth of high-quality dielectric films from Electron Cyclotron Resonance (ECR) plasmas provides for low-temperature surface passivation of compound semiconductors. Silicon nitride (SiN{sub x}) films were grown at temperatures from 30 to 250 C on GaAs substrates. Stress in films was measured as a function of bias applied during growth (varied from 0 to 200 V), and of sample annealing treatments. Composition profiles of the samples were measured using ion beam analysis. The GaAs photoluminescence (PL) signal after SiN{sub x} growth without an applied bias (ion energy {congruent}30 eV) was twice as large as the PL signal from the cleaned GaAs substrate. The PL signal from samples biased at -50 and -100 V indicated that damage degraded the passivation quality, while atomic force microscopy of these samples showed a three fold increase in rms surface roughness relative to unbiased samples. The sample grown with a bias of -200 V showed the largest reduction in film stress but also the smallest PL signal.