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Development of biaxially textured buffer layers on rolled-Ni substrates for high current YBa{sub 2}Cu{sub 3}O{sub 7{minus}y} coated conductors

Description: This paper describes the development of 3 buffer layer architectures with good biaxial textures on rolled-Ni substrates using vacuum processing techniques. The techniques include pulsed laser ablation, e-beam evaporation, dc and rf magnetron sputtering. The first buffer layer architecture consists of an epitaxial laminate of Ag/Pd(Pt)/Ni. The second buffer layer consists of an epitaxial laminate of CeO{sub 2}/Pd/Ni. The third alternative buffer layer architecture consists of an epitaxial laminate of YSZ/CeO{sub 2}/Ni. The cube (100) texture in the Ni was produced by cold rolling followed by recrystallization. Crystallographic orientations of the Pd, Ag, CeO{sub 2}, and YSZ films grown were all (100). We recently demonstrated a critical- current density of 0.73x10{sup 6} A/cm{sup 2} at 77 K and zero field on 1.4 {mu}m thick YBa{sub 2}Cu{sub 3}O{sub 7-y} (YBCO) film. This film was deposited by pulsed laser ablation on a YBCO/YSZ/CeO{sub 2}/Ni substrate.
Date: December 31, 1996
Creator: Paranthaman, M.; Goyal, A. & Norton, D.P.
Partner: UNT Libraries Government Documents Department

Formation of artificially-layered superconducting materials by pulsed-laser deposition

Description: Artificially-layered structures, consisting of (Sr,Ba,Ca)CuO{sub 2} layers in the tetragonal, {open_quotes}infinite layer{close_quotes} crystal structure, have been grown by pulsed-laser deposition. Superlattice chemical modulation is observed for structures with SrCuO{sub 2} and (Sr,Ca)CuO{sub 2} layers as thin as a single unit cell ({approximately}3.4 {Angstrom}). In addition, novel thin-film superconductors were formed by using the constraint of epitaxy to stabilize SrCuO{sub 2}/BaCuO{sub 2} superlattices in the infinite layer structure. Using this approach, two new structural families, Ba{sub 2}Sr{sub n-1}Cu{sub n+1} O{sub 2n+2+{gamma}} and Ba{sub 4}Sr{sub n-1}Cu{sub n+3}O{sub 2n+6+{gamma}}, have been synthesized which superconduct at temperatures as high as 70 K. These results represent not only the synthesis of new structural families of superconductors, but also demonstrate that pulsed-laser deposition and epitaxial stabilization can be effectively used to engineer artificially-layered thin-film materials.
Date: September 1, 1994
Creator: Norton, D.P.; Chakoumakos, B.C. & Budai, J.D.
Partner: UNT Libraries Government Documents Department

Formation and properties of novel artificially-layered cuprate superconductors using pulsed-laser deposition

Description: Pulsed-laser deposition and epitaxial stabilization have been effectively used to engineer artificially-layered thin-film materials. Novel cuprate compounds have been synthesized using the constraint of epitaxy to stabilize (Ca,Sr)CuO{sub 2}/(Ba,Ca,Sr)CuO{sub 2} superconducting superlattices in the infinite layer structure. Superlattice chemical modulation can be observed from the x-ray diffraction patterns for structures with SrCuO{sub 2} and (Ca, Sr)CuO{sub 2} layers as thin as a single unit cell ({approximately}3. 4 {angstrom}). X-ray diffraction intensity oscillations, due to the finite thickness of the film, indicate that (Ca,Sr)CuO{sub 2} films grown by pulsed-laser deposition are extremely flat with a thickness variation of only {approximately}20 {angstrom} over a length scale of several thousand angstroms. This enables the unit-cell control of (Ca, Sr)CuO{sub 2} film growth in an oxygen pressure regime in which in situ surface analysis using electron diffraction is not possible. With the incorporation of BaCuO{sub 2} layers, superlattice structures have been synthesized which superconduct at temperatures as high as 70 K. Dc transport measurements indicate that (Ca, Sr)CuO{sub 2}/BaCuO{sub 2} superlattices are two dimensional superconductors with the superconducting transition primarily associated with the BaCuO{sub 2} layers. Superconductivity is observed only for structures with BaCuO{sub 2} layers at least two unit cells thick with {Tc} decreasing as the (Ca,Sr)CuO{sub 2} layer thickness increases. Normalized resistance in the superconducting region collapse to the Ginzburg-Landau Coulomb gas universal resistance curve consistent with the two-dimensional vortex fluctuation model.
Date: March 1, 1996
Creator: Norton, D.P.; Chakoumakos, B.C. & Budai, J.D.
Partner: UNT Libraries Government Documents Department

Fabrication of high critical current density superconducting tapes by epitaxial deposition of YBCO thick films on biaxially textured metal substrates

Description: High critical current density YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} (YBCO) tapes were fabricated by epitaxial deposition on rolling- assisted-biaxially-textured-substrates (RABiTS). The RABiTS technique uses well established, industrially scaleable, thermomechanical processes to impart a strong biaxial texture to a base metal. This is followed by vapor deposition of epitaxial buffer layers to yield chemically and structurally compatible surfaces. Epitaxial YBCO films grown on such substrates have critical current densities approaching 10{sup 6} A/cm{sup 2} at 77K in zero field and have field dependences similar to epitaxial films on single crystal ceramic substrates. Deposited conductors made using this technique offer a potential route for fabricating long lengths of high J{sub c} wire capable of carrying high currents in high magnetic fields and at elevated temperatures.
Date: December 31, 1996
Creator: Goyal, A.; Norton, D.P. & Paranthaman, M.
Partner: UNT Libraries Government Documents Department

Epitaxial YBa{sub 2}Cu{sub 3}O{sub 7} on biaxially textured (001) Ni: An approach to high critical current density superconducting tapes

Description: In-plane aligned, c-axis oriented YBa{sub 2}Cu{sub 3}O{sub 7} (YBCO) films with superconducting critical current densities, J{sub c}, as high as 700,000 amperes per square centimeter at 77 kelvin have been grown on thermo-mechanically, rolled-textured (001) Ni tapes using pulsed-laser deposition. Epitaxial growth of oxide buffer layers directly on biaxially textured Ni, formed by recrystallization of cold-rolled pure Ni, enables the growth of 1.5 micrometer-thick YBCO films with superconducting properties that are comparable to those observed for epitaxial films on single crystal oxide substrates. This result represents a viable approach for producing long-length superconducting tapes for high current, high field applications at 77 kelvin.
Date: June 1, 1997
Creator: Norton, D.P.; Goyal, A. & Budai, J.D.
Partner: UNT Libraries Government Documents Department

Epitaxial growth of oxide thin films on (001) metal surfaces using pulsed-laser deposition

Description: The epitaxial growth of CeO{sub 2} on various (001) metal surfaces using pulsed-laser deposition is discussed. In particular, the growth of (001) CeO{sub 2} on (001) Pd, Ag, and Ni is described. Emphasis is given to the specific deposition conditions which successfully alleviate the formation of native oxides at the metal/metal oxide interface. The control of the epitaxial relationships between the oriented oxides films and the underlying noble and oxidizing metal surfaces is addressed. In addition, recent use of these heterostructures in the epitaxial growth of high temperature superconducting films on biaxially-textured metal substrates for superconducting wire development is described.
Date: March 1, 1997
Creator: Norton, D.P.; Park, C. & Saffian, B.
Partner: UNT Libraries Government Documents Department

Fabriacation and properties of high-J{sub c}, biaxially aligned YBa{sub 2}Cu{sub 3}O{sub 7-delta} thick films on metallic tape ...

Description: We report the synthesis and properties of high-J{sub c}, biaxially aligned YBCO films deposited on thermo-mechanically textured Ni tapes. Sharply cube-textured Ni tapes, 125 {mu}m thick, were produced by mechanical rolling followed by recrystallization anneal. Short segments were coated with epitaxial oxide buffer layers, followed by fully aligned YBCO films to thicknesses of 1 to 3 {mu}m. In-plane textures of 7-10{degree} FWHM are achieved, with c-perpendicular alignment to 1{degree} FWHM. Typical zero-field J{sub c} values are in the range 5-9x10{sup 5} A/cm{sup 2} at 77 K, with strong behavior in magnetic fields comparable to that of epitaxial films on single crystal oxides. Assessment of properties necessary for a tape conductor technology are discussed.
Date: March 1, 1997
Creator: Christen, D.K.; Norton, D.P. & Goyal, A.
Partner: UNT Libraries Government Documents Department

Epitaxial Electronic Oxides on Semiconductors Using Pulsed-Laser Deposition

Description: We describe the growth and properties of epitaxial (OO1) CeO{sub 2} on a (001) Ge surface using a hydrogen-assisted pulsed-laser deposition method. Hydrogen gas is introduced during film growth to eliminate the presence of the GeOs from the semiconductor surface during the initial nucleation of the metal oxide film. The hydrogen partial pressure and substrate temperature are selected to be sufficiently high such that the germanium native oxides are thermodynamically unstable. The Gibbs free energy of CeO{sub 2} is larger in magnitude than that of the Ge native oxides, making it more favorable for the metal oxide to reside at the interface in comparison to the native Ge oxides. By satisfying these criteria. the metal oxide/semiconductor interface is shown to be atomically abrupt with no native oxide present. Preliminary structural and electrical properties are reported.
Date: December 1, 1999
Creator: Norton, D.P.; Budai, J.D. & Chisholm, M.F.
Partner: UNT Libraries Government Documents Department

Epitaxial YBa{sub 2}Cu{sub 3}O{sub 7} films on rolled-textured metals for high temperature superconducting applications

Description: The epitaxial growth of high temperature superconducting (HTS) films on rolled-textured metal represents a viable approach for long-length superconducting tapes. Epitaxial, 0.5 {micro}m thick YBa{sub 2}Cu{sub 3}O{sub 7} (YBCO) films with critical current densities, J{sub c}, greater than 1 MA/cm{sup 2} have been realized on rolled-textured (001) Ni tapes with yttria-stabilized zirconia (YSZ)/CeO{sub 2} oxide buffer layers. This paper describes the synthesis using pulsed-laser deposition (PLD) of epitaxial oxide buffer layers on biaxially-textured metal that comprise the so-called rolling-assisted biaxially-textured substrates (RABiTs{trademark}). The properties of the buffer and YBa{sub 2}Cu{sub 3}O{sub 7} films on rolled-textured Ni are discussed, with emphasis given to the crystallographic and microstructural properties that determine the superconducting properties of these multilayer structures.
Date: April 1, 1998
Creator: Norton, D.P.; Park, C. & Prouteau, C.
Partner: UNT Libraries Government Documents Department

Formation of artificially-layered high-temperature superconductors using pulsed-laser deposition

Description: Pulsed-laser deposition has been used to synthesize artificially-layered high-temperature superconductors. Novel thin-film SrCu0{sub 2}/BaCu0{sub 2} superlattices have been synthesized which superconduct at temperatures as high as 70 K. These results demonstrate that pulsed-laser deposition and epitaxial stabilization can be effectively used to engineer artificially-layered thin-film materials.
Date: March 1, 1995
Creator: Norton, D.P.; Chakoumakos, B.C. & Budai, J.D.
Partner: UNT Libraries Government Documents Department

Formation of artificially-layered thin-film compounds using pulsed-laser deposition

Description: Superlattice structures, consisting of SrCuO{sub 2}, (Sr,Ca)CuO{sub 2}, and BaCuO{sub 2} layers in the tetragonal, ``infinite layer`` crystal structure, have been grown by pulsed-laser deposition (PLD). Superlattice chemical modulation is observed for structures with component layers as thin as a single unit cell ({approximately}3.4 {angstrom}), indicating that unit-cell control of (Sr,Ca)CuO{sub 2} growth is possible using conventional pulsed-laser deposition over a wide oxygen pressure regime. X-ray diffraction intensity oscillations, due to the finite thickness of the film, indicate that these films are extremely flat with a thickness variation of only {approximately}20 {angstrom} over a length scale of several thousand angstroms. Using the constraint of epitaxy to grow metastable cuprates in the infinite layer structure, novel high-temperature superconducting structural families have been formed. In particular, epitaxially-stabilized SrCuO{sub 2}/BaCuO{sub 2} superlattices, grown by sequentially depositing on lattice-matched (100) SrTiO{sub 3} from BaCuO{sub 2} and SrCuO{sub 2} ablation targets in a PLD system, show metallic conductivity and superconductivity at {Tc}(onset) {approximately}70 K. These results show that pulsed-laser deposition and epitaxial stabilization have been used to effectively ``engineer`` artificially-layered thin-film materials.
Date: April 1, 1995
Creator: Norton, D.P.; Chakoumakos, B.C.; Lowndes, D.H. & Budai, J.D.
Partner: UNT Libraries Government Documents Department

Branches on the family tree: Superconductivity in epitaxial films of {open_quotes}first-in-line{close_quotes} descendants of the parent compound SrCuO{sub 2}

Description: The defect-induced carrier doping in epitaxial films of the parent compound SrCuO{sub 2} is reviewed. Two superconducting descendants were synthesized by expanding the role of natural defects in the parent SuCuO{sub 2} films, namely, the Ruddlesheim-Popper phases Sr{sub n+1}Cu{sub n}O{sub 2n+1} and a nonstoichiometric variant of the oxycarbonate Sr{sub 2}CuO{sub 2}(CO{sub 3}). The synthesis and structures are described.
Date: April 1, 1995
Creator: Feenstra, R.; Budai, J.D. & Norton, D.P.
Partner: UNT Libraries Government Documents Department

Morphology and microstructure of (111) crystalline CeO{sub 2} films grown on amorphous SiO{sub 2} substrates by pulsed-laser ablation

Description: The surface morphology and microstructure of (111)-oriented CeO{sub 2} thin films, grown on amorphous fused silica (SiO{sub 2} substrates by low-energy-ion-beam assisted pulsed laser ablation, have been studied by atomic force microscopy (AFM) and x-ray diffraction (XRD). These CeO{sub 2} films are aligned with respect to a single in-plane axis despite being deposited on an amorphous substrate. There is a honeycomb-like growth morphology to the films and island-growth can be observed in thicker films. These islands, inside of which are high density of honeycomb-like clusters, are separated by a void network with {approximately}700 nm width. However, on the surface of the thinnest film ({approximately}3 nm), only very small clusters (diameter <60 nm) appear, and the boundaries of the void network are undefined, which implies that the film is just beginning to coalesce into clusters (grains). The combined AFM images and XRD pattern suggest these clusters probably are the initial seeds for the subsequent island growth. Based on these results, the growth mechanism of oriented CeO{sub 2} films on amorphous fused silica substrates is discussed.
Date: November 1, 1994
Creator: Zhu, S.; Lowndes, D.H.; Budai, J.D.; Thundat, T.; Norton, D.P. & Warmack, R.J.
Partner: UNT Libraries Government Documents Department

AnGa{sub 2}O{sub 4} Thin-Film Phosphors Grown by Pulsed Laser Ablation

Description: The growth and properties of undoped and Mn-doped ZnGa{sub 2}O{sub 4} thin-film phosphors on (100) MgO and glass substrates using pulsed laser ablation were investigated. Blue-white and green emission were observed for as-deposited undoped and Mn-doped films, respectively. Luminescent properties as well as crystallinity were considerably affected by processing conditions and film stoichiometry. Films with enhanced luminescent characteristics were obtained on single crystal substrates without post-annealing.
Date: April 5, 1999
Creator: Lee, Y. E.; Rouleau, C. M.; Park, C. & Norton, D. P.
Partner: UNT Libraries Government Documents Department

Epitaxial growth of metal fluoride thin films by pulsed-laser deposition

Description: We have studied growth of GdLiF4 thin films for optical waveguide applications. Epitaxial, c-axis oriented GdLiF4 films wer grown from undoped GdLiF4 targets in an on-axis Pulsed-laser deposition geometry on (100) CaF2. These films exhibit a high density of particulates on the surface which are ejected from the target in the ablation process. Growth from Nd-doped polycrystalline GdLiF4 ablation targets results in smooth films with lower particulate densities, as Nd doping increases the optical absorption of GdLiF4 at the ablation laser wavelength 193 nm and permits efficient pulsed-laser deposition. Optical emission spectra of the ablation pume reveals the presence of atomic F, Gd, and Li, indicating the dissociation of the metal-fluorine bonds in the ablation process. In addition, we find that the residual background oxygen pressure must be reduced to avoid formation of Gd4O3F6 as an impurity oxyfluoride phase in the films.
Date: December 1, 1995
Creator: Norton, D.P.; Budal, J.D.; Chakoumakos, B.C.; Geohegan, D.B. & Puretzky, A.
Partner: UNT Libraries Government Documents Department

Enhanced Luminescence in Epitaxial Oxide Thin-Film Phosphors

Description: Undoped and Mn-doped ZnGa{sub 2}O{sub 4} thin-film phosphors were grown using pulsed laser ablation on (100) MgO single crystal and glass substrates. X-ray results showed the films on (100) MgO are well aligned both out-of plane and in-plane. Epitaxial films show superior photoluminescent intensity as compared to randomly oriented polycrystalline films, indicating that intragranular crystallinity strongIy influences luminescent properties. Li-doped ZnGa{sub 2}O{sub 4} exhibited significantly enhanced photoluminescence intensity.
Date: November 8, 1999
Creator: Lee, Y.E.; Norton, D.P.; Budai, J.D.; Park, C.; Kim, M.; Pennycook, S.J. et al.
Partner: UNT Libraries Government Documents Department

Alternative Gate Dielectrics on Semiconductors for MOSFET Device Applications

Description: We have investigated the synthesis and properties of deposited oxides on Si and Ge for use as alternative gate dielectrics in MOSFET applications. The capacitance and leakage current behavior of polycrystalline Y{sub 2}O{sub 3} films synthesized by pulsed-laser deposition is reported. In addition, we also discuss the growth of epitaxial oxide structures. In particular, we have investigated the use of silicide termination for oxide growth on (001) Si using laser-molecular beam epitaxy. In addition, we discuss a novel approach involving the use of hydrogen to eliminate native oxide during initial dielectric oxide nucleation on (001) Ge.
Date: December 6, 1999
Creator: Norton, D.P.; Budai, J.D.; Chisholm, M.F.; Pennycook, S.J.; McKee, R.; Walker, F. et al.
Partner: UNT Libraries Government Documents Department

Investigation of the local superconducting properties at grain boundaries in high-Tc superconductors

Description: Developing an atomic scale study of the structure-property relationships of grain boundaries in high-{Tc} superconductors is essential to understand their current dissipation mechanism and for incorporating these materials into viable devices. Thin YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} films have been deposited by pulsed laser deposition (PLD) on SrTiO{sub 3} symmetric bicrystals. Transport measurements in a magnetic field have been conducted across the grain boundaries through a wide bridge. The data obtained are consistent with microstructural observation in a VG Microscopes HB603 U and a VG HB501 UX dedicated STEM. Of particular interest in the study of high-{Tc} materials is the use of EELS, which can highlight the presence of non-superconducting regions through interpretation of the onset positions and fine-structure (ELNES) of characteristic core-edge features. This preliminary investigation has shown that the study of the grain boundary electronic microstructure in parallel with transport properties study should lead to a better understanding of the transport properties of the superconducting materials. In particular, it has been shown that transport measurements are consistent with a model where the grain boundaries are composed of weak links separated by a very small fraction of much strong links although more boundaries need to be studied to draw definitive conclusions and determine the real nature of these strong links.
Date: February 1, 1998
Creator: Prouteau, C.; Duscher, G.; Browning, N.D.; Pennycook, S.J.; Verebelyi, D.; Christen, D.K. et al.
Partner: UNT Libraries Government Documents Department

Defect formation and carrier doping in epitaxial films of the ``parent`` compound SrCuO{sub 2}: Synthesis of two superconductors descendants

Description: The infinite layer or parent compounds ACuO{sub 2} (A: Ca-Sr-Ba) constitute the simplest copper oxygen perovskites that contain the CuO{sub 2} sheets essential for superconductivity. The stabilization of these basic ``building blocks`` as epitaxial films, therefore, provides alluring opportunities towards the search for new superconducting compounds and elucidation of the underlying mechanisms. In this work, general trends of the defect formation and carrier doping for epitaxial films of the intermediate endmember SrCuO{sub 2} are reviewed. First results are presented from successful attempts to induce hole-doped superconductivity via the processing-controlled incorporation of charge reservoir layers.
Date: April 1, 1995
Creator: Feenstra, R.; Norton, D.P.; Budai, J.D.; Jones, E.C.; Christen, D.K. & Kawai, T.
Partner: UNT Libraries Government Documents Department

Growth and transport properties of Y-Ba-Cu-O/Pr-Ba-Cu-O superlattices

Description: The pulsed-laser deposition method has been used to fabricate epitaxial, nonsymmetric M(Y) {times} N(Pr) superlattices in which YBa{sub 2}Cu{sub 3}O{sub 7-x} (YBCO) layers either M = 1,2,3,4,8, or 16 c-axis unit cells thick are separated by insulating PrBa{sub 2}Cu{sub 3}O{sub 7-x} (PBCO) layers N unit cells thick (N = 1 to {approximately}32). The zero-resistance superconducting transition temperature, T{sub c0}, initially decreases rapidly with increasing PBCO layer thickness, but then saturates at T{sub c0} {approximately} 19 K, 54 K, 71 K, or 80 K, or structures containing 1-,2-,3-, or 4-cell-thick YBCO layers, respectively. Critical current density measurements carried out on structures with 16- or 32-cell thick YBCO layers show that the magnitude of J{sub c}(H = 0) {approximately} 1-2 MA/cm{sup 2}, as well as the magnetic field dependence and the anisotropy of J{sub c}(H) all are in good agreement with corresponding measurements on thicker, single-layer YBCO films. Thus, there is no evidence of an enhanced J{sub c}(H) due to the multi-layered structure, for the layer thickness investigated to date. The systematic variation of T{sub c0}, as a function of the YBCO and PBCO layer thickness, is discussed in light of other recent experiments and theoretical model calculations. The superlattices' structural and compositional order are characterized using x-ray diffraction, transmission electron microscopy, and scanning tunneling microscopy, and details of the pulsed-laser deposition process are reported. 42 refs., 7 figs.
Date: January 1, 1990
Creator: Lowndes, D.H.; Norton, D.P.; Budai, J.D.; Christen, D.K.; Klabunde, C.E.; Warmack, R.J. et al.
Partner: UNT Libraries Government Documents Department

Properties of doped and undoped (Ca,Sr)CuO{sub 2} thin films

Description: The authors have studied the transport and structural properties of (Ca,Sr)CuO{sub 2}, Sr{sub 1{minus}y}Nd{sub y}CuO{sub 2}, and Sr{sub 1{minus}x}CuO{sub 2{minus}{delta}} thin films grown by pulsed-laser deposition. Stoichiometric {open_quotes}infinite layer{close_quotes} (Ca,Sr)CuO{sub 2} thin films grown over a large range of growth conditions are insulators, while superconductivity is observed in Sr{sub 1{minus}y}Nd{sub y}CuO{sub 2} films with {Tc}(onset) {approximately}28 K for y = 0.10. A Nd solubility limit of y = 0.10 is observed with the appearance of a new phase with c {approximately} 0.37 nm for y > 0.10. In addition, the transport and structural properties of Sr{sub 1{minus}x}CuO{sub 2{minus}{delta}} thin films grown by pulsed-laser deposition support the contention that the tetragonal phase is capable of accommodating a significant density of alkaline-earth deficiencies up to x {le} 0.3. Resistivity measurements indicate a significant change in the carrier density of the CuO{sub 2} planes as Sr vacancies are introduced. In addition, an enigmatic anomaly in resistivity at 185 K is observed for Sr{sub 0.85}CuO{sub 2{minus}{delta}} thin films. Magnetic measurements on these samples indicate that, although a significant drop in resistivity at 185 K is observed, it is not due to a superconducting transition. Hall measurements, as well as changes in resistivity with film growth conditions, suggest that the majority carriers in these Sr{sub 1{minus}x}CuO{sub 2{minus}{delta}} thin films are electrons even with the Sr-vacancies present.
Date: December 1, 1993
Creator: Norton, D. P.; Chakoumakos, B. C.; Budai, J. D.; Jones, E. C.; Christen, D. K. & Lowndes, D. H.
Partner: UNT Libraries Government Documents Department

Laser ablation synthesis and properties of epitaxial YBa sub 2 Cu sub 3 O sub 7-. delta. /PrBa sub 2 Cu sub 3 O sub 7-. delta. superconducting superlattices

Description: Pulsed laser ablation has been used to fabricate epitaxial superlattices in which c-axis-perpendicular YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} (YBCO) layers as thin as a single unit cell are separated by semiconducting PrBa{sub 2}Cu{sub 3}O{sub 7-{delta}} (PBCO) layers. The superlattice {Tc} (R=0) decreases rapidly with increasing PBCO thickness, but then saturates at {Tc}{approximately}19 K, 54 K, 71 K, or 80 K, for structures containing isolated, 1-,2-,3-, or 4-cell-thick YBCO layers, respectively. Recent theoretical analyses suggest that the decrease of {Tc} and broadening of the resistive transition for the thinnest YBCO layers is most likely due either to a crossover from 3D to 2D resistive behavior, or to hole-filling in the YBCO layers caused by electron transfer from the PBCO. We find that the resistance in the superconducting transition region scales with temperature as expected for dissipation by characteristically 2D (vortex) excitations. Departures from the universal'' resistance behavior expected for a 2D Ginzburg-Landau Coulomb gas, as the YBCO thickness is increased or the PBCO thickness is decreased, can be attributed to the onset of phase coupling between the YBCO layers. Thus, the experiments show that YBCO's effective anisotropy can be greatly increased by separating very thin YBCO layers by thicker PBCO layers in superlattice structures. As a results, characteristic 2D dissipation can be observed over a greatly expanded temperature range, relative to both thicker-film and single-crystal YBCO specimens. Experiments also were carried out using two additional isostructural sets of superlattices in which the PBCO layers were replaced by more conductive Y- or Ca-doped PBCO layers, in order to alter any electron transfer from PBCO to YBCO.
Date: September 1, 1991
Creator: Lowndes, D.H.; Norton, D.P.; Zheng, X.Y. (Oak Ridge National Lab., TN (United States)) & Zhu, Shen (Tennessee Univ., Knoxville, TN (United States). Dept. of Physics and Astronomy)
Partner: UNT Libraries Government Documents Department

The Use of X-Ray Microbeams in Materials Science

Description: Most materials are heterogeneous on mesoscopic length scales (tenths-to-tens of microns), and materials properties depend critically on mesoscopic structures such as grain sizes, texture, and impurities. The recent availability of intense, focused x-ray microbeams at synchrotron facilities has enabled new techniques for mesoscale materials characterization. We describe instrumentation and experiments on the MHATT-CAT and UNICAT undulator beamlines at the Advanced Photon Source which use micron and submicron-size x-ray beams to investigate the grain orientation, local strain and defect content in a variety of materials of technological interest. Results from a combinatorial study on epitaxial growth of oxide films on textured metal substrates will be described to illustrate x-ray microbeam capabilities.
Date: October 13, 1998
Creator: Budai, J.D.; Chung, J.-S.; Ice, G.E.; Larson, B.C.; Lowe, W.P.; Norton, D.P. et al.
Partner: UNT Libraries Government Documents Department