Showing papers by "David P. Norton published in 1991"

Superconductivity and hole doping in Pr0.5Ca0.5Ba2Cu3O7- delta thin films.

Abstract: We have grown superconducting Pr{sub 0.5}Ca{sub 0.5}Ba{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} epitaxial thin films that exhibit a superconducting onset temperature of 43 K with {ital T}{sub {ital c}}({ital R}=0)=34.9 K. This is the first time, to our knowledge, that 1:2:3-phase superconductivity has been achieved by substituting Ca for Pr, without the presence in the alloy of {Upsilon} or any other rare-earth element {ital R} for which {ital R}Ba{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} is superconducting. This result supports the view that hole localization/filling contributes substantially to the suppression of superconductivity by Pr in PrBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}, and clearly demonstrates that this suppression can be compensated by appropriate hole doping with Ca.

Interdiffusion, growth mechanisms, and critical currents in YBa2Cu3O7-x/PrBa2Cu3O7-x superlattices.

Abstract: {ital Z}-contrast electron microscopy demonstrates that interdiffusion is not affecting the resistive transitions observed in {ital M}{times}{ital N} YBa{sub 2}Cu{sub 3}O{sub 7{minus}2}/PrBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} superlattices. At moderate supersaturations, island growth is observed, the islands comprising one Ba-Y-Ba structural unit in height terminated at the Cu-chain plane, having sides 20--30 nm long along (100) and (010). The associated gradual roughening of the growing surface has no apprent effect on critical currents for {ital M}{ge}2, the temperature and field dependence being comparable to that of a film.

Depression and broadening of the superconducting transition in superlattices based on YBa 2 Cu 3 O 7 − δ : Influence of the barrier layers

Abstract: The superconducting properties of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$-based superlattices are shown to depend strongly on the electronic properties of the barrier layers. The resistive transition width decreases significantly as the hole carrier density in the barrier layers is increased. However, ${\mathit{T}}_{\mathit{c}}$(onset) does not change, contrary to predictions of hole-filling models. ${\mathit{T}}_{\mathit{c}}$(onset) is apparently determined by the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ layer thickness, while the transition width, determined by long-range phase coherence of the superconducting wave function, depends on the electronic properties of the isolating barrier layers.

Preferred alignment of twin boundaries in YBa2Cu3Ox thin films and YBa2Cu3Ox/PrBa2Cu3Ox superlattices on SrTiO3

Abstract: The structure of YBa2Cu3Ox thin films and YBa2Cu3Ox/PrBa2Cu3Ox superlattice films grown on (00L) SrTiO3 substrates has been studied using x‐ray diffraction and transmission electron microscopy. The films consist of four symmetry‐equivalent domains related by mutually perpendicular (hh0) and (hh0) twin boundaries. One twin orientation is often highly favored over the other. We have correlated this in‐plane symmetry breaking with small miscuts of the substrate surface towards the [HH0] direction and propose that interfacial steps act as nucleation sites for twins. Thus, by controlling the surface normal, a technique is described for producing films containing aligned twin boundaries.

Flux creep in the Josephson mixed state of granular‐oriented YBa2Cu3O7−x thin thin films

Abstract: A self‐consistent critical current model in the Josephson mixed state is proposed for a series of c‐oriented, polycrystalline and for a series of epitaxial triaxially oriented YBCO thin films. The flux pinning activation energies were experimentally determined from electrical transport measurements over a wide range of temperatures and were found to behave quite differently for the two types of granular film. With these activation energies applied to a superconductor‐normal metal‐superconductor weak‐link system, thermally activated flux motion is shown to reproduce the experimentally measured critical current densities.

Epitaxial YBa2Cu3O7−x Thin Films: Scanning Tunneling Microscope Study of the Initial Stages of Epitaxial Growth, Growth Mechanism, and Effects of Substrate Temperature

Abstract: The surface microstructure of epitaxial YBa{sub 2}Cu{sub 3}O{sub 7-x} films grown by pulsed laser ablation on (001) MgO and SrTiO{sub 3} substrates has been studied at various growth stages, ranging in thickness from eight c-axis perpendicular unit cells to {approximately}220 nm. On MgO (lattice mismatch {approximately}9%) even the thinnest films grow unit cell-by-unit cell by an island growth mechanism. However, on SrTiO{sub 3} (mismatch {approximately}1%), a transition from a layer-like growth mode to island growth is observed as the film thickness increases. Islands with clear spiral growth structures are observed in even the thinnest films on MgO, but for films grown on SrTiO{sub 3} the spiral growth features are found only for film thicknesses slightly greater than the critical thickness for the switch to an island growth mode. The islands consist of stacks of atomically flat terraces whole step heights are multiples of the c-axis lattice parameter. This island density decreases significantly with increasing film thickness, while their diameters range from 50--400 nm, increasing with growth temperature. The terraced island grain morphology causes a surface roughness of from 10 to 30 nm (depending on growth temperature) in films {approximately}200 nm thick.

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

Abstract: The pulsed-laser deposition method has been used to fabricate epitaxial, nonsymmetric M(Y) x N(Pr) superlattices in which YBa2Cu3O7 (YBCO) layers either M = 1, 2, 3, 4, 8, or 16 c-axis unit cells thick are separated by insulating PrBa2Cu3O7 (PBCO) layers N unit cells thick (N = I to -32). The zero-resistance superconducting transition temperature, Tc0, initially decreases rapidly with increasing PBCO layer thickness, but then saturates at TcO 19 K, 54 K, 71 K, or 80 K, for 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 Jc(H 0) 12 MA/cm2, as well as the magnetic field dependence and the anisotropy of Jc(H) all are in good agreement with corresponding measurements on thicker, single-layer YBCO films. Thus, there is no evidence of an enhanced Jc(H) due to the multi-layered structure, for the layer thicknesses investigated to date. The systematic variation of Tc0, as a function of the YBCO and PBCO layer thicknesses, 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.

Kosterlitz-Thouless-Like Behavior Over Extended Ranges of Temperature and Layer Thickness in Crystalline YBa2Cu3O7-x/PrBa2Cu3O7-x Superlattices

Abstract: High-temperature superconductivity is associated with layered, quasi-two-dimensional (2D) crystal structures and with carriers moving in CuO2 planes. Because the c-axis superconducting coherence length is very short (e.g., ξc ~ 3–6 A, vs. a lattice constant c ~ 11.7 A in YBa2Cu3O7-x), questions arise whether isolated single-cell-thick layers of these materials are superconducting and, if so, how their superconductivity is affected, first, by their extreme anisotropy (including possible reduced dimensionality) and, second, by residual interlayer coupling or other interactions. We1,2 and several other groups3–5 recently fabricated epitaxial M×N “123”-family superlattices in which c-axis perpendicular (c⊥) YBa2Cu3O7-x (YBCO) layers M unit cells in thickness are separated by N unit-cell-thick layers of semiconducting PrBa2Cu3O7-x (PBCO). The superlattice Tco(R = 0) was found to decrease rapidly with increasing PBCO layer thickness, but then to saturate at Tco ~ 19 K, 54 K, 71 K, or 80 K, for structures containing 1-, 2-, 3-, or 4-cell-thick YBCO layers, respectively.1,2 In contrast, recent measurements for Bi2Sr2(Ca1-xYx)1Cu2O8-based superlattices, in which superconducting (x=0.15) and semiconducting (x=0.5) layers also alternate, show almost no change in Tc(mid) ~65 K with decreasing superlattice period.6 These results imply that high-Tc is an intrinsic property of CuO2 bilayers, but that 3-dimensional interactions are present, at least for the “123”-family materials.

Superconducting transport properties and surface microstructure for YBa2Cu3O7-δ-based superlattices grown by pulsed laser deposition

Abstract: YBa2Cu3O7−δ/PrBa2Cu3O7−δ, YBa2Cu3O7−δ/Pr0.7Y0.3Ba2Cu3O7−δ, and YBa2Cu3O7−δ/Pr0.5Ca0.5Ba2Cu3O7−δ superlattice structures have been grown by pulsed laser deposition. The superconducting properties of these YBa2Cu3O7−δ-based superlattices are shown to depend on the electronic properties of the barrier layers. In particular, the superconducting transition width decreases as the hole carrier density in the barrier layers is increased, while T c(onset) does not change with barrier layer carrier density. Tc(onset) is apparently determined by the YBa2Cu3O7−δ layer thickness, while the transition width, determined by long-range phase coherence of the superconducting wave function, depends on the electronic properties of the isolating barrier layers. Using scanning tunneling microscopy, we also have investigated the surface microstructure of epitaxial YBa2Cu3O7−δ thin films grown by pulsed laser deposition, and find a highly terraced, granular morphology. This morphology suggests that multilayer thin film structures possess a high density of steps within the individual layers. These steps should be quite important in the transport properties of these superconducting structures.