Search

We report the basal-plane critical current and superfluid density of magnetically aligned ${\text{NdFeAsO}}_{0.86}{\text{F}}_{0.14}$ powder. This sample has individual crystallite grains permanently oriented with their $c$ axis along the external measuring field. Magnetic irreversibilities at high field suggest strong flux pinning of basal-plane critical currents, with monotonic field dependence and no evidence of the ``fishtail'' effect. The small particles provide a sensitive indicator of dc flux penetration and allow analysis of the temperature dependence of $ab$-plane London penetration depth ${\ensuremath{\lambda}}_{ab,\text{L}}$, which is quadratic at low $T$. This feature may not necessarily be due to the nodes in the gap but may be rather a sign of a strong pair breaking. A quantitative determination of the absolute magnitude of ${\ensuremath{\lambda}}_{ab,\text{L}}$ is hindered by the need for accurate knowledge of the particle size distribution.

High-purity niobium exhibits a surprisingly rich assortment of vortex lattice (VL) structures for fields applied parallel to a fourfold symmetry axis, with all observed VL phases made up of degenerate domains that spontaneously break some crystal symmetry. Yet a single regular hexagonal VL domain is observed at all temperatures and fields parallel to a threefold symmetry axis. We report a detailed investigation of the transition between these lush and barren VL landscapes, discovering new VL structures and phase transitions at high fields. We show that the number and relative population of VL domains is intrinsically tied to the underlying crystal symmetry. We discuss how subtle anisotropies of the crystal may generate the remarkable VLs observed.

We have studied the pinning force {ital F}{sub {ital p}}({ital B},{ital T}) in YBa{sub 2}Cu{sub 3}O{sub 7} crystals as a function of fluence of 3-MeV protons. For a given ion-damage level, we could reduce all {ital F}{sub {ital p}} data to a single function of reduced field {ital b}={ital B}/{ital B}{sup *}, where the scaling field {ital B}{sup *}({ital T}){much lt}{ital H}{sub {ital c}2}({ital T}) is related to the irreversibility line. This result rules out the possibility of matching effects between the vortex lattice and the distribution of pinning sites. We emphasize the influence of thermally activated relaxation in the determination of {ital F}{sub {ital p}} and compare the data with simple pinning models.

Vortex pinning in textured Bi2Sr2CaCu2Ox/Ag superconducting tapes was enhanced by irradiation with 0.8 GeV protons, creating randomly oriented columnar defects. Measurements of the current density J versus time show that thermally-activated current decay was greatly decreased, compared with unirradiated tapes. However, temperature-independent quantum tunneling of vortices considerably limits J and its temporal stability, for a significant fraction of the finite-Jc region.

A low-cost electrodeposition (ED) method is reported for fabrication of high-temperature Tl-oxide-based superconductors. Recent studies on the development of biaxially textured ED films on single-crystal and rolling-assisted biaxially textured Ni substrates are presented. Pole-figure measurements of electrodeposited (Tl,Bi)-(Sr,Ba)-Ca-Cu-Ag-O (TBSBCCO) films, with thicknesses between 1-5 /spl mu/m, clearly show strong biaxial texturing. The rocking curve and phi scan of TBSBCCO on 300 /spl Aring/ Ag/LaAlO/sub 3/, show a full width half maximum (FWHM) of only 0.9/spl deg/ and 1.2/spl deg/, respectively, consistent with a very high-quality. Transport measurements for the ED films show J/sub c/ values up to 7.84/spl times/10/sup 5/ A/cm/sup 2/ at 77 K in zero magnetic field. The preliminary results of electrodeposited TBSBCCO film on CeO/sub 2//YSZ/CeO/sub 2/ on textured Ni showed primarily 1212 phase with a transport critical current density of 1.36/spl times/10/sup 5/ A/cm/sup 2/ at 77 K in zero field.

The synthesis of the novel superconductor LaFeAsO0.89F0.11 with transition temperature Tc ∼ 26 K has been quickly followed by reports of even higher Tcs. These discoveries have generated much interest in the mechanisms and manifestations of unconventional superconductivity in the family of doped quaternary layered oxypnictides, because many features of these materials set them apart from other superconductors. Hunte et al. report very high-field resistance measurements of LaFeAsO0.89F0.11 up to 45 T, which show a remarkable enhancement of the upper critical field Bc2 as compared to values expected from the slopes dBc2/dT ≈ 2 T/K near Tc, particularly at low temperatures where the deduced Bc2(0) ≈ 63–65 T exceeds the paramagnetic limit. The recent synthesis of the superconductor LaFeAsO0.89F0.11 with transition temperature Tc ≈ 26 K (refs 1–4) has been quickly followed by reports of even higher transition temperatures in related compounds: 41 K in CeFeAsO0.84F0.16 (ref. 5), 43 K in SmFeAsO0.9F0.1 (ref. 6), and 52 K in NdFeAsO0.89F0.11 and PrFeAsO0.89F0.11 (refs 7, 8). These discoveries have generated much interest9,10 in the mechanisms and manifestations of unconventional superconductivity in the family of doped quaternary layered oxypnictides LnOTMPn (Ln: La, Pr, Ce, Sm; TM: Mn, Fe, Co, Ni; Pn: P, As), because many features of these materials set them apart from other known superconductors. Here we report resistance measurements of LaFeAsO0.89F0.11 at high magnetic fields, up to 45 T, that show a remarkable enhancement of the upper critical field Bc2 compared to values expected from the slopes dBc2/dT ≈ 2 T K-1 near Tc, particularly at low temperatures where the deduced Bc2(0) ≈ 63–65 T exceeds the paramagnetic limit. We argue that oxypnictides represent a new class of high-field superconductors with Bc2 values surpassing those of Nb3Sn, MgB2 and the Chevrel phases, and perhaps exceeding the 100 T magnetic field benchmark of the high-Tc copper oxides.

We compare magnetotransport of the three iron-arsenide-based compounds ReFeAsO ($\text{Re}=\text{La}$, Sm, Nd) in very high DC and pulsed magnetic fields up to 45 and 54 T, respectively. Each sample studied exhibits a superconducting transition temperature near the maximum reported to date for that particular compound. While high magnetic fields do not suppress the superconducting state appreciably, the resistivity, Hall coefficient, and critical magnetic fields, taken together, suggest that the phenomenology and superconducting parameters of the oxypnictide superconductors bridges the gap between ${\text{MgB}}_{2}$ and YBCO.

The synthesis of the novel superconductor LaFeAsO0.89F0.11 with transition temperature Tc ∼ 26 K has been quickly followed by reports of even higher Tcs. These discoveries have generated much interest in the mechanisms and manifestations of unconventional superconductivity in the family of doped quaternary layered oxypnictides, because many features of these materials set them apart from other superconductors. Hunte et al. report very high-field resistance measurements of LaFeAsO0.89F0.11 up to 45 T, which show a remarkable enhancement of the upper critical field Bc2 as compared to values expected from the slopes dBc2/dT ≈ 2 T/K near Tc, particularly at low temperatures where the deduced Bc2(0) ≈ 63–65 T exceeds the paramagnetic limit. The recent synthesis of the superconductor LaFeAsO0.89F0.11 with transition temperature Tc ≈ 26 K (refs 1–4) has been quickly followed by reports of even higher transition temperatures in related compounds: 41 K in CeFeAsO0.84F0.16 (ref. 5), 43 K in SmFeAsO0.9F0.1 (ref. 6), and 52 K in NdFeAsO0.89F0.11 and PrFeAsO0.89F0.11 (refs 7, 8). These discoveries have generated much interest9,10 in the mechanisms and manifestations of unconventional superconductivity in the family of doped quaternary layered oxypnictides LnOTMPn (Ln: La, Pr, Ce, Sm; TM: Mn, Fe, Co, Ni; Pn: P, As), because many features of these materials set them apart from other known superconductors. Here we report resistance measurements of LaFeAsO0.89F0.11 at high magnetic fields, up to 45 T, that show a remarkable enhancement of the upper critical field Bc2 compared to values expected from the slopes dBc2/dT ≈ 2 T K-1 near Tc, particularly at low temperatures where the deduced Bc2(0) ≈ 63–65 T exceeds the paramagnetic limit. We argue that oxypnictides represent a new class of high-field superconductors with Bc2 values surpassing those of Nb3Sn, MgB2 and the Chevrel phases, and perhaps exceeding the 100 T magnetic field benchmark of the high-Tc copper oxides.

In situ growth of highly oriented YBa2Cu3O7−x thin films (200–500 nm in thickness) has been obtained by pulsed KrF (248 nm) laser ablation on both rigid and flexible randomly oriented polycrystalline yttria‐stabilized zirconia substrates. It is shown that c‐axis‐perpendicular YBa2Cu3O7−x films with a mosaic spread of only 1.0° can be grown on these randomly oriented polycrystalline substrates. Superconducting thin films were obtained with Tc(R=0)∼89 K on well‐polished substrates. For the films deposited on the flexible substrates, the superconducting Tc is not degraded by repeated bending of the flexible substrate/film composite over a 2.25‐cm‐radius arc although the normal‐state resistivity increases slightly, suggesting the creation of microcracks. The YBa2Cu3O7−x films grown on rigid polycrystalline yttria‐stabilized zirconia substrates have a critical current density Jc(H=0)∼1400 A/cm2 at 77 K.

Magnetic penetration depths {lambda}({ital T}) have been determined as a function of temperature {ital T} for the high-temperature superconductor Tl{sub 2}Ca{sub 2}Ba{sub 2}Cu{sub 3}O{sub 10+{delta}}. Magnetization measurements were performed on an aligned, composite powder sample ({ital T}{sub {ital c}}=123 K) with the magnetizing field parallel or perpendicular to the common {ital c} axes of the dispersed crystallites. The magnetization in the vortex state was analyzed using the theory of Kogan {ital et} {ital al}. (Phys. Rev. B 38, 11 958 (1988)) to obtain values for {lambda}. The results were best described by BCS weak-coupling theory in the clean limit, with deduced values {lambda}(0) of 173 and 480 nm for the components of the penetration-depth tensor.

We have determined the stability line in the 1/T−log[P(O2)] phase space for the synthesis of Nd1+xBa2−xCu3Oy (NdBCO) films. A systematic study of Tc, Jc, and ρ(T) dependence on oxygen partial pressure and temperature for the deposition of thin NdBCO films grown by pulsed-laser deposition was performed. The conditions for optimal NdBCO film growth were determined by varying oxygen partial pressure from 0.02 to 400 mTorr, and substrate temperature between 730 and 800 °C. The results show that the best NdBCO films are obtained at oxygen pressures in the range of 0.2–1.2 mTorr, depending on the substrate temperature. This is more than two orders-of-magnitude lower than the correspondent oxygen pressure appropriate for YBa2Cu3O7−δ film growth.

We report measurements of the temperature dependence of the magnetic penetration depth {lambda}(T) in noncentrosymmetric superconductor Re{sub 3}W. We employed two experimental techniques: extraction of {lambda}(T) from magnetic dc susceptibility, measured on a powder sample, and the rf tunnel diode resonator technique, where a bulk polycrystalline sample was used. The results of both techniques agree: the temperature dependence of the penetration depth can be well described by weak-coupling, dirty-limit, s-wave BCS theory where we obtain {Delta}(0)/k{sub B}T{sub C}=1.76{+-}0.05. No evidence for unconventional pairing resulting from the absence of the inversion symmetry is found.

This article surveys the formation and effects of “tailored” defects, having controlled numbers and several different morphologies, in high temperature superconductors. Defects can affect the equilibrium properties, such as the superconducting length scales ξ (the coherence length) and λ (the London penetration depth). Very importantly, defects provide vortex pinning that supports the conduction of a macroscopic current density. The article introduces these topics and illustrates them with specific examples.

Superconducting YBa2Cu3O7/CeO2/YSZ/CeO2 multilayer structures were grown on rolled-textured (001)Ni using pulsed-laser deposition. Critical current densities greater than 1 MA/cm2 were obtained for relatively thick YBa2Cu3O7 films. The compressive and tensile bend strain tolerance of critical currents for the YBa2Cu3O7 films deposited on these rolling-assisted biaxially textured substrates was also determined. These conductors retained up to 80% of their unstrained critical currents for applied compressive bend diameters as small as 1.5 cm and tensile bend diameters of 3.2 cm. The degradation of Jc is caused by the formation and propagation of transverse cracks. The results also suggest a correlation between bend-strain tolerance for these coated conductors and total oxide layer thickness.

Epitaxial film growth of Tl0.78Bi0.22Sr1.6Ba0.4Ca2Cu3O9 ((Tl,Bi)-1223) on rolling assisted biaxially textured substrates with YSZ and CeO2 buffer layers (RABiTS) has been successfully demonstrated by laser ablation and post-deposition annealing in flowing argon. X-ray diffraction (XRD) θ-2θ spectra showed that the films consisted mainly of c-axis aligned 1223 phase with some intergrown 1212 phase, while XRD Φ-scans of (102) pole figure revealed that the films are also a- and b-axes aligned, with an epitaxy of the «100» of (Tl,Bi)-1223 film on the «110» of the top YSZ buffer layer. Four-terminal electrical transport measurements showed that the zero-resistance transition temperature (Tc) was in the range of 106 - 110 K, and the critical current density (Jc) at 77 K and zero field was about 105 A/cm2 for the entire film width (3 mm) of a longer film (14 mm) which was processed differently from the shorter films (7 mm). For a shorter film (7 mm) that showed better ab-in-plane alignment, the magnetization Jc, at 77 K and extrapolated to zero field, calculated from Bean's model using the full film width (3.5 mm) as the appropriate lateral dimension, was 2 × 105 A/cm2.

Enhanced in-field critical current density ${J}_{c}$ has been achieved in $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) films grown on single crystal and biaxially textured metal substrate surfaces that were pretreated with second-phase nanoscale MgO and $\mathrm{Ba}\mathrm{Zr}{\mathrm{O}}_{3}$ particles. Nanoparticles were applied using wet chemical solution-based approaches. Results obtained in this study are compared with our previous investigations on surface modified substrates with sputter processed Ir nanoparticles. Cross-sectional TEM analysis and details of the field-orientation dependence of ${J}_{c}$ are found to be similar for both sputtered and solution processed nanoparticle species. Present results show a more uniform dependence of ${J}_{c}$ over all orientations of magnetic field, along with improved irreversibility behavior. Irrespective of the decoration technique, scaling behavior of the normalized volume pinning force density, ${F}_{p}$, demonstrates the similarity of pinning mechanisms for these YBCO films deposited on artificially modified substrates.

We report an extensive investigation of magnetic vortex lattice (VL) structures in single crystals of pure niobium with the magnetic field applied parallel to a fourfold symmetry axis, so as to induce frustration between the cubic crystal symmetry and hexagonal VL coordination expected in an isotropic situation. We observe new VL structures and phase transitions; all the VL phases observed (including those with an exactly square unit cell) spontaneously break some crystal symmetry. One phase even has the lowest possible symmetry of a two-dimensional Bravais lattice. This is quite unlike the situation in high-${T}_{c}$ or borocarbide superconductors, where VL structures orient along particular directions of high crystal symmetry. The causes of this behavior are discussed.

The critical current density ${J}_{c}$ flowing in thin $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) films of various thicknesses $d$ has been studied magnetometrically, both as a function of applied field $H$ and temperature $T$, with a central objective to determine the dominant source of vortex pinning in these materials. The films, grown by a $\mathrm{Ba}{\mathrm{F}}_{2}$ ex situ process and deposited on buffered rolling assisted biaxially textured substrates (``RABiTS'') substrates of $\mathrm{Ni}\text{\ensuremath{-}}5%\phantom{\rule{0.3em}{0ex}}\mathrm{W}$, have thicknesses $d$ ranging from $28\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ to $1.5\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$. Isothermal magnetization loops $M(H;T)$ and remanent magnetization ${M}_{\mathit{rem}}(T)$ in $H=0$ were measured with $H\ensuremath{\Vert}c$-axis (i.e., normal to film plane). The resulting ${J}_{c}(d)$ values (obtained from a modified critical state model) increase with thickness $d$, peak near $d\ensuremath{\sim}120\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, and thereafter decrease as the films get thicker. For a wide range of temperatures and intermediate fields, we find ${J}_{c}\ensuremath{\propto}{H}^{\ensuremath{-}\ensuremath{\alpha}}$ with $\ensuremath{\alpha}\ensuremath{\sim}(0.56--0.69)$ for all materials. This feature can be attributed to pinning by large random defects, which theoretically has power-law exponent $\ensuremath{\alpha}=5∕8$. Calculated values for the size and density of defects are comparable with those observed by TEM in the films. As a function of temperature, we find ${J}_{c}(T,sf)\ensuremath{\sim}{[1\ensuremath{-}{(T∕{T}_{c})}^{2}]}^{n}$ with $n\ensuremath{\sim}1.2--1.4$. This points to ``$\ensuremath{\delta}{T}_{c}$ pinning'' (pinning that suppresses ${T}_{c}$ locally) in these YBCO materials.

YBa2Cu3O7−δ films grown on rolling-assisted biaxially textured substrates carry critical current densities 105–106 A/cm2 at 77 K and low applied magnetic fields. In the low-field and low-current regime, ac transport current studies show hysteresis energy loss (per cycle and per unit length) roughly the value expected for a superconductor of elliptic cross section. The critical current Ic was deduced from observed dc and dynamic current-voltage relations. The power loss rises sharply as I0 (the maximum current in each cycle) is raised above Ic. In the present configuration, ferromagnetic hysteresis of the Ni substrate contributes little or no loss.

In several Tl- and Hg-based high-Tc superconducting materials, vortex-pinning defects were formed by irradiation with 0.8 GeV protons. The protons cause heavy constituent nuclei (Tl, Hg, Bi, …) to fission, which generates randomly oriented tracks. These columnar defects lead to significant enhancements in the current-conducting properties. We investigated bulk materials and thin films, irradiated with or without Pb or Au “amplifier foil” overlayers, which increase the number of column-creating fission fragments.

Magneto-optical imaging (MO) and electromagnetic studies have been applied to measure anisotropy and critical currents (J{sub c}) of YBa{sub 2}Cu{sub 3}O{sub 7} (YBCO) films with variable thickness of 0.2, 0.5, 1.0, 2.0, and 3.0 {micro}m deposited on flat and vicinal SrTiO{sub 3} substrates of 10 degree miscut angle ({theta}{sub s}). The flat films nucleated in island-type mode and developed with a dominant c-axis orientation with minority misoriented grains at larger thickness of overall dense structure. The vicinal films, on the other hand, nucleated in step-flow mode and developed with a highly porous structure and minimal density of misoriented grains and impurity phases at large thickness. The difference in the microstructures of these two types of films results in different J{sub c} vs. thickness behaviors. The MO images showed that the magnetic flux penetration in the flat samples is isotropic at all thicknesses, while it is highly anisotropic in the vicinal samples. The anisotropy decreases with film thickness and temperature. These results correlate with distinctive patterns of microstructural evolution in flat and vicinal YBCO films with increasing thickness.

In ${\mathrm{V}}_{3}\mathrm{Si}$, a first-order structural phase transition from hexagonal to square flux-line lattice occurs at approximately 1 T with $H\ensuremath{\Vert}$ to the $a$ axis. In this paper, we demonstrate the reentrant structural transition in the flux-line lattice, which reverts to hexagonal symmetry as the magnetic field approached ${H}_{c2}(T)$. This behavior is described very well by a nonlocal London theory with thermal fluctuations. The phase diagram of the flux lattice topology is mapped out for this geometry.

The critical current density of epitaxial $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) films grown on vicinal $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ substrates was investigated by electrical transport measurements along and across the steps of the $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ surface for a range of temperatures of $10\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}85\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and in applied magnetic fields varying from $0\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}14\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. For vicinal angles of 4\ifmmode^\circ\else\textdegree\fi{} and 8\ifmmode^\circ\else\textdegree\fi{}, we found evidence of enhanced pinning in the longitudinal direction at low magnetic fields for a wide region of temperatures and attribute this phenomenon to antiphase boundaries in the YBCO film. The transverse ${J}_{c}$ data showed a peak in the ${J}_{c}(\mathrm{H})$ curve at low magnetic fields, which was explained on the basis of magnetic interaction between Abrikosov and Abrikosov-Josephson vortices. The in-plane ${J}_{c}$ anisotropy observed for vicinal angles of 0.4\ifmmode^\circ\else\textdegree\fi{} was reversed with respect to the 8\ifmmode^\circ\else\textdegree\fi{} and 4\ifmmode^\circ\else\textdegree\fi{} samples. This phenomenon was interpreted on the basis of strain induced in the YBCO film by the stepped substrate's surface.

In-plane—aligned, c axis—oriented YBa2Cu3O7 (YBCO) films with superconducting critical current densities Jc as high as 700,000 amperes per square centimeter at 77 kelvin have been grown on thermomechanically rolled-textured nickel (001) tapes by pulsed-laser deposition. Epitaxial growth of oxide buffer layers directly on biaxially textured nickel, formed by recrystallization of cold-rolled pure nickel, made possible the growth of YBCO films 1.5 micrometers thick with superconducting properties that are comparable to those observed for epitaxial films on single-crystal oxide substrates. This result represents a viable approach for the production of long superconducting tapes for high-current, high-field applications at 77 kelvin.

A method to obtain long lengths of flexible, biaxially oriented substrates with smooth, chemically compatible surfaces for epitaxial growth of high‐temperature superconductors is reported. The technique uses well established, industrially scalable, thermomechanical processes to impart a strong biaxial texture to a base metal. This is followed by vapor deposition of epitaxial buffer layers (metal and/or ceramic) to yield chemically compatible surfaces. Epitaxial YBa2Cu3Ox films grown on such substrates have critical current densities exceeding 105 A/cm2 at 77 K in zero field and have field dependencies similar to epitaxial films on single crystal ceramic substrates. Deposited conductors made using this technique offer a potential route for the fabrication of long lengths of high‐Jc wire capable of carrying high currents in high magnetic fields and at elevated temperatures.

We report significant alteration of the equilibrium properties of the superconductor ${\mathrm{HgBa}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{x}$ when correlated disorder in the form of randomly oriented columnar tracks is introduced via induced fission of Hg nuclei. From studies of the equilibrium magnetization ${M}_{\mathrm{eq}}$ and the persistent current density over a wide range of temperatures, applied magnetic fields, and track densities up to a ``matching field'' of 3.4 T, we observe that the addition of more columnar tracks acting as pinning centers is progressively offset by reductions in the magnitude of ${M}_{\mathrm{eq}}.$ Invoking anisotropy-induced ``refocusing'' of the random track array and incorporating vortex-defect interactions, we find that this corresponds to increase in the London penetration depth $\ensuremath{\lambda};$ this reduces the vortex line energy and consequently reduces the pinning effectiveness of the tracks.

The critical current density flowing across low angle grain boundaries in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ thin films has been studied magnetometrically. Films (200 nm thickness) were deposited on ${\mathrm{SrTiO}}_{3}$ bicrystal substrates containing a single [001] tilt boundary, with angles of 2, 3, 5, and 7 degrees, and the films were patterned into rings. Their magnetic moments were measured in applied magnetic fields up to 30 kOe at temperatures of 5\char21{}95 K; current densities of rings with or without grain boundaries were obtained from a modified critical state model. For rings containing 5 and 7 degree boundaries, the magnetic response depends strongly on the field history, which arises in large part from self-field effects acting on the grain boundary.

Buffer layers play a key role in the YBa2Cu3O7−δ (YBCO) coated conductor composite wire technology. The important buffer layer characteristics are: these buffers should prevent metal diffusion into the superconductor from the substrate; possibly act as oxygen diffusion barriers; should be smooth, continuous, crack‐free, highly aligned and dense. In order to develop low‐cost alternative buffer layer architectures to the standard architecture of YBCO/CeO2/YSZ/Y2O3/Ni/Ni‐W, LaMnO3 (LMO) and La2Zr2O7 (LZO) buffers were chosen for this study. These buffers were grown epitaxially on biaxially textured and strengthened Ni‐W3% metal tapes by scalable vapor and/or solution techniques. LaMnO3 buffers were grown epitaxially on Ni‐W substrates by rf sputtering, and La2Zr2O7 buffers were grown by a Metal‐Organic Deposition (MOD). Detailed microstructural characterizations indicate that both of these buffers were excellent Ni diffusion barrier layers and high current YBCO films were grown on both LMO and LZO buffer tem...

The conversion of semiconductive SrCuO2 ‘‘infinite layer’’ epitaxial films into a metallic and superconducting (Tc≂30 K) oxycarbonate phase related to Sr2CuO2(CO3) has been induced by thermal processing in oxidizing ambient containing a small amount of CO2. Ion beam analysis using the resonant 12C(p,p)12C elastic backscattering cross section for protons incident at 1.73 MeV confirms the presence of substitutional C in the converted perovskite lattice. Comparison of the threshold CO2 partial pressure with thermodynamic stability data for SrCO3 suggests that the conversion is initiated by the high affinity of Sr(O) for CO2.

This work establishes the stability at elevated temperatures of columnar defects, artificially formed in the Bi‐based cupric oxide superconductor Bi2Sr2Ca1Cu2O8 for enhanced vortex pinning. Isochronal anneals, conducted in air, led to losses of critical current density in two stages. The defects were relatively stable up to ∼550 °C, where second stage annealing began; above this, the pinning diminished rapidly. The recrystallization and loss of columnar defects were corroborated by transmission electron microscopy.

A pulsed‐current technique was used to measure the extended I–V characteristics of a wide variety of prototype high‐temperature‐superconductor (HTS) leads. It was found that the average resistivity rises with J(≳Jc) more gradually than in conventional superconductors—often remaining very small compared to silver, for values of J(≫Jc) that are high enough to be practically useful. This observation, combined with the low thermal conductivity (∼50 times smaller than Ag), should extend the utility of HTS leads to the dissipative regime where J/Jc≫1.

Thick films of the high‐temperature superconductor Tl1Ba2Ca2Cu3Oy with high current density Jc have been deposited on polycrystalline Ag substrates by a spin‐coating method. The spin‐coated precursor films were annealed in a two‐zone Tl reaction furnace. Film thicknesses ranged from 10 to 30 μm, leading to good total current capabilities. The films have highly‐textured c‐orientation as evidenced by rocking curves with FWHM of 4° for the (006) peak. Scanning electron micrographs revealed a dense, plate‐like layered structure and almost no reaction between the film and the Ag substrate. Values of Jc (77 K) up to 2.5×104 A/cm2 in zero field and more than 1000 A/cm2 in a 1 tesla field (H∥c axis).

Superconductivity with Tc≂25–45 K has been induced in films based on the ‘‘infinite layer’’ structure though high temperature annealing in an oxygen ambient. The films had compositions Srn+1CunO2n+1+δ (n=3–9) and were previously grown by laser molecular beam epitaxy by periodic insertion of SrO defect layers into the SrCuO2 infinite layer matrix. A marked dependence on the cooling rate at the end of the postgrowth anneals indicates that preservation of a high temperature oxygen sublattice forms the key to enhancing the carrier density in these films, opposite to the situation with YBa2Cu3O7−δ .

The peak effect in critical current density ${J}_{c}$ is investigated by studying the flux dynamics in ${\mathrm{V}}_{3}\mathrm{Si}$ using bulk magnetometry, small-angle neutron scattering, and transport measurements on clean single-crystal samples from the same ingot. For a field-cooled history, well-defined structure in the vortex lattice was found for fields and temperatures (H,T) below the peak-effect line ${H}_{P}(T);$ above this line, the structure disappeared. History-dependent, metastable disorder is found only for (H,T) below ${H}_{P}(T)$ but the vortex system is reproducibly re-ordered either by field cooling or a low-frequency, pulsed ``shaking'' transport current. The latter is shown to attain Bardeen-Stephen flux flow. In addition, flux flow is observed at ${H}_{P}(T)$ at high current levels. The results support the traditional picture of ${H}_{P}(T)$ as an order-disorder transition due to the competition between elasticity and pinning.

The electrochemical properties of (0.3 μm thick) yttrium-barium-copper-oxide (YBCO) coatings on buffered pure-nickel rolling-assisted-biaxially-textured substrates (RABiTS) were measured at 76 K and self magnetic field. YBCO coatings on buffered substrates both 50 μm and 125 μm thick were tested. Significant degradation of transport critical-current density (Jc) can occur from transverse compressive stress unless the in-plane yielding of the substrate is prevented by providing frictional support to the sample or by using substrate materials of high yield-strength. Scanning electron microscopy was used to investigate the microstructure of the samples after electromechanical testing. Isolated defective regions were found with a mixed pattern of cracks, both longitudinal and transverse to the direction of electrical-current flow. The more the degradation of Jc from electromechanical testing, the greater the density of the observed cracked regions. Also, it was found that the use of magnetic substrates in the...

Pulsed‐laser ablation has been used to grow epitaxial films of Bi2Sr2CaCu2O8−x (Bi‐2212) and Bi2Sr2CuO6−x (Bi‐2201) on (001) MgO with entirely in situ processing. The films’ layer‐stacking sequence, microstructure, and superconducting Tc are highly sensitive to the growth temperature and deposition rate. Pure Bi‐2212 films exist over only a narrow temperature range. Pure Bi‐2201 films appear at higher growth temperature, while lower growth temperature enhances the Bi‐2223 phase, though it is mixed with Bi‐2212. The phase homogeneity, in‐plane connections among grains, and superconducting Tc of Bi‐2212 grown at a given temperature are greatly improved by reducing the deposition rate. Epitaxial Bi‐2212 films grown at 740 °C and 0.5 Hz (∼0.05 nm/s) have Tc0 (R=0)=71 K, with Jc∼8×105 A/cm2 at 50 K and Jc∼5×106 A/cm2 at 4.2 K.

Spray pyrolyzed T1(1223) films deposited on polycrystalline YSZ substrates are characterized before and after heavy ion irradiation. A factor of 2 decrease in zero field critical current is observed. However, significantly improved critical current is found at fields above 1 T, where intragranular effects dominate. The irreversibility line at 5 T is shifted by ∼20 K to higher temperatures. Scaling of the data before and after irradiation and at different temperatures is modeled by an expression which interpolates between single vortex pinning and collective creep.

By exploiting phase-separation in oxide materials, we present a simple and potentially low-cost approach to create exceptional superhydrophobicity in thin-film based coatings. By selecting the TiO2–Cu2O system and depositing through magnetron sputtering onto single crystal and metal templates, we demonstrate growth of nanostructured, chemically phase-segregated composite films. These coatings, after appropriate chemical surface modification, demonstrate a robust, non-wetting Cassie–Baxter state and yield an exceptional superhydrophobic performance, with water droplet contact angles reaching to ~172° and sliding angles

We have performed accurate measurements of the temperature dependence of the magnetic penetration depth of the new intermetallic superconductor ${\mathrm{MgB}}_{2}$ using a high resolution single coil mutual inductance technique. Both sintered pellets and thin films exhibiting critical temperature values ranging between 37 and 38.4 K have been studied. In the case of the film, for $Tl{T}_{c}/2$ a clear exponential behavior is observed, which can be explained by a simple BCS s-wave model with a reduced value of the energy gap. On the contrary, pellets show no evidence of saturation, and the experimental results strictly follow a quadratic dependence down to the lowest temperatures. The experimental data on sintered samples can be explained in the framework of a conventional s-wave BCS theory using a two-band model.

We have investigated the temperature and magnetic field dependence of the Hall coefficient of two well-characterized superconducting ${\mathrm{MgB}}_{2}$ films ${(T}_{c0}=38.0\mathrm{K})$ in both the normal and superconducting states. Our results show that the normal-state Hall coefficient ${R}_{H}$ is positive and increases with decreasing temperature, independent of the applied magnetic field (to 8 T). We find that ${R}_{H}^{\ensuremath{-}1}\ensuremath{\propto}T(40--300\mathrm{K})$ and $\mathrm{cot}{\ensuremath{\theta}}_{H}\ensuremath{\propto}{T}^{2}(100--300\mathrm{K}).$ As the sample is cooled below ${T}_{c}(H),$ ${R}_{H}$ decreases rapidly with temperature and changes sign before it reaches zero. The position and magnitude at which ${R}_{H}$ shows a minimum depends on the applied field.

Extensive work has shown that irradiation with 0.8 GeV protons can produce randomly oriented columnar defects (CD's) in a large number of HTS materials, specifically those cuprates containing Hg, Tl, Pb, Bi, and similar heavy elements. Absorbing the incident proton causes the nucleus of these species to fission, and the recoiling fission fragments create amorphous tracks, i.e., CD's. The superconductive transition temperature Tc decreases linearly with proton fluence and we analyze how the rate depends on the family of superconductors. In a study of Tl-2212 materials, adding defects decreases the equilibrium magnetization Meq(H) significantly in magnitude and changes its field dependence; this result is modeled in terms of vortex pinning. Analysis of the irreversible magnetization and its time dependence shows marked increases in the persistent current density and effective pinning energy, and leads to an estimate for the elementary attempt time for vortex hopping, tau ~ 4x10^(-9) s., Submitted to Physica C; presentation at ISS-2001. PDF file only, 13 pp. total

A series of biaxially textured Ni(1-x)Cr(x) materials, with compositions x = 0, 7, 9, 11, and 13 at % Cr, have been studied for use as substrate materials in coated conductor applications with high temperature superconductors. The magnetic properties were investigated, including the hysteretic loss in a Ni-7 at % Cr sample that was controllably deformed; for comparison, the loss was also measured in a similarly deformed pure Ni substrate. Complementary X-ray diffraction studies show that thermo-mechanical processing produces nearly complete {100} cube texturing, as desired for applications., PDF only; 19 pp., incl 10 figures

Bulk superconducting samples of MgB2 were prepared by solid state reaction of stoichiometric quantities of Mg turnings and B in a sealed Ta cylinder at 890 C for 2 hours. The as-synthesized MgB2 samples had a Tc of 39 K, as defined as the onset of diamagnetism. The crystal symmetry was found to be hexagonal with lattice parameters, a=3.0856 A, and c=3.5199 A, similar to the literature values. To study the effect of carbon doping in MgB2, various C-containing samples of x varying from 0 to 1.00 in MgB2-xCx were prepared. Magnetic characterizations indicate that the Tc onset is same for pure and C-doped samples for x = 0.05, and 0.10. However, the shielding signal decreased monotonically with C content, apparently due to the presence of carbon on the grain boundaries that isolates grains and prevents flow of supercurrents on the perimeter., 10 pages, 3 figures

Superconducting magnesium diboride films with Tc0 ~ 24 K and sharp transition \~ 1 K were successfully prepared on silicon substrates by pulsed laser deposition from a stoichiometric MgB2 target. Contrary to previous reports, anneals at 630 degree and a background of 2x10^(-4) torr Ar/4%H2 were performed without the requirement of Mg vapor or an Mg cap layer. This integration of superconducting MgB2 films on silicon may thus prove enabling in superconductor-semiconductor device applications. Images of surface morphology and cross-section profiles by scanning electron microscopy (SEM) show that the films have a uniform surface morphology and thickness. Energy dispersive spectroscopy (EDS) reveals these films were contaminated with oxygen, originating either from the growth environment or from sample exposure to air. The oxygen contamination may account for the low Tc for those in-situ annealed films, while the use of Si as the substrate does not result in a decrease in Tc as compared to other substrates., 5 pages, 4 figures, 15 references; due to file size limit, images were blured

Fine grained polycrystalline samples of MgB(2) superconductor were synthesized from the elements to contain < 5 % of impurity phases, according to X-ray powder diffractometry. The superconductive transition was sharp with a midpoint Tc = 38.5 K. The magnetization in the vortex state was studied as a function of applied field H, temperature T, and time t. From the equilibrium magnetization, the London penetration depth was obtained. The supercurrent density J(T,H,t) in the vortex state (derived from the irreversible magnetization) decreases approximately linearly with T, in contrast to the quasi-exponential falloff in high-Tc superconductors. The current is highly stable in time, with normalized decay rates S = -d ln(J)/d ln(t) well below those in high-Tc materials. These results are compared with those of other superconductors., PDF file containing text and 6 figures, 12 pages total

A study has been perfomed on the superconducting critical current density J_{c} flowing across low angle grain boundaries in epitaxial thin films of YBa{2}Cu{3}O{7-d} (YBCO). The materials studied were dual grain boundary rings deposited on SrTiO{3} and containing 2, 3, 5 and 7 degree tilt boundaries. The current density in self-field was determined by magnetometric methods at temperatures from 5 K to T_c. We conclude that at the higher temperatures of coated conductor applications, there is limited potential for improving J_{c} by reducing the grain boundary angle below about 3 degrees., 10 pp. total, including REVTEX text, 1 table, and 3 figures

We present a study of the {100} biaxially textured Ni (001) surface and oxide seed layer nucleation by in situ reflection high-energy electron diffraction and Auger electron spectroscopy. Our observations revealed the existence of a c(2×2) superstructure on the textured Ni surface due to segregation of sulfur contained in the bulk metal. The sulfur superstructure promotes the epitaxial (002) nucleation of seed layers such as Y2O3-stabilized ZrO2 (YSZ) and CeO2 on the metal and optimizes the biaxial texture necessary for high Jc superconductors on RABiTS.

In an effort to develop alternative single buffer layer technology for YBa2Cu3O7-δ (YBCO) coated conductors, we have investigated LaMnO3 (LMO) as a potential buffer layer. High-quality LMO films were grown directly on biaxially textured Ni and Ni-W (3%) substrates using rf magnetron sputtering. YBCO films were then grown on LMO buffers using pulsed laser deposition. Detailed X-ray studies have shown that both YBCO and LMO layers were grown with a single epitaxial orientation. Rutherford backscattering spectroscopy (RBS) analyses have indicated the ratio of La to Mn ratio is 1:1. SEM micrographs indicated that 3000-Å-thick LMO films on biaxially textured Ni (100) substrates were dense, continuous and crack-free. A high Jc of over 1 MA/cm2 at 77 K and self-field was obtained on YBCO films grown on LMO-buffered Ni or Ni-W substrates. We have identified LaMnO3 as a good diffusion barrier layer for Ni and it also provides a good template for growing high current density YBCO films.

We have investigated the diffusion mechanism of Hg and Tl cations in cation-exchange process that has been used successfully for fabrication of Hg-based high-Tc superconducting films. Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) was employed to map the distribution of Tl and Hg cations in films quenched at different stages of the cation exchange process. SEM/EDS mappings showed that the nonuniform distribution of Hg is visible in micrometer size on the surface of samples quenched after short-time (∼10min) Hg-annealing, but this nonuniformity disappears for longer-time (∼45 min) Hg-annealing samples. This change could be ascribed to different stages in Hg diffusion -- the former is the early stage when Hg is concentrated in the channels and the latter Hg-cations have already diffuse to grains. Our experimental results hence suggest that Hg-cations channel through defects in the films then diffuse into grains along a-b planes and vise versa for Tl-cations. The diffusion mechanism of anions (oxygen) in post annealing has also been discussed. Fluorine-doped Hg-1212 films were post annealed in flowing oxygen at 300 °C for several hours. Magnetic measurement has shown these samples are comprised pure overdoped Hg-1212 phase with smooth and sharp transition below 120 K, but resistivity vs. temperature measurement shows a kink at ∼123 K. This implies that the optimally doped Hg-1212 phase with higher Tc might be surrounded by overdoped Hg-1212 phase and their contribution to magnetization is minimized, thus the possible diffusion mechanism for anions is also through grain boundaries at a much larger time scale compared to the cations.