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Detailed measurement of electrical resistivity and V-I curves as a function of temperature and magnetic field up to 9 T were performed on several superconducting Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+/spl delta// (Bi-2212) bicrystals with synthetic high angle [001] twist grain boundaries. We found that the resistive onset temperature and field (at level of 10 nV) for all of the measured high angle twist boundaries were similar to those for their constituent single crystals. Furthermore, these boundaries were able to carry critical current as high as the single crystals. Our results suggested absence of weak-link behavior in the high angle [001] twist grain boundaries of Bi-2212 bicrystals.

Hard x-rays from a synchrotron source were utilized in diffraction experiments performed at elevated temperatures (up to ˜ 870 °C) on (Bi, Pb)2Sr2Ca2Cu3O10 (Bi-2223) tapes completely encased in silver. The general behavior of the phase and texture development under typical processing conditions was determined, and the effects that several variations in processing conditions had on the phase and texture development were examined. These results and their implications for improving processing conditions are discussed.

We report a study of critical current density J/sub c/ as a function of temperature T, magnetic field H, and film thickness d in a series of high quality YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films. A crossover in the thickness dependence of J/sub c/(H) curves was observed for T>50 K, where thinner film carries significantly lower J/sub c/(H) than thicker film at high fields, while the opposite was observed at low fields. This J/sub c/(H) crossover behavior seems to be a general feature for a superconducting system with a spectrum of pinning density, whose trend opposes to the effective volume faction for J/sub c/. This generalization is supported by examining J/sub c/(H) crossover behaviors observed earlier in the Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10+/spl delta//(Bi2223)/Ag tapes irradiated with 5.8 GeV Pb ions, where enhanced J/sub c/(H) at high fields is attributed to the density increase in the columnar flux pinning centers.

An expression P=2R/sub c/V/sub L rms/H/sub a rms/ was theoretically derived for the ac power losses in a circular superconducting disk in a perpendicular ac applied magnetic field when the losses are measured by a planar circular pickup coil surrounding the disk. Here, R/sub c/ is the radius of the pickup coil, which is coplanar with the disk, and V/sub L rms/ and H/sub a rms/ are rms values of the loss voltage and applied magnetic field. This method of measuring losses has an advantage over a susceptibility technique, since the coil need not be calibrated. However, this expression is valid only for large values of R/sub c/ relative to the radius R of the disk, since the dipole approximation was used in the derivation. We experimentally evaluated the minimum value of the diameter 2R/sub c/ for which this method provided approximately correct losses. A YBa/sub 2/Cu/sub 3/O/sub 7/ disk with diameter 2R/spl ap/5.3 mm was used for the measurements. The electronically determined apparent losses at all field values decreased with increasing coil radius, and asymptotically approached a constant value for R/sub c//R>/spl sim/3.

A quantitative comparison has been made for ac losses at power frequencies, which were determined by a magnetically induced technique and a transport current technique, for silver-sheathed (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10 tapes. A meaningful comparison of these losses can be made based on the normalized currents (with respect to the respective deduced critical current) rather than the amplitudes of surface magnetic fields. Then, the intrinsic difference in the losses is shown to originate from a geometrical factor and a difference in the deduced critical currents. The discrepancy in the critical currents arises from the facts that the directions of penetrating magnetic field to the tape face are different in these cases and the critical currents are highly dependent on the field direction.

Powder-in-the Ag tube processed ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ and (Bi,Pb${)}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$ tapes were irradiated with 5.8-GeV Pb ions along the tape normal for the ion fluences of 0.68, 1.0, and 2.0\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. The angular (\ensuremath{\theta}) dependence of critical current densities ${\mathit{J}}_{\mathit{c}}$(\ensuremath{\theta},B,T) for these tapes was measured at temperature T=27 K and 55--80 K. Cusplike sharp peaks in ${\mathit{J}}_{\mathit{c}}$(\ensuremath{\theta},B,T) were observed for T\ensuremath{\geqslant}55 K when the applied magnetic field B is oriented parallel to the direction of the irradiation (\ensuremath{\theta}=90\ifmmode^\circ\else\textdegree\fi{}), and ${\mathit{J}}_{\mathit{c}}$(\ensuremath{\theta}) did not scale with the perpendicular component of B, (${\mathit{B}}_{\mathrm{\ensuremath{\perp}}}$=B sin\ensuremath{\theta}). Based on this evidence, it was concluded that the magnetic vortices behave as linelike (three-dimensional) rather than the pancakelike [two-dimensional (2D)] objects in these irradiated superconductors. However, at 27 K, neither the peaks in ${\mathit{J}}_{\mathit{c}}$(\ensuremath{\theta}) nor the scaling of ${\mathit{J}}_{\mathit{c}}$(\ensuremath{\theta}) with ${\mathit{B}}_{\mathrm{\ensuremath{\perp}}}$ was observed. We attribute this to the increased strength of the isotropic pinning by the small irradiation-induced defect debris relative to that of the columns and due to the increased interlayer coupling strength at reduced temperature, but not as evidence for the pancakelike 2D vortices at low temperatures. Also, for T\ensuremath{\geqslant}55 K, the enhancement in the pinning strength qualitatively followed the theory by Nelson and Vinokur [Phys. Rev. Lett. 68, 2398 (1992); Phys. Rev. B 48, 13 060 (1993)]. \textcopyright{} 1996 The American Physical Society.

{ital c}-axis-oriented superconducting Bi{sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub 10+{delta}} (Bi-2223) tapes have been irradiated with 5.8-GeV Pb ions to produce columnar defects along the {ital c} axis. The magnetization {ital M} as a function of field and temperature were measured for both the unirradiated and irradiated samples at matching field {ital B}{sub {phi}} values of 1.28, 2.0, and 4 T. In striking contrast to the conventional log({ital B}) dependence of the reversible {ital M} in the London regime, the field dependence of the reversible {ital M} for the irradiated samples exhibits some unusual characteristics. A large reduction in the value of {ital M} was observed at {ital B}{le}{ital B}{sub {Phi}}, while a maximum appears at {ital B}{approx_gt}{ital B}{sub {Phi}}. We explain this field dependence in terms of the London model modified by the localization of the vortex lines in the columnar defects. Based on this study, we obtained the single vortex pinning energy for Bi-2223 by these columnar defects. {copyright} {ital 1996 The American Physical Society.}

Effects of oxygen partial pressure PO2 during low temperature postheat treatments on the superconducting properties of silver‐sheathed Bi2Sr2CaCu2Oy tapes were investigated. The critical temperature Tc of the resulting tapes changes systematically with PO2. This variation of Tc is attributed to the change of electronic carrier (hole) concentration by the observation of oxygen K‐edge absorption spectra. The intragranular critical current density Jc is essentially unaffected by the heat treatment, while deterioration in grain connectivity is observed in the specimens treated in reduced‐oxygen atmospheres. The change of anisotropy in Jc with respect to crystallographic orientations is also quite small for the variation of PO2.

The structural properties of superconducting (Bi,Pb)2Sr2Ca2Cu3O10–Ag (2223) tapes have been measured using synchrotron x‐ray scattering techniques. The x‐ray photon energy was tuned just below the silver K absorption edge so the penetration depth was large, which allowed the measurements to be performed in a transmission geometry without removing the silver cladding. Analysis of the peaks in 2θ scans indicates that residual (Bi,Pb)2Sr2CaCu2O8 (2212) superconductor starting material is present in all samples studied. The amount of 2212 varied widely among the tapes, and was not homogeneous along the length of each individual tape. Residual 2212 content increased near the ends of most samples, suggesting that 2223 phase development is sensitive to whether the superconducting material is encased in silver or not. The bulk c‐axis alignment was measured in ∼100 mono‐ and multifilament samples, and correlations between c‐axis alignment and current carrying capacity at 77 K were found. Multifilament samples generally had better alignment than monofilament samples. The c‐axis alignment along the length of the tapes was uniform, and the superconducting material within ∼1 μm of the Ag was better textured than the bulk of the sample. Intermediate pressings were directly shown to have an adverse affect on c‐axis alignment. Finally, the evolution of texture and phase development was examined in a series of samples annealed for varying times. The 2212 starting material acquired the final c‐axis alignment state after brief heating times, and only after much longer heating times did the 2212 transform into the 2223 phase. These results and their implications for improving processing procedures are discussed.The structural properties of superconducting (Bi,Pb)2Sr2Ca2Cu3O10–Ag (2223) tapes have been measured using synchrotron x‐ray scattering techniques. The x‐ray photon energy was tuned just below the silver K absorption edge so the penetration depth was large, which allowed the measurements to be performed in a transmission geometry without removing the silver cladding. Analysis of the peaks in 2θ scans indicates that residual (Bi,Pb)2Sr2CaCu2O8 (2212) superconductor starting material is present in all samples studied. The amount of 2212 varied widely among the tapes, and was not homogeneous along the length of each individual tape. Residual 2212 content increased near the ends of most samples, suggesting that 2223 phase development is sensitive to whether the superconducting material is encased in silver or not. The bulk c‐axis alignment was measured in ∼100 mono‐ and multifilament samples, and correlations between c‐axis alignment and current carrying capacity at 77 K were found. Multifilament samples gene...

Long lengths of mono-and multifilament Ag-clad BSCCO superconductors were fabricated by the powder-in-tube technique. Critical current density (Jc) up to 12,000 A/cm2 has been achieved in an 850 m long multicore conductor. Long length conductors were formed into pancake-shaped coils by the wind-and-react approach. Test magnets were then fabricated by stacking the pancake coils and connecting them in series. The magnets were characterized as a function of applied magnetic field at various temperatures. A test magnet, fabricated with ≈770 m of BSCCO tape, generated fields of ≈1 T at 4.2K and ≈ 0.6 T at 27K, both in an applied background field of 20 T. Additionally, the strain tolerance of both mono-and multifilament conductors at 77K in 0.5 T applied field has been studied. We observed that multifilament conductors have better strain tolerance than monofilament tapes, retaining more than 90% of the initial critical current (at 0.5 T) with strain ≥1%.

We demonstrate, for the first time, that even in highly anisotropic, quasi-two-dimensional Bi2Sr2Ca1Cu2O8+δ, electron energy-loss spectroscopy (EELS) performed in a transmission electron microscope (TEM) is a very valuable technique for understanding local changes in the oxygen-hole density on a nanometer scale. After optimizing both the scattering angle of the incident beam and the collection angle of the spectrometer for a particular crystal orientation, a TEM-EELS experiment is capable of documenting local (∼ several nm) variations in strength of the pre-peak of oxygen absorption K edge spectrum. This investigation reveals the hole-concentration profile across grain boundaries, and hence, furthers understanding of the superconducting behavior of the grain boundary in this technologically important material.

Alternating current (ac) losses in silver sheathed mono‐ and multicored (Bi,Pb)2Sr2Ca2Cu3O10 superconducting tapes were investigated at liquid neon temperature (27 K) under various direct current (dc) background magnetic fields. The loss behavior was well accounted for by the Bean critical state model with the inclusion of eddy current loss in the silver sheath. The interfilamentary coupling loss was found to be negligibly small for the present orientation of the tape. In addition, a severe distortion of the loss signal and the B‐H hysteresis curve was observed at 27 K when a high dc magnetic field was applied simultaneously with ac fields. The losses of the tapes were also measured at 77 K and compared with the 27 K values.

We report our observations on and identification of an intermediate phase, (Bi,Pb)4Sr4Ca3Cu5O18+δ (denoted as Bi/4435), during the phase transformation of (BiPb)2Sr2CaCu2O8+δ (Bi/2212) to (Bi,Pb)2Sr2Ca2Cu3O10+δ (Bi/2223) in powder-in-tube processed tapes. The until cell of the phase consists of one half each of the Bi/2212 and the Bi/2223 cell, with lattice parameters of a ≈ b = 5.4 A and c = 33.9 A . The space group of the new phase was determined to be Pbmn using the extinction rule of the selected-area electron diffraction, as well as convergent beam electron diffraction and high-resolution electron microscopy. The presence of the Bi/4435 phase and the abundance of its local structure during the early stage of the transformation suggests that the Bi/2223 phase is formed by the rapod insertion of two extra Ca/CuO bi-layers, one at a time, into the two identical CuO/Ca/CuO building blocks of the Bi/2212 matrix. This finding may shed light on the mechanism of structural phase transformation in layered cuprate superconductors.

We present our study of the vortex solid-liquid phase transition of superconducting Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10/ in the mixed state, based on our detailed transport measurements of a Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10//Ag tape (J/sub c//spl cong/2/spl times/10/sup 4/ A/cm/sup 2/ at 77 K and zero field) immersed in liquid neon (27 K) and in liquid oxygen (54 K-90 K) as a function of applied magnetic field up to 17 T. A comprehensive vortex phase diagram is obtained. We show that the vortex. Solid-liquid transition is the fundamental limiting factor for J/sub c/ in this tape at T/spl ges/27 K in magnetic field (except at very low field). In addition, the field-dependence of the flux pinning force in both solid and liquid regime is discussed. >

We report detailed measurements of current-voltage characteristics for a ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$/Ag tape immersed in liquid neon (27 K) and in liquid oxygen (54\char21{}90 K) as a function of applied magnetic field up to 17 T. It is found that the electric-field-versus-current-density (E-J) curves can be well described by the relation E/J\ensuremath{\sim}exp[-(${\mathit{J}}_{0}$/J${)}^{\mathrm{\ensuremath{\mu}}}$], where ${\mathit{J}}_{0}$ is a constant. The exponent \ensuremath{\mu} at elevated temperature (T\ensuremath{\ge}54 K) is less than 0.9, and is inconsistent with the vortex-glass description, which predicts \ensuremath{\mu}1. At low-temperature (T=27 K), a plateau in the value of \ensuremath{\mu}(\ensuremath{\approxeq}1.45) is found for 2\ensuremath{\le}H\ensuremath{\le}8 T, which may be described by the collective creep theory at this temperature and field range.

The removal of hydrogen fluoride gas (HF) is a significant problem for the ex situ processing of large area YBCO precursor coated conductors employed in the BaF2 process. We propose a new solution based on the in situ chemical absorption of HF. This approach allows the design of a simple ex situ reactor, which, in theory, has an extremely large capacity to uniformly absorb HF and permit the uniform growth of crystalline YBCO.

Some coated conductor applications require a YBCO layer at least 5 /spl mu/m thick with high J/sub c/. The growth of thick c-axis oriented epitaxial layers using the barium fluoride ex-situ technology is not always possible. Films over 3 /spl mu/m thick have more undesirable a-oriented and random grains than thinner ones. We present an analysis of thick film nucleation and conclude that thick precursor layers impede out-diffusion of the ex-situ reaction product, HF. High impedance for HF diffusion results in large variations of the chemical potential of the growth reaction and disruption of the nucleation process. We conclude that a solution for c-axis growth is control of the precursor permeability. A method is described for controlling the permeability of precursor layers. Using this technique we were able to grow c-axis oriented 5-/spl mu/m thick films on oxide-buffered Ni tape with J/sub c/(0 T) = 4 /spl times/ 10/sup 5/ A/cm/sup 2/ and J/sub c/(1 T) = 8 /spl times/ 10/sup 4/ A/cm/sup 2/.

Local lattice parameters and oxygen (hole) densities in the vicinity of large-angle grain boundaries in bulk YBa2Cu3O7−δ were measured simultaneously using convergent-beay electron diffraction and electron-energy-loss spectroscopy with a less than 20 A field emission probe respectively. The oxygen (hole) contents were quantified by Gaussian fitting to the observed oxygen K-edge absorption spectra and comparing them with results for a set of standard samples with different oxygen contents. The local lattice parameters were determined by matching the simulated high-order Laue zone patterns with experimental patterns by a least-squares algorithm. We found that large-angle grain boundaries in YBa2Cu3O7−δ often exhibit volume expansion. The increase of the c-lattice parameter near the boundary core can be attributed to lattice distortion and/or to an oxygen deficiency. The lattice distortion extends less than 20 A, while the deficiency can extend more than 100 A. Comparison of the c/a ratio of the lat...

Below \ensuremath{\sim}60 K ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ba}}_{x}\mathrm{Cu}{\mathrm{O}}_{4} (0.10\ensuremath{\le}x\ensuremath{\le}0.15)$ reveals superstructure reflections consistent with the tetragonal space group $\frac{P{4}_{2}}{\mathrm{ncm}}$ [low temperature tetragonal (LTT)]. However, twins, characteristic of the related orthorhombic phase [low temperature orthorhombic (LTO)], persist in this material. Electron microscopy shows that the LTT superstructure reflections originate from the twin-boundary region. This suggests a phase mixture of orthorhombic and tetragonal with a gradual change from LTO (space group $\mathrm{Bmab}$) in the interior of the twin domains, via the $\mathrm{Pccn}$, to LTT at the twin boundary. This is modeled as a spatial modulation, the tipping angle of Cu${\mathrm{O}}_{6}$ octahedra being an order parameter.

We report a detailed measurement of the magnetization for a high-quality ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ single crystal as a function of magnetic field (H) and the angle between H and the c-axis, near the transition temperature ${\mathit{T}}_{\mathit{c}}$. The fluctuation magnetization for H parallel to the c-axis was found to be in excellent agreement with the recently developed vortex-fluctuation theory by Bulaevskii et al. and the order-parameter fluctuation theory by Te\ifmmode \check{s}\else \v{s}\fi{}anovi\ifmmode \acute{c}\else \'{c}\fi{} et al. However, our fluctuation magnetization data for the fields, applied at a large angle with respect to the c-axis, were found inconsistent with the expectations from the above theories.

The field ([ital B]) and temperature dependence of reversible magnetization [ital M] in Bi (2:2:1:2) single crystals is measured for the orientation of the field along the [ital c] axis. The slope of the curve [ital M] vs ln[ital B] decreases remarkably with field below 70 K. In the mean-field approach this slope should be almost [ital B] independent. We explain this change of slope by the quantum fluctuations of vortices. We show that for magnetization quantum fluctuations are important at all temperatures except in a narrow region near [ital T][sub [ital c]].

We report a series of measurements of [ital I]-[ital V] curves for a [ital c]-axis-oriented Bi[sub 2]Sr[sub 2]Ca[sub 2]Cu[sub 3]O[sub 10]/Ag tape immersed in liquid oxygen. By applying the critical scaling analysis to the [ital I]-[ital V] characteristics measured in various magnetic fields, a vortex-glass--liquid phase-transition line was identified in the mixed state of this system for [ital H][le]1.2 T, with critical exponents [nu]=1.25[plus minus]0.15 and [ital z]=8.5[plus minus]1.5. We also show that this transition boundary in the [ital H]-[ital T] plane defines the upper limit of useful current carrying capacities for the Bi[sub 2]Sr[sub 2]Ca[sub 2]Cu[sub 3]O[sub 10]/Ag tapes at a given temperature and magnetic field.

The mechanisms have been identified for the hetero-epitaxial nucleation of YBa2Cu3O∼6.1 (YBCO) at the buried interface between a precursor film and SrTiO3 for the so-called BaF2 process which is a postdeposition reaction process for the synthesis of epitaxial YBCO films. It is shown that the preferential nucleation of YBCO at the interface is due to (1) the strong chemical affinity of the (Y, Ba)-oxy-fluoride, an intermediate phase, to SrTiO3 and (2) the epitaxial alignment of its (111) planes with the (001) surface of the SrTiO3 which reduces the activation barrier for the formation of YBCO. In thin films (

We report a detailed study of the temperature and magnetic-field dependence of free energy, magnetization, and specific heat of superconducting ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$ in the critical fluctuation region near ${\mathit{H}}_{\mathit{c}2}$(T) line. Our approach is to apply the recently developed nonperturbative scaling theory (Te\ifmmode \check{s}\else \v{s}\fi{}anovi\ifmmode \acute{c}\else \'{c}\fi{} et al.) in high fields to describe the reversible magnetization data taken on a c-axis-oriented superconducting ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$ thin tape. We find good agreement on some of the superconducting parameters obtained in the present study with those derived by applying the Ginzburg-Landau theory to our reversible magnetization data taken outside the critical fluctuation region.

Irradiation of high Tc superconductors by heavy ions generates amorphous columns. The creation of the columnar defects in YBa2CU3O7-δ often leads to the formation of wing-like planar faults attached to the columns. Such planar faults, which were characterized by electron microscopy combining nano-probe chemical analysis and high-resolution imaging and image simulation, were found to have various local structures. The majority of faults consists of one or two extra CuO planes inserted at the location of the original CuO single layer, resulting in a local structure of YBa2Cu4O8(124) or YBa2Cu5O9(125). Since these faults are chemical faults and are often bounded by sessile dislocations, the existence of such defects supports the idea that the formation of the amorphous columns involves partial epitaxial regrowth of the ion-induced molten region upon the heavy-ion irradiation.

The magnetization measurements on ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ single crystals with H parallel to the c axis are presented to show that the critical fluctuations in the magnetization near the ${\mathit{H}}_{\mathit{c}2}$(T) line can be well studied within the two-dimensional nonperturbative scaling theory developed by Te\ifmmode \check{s}\else \v{s}\fi{}anovi\ifmmode \acute{c}\else \'{c}\fi{} et al. By using this scaling theory and basic thermodynamic arguments, we obtained the Ginzburg-Landau parameter \ensuremath{\kappa} (\ensuremath{\simeq}115) and ${\mathit{dH}}_{\mathit{c}2}$/dT (\ensuremath{\simeq}-2.7 T/K) near ${\mathit{T}}_{\mathit{c}}$ for this material. In addition, we found that these derived parameters are in good agreement with those determined from the study of reversible magnetization in an intermediate field at low temperature.

Highly textured (c axis perpendicular to the plane of the substrate tape) thick films of YBa2Cu3O7 grown on Ag0.9Pd0.1 and pure silver substrates have been made by a traveling heated zone method. The speed of the traveling zone can be as fast as ∼5 cm/h and the temperatures of the zone, 930–975 °C, were well below the peritectic temperature of YBa2Cu3O7. A precursor layer, which consisted of a nitrate solution of yttrium, barium, and copper cations, was deposited by spray pyrolysis directly onto a heated substrate. The values of the resistively measured Tc were 83–91 K. The transport critical current density of these thick films was in the range of 2–8×103 A/cm2 at 77 K and 0 T.

This paper reports a study of the nature and systematic variation of radiation damage to cuprate superconductors caused by several-hundred-MeV heavy ions. While irradiation of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ with 300-MeV ${\mathrm{Au}}^{24+}$ and 276-MeV ${\mathrm{Ag}}^{21+}$ ions produces columns of amorphous material along the ion trajectories, such defects are only created occasionally during irradiation with 236-MeV ${\mathrm{Cu}}^{18+}$ and not induced with 182-MeV ${\mathrm{Si}}^{13+}$. A comparative study of the defect formation in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$ and oxygen-reduced and ozone-treated ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$, shows that the degree of the radiation damage by the heavy ions depends on (a) the rate at which ions lose their energy in the target; (b) the crystallographic orientations with respect to the incident ion beam; (c) thermal conductivity and chemical state (oxygen concentration for ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$) of the sample; and (d) the extent of preexisting defects in the crystal. A theoretical model based on ion-induced localized melting and the effects of anisotropic thermal conductivity of these materials provides a basis for understanding the size and shape of the amorphous tracks. Measurements of the superconducting properties of ${\mathrm{Au}}^{24+}$- and ${\mathrm{Ag}}^{21+}$-irradiated ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ thin films show a universal linear scaling between the fractional areal damage versus the superconducting transition temperature and the normal-state resistivity.

Computer-simulation techniques and TEM analysis were employed to study the dependence of the twin and tweed microstructures in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{M}}_{\mathit{x}}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ on doping by trivalent atoms (like Fe, Co, or Al). Since the trivalent atoms substituting the bivalent Cu(I) atoms have a greater number of the nearest-neighbor oxygen atoms, it was suggested that doping can be described by the M-O nearest-neighbor attractive interaction. This interaction generates a local oxygen disorder which plays an important role in the formation of the tweed structure. The M-O interaction and the long-range O-O interaction (screened Coulomb and strain induced) were taken into account to simulate the oxygen ordering kinetics. The simulation is based on equations describing the microscopic diffusion of oxygen atoms. The obtained simulated microstructures are in qualitative and even quantitative agreement with electron microscopic observations for ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ and with previous electron microscopic data. Both the computer simulations and the TEM results have shown that the usual twin structure, formed through coarsening and refining of the transient tweed structure, is produced at small doping (x\ensuremath{\le}\ensuremath{\sim}0.08). At larger x(\ensuremath{\sim}0.08x\ensuremath{\sim}0.2--0.3), the dopant atoms prevent coarsening and a metastable (or stable) mesoscopic tweed pattern appears. Although the crystal lattice is locally distorted by ultrafine orthorhombic domains forming the tweed pattern, the average crystal lattice determining the diffraction spot pattern is tetragonal. At higher x, the ultrafine orthorhombic domains producing the tweed structure disappear and a disordered tetragonal phase is formed. It is found that doping does not affect the microstructure if the long-range model for the M-O interactions is assumed.

Twinning on the (110) and the equivalent (110) planes is frequently encountered within one crystal grain in YBa2Cu3O7-δ. Using high-resolution electron microscopy and electron diffraction analysis, we studied the interface and its vicinity where the (110) and (110) twins meet. We found that the crystal lattice of the impinging set of twins rotates around the c axis when they confront the orthogonally oriented twins. Every second one of the impinging twins is pointed, but does not taper off to zero width, yielding a small-angle grain boundary; the other twin forms a coherent boundary with a twin belonging to the orthogonal set. Thus, the interface of the orthogonally oriented twins comprises of an alternation of twin boundaries and grain boundaries. Furthermore, we found that there is a limiting wedge-angle for the tapered twins, and if this angle is exceeded, the tip of the twin splits. The structure of the interface and its vicinity is determined by the equilibrium of elastic strain energy due to the tet...

Systematic magnetization measurements were made to study reversible magnetic properties of a series of high-quality, large single crystals of (La[sub 1[minus][ital x]]Sr[sub [ital x]])[sub 2]CuO[sub 4] over a composition range of [ital x]=0.05--0.10, with magnetic fields parallel to the [ital c] axis. A method of data analysis, based on the variational model developed by Hao and Clem, was used for the determination of various superconducting parameters of this family. We fit the temperature dependence of the thermodynamic critical field [ital H][sub [ital c]]([ital T]) data, which was obtained in the temperature region where the thermodynamic fluctuation is insignificant, with the BCS or an empirical function for [ital H][sub [ital c]]([ital T]). We found that a crystal ([ital x]=0.077), which has the highest [ital T][sub [ital c]] ([congruent]35.0 K), shows a maximum in the value of [ital H][sub [ital c]](0) (=2510[plus minus]80 Oe) and a minimum in the value of [lambda][sub [ital a][ital b]](0) (=2545[plus minus]40 A) for the range of [ital x] that we have studied. We also observed rather close values of coherence length in the [ital ab] plane [[xi][sub [ital a][ital b]](0)[similar to]32 A] for all (La[sub 1[minus][ital x]]Sr[sub [ital x]])[sub 2]CuO[sub 4] single crystals that we studied.

We report the effects of ozone oxygenation of YBa2Cu3O7−δ thin crystals for eliminating local fluctuation in oxygen content, which is known to exist even in normal δ≊0 samples. An optimum ozone annealing condition was determined based on the microstructure and the uniformity of oxygen content of the samples. We observed the formation of fine‐spacing twins running perpendicular to the original twins at thin areas and attributed this to accommodating the local strain due to oxygen uptake during ozone annealing.

The authors report a comparative study of superconducting behavior of YBa/sub 2/Cu/sub 3/O/sub 7/ epitaxial films irradiated with Ag and Au ions of several hundred megaelectronvolt energy. The irradiation results in columnar defects of average diameter approximately 55 AA and approximately 110 AA for Ag/sup +21/ and Au/sup +24/ ions, respectively. A universal linear scaling is observed between the fractional areal damage vs. the superconducting transition temperature and normal state resistivity. The results also show that in these films, which have intrinsically strong pinning, a marginal enhancement in the critical current density (J/sub c/) occurs when the defect density is >

The tweed contrast from YBa2Cu3O7-δ alloyed with Fe, Co or Al, observed in the electron microscope, was analysed under different diffraction conditions. The characteristic features of the tweed image were reproduced in calculations of simple models based on the contrast originating from structures with twinning or displacive modulation, stacked on top of each other along the c axis.

A large shift of the onset of flux-flow resistivity and the irreversibility line H(irr)(T) to higher temperatures is observed in Tl2Ba2Ca2Cu3O10 films containing linear defects created by Ag(+21) ion irradiation. The H(irr)(T), which has a characteristic L shape in highly anisotropic Tl and Bi based cuprates, becomes more like that of YBa2Cu3O7 in the presence of these defects. The Jc at 77 K also shows a large increase as a result of flux localization at the defects. The transport data indicate that in the H-T plane above H(irr)(T) of the unirradiated material, an ensemble of unoccupied defects is required for effective pinning of each flux line in the system.

We report the behavior of the Ohmic resistivity and the current-dependent dissipation in the mixed state of YBa2Cu3O7 films irradiated with 300 MeV gold ions. The transport in both irradiated and unirradiated states of the samples agrees with the model of a second order phase transition to a vortex glass state. While the zero-field critical temperature and current are reduced by the irradiation, the vortex-glass description of the mixed state remains unchanged up to a dosage of ∼9 × 1010 ions/cm2. These results indicate that in these high Jc films the vortex dynamics is controlled by pinning centers distributed at a length scale much shorter than the gold-ion induced defects.

Structural images of twinning steps associated with an |a — b|[ — 10] twinning dislocation at a (110) twin boundary of YBa2Cu3O7 −δ (δ ≈ 0·0) were observed for the first time. High-resolution electron microscopy (HREM) studies suggest that there are two types of twin boundaries: one with a lattice translation along the boundary for fully oxygenated YBa2Cu3O7 −δ, and the other, without a lattice translation, for the oxygen-deficient one. High resolution in situ experiments indicate that the structure of the twin boundary changes from one type to the other under electron beam irradiation. Dislocations were observed from regions where the two types of twin boundary merge together. Dislocation models are proposed for interpreting the kinetics of the transition of the boundary structures. The geometry of the twin boundary and twinning dislocation using the concepts of coincidence site lattice (CSL) and of displacement shift complete lattice (DSCL) is also demonstrated.

Magnetization measurements on {ital c}-axis-oriented bulk Bi{sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub 10} with the magnetic field parallel to the {ital c} axis are reported. The results show large fluctuation effects, which can be explained by Ginzburg-Landau fluctuation theory for a two-dimensional (2D) system. The magnetization in high fields displays good scaling behavior as a function of ({ital T}{minus}{ital T}{sub {ital c}}({ital H}))/({ital TH}){sup 1/2}. The weak-field fluctuation diamagnetic susceptibility {chi}({ital T}) above {ital T}{sub {ital c}} can be fitted well in terms of the 2D Lawrence-Doniach model. Values of {xi}{sub {ital a}{ital b}}(0) and {ital dH}{sub {ital c}2}/{ital dT} derived from our fluctuation analysis are consistent with those obtained by fitting the Hao-Clem variational model to the reversible magnetization data below {ital T}{sub {ital c}}.

Magnetization measurements on {ital c}-axis-oriented bulk superconducting Bi{sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub 10} have been carried out with the magnetic field applied parallel to the {ital c} axis. Fitting the results to the variation model for magnetization suggested by Hao and Clem shows that the Ginzburg-Landau parameter {kappa} is nearly constant for 74{le}{ital T}{le}86 K, with the value of 170 at 74 K, and slowly increases with temperature from {similar to}86 to {similar to}100 K. {kappa} diverges in the vicinity of {ital T}{sub {ital c}}. This unusual temperature dependence of {kappa}({ital T}) could come from extended fluctuation effects and a limitation of the Hao-Clem theory, which may fail to describe this extremely anisotropic Bi-based superconductor. By fitting the data with the value of {kappa} equal to 170, the deduced values of some of the superconducting parameters are {ital H}{sub {ital c}}(0)=10 600 Oe, {xi}(0){sub {ital a}{ital b}}=10.5{plus minus}0.6 A, {lambda}(0){sub {ital a}{ital b}}=1940{plus minus}13 A, {ital dH}{sub {ital c}2}/{ital dT}{congruent}{minus}3.68 T/K near {ital T}{sub {ital c}}.

Calculations predict that orthorhombic microtwins of the same orthorhombicity as pure YBa2Cu3O7−δ result in an apparent tetragonal symmetry in diffraction experiments when the twin period is reduced below about 20 nm. The diffuse electron scattering observed around the fundamental refiections from YBa2(Cu1−x M x)3O7+δ, where M = Fe, Al, or Co and x ≳ 0.03, is also well described by this model of non-periodic orthorhombic microtwins, suggesting that these trivalent cations on the chain sites promote the formation of chemical twins.

Current-Voltage (I–V) characteristics and resistivity measurements on NbxZr1−x (x ∼ 0.15) thin films are reported over a broad range of temperature and magnetic field. In the vicinity of the critical temperature (field) the I–V curves show a transition from the Ohmic to a strongly non-linear behavior on increasing the excitation current density. The linear resistivity is thermally activated with a field dependent barrier height. Although the Ohmic and non-linear data can be described in the framework of the glass model for the mixed state in hard superconductors, it does not yield a universal set of exponents for the glass to fluid transition. The overall response of the mixed state can be understood in the framework of the standard flux-creep theory with inter-vortex interaction.

The ex situ growth of thick textured YBa2Cu3O7 films on SrTiO3 substrates was investigated in order to find the factors, which influence growth kinetics. It was found that the growth rate of YBa2Cu3O7 films remains constant during heat treatment and is proportional to the square root of the H2O partial pressure in the processing atmosphere. Using transmission electron microscopy, we observed that the growth of the YBa2Cu3O7 films proceeds from a thin, ∼7-nm-thick, layer of liquid located at the growth front. Chemical analysis of this layer and the adjacent material suggests that decomposition of an oxifluoride compound in the precursor film occurs at the liquid layer-precursor film interface. The rate of this decomposition reaction defines the growth rate of YBa2Cu3O7. Using a simple model we show that rate of this reaction is limited by the out-diffusion of the decomposition product HF.

The misorientations of over 200 pairs of adjacent grains separated by grain boundaries in textured YBa2Cu3O7−δ were measured using a transmission electron microscopy technique. The results indicate that there exist discrete preferred rotation angles and rotation axes. The existence of low-energy boundaries is inferred. The results are analyzed based on the Constrained Coincidence Site Lattice (CCSL) and O2-lattice theories and imply the applicability of such theories for the case of large-angle grain boundaries in a complex crystal structure such as YBa2Cu3O7−δ. The results of analysis also show that some boundaries are likely to be reduced in oxygen near the boundary to satisfy the constraint of the coincidence site lattice.

We have investigated YBa{sub 2}(Cu{sub 1{minus}{ital x}M{ital x}}){sub 3}O{sub 7{minus}{delta}} (1:2:3) ({ital M}=Fe, with preliminary results for Co and Al) prepared using three different heat treatments. Standard samples were prepared using sintering temperatures up to 960 {degree}C, followed by slow cooling in oxygen. Oxygen was removed by heating the standard samples in flowing argon near 830 {degree}C, the resulting specimens being called reduced.'' Reoxidized samples, in turn, were prepared from reduced samples that were heated in flowing oxygen near 400 {degree}C. The Fe content of selected standard, reduced, and reoxidized samples was determined by electron microprobe. Transmission electron microscopy and x-ray diffraction for reoxidized 1:2:3 {ital M}=Fe are consistent with an orthorhombic phase extending to {ital x}{ge}0.12. Superconducting transition temperatures as measured magnetically are similar for standard and reoxidized cases (except {ital M}=Al). X-ray-absorption measurements were performed on samples {ital M}=Fe at the Fe {ital K} edge to probe the Fe near-neighbor environment. Our analysis suggests that Fe atoms remain in the chain layer regardless of heat treatment. This result is discussed in terms of proposed models involving migration of Fe, clustering in the Cu-O chain layer, and/or movement of Fe to the CuO{sub 2} plane layer. Preliminary oxygen {italmore » K}-edge studies utilizing fluorescence yield were used to investigate oxygen electronic structure.« less

Microstructure and diffuse scattering of electron diffraction in YBa2(Cu1-xMx)3O7-δ (M=Fe, Co, Al, Ga, Zn and Ni, 0≤x≤0.33) were studied. It was found that the substitution for Cu by 3-valent Fe, Co and Al at a level x≥0.03 causes a structural transition from twin to tweed. For 0.03≤x≤0.06, the geometry of the reciprocal spot associated with the tweed structure has a cross-platelet shape extended in 2 equivalent 〈hh0〉 directions, which corresponds to the displacive modulation in real space. For x≥0.10, the observed diffuse satellites and short-square-pipe intensity suggest the existence of a three-dimensional short-range ordering in these materials. Oxygen-reduction and reoxidization experiments indicate that development of tweed depends on the oxygen distribution. We believe that the configuration of oxygen in the Cu(1) layer is responsible for the formation of tweed structure in YBa2Cu3O7-δ system.

High-resolution electron microscopy and electron diffraction of YBa{sub 2}(Cu{sub 0.98}{ital M0.02}){sub 3}O{sub 7{minus}{delta}} with {ital M}=Zn, Ni, Al, Fe, Cu indicate that twin boundaries are centered on (110) planes through the O atoms in the CuO layer for {delta}{approx}0 and {ital M}=Ni, Zn, or Cu, accompanied by a displacement of the cation sublattices across the boundary. Oxygen deficiency causes a shift of the center of twin boundaries to (110) planes through the cations and eliminates the cation-sublattice displacement, while solutes {ital M}=Al or Fe (even for oxygen content in excess of 7.00) substantially reduce the magnitude of the cation-sublattice displacement.

The diffuse electron scattering accompanying the tweed contrast in oxygen deficient and Fe, Co and Al doped YBa2Cu3O7-δ has been qualitatively corrected for double scattering effects. The kinematical diffuse scattering which results from this work suggests that shear or twinning displacement causes the tweed contrast, and may provide a useful basis for a better insight into the detail microstructure of these phases which are tetragonal on a macroscopic scale.

For fine multifilamentary ${\mathrm{Nb}}_{3}$Sn and Nb-Ti wires, it was found that there exists a surprisingly large temperature region (e.g., \ensuremath{\sim}1 and 0.5 K at 2 T for a ${\mathrm{Nb}}_{3}$Sn and Nb-Ti wire, respectively) below the mean-field critical-magnetic-field line ${\mathit{H}}_{\mathit{c}2}$(T) where the magnetization is reversible during a cycle of warming and cooling. The magnetic-field dependence of the lower boundary of this temperature region, i.e., the irreversibility temperature ${\mathit{T}}_{\mathit{r}}$(H), was shown to follow theoretical predictions for flux-lattice melting based on an analysis of thermally induced flux-line fluctuations calculated with nonlocal lattice elasticity and the Lindemann melting criterion.

Critical current and magnetization data are presented for some multifilamentary Nb/sub 3/Sn wires that have been produced by the internal-tin method. A comparison of magnetization and transport critical current measurements shows that filament bridging during heat treatment is a common occurrence leading to effective filament diameters that are sometimes an order of magnitude larger than the geometrical filament size. At present J/sub c/'s (in the noncopper region) greater than 1300 A/mm/sup 2/ at 10 T have been achieved in some conductors, which also exhibit high losses. Low losses have only been seen in conductors with a high local ratio of copper to niobium. The use of (Nb-1%Ti) alloy instead of pure Nb helps to reduce low-field loss and increase high-field J/sub c./ Measurements of the temperature dependence of hysteretic loss to 5 T indicate that loss decreases linearly with increasing temperature.

The magnetic-field dependence of the irreversibility temperatures follows an H=a[1-${\mathit{T}}_{\mathit{r}}$(H)/${\mathit{T}}_{\mathit{c}}$(0)${]}^{\mathit{n}}$ relationship with n\ensuremath{\simeq}1.5, for pure and alloyed ${\mathrm{YBa}}_{2}$(${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{M}}_{\mathit{x}}$${)}_{3}$${\mathrm{O}}_{7+\mathrm{\ensuremath{\delta}}}$ with x=0 and 0.02, where M=Al, Fe, Ni, and Zn, measured for an applied field parallel to the c axis. However, for M=Ni and x=0.04 and 0.06, n\ensuremath{\simeq}2.0. This relationship is not applicable for either ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ or (Bi,Pb${)}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$ powders. It is also shown that the irreversibility temperature is a strong function of the magnetic hysteresis width \ensuremath{\Delta}M for pure and alloyed ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. These results and the measurements of the flux creep \ensuremath{\Delta}M(t) for these specimens suggest that ${\mathit{T}}_{\mathit{r}}$(H) is a depinning line rather than a lattice melting or glass-to-liquid phase-transition temperature. However, the conventional flux-creep model cannot account for all of the observed temporal dependences of \ensuremath{\Delta}M(t).