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Showing papers in "Physica C-superconductivity and Its Applications in 1994"


Journal ArticleDOI
TL;DR: In this article, small sections cut from within individual superconducting filaments of Ag sheathed (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O y (BSCCO-2223) tapes have critical current densities (J c ) which depend very much on the local microstructure and which can be as much as five times higher than the average J c.
Abstract: Small sections cut from within individual superconducting filaments of Ag sheathed (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O y (BSCCO-2223) tapes have critical current densities ( J c ) which depend very much on the local microstructure and which can be as much as five times higher than the average J c . Tapes having average J c values of 12–15 000 A/cm 2 (77 K, 0 T) had local J c values up A/cm 2 , a value larger than hitherto reported for any bulk sample. Close to the Ag sheath the conversion to the 2223 phase is more complete and the grains are larger and better aligned and the J c is then much higjer and less field sensitive. Large variations are also found in more uniformly reacted tapes, perhaps due to variations of the local crack density. The microstructure of the highest J c regions is still far from ideal, thus showing that the limits of this technologically very important system are far from being achieved.

173 citations


Journal ArticleDOI
John Loram1, K.A. Mirza1, J. M. Wade1, J. R. Cooper1, W.Y. Liang1 
TL;DR: In this paper, the electronic specific heat coefficient γ(T) for several cuprate series determined from high resolution differential measurements is presented, which is consistent with Fermi statistics, a low degeneracy temperature and at most a modest low temperature mass enhancement.
Abstract: We present the electronic specific heat coefficient γ(T) for several cuprate series determined from high resolution differential measurements. γ(T) is consistent with Fermi statistics, a low degeneracy temperature and at most a modest low temperature mass enhancement. We show the progression of γ(T) with hole doping for La 2-y Sr y CuO 4 and the overdoped system Tl 2 Ba 2 CuO 6+δ , and demonstrate that the rapid fall in condensation energy in underdoped YBa 2 Cu 3 O 6+x results from the growth of a pseudogap in the normal state quasiparticle spectrum. We discuss the effects of Zn doping on superconducting and pseudogap correlations in YBa 2 Cu 3 O 6+x .

171 citations


Journal ArticleDOI
TL;DR: In this paper, the Nd-Ba-Cu-O system exhibits a solid solution, Nd1+xBa2−xCu3O7+δ (Nd123ss), for 0.04⩽x ⩽0.6.
Abstract: Unlike YBa2Cu3O7−δ (Y123), the Nd-Ba-Cu-O system exhibits a solid solution, Nd1+xBa2−xCu3O7+δ (Nd123ss), for 0.04⩽x⩽0.6. However, the superconducting properties decrease nonlinearly for increasing x, and Tc can also be varied by lowering the annealing temperature without changing the low-temperature oxygen soak. The changes in microstructure and Tc with increasing x are analogous with Y123 with increasing δ, yet for fully oxygenated Nd123ss samples, electron energy loss spectroscopy (EELS) demonstrates that the total unoccupied density of states below the unfilled band is constant for all x. The neutron diffraction and the inert gas fusion analysis support an overall charge balance for all x. Tc is modeled in terms of a forced oxygen disorder resulting from Nd3+ atoms on the Ba sites relocating chain oxygens to anti-chain sites. The variability in Tc as a function of x and heat treatments can be explained by the number of four-fold coordinated Cu's on the chains which can be varied by differing the amounts of paired and unpaired Nd3+ substituting for Ba2+, resulting in more or less charge transfer from the planes to the chains.

158 citations


Journal ArticleDOI
Bertram Batlogg1, H. Y. Hwang1, Hidenori Takagi1, Robert J. Cava1, H. L. Kao1, J. Kwo1 
TL;DR: In this article, the normal state of the prototypical cuprate La 2-x Sr x CuO 4 is described and significant progress in elucidating the normal status of the cuprate is described.
Abstract: The significant progress in elucidating the normal state of the prototypical cuprate La 2-x Sr x CuO 4 is described. New insight is gained from quantitative charge dynamics studies (resistivity, Hall effect) over wide temperature and hole concentration ranges. Characteristic temperatures T ∗ have been discovered and corresponding T ∗ 's can also be identified in the magnetic response. At lowest carrier concentrations, T ∗ 's start out high (700K–800K) and drop rapidly to ∼ 100K when superconductivity disappears at high doping levels. From comparisons with other compounds, we suggest that the T ∗ marks the development of a pseudogap in the excitation spectrum and we present a T ∗ -x diagram for LSCO, which we suggest is generic to all hole doped layered cuprates.

150 citations


Journal ArticleDOI
TL;DR: In the process of searching for the phases of the Ag-Ba-Ca-Cu-O system at 1200°C under 5 GPa, the new oxycarbonate superconductors (Cu 0.5C0.5)Ba2Can-1CunO2n+3.
Abstract: In the process of searching for the phases of the Ag-Ba-Ca-Cu-O system at 1200°C under 5 GPa, we found the new oxycarbonate superconductors (Cu0.5C0.5)Ba2Can-1CunO2n+3. Two members of the series with n=3 and n=4 were isolated as almost single phases; their structures are closely related to those of Tl (orHg) based 1223 and 1234, respectively. The Tl (Hg) sites are, in the present series, replaced by Cu and C. Disregarding ordering of Cu and C, the present compounds take tetragonal subcells; at=3.859(1), ct=14.766(5) A for n=3 and at=17.930(6) A for n=4. Electron microscopic observation indicated that Cu and C are, in both cases, located alternately along the a-axes of the subcells resulting in superstructures having as=2at, bs=bt and cs=2ct. The n=3 member showed a superconducting transition at 67 K, while the n=4 member at 117 K which is the highest Tc among the oxycarbonate superconductors found thus fur.

136 citations


Journal ArticleDOI
TL;DR: In this paper, a large portion of the magnetic field-temperature plane on a single crystal of YNi 2 B 2 C for field applied parallel to the c -axis in order to determine the range of thermodynamic reversibility and the critical magnetic fields was analyzed.
Abstract: Single crystals of YNi 2 B 2 C have been grown via high-temperature flux growth, using Ni 2 B as a solvent. Magnetization measurements have been carried out over a large portion of the magnetic field-temperature plane on a single crystal of YNi 2 B 2 C for field applied parallel to the c -axis in order to determine the range of thermodynamic reversibility and the critical magnetic fields. The magnetization is thermodynamically reversible within 2% of the magnetization for fields above 0.5 T and temperatures above 3 K. Near T c , the upper critical field has a slope of -0.32 T/K. The zero-temperature Ginzburg-Landau parameter κ is 13≈15, the penetration depth λ ab is about 150 nm, and the coherence distance λ ab is about 10 nm. The irreversibility line H irr ( T ) obeys H irr ( T ) = H irr (0) (1- T / T c ) 1.16 . The magnetization data below T c closely follow a logarithmic dependence on applied field, in the intermediate region, as predicted by the London model. The magnetization versus temperature curves shift uniformly to lower temperature as the field increases. Based on these measurements, YNi 2 B 2 C seems more similar to conventional low- T c superconductors than to high- T c superconductors.

119 citations


Journal ArticleDOI
TL;DR: In this paper, the authors reviewed the progress of the quench and melt growth (QMG) process and found that the critical current density of the QMG crystal that has no high angle grain boundaries has been exceeding 104A/cm2 at 77K and 1T.
Abstract: Recent progress of Quench and Melt Growth (QMG) process is reviewed. QMG process is one of the melt processes for Y based superconductor. A pseudo single crystal (QMG crystal) which includes finely dispersed 211 phase in the matrix of 123 phase was produced by the original QMG process. The critical current density of the QMG crystal that has no high angle grain boundaries has been exceeding 104A/cm2 at 77K and 1T. The original QMG process was improved to make larger QMG crystals of which crystallographic orientation was controlled. Modified QMG process can be characterized by Pt addition in precursor preparation and seeding technique in growth process. Crystallinity of the QMG crystal made by this modified process was investigated. It appeared that there were two kinds of domain structures that depend on the growth direction. QMG crystals enlarged by this modified process are applicable to various fields. For instance, QMG crystal can operate as a bulk magnet. The maximum value of flux density was 4.5T at 40K on the surface. It was found that QMG crystal is thermally stable above 40K at least.

112 citations


Journal ArticleDOI
TL;DR: In this paper, the phase assemblages were examined by optical, electron diffraction and X-ray diffraction techniques, and their stoichiometries verified by electron microprobe.
Abstract: High-pressure methods have been used to synthesize multiphase compositions in the Hg12{ n −1} n homologous series. The phase assemblages were examined by optical, electron diffraction and X-ray diffraction techniques, and their stoichiometries verified by electron microprobe. Transport and magnetic susceptibility measurements were combined with the results of the phase analysis to establish superconducting transition temperatures for both as-prepared and O 2 - or Ar-annealed materials. It was found that the transition temperature peaks at T c = 134 K for n = 3 and then decreases abruptly for n >4, reaching T c K for n ⪖7.

110 citations


Journal ArticleDOI
TL;DR: In this article, superconductivity was observed at 117 K in the BaCaCuO samples synthesized at 950°C under 5.0 GPa for 1 h. The samples were multiphase.
Abstract: Superconductivity was observed at 117 K in the BaCaCuO samples synthesized at 950°C under 5.0 GPa for 1 h. The samples were multiphase. Electrical resistivity and DC magnetic susceptibility measurements revealed bulk superconductivity at Tconset = 117 K, Tczero = 107.6 K and Tcmag = 116.0 K. Crystals having primitive tetragonal unit cells with the lattice parameters of a=3.88 A , and c=15.0 A , 18.3 A and 21.6 A found by electron diffraction. The dominant peaks of the X-ray powder diffraction pattern were tentatively assigned to the hypothetical Cu-1234 ( a=3.85 A and c=18.30 A ) and Cu-1223 ( a=3.88 A and c=14.94 A ). structures. It is likely that these are new high-Tc superconducting phases.

110 citations


Journal ArticleDOI
TL;DR: In this paper, the electronic specific heat was studied at T ⩽ 10 K both on La 2− x Sr x CuO 4 samples and on La 1− y Zn y O 4 ( y y c samples, and its coefficient, γ, increases markedly with Zn content.
Abstract: The electronic specific heat C el was studied at T ⩽ 10 K both on La 2− x Sr x CuO 4 samples and on La 2− x Sr x Cu 1− y O 4 samples below and above y c at which the superconductivity disappears. In superconducting La 2− y Sr x CuO 4 samples, the C el contains only a T 2 component at T « T c in a x range between 0.14 and 0.22, suggesting a gapless superconductor with line nodes. A T linear component of C el appears in superconducting La 2− x Sr x Cu 1− y Zn y O 4 ( y y c samples, and its coefficient, γ, increases markedly with Zn content. In samples with various Zn contents, the relation between T c / T c ( y = 0) and γ/γ( y ≥ y c ) can be understood in terms of strong impurity scattering in a d wave superconductor.

108 citations


Journal ArticleDOI
TL;DR: The magnetization of single-crystal HoNi 2 B 2 C has been measured as a function of applied field (H ) and temperature in order to probe the interplay between superconductivity and magnetism in this complex layered system as discussed by the authors.
Abstract: The magnetization of single-crystal HoNi 2 B 2 C has been measured as a function of applied field ( H ) and temperature in order to probe the interplay between superconductivity and magnetism in this complex layered system. The normal-state magnetic susceptibility of HoNi 2 B 2 C is highly anisotropic with a Curie-Weiss-like temperature dependence for H applied perpendicular to the c -axis and with a much weaker temperature dependence for H applied parallel to the c -axis, indicating that the Ho +3 magnetic moments lie predominately in the tetragonal a − b plane below 20 K. High-field magnetization (2000 Oe), low-field magnetization (20 Oe) and zero-field specific heat all give an antiferromagnetic ordering temperature of T N =5.0 K. Remarkably, in 20 Oe applied field both superconductivity ( T c =8.0 K) and antiferromagnetism ( T N =5.0 K) clearly make themselves manifest in the magnetization data. From these magnetization data a phase diagram in the H − T plane was constructed for both directions of applied field. This phase diagram shows a non-monotonic temperature dependence of H c2 with a deep minimum at T N =5 K. The high-field magnetization data for H applied perpendicular to the c -axis also reveal a cascade of three phase transitions for T H H versus T phase diagram for HoNi 2 B 2 C at low temperatures.

Journal ArticleDOI
TL;DR: In this paper, the crystal structures of superconducting HgBa2CuO4+δ and Hg Ba2CaCu2O6+ Δ have been investigated with a pressure up to 0.6 GPa by neutron powder diffraction.
Abstract: The crystal structures of superconducting HgBa2CuO4+δ and HgBa2CaCu2O6+δ have been investigated with a pressure up to 0.6 GPa and HgBa2Ca2Cu3O8+δ to 9.2 GPa by neutron powder diffraction. The compressibility along the c-axis is nearly the same for the three compounds and up to two times larger than the compressibility along the a-axis. The one-layer compound, HgBa2CuO4+δ, shows the largest a-axis compressibility, while HgBa2Ca2Cu3O4+δ shows the smallest compressibility. The bond compressibilities of HgBa2CuO4+δ and HgBa2CaCu2O6+δ are significantly different from HgBa2Ca2Cu3O8+δ bond compressibilities. In the one- and two-layer compounds the largest bond compressibility was the Cu-O2 (apical) bond distance, while for the three-layer compound it was the Hg-O2 bond distance.

Journal ArticleDOI
TL;DR: In this paper, the authors measured at 4.2 K and partly also at 77 K the hysteresis and coupling losses of three different superconducting c-axis oriented Bi-2223/Ag multifilament tapes.
Abstract: We have measured at 4.2 K and partly also at 77 K the hysteresis and coupling losses of three different superconducting c -axis oriented Bi-2223/Ag multifilament tapes. The inter- and intragranular contributions to the hysteresis losses were analyzed and their dependence on d B /d t was investigated. The penetration field of a very flat filament was calculated. We could separate the measured coupling losses from the also d B /d t dependent hysteresis losses. Further we calculated the geometry dependence of the coupling losses of a very flat multifilament tape. The measured coupling losses can be described by a general scaling law with a generalized frequency ωτ. A skin-effect-like behavior has been found for ωτ>1 and AC loss saturation occurs for large AC field amplitudes.

Journal ArticleDOI
TL;DR: In this paper, various transport quantities have been measured for La1.92Sr0.08Cu1-xMxO4 and Nd2-yCeyCuO4 over a wide temperature region to study the crossover-like change of the electronic state previously found in high-Tc oxides.
Abstract: Various transport quantities have been measured for La1.92Sr0.08Cu1-xMxO4 (M=Ni and Zn) and Nd2-yCeyCuO4 over a wide temperature region to study the crossover-like change of the electronic state previously found in high-Tc oxides. The change shows a formation process of the anomalous low carrier concentration state near the Mott metal-insulator phase boundary. The Zn doping produces only a weak change of the T-dependence of the transport properties, except for electron localization effects at very low temperatures, while the Ni doping introduces a rather significant change of their T-dependence. Results of the measurements on Nd2-yCeyCuO4 have revealed that a rough relation of the electron-hole symmetry with respect to the “half-filled” Mott-insulating phase holds in the transport behaviors.

Journal ArticleDOI
TL;DR: In this paper, a rotating disk holder was used to separate oxidation and deposition, enabling a reactive high oxygen pressure zone in a surrounding high vacuum background, which achieved zero resistance temperatures around 86 K and critical current densities of 2 × 10 6 A/cm 2.
Abstract: The fabrication of homogeneous high quality YBa 2 Cu 3 O 7 films on large area substrates is the first essential step on the way to commercial high temperature superconductor devices. We have succeeded in growing YBa 2 Cu 3 O 7 films with excellent electronic properties and outstanding homogeneity in thickness and composition up to 4″ diameter. This was possible by employing reactive co-evaporation in combination with a special substrate heater design: a rotating disk holder separates oxidation and deposition enabling a reactive high oxygen pressure zone in a surrounding high vacuum background. Epitaxial YBa 2 Cu 3 O 7 films were grown either directly on MgO (30 × 30 mm 2 ) and LaAlO 3 (3″) or with intermediate buffer layers on sapphire (4″), silicon (4″) and GaAs (3″). For GaAs substrates we developed a chemical encapsulation to avoid contamination by volatile arsenic. On all of these substrates we obtained zero resistance temperatures around 86 K and critical current densities j c (77 K) ⩾ 2 × 10 6 A/cm 2 .

Journal ArticleDOI
TL;DR: In this article, a reflection high-energy electron diffraction investigation of the surface structures of the (001) and (110) crystal faces of SrTiO 3 given various surface treatments was performed.
Abstract: This paper reports a reflection high-energy electron diffraction investigation of the surface structures of the (001) and (110) crystal faces of SrTiO 3 given various surface treatments. The as-polished surface has a (1×1) unreconstructed structure on both the (001) and (110) crystal faces. The vacuum annealed surface has a two-domain (2×1) superstructure on the (001) face and a (4×4) structure on the (110) face. The oxygen annealed surface has two superstructures on the (001) face: a two-domain (√13×√13-R33.7°) structure and a c(6×2) structure, and a (4×2) or a (4×4) structure on the (110) face. Of these surfaces, the oxygen annealed surface has the highest quality.

Journal ArticleDOI
TL;DR: In this article, the authors studied the growth of disks of single-grain YBa 2 Cu 3 O 7 (3BCO) with diameter to 5 cm using a top-seeded melt-texture technique.
Abstract: We have studied the growth of disks of single-grain YBa 2 Cu 3 O 7 (3BCO) with diameter to 5 cm using a top-seeded melt-texture technique. The quality of these YBCO disks was found to depend critically on the quality and size of the seed used, the temperature schedule of the growth adopted, the amount of Y 2 BaCuO 5 and Pt added, and the size of the final disks processed. Samples of a diameter of 2.5 cm so-prepared have been observed to shield a magnetic field greater than 20 T at 4.2 K and to trap a field of 1.3 T at 77 K. However, no significant increase in the trapped field has been achieved by increasing the diameter of the disk to beyond ∼ 3 cm.

Journal ArticleDOI
TL;DR: In this paper, the authors measured AC losses in (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x silver-sheathed tape as a function of frequency and amplitude of a sinusoidall y varying transport current.
Abstract: AC losses in (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x silver-sheathed tape were measured as a function of frequency and amplitude of a sinusoidall y varying transport current. Experimental results show that energy losses are purely hysteretic in nature in the range of applied frequencies for transport currents below the critical value I c . The lossy component of the measured voltage is very dependent on the position of the potential taps placed onto the surface of the tape. This can be explained using the theory of Norris [J. Phys. D 3 (1970) 489] for ellipsoidal conductors. It means that neither voltage gives an accurate measurement of the loss. The error is least for the side contacts but still varies from 40% at I c to a factor of four at 0.1 I c .

Journal ArticleDOI
TL;DR: In this article, a generalized inversion scheme (GIS) was used to determine the current dependent activation energy U ( j, T, B e ) for thermally activated flux creep.
Abstract: By means of high-sensitivity capacitance torque magnetometers we have measured the superconducting current j s and the dynamic magnetic-moment relaxation of YBa 2 Cu 3 O 7 and YBa 2 Cu 4 O 8 films of typically 100 nm thickness at temperatures between 2 K and T c in magnetic fields up to 6 T. For the measurements of the dynamic relaxation rate Q ≡d ln j s /d ln (d B e /d t ) magnetic-field sweep rates were varied between 0.5 and 40 mT/s. At low fields (typically 0.5 T) the dynamical relaxation rate exhibits a plateau at Q ≈0.06 in YBa 2 Cu 3 O 7 and 0.04 in YBa 2 Cu 4 O 8 . At high fields ( B e = μ 0 H e ≈ 6 T) the plateaus have completely disappeared and Q increases almost linearly with increasing temperature. At all fields a sharp increase up to Q≊1 is observed when the irreversibility line is approached. By means of the generalized inversion scheme (GIS), the j s ( T , B e ) and Q ( T , B e ) data are used to determined the current dependent activation energy U ( j , T , B e ) for thermally activted flux creep. Although the GIS does not make any a priori assumptions about the explicit functional dependences on T and j , the U ( j , T =0, B e ) function derived from the experimental data by means of the GIS can remarkably well be described with the collective-creep interpolation formula U ( j )=( U c / μ )[( j c / j ) μ −1] with μ ≈0.6 for currents j >0.15 j c ( T =0, B e ) where J c ( T =0, B e ) is the critical current at T =0, and wi U c depending on B e . At lower current densities U ( j , T =0, B e ) does not diverge as j −0.6 but shifts gradually to a weaker ln( j c / j ) dependence. At low temperatures the current and relaxation data cannot be explained in terms of a thermally activated flux-motion model. Quantum creep has an influence up to ∼13 K.

Journal ArticleDOI
TL;DR: In this paper, a new family of superconducting oxides has been obtained at high pressures in multiphasic samples within the Ba-Ca-Cu-O system, which corresponds to the substitution by copper of the Tl or Hg atoms in the so-called single-layer superconductors.
Abstract: A new family of superconducting oxides has been obtained at high pressures in multiphasic samples within the Ba-Ca-Cu-O system. It corresponds to the substitution by copper of the Tl or Hg atoms in the so-called single-layer superconductors. At least four members have been detected, corresponding to n=3−6, in CuBa2Can-1CunO2n+2+x. Tc values of up to 120 K have been measured.

Journal ArticleDOI
Jun-ichi Shimoyama1, S. Hahakura1, K. Kitazawa1, K. Yamafuji1, K. Kishio1 
TL;DR: In this paper, a new mercury-based cuprate superconductor, (Hg 1- x Cr x )Sr 2 CuO y (0⩽ x ⩽0.8), having T c of 58 K ( x = 0.3 and 0.4), was synthesized.
Abstract: A new mercury-based cuprate superconductor, (Hg 1- x Cr x )Sr 2 CuO y (0⩽ x ⩽0.8), having T c of 58 K ( x =0.3 has been synthesized. While a chromium-free sample ( x =0) was multi-phased and the superconducting volume fraction was less than 1%, samples for x =0.3 and 0.4 were of nearly single phase and their superconducting shielding volume fraction reached 100% of perfect diamagnetism. X-ray diffraction analysis revealed that its crystal symmetry was tetragonal with lattice parameters a 0 =3.840−3.854 A and c 0 =8.639−8.705 A which slightly varied with x . The observed c 0 value, corresponding to the interlayer distance of superconducting CuO 2 planes, was found to be the shortest among all the known mercury-based “12( n -1) n ” superconductors. Compared to the barium-containing compounds, the present barium-free materials have been found to be insensitive to the ambient air atmosphere during the synthesis and also to be quite stable after storage in air.

Journal ArticleDOI
TL;DR: In this paper, the specific heat of single crystals of superconducting YNi 2 B 2 C and TmNi 2B 2 C was measured in zero and 5 T magnetic fields and it was shown that superconductivity coexists with antiferromagnetic order below 1.5 K.
Abstract: We have measured the specific heat of single crystals of superconducting YNi 2 B 2 C and TmNi 2 B 2 C. Analysis of YNi 2 B 2 C data in zero and 5 T magnetic fields indicates modestly strong electron-phonon coupling and a superconducting energy gap Δ ( O ) = 29 K . Superconductivity coexists with antiferromagnetic order below 1.5 K in TmNi 2 B 2 C. An interpretation of very low temperature specific heat indicates substantial magnetic anisotropy, with ferromagnetic planes weakly coupled antiferromagnetically to each other. Two-dimensional ferromagnetic spin waves produce a large linear-in-temperature contribution to the specific heat. We find no compelling evidence for an unconventional pairing mechanism in these systems.

Journal ArticleDOI
TL;DR: In this paper, the phase conversion reactions that occur when a fine, micron-size powder of overall composition Bi 1.8 Pb 0.4 Sr 2.0 Ca 2.2 Cu 3.0 O y (2223) is heat treated in a silver tube have been studied.
Abstract: The phase conversion reactions that occur when a fine, micron-size powder of overall composition Bi 1.8 Pb 0.4 Sr 2.0 Ca 2.2 Cu 3.0 O y (2223) is heat treated in a silver tube have been studied. We find that non-superconducting phases, particularly alkaline earth cuprates, grow more readily than the desired 2223 phase, because the activation energy for their growth (∼300 kJ/mole in air) is about a factor of five less than for the Bi 2 Sr 2 CaCu 2 O x (2212) to 2223 conversion (∼1.5 MJ/mole). A reduced oxygen partial pressure (7.5% O 2 /Ar) is found to favor the 2223 phase and CuO. The reaction to 2223 occurs preferentially at the Ag interface; continued growth occurs preferentially from the initial 2223 interface plates. The study shows that an aligned 2223 microstructure requires a smooth Ag-oxide interface and very close attention to thermal processing.

Journal ArticleDOI
TL;DR: In this paper, structural disorder in expanded "c" parameter ReBa 2 Cu 3 O 7−x (Re=rare earth) thin films grown by radio-frequency magnetron sputtering has been quantified by undertaking Rietveld refinements of X-ray data.
Abstract: Structural disorder in expanded “ c ” parameter ReBa 2 Cu 3 O 7− x (Re=rare earth) thin films grown by radio-frequency magnetron sputtering has been quantified by undertaking Rietveld refinements of X-ray data In order to understand why the disorder occurs, we have focused on determining the factors which influence the amount of disorder, such as growth conditions, rare-earth ion size, and overall composition To understand more about the disordering mechanism, some additional comparative growth experiments with bulk samples have been carried out For YBa 2 Cu 3 O 7− x films, low oxygen growth pressures and temperatures give rise to disordered films with enlarged “ c ” parameters and degraded superconducting properties There is a correlation between (00 l ) to (006) peak intensity ratios and the “ c ” parameter with the amount of disorder in the films In the most disordered YBa 2 Cu 3 O 7− x films, as much as 14% Y and Ba cation exchange, and a large fraction of apical oxygen vacancies have been found The effect of smaller rare-earth ions is to decrease the amount of cation disorder However, as was shown for YbBa 2 Cu 3 O 7− x films, there are structural vacancies in the rare-earth site and T c is reduced A model for the disordering mechanism is proposed based on the kinetic and thermodynamic driving forces for the growth of YBa 2 Cu 3 O 7− x from the disordered, precursor phase, Re 033 Ba 066 CuO y

Journal ArticleDOI
TL;DR: In this article, the crystal structure of the superconductor HgBa2Ca4Cu5O12+δ has been analyzed by neutron powder diffraction at room temperature and at 10 K.
Abstract: The crystal structure of the superconductor HgBa2Ca4Cu5O12+δ has been analyzed by neutron powder diffraction at room temperature and at 10 K. The compound crystallizes with the symmetry of space group P4/mmm and lattice parameters a=3.85145(8) and c=22.1066(9) A. The unit cell contains three (CuO2) layers with copper in four-fold planar coordination and two (CuO2) layers with copper in five-fold pyramidal coordination. The Cu-O apical distance for this last copper is 2.85(1) A, i.e. of the same order as those found in other members of the homologous series. The compound has an oxygen composition corresponding to δ=0.32(3). This extra oxygen is located on the layer of the mercury atoms at the sites ( 1 2 0). The superconducting transition temperature for the “as-prepared” sample with this composition is 101 K. In the structure of Hg-1245 there is no evidence for substitution of mercury by copper and for the presence of oxygen at the sites ( 1 2 , 0, z). The copper and oxygen atoms are coplanar for the (CuO2) layer at z= 1 2 , and almost coplanar for the other layers. No phase transition has been observed down to 10 K. The AC susceptibility versus T curve of the sample used in the experiment shows the presence of a superconducting minority phase with a Tc≈114 K. This phase could be Hg-1256, whose presence in the sample has been established by neutron and electron diffraction.

Journal ArticleDOI
TL;DR: In this article, the flux pinning in superconductors is attributed to a finely distributed RE(Ba 1−x, RE x ) 2 Cu 3 O y phase in a good superconductive matrix.
Abstract: REBa 2 Cu 3 O y (RE: Nd, Sm) superconductors exhibit relatively high J c values even at 77K when melt-processed in a reduced oxygen atmosphere. The J c values of these superconductors are higher than those of melt processed YBa 2 Cu 3 O y with fine Y 2 BaCuO 5 inclusions in a high field region. The irreversibility field with H ∥ c axis was dramatically increased to 8T at 77K. We believe that flux pinning in these superconductors is ascribable to a finely distributed RE(Ba 1−x , RE x ) 2 Cu 3 O y phase in a good superconductive matrix.

Journal ArticleDOI
TL;DR: In this article, single-phase polycrystalline samples of HgBa2Ca2Cu3O8+δ were successfully synthesized under relatively simple and unprecedented synthesis conditions, i.e. at a low temperature (<700°C) and for a long period of time (5-168h).
Abstract: Nearly single-phase polycrystalline samples of HgBa2Ca2Cu3O8+δ were successfully synthesized under relatively simple and unprecedented synthesis conditions, i.e. at a “low” temperature (<700°C) and for a “long” period of time (5–168h), using mono-oxide powders of Hg, Ba, Ca, and Cu as starting materials. A pressed powder mixture was heated at 665°C for ≈75 h in an evacuated fused quartz tube, and then post-annealed in flowing O2 gas to exhibit a Meissner signal below 135.4 K. Zero resistance of this sample was observed at 133.9 K.

Journal ArticleDOI
TL;DR: The field dependence of the lower temperature transition (T' (H)) exhibited a marked kink at 80 K and was always very sensitive to the applied field above 80 K, indicating an electromagnetically granular behavior.
Abstract: Field-dependent AC susceptibility measurements are correlated with the transport critical current density (Jct) and microstructure for some 20 Ag-clad (Bi, Pb)2Sr2Ca2Cu3Ox (2223) tapes having Jct (0 T, 77 K) values ranging from 0–20 000 A/cm2. Fields of 0.1-1 mT induced a double transition in the real component of the susceptibility of all samples. The field dependence of the lower temperature transition (T' (H)) exhibited a marked kink at 80 K and was always very sensitive to the applied field above 80 K, indicating an electromagnetically granular behavior. High-resolution transmission electron microscopy often revealed (Bi, Pb)2Sr2CaCu2Ox (2212 phase) intergrowths at the numerous (001) twist grain boundaries. Samples with a high Jct (0 T, 77 K) had only occasional half layers of 2212 at the (001) boundaries and required correspondingly higher fields to induce a kink in the T'(H) characteristic. The universal appearance of a kink at 80 K in all samples and the strong correlation of the field required to produce the kink to the zero-field, 77 K transport critical current density strongly suggests that c-axis transport across (001) twist boundaries within grain colonies controls the critical current density, even in relatively high Jc BSCCO-2223 tapes.

Journal ArticleDOI
TL;DR: In this paper, a linear relation between the Vickers microhardness (Hv) of reacted tapes and the critical current density (Jc) of Ag sheathed (Bi, Pb)2Sr2Ca2Cu3Ox tapes was found.
Abstract: Because there are many parameters influencing the critical current density (Jc) of Ag sheathed (Bi, Pb)2Sr2Ca2Cu3Ox tapes, good and easily measured correlation variables to Jc are highly desirable. Yamada et al. have recently shown that very high Jc values can be obtained when the BSCCO core is rolled to a high Vickers microhardness (Hv) prior to heat treatment. The present experiment extends the correlation by showing that there is a linear relation between the Hv of reacted tapes and Jc. Measurement of the Knoop microhardness (HK) shows that the hardness is highly anisotropic. HK measured with the long indentor axis parallel to the a-b planes shows no dependence on reaction time or Jc, while HK measured parallel to the c-axis is strongly correlated. A high microhardness appears to be directly correlated to a high-density BSCCO core with a small crack density. Such a core has good connectivity which directly translates to a high Jc value.

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TL;DR: In this article, the defect cascades produced by fast neutron irradiation of YBa 2 Cu 3 O 7−δ single crystals were studied by transmission electron microscopy and the defect density scales linearly with fluence and amounts to 1x10 22 m −3 at a neutron fluence of 2x10 21 m −2.
Abstract: Defect cascades produced by fast neutron irradiation of YBa 2 Cu 3 O 7−δ single crystals were studied by transmission electron microscopy. The visible defects were found to have sizes between 1 and 5 nm. Defect densities were obtained as a function of neutron fluence between 2x10 21 and 8x10 21 m −2 ( E >0.1 MeV) and compared to damage calculations. The measured defect density scales linearly with fluence and amounts to 1x10 22 m −3 at a neutron fluence of 2x10 21 m −2 . The defect stability was studied at room temperature and through annealing to 400°C.