Showing papers in "Physica C-superconductivity and Its Applications in 1995"
TL;DR: In this article, the authors synthesized layered cuprates where the intermediate layer between the CuO 2 blocks consists of vertex sharing RuO 6 octahedra, where the analogy to the isostructural Nb based compounds, as well as simple valence counting, suggest that Ru is pentavalent in these compounds.
Abstract: We synthesized layered cuprates where the intermediate layer between the CuO 2 blocks consists of vertex sharing RuO 6 octahedra. The (idealized) stoichiometries of these compounds are RuSr 2 LnCu 2 O 8 (Ru-1212) and RuSr 2 (Ln 1+ x Ce 1− x ) Cu 2 O 10 (Ru-1222), where Ln is one of the three neighboring lanthanides Sm, Eu and Gd. The analogy to the isostructural Nb based compounds, as well as simple valence counting, suggest that Ru is pentavalent in these compounds. Both 1212 and 1222 compounds are semiconducting when prepared in air and rapidly cooled to room temperature. Metallic behavior and superconductivity can be achieved for various compositions upon appropriate annealing.
382 citations
TL;DR: In this article, a mini-magnet fabricated from four melt-textured YBa2Cu3O7 - delta (MT-YBCO) discs activated at 49 K using an applied field of 14 T was observed to crack.
Abstract: Quasi-permanent magnets made of melt-textured YBa2Cu3O7 - delta (MT-YBCO) superconductor can now trap multi-tesla fields, B sub t. The interaction of the trapped field and the critical current causes an outward pressure, proportional to B sub t (2), which can crack the magnet. The authors have done an experiment to observe such cracking in a mini-magnet fabricated from four MT-YBCO discs activated at 49 K using an applied field of 14 T. They have compared the results to existing theories which describe magnetic pressure in a trapped-field magnet (TFM) previously activated. They find that a modification is needed to describe magnetic pressure during the process of activation. They present the experimental results and the expanded theory, based on the simple Bean model. Theory and experiment show good agreement. The authors find that cracking is more likely during activation, and conclude that 10 T is achievable in TFM`s composed of present materials. Cracking is most probable at the center of a TFM, with the cracks running radially outward.
308 citations
TL;DR: In this article, the authors reported the solution growth of single crystals of YBa2Cu3O7−δ and PrBa2cu3O 7−ε in a new type of home-made crucible, namely made of BaZrO3, which has the advantage that it does not react with the occurring melts which are commonly used as a flux.
Abstract: We report on the solution growth of single crystals of YBa2Cu3O7−δ and PrBa2Cu3O7−δ in a new type of home-made crucible, namely made of BaZrO3, which has the advantage that it does not react with the occurring melts which are commonly used as a flux. These new crucibles have several advantages: not only the crystals obtained after the growth experiment contain no impurities from the crucible material, but also an important problem is solved, e.g. the corrosion of the crucible does no longer change the composition of the melt during the growth experiment, thus avoiding uncontrollable growth conditions. Hence, due to this novel inert crucible material the use of more sophisticated crystal growth techniques such as top seeded solution growth (TSSG) are now possible.
178 citations
TL;DR: In this paper, the magneto-optical signal was converted into two-dimensional (2D) field and current distributions and the current patterns were very non-uniform and sensitive to weak magnetic fields of ∼ 400-800 Oe.
Abstract: Magneto-optical imaging of the flux penetration into a Ag sheathed Bi1.8Pb0.4Sr2Ca2Cu3O8+x tape has been used to extract the current flow paths in a magnetic field applied perpendicular to the c-axis. Using the large aspect ratio of the plate-like grain structure and the slab geometry of the sample to simplify the current-flow geometry, we converted the magneto-optical signal into two-dimensional (2D) field and current distributions. We found that the current patterns were very non-uniform and sensitive to weak magnetic fields of ∼ 400–800 Oe, even at 10 K. Current streamlines show that the effective current-carrying cross-section of the tape strongly depends on the field. Magnetization currents flow preferentially near the silver sheath, while the tape center supports mainly percolative and granular current patterns consisting of an array of macroscopic current loops whose long dimensions are of the order of the tape thickness. By comparing contour maps of the local Jc values with the microstructural images, we found that the high-Jc regions correlate with colonies of well-aligned long grains which are preferentially located near the silver interface, while the less aligned structure of smaller grains in the central part of the tape is associated with the granular behavior and much lower current-carrying capability. The wide distribution of the local Jc(x, y) revealed by magneto-optical imaging indicates that the performance of BSCCO-2223 tapes can be significantly improved if a larger fraction of well aligned grains can be produced more uniformly throughout the tape cross-section.
123 citations
TL;DR: In this article, the lateral distribution of the transport critical current density of Ag sheathed (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x tapes was measured by using a strip-cutting technique.
Abstract: By using a strip-cutting technique, we directly measured the lateral distribution of the transport critical current density of Ag sheathed (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x tapes. For various long tapes prepared by the rolling technique with a critical current density of 23 000 A/cm 2 at 77 K and 0 T, the local critical current density of longitudinal 0.2 mm wide strips was reproducibly found to vary from 18 000 A/cm 2 at the center to 46 000 A/cm 2 at the sides of the filament. In all cases, a symmetrical behavior of j c at both sides of the central axis was observed. The value of the transport j c of 46 000 A/cm 2 at 77 K and 0 T in the external strips of Bi(2223) tapes produced by rolling processes comes quite close to the highest reproducible values reported so far on short pressed tapes. The field dependence of the critical current densities of the individual strips does not depend on their position inside the tape. Thus the observed difference in j c between central and external strips is not primarily due to a variation of the degree of texturing. A higher degree of compression and a lower amount of secondary phases were found for the external strips with the highest critical current density.
115 citations
TL;DR: In this paper, the reflectance of high-quality, unpolished single crystals of YBa2Cu3O6+x, for the doping range x = 0.50 → 0.95, has been measured with radiation polarized along the c-axis from ≈ 50 cm −1 to 5000 cm−1 between 10 K and 300 K.
Abstract: The reflectance of high-quality, unpolished single crystals of YBa2Cu3O6+x, for the doping range x = 0.50 → 0.95, has been measured with radiation polarized along the c-axis from ≈ 50 cm−1 to 5000 cm−1 between 10 K and 300 K. In highly doped (x = 0.95) material, the normal-state conductivity shows a metallic response. For intermediate dopings (x = 0.85 → 0.90) the conductivity is no longer metallic, increasing with temperature, and for low dopings (x = 0.50 → 0.80) this behavior is clearly seen to be caused by a broad pseudogap at ≈ 290 cm−1, which develops well above Tc. In the superconducting state, in the optimally doped material, a gap-like depression develops below Tc but there is residual conductivity to very low frequency indicating either an anisotropic gap, or a gap with nodes. In the underdoped materials the superconductivity-induced changes to the conductivity are harder to see, but it appears that the weight of the condensate originates at frequencies much higher than the conventional weak-coupling BCS value of 3.5kBTc.
110 citations
TL;DR: Based on phase diagram considerations, it is shown that the process window for stable growth conditions can be considerably increased as discussed by the authors, and a lower process temperature (T a =960°C to 985°C) is possible.
Abstract: Y 2 O 3 admixed to 123 significantly influences the mechanism of melt texturing of YBCO. Based on phase diagram considerations, it is shown that the process window for stable growth conditions can be considerably increased. Furthermore, a lower process temperature ( T a =960°C to 985°C) becomes possible. Samples have been grown by both directional solidification and zone melting techniques. The superconducting properties are determined by local and integral levitation force measurements.
90 citations
TL;DR: In this paper, the authors investigated the formation mechanism of the (Bi,Pb)2Sr2Ca2Cu3O10 phase from the precursor with Bi-2212 phase as the main phase.
Abstract: X-ray diffraction method was used to characterise the phase composition and investigate the formation mechanism of the (Bi,Pb)2Sr2Ca2Cu3O10 phase from the precursor with (Bi,Pb)2Sr2CaCu2O8 as the main phase. The reaction is found to be a two-dimensional nucleation (random)-growth type, −[ ln (1 − F)] 1 2 = kt , where F is the conversional fraction of (Bi,Pb)2Sr2CaCu2O8 and t is the sintering time. The size of the critical current of the tape is quantitatively related to the conversional fraction of (Bi,Pb)2Sr2CaCu2O8 to (Bi,Pb)2Sr2Ca2Cu3O10 phase. At low fraction regime of (Bi,Pb)2Sr2CaCu2O8, the critical current of a tape shows no clear dependence on the remaining Bi-2212 phase, other factors such as grain alignment, colony size, contact between colonies, and fine nonsuperconducting particles become important in controlling the Jc. Also, in this regime, the predominant weak links to be the colony boundaries rather than the Bi-2212 phase.
85 citations
TL;DR: In this article, the DC Josephson effect is theoretically studied in a planar junction in which a two-dimensional electron gas (2DEG) infinite in lateral directions is in contact with two superconducting electrodes placed on top of the 2DEG.
Abstract: The DC Josephson effect is theoretically studied in a planar junction in which a two-dimensional electron gas (2DEG) infinite in lateral directions is in contact with two superconducting electrodes placed on top of the 2DEG. An energy gap in the excitation spectrum is created in the 2DEG due to the proximity effect. It is shown that under certain conditions, the region of the 2DEG underneath the superconductors is analogous to a superconducting region with an order parameter ϵ g exp (i∅), where ϵ g ( ϵ g Δ ) depends on the interface transmittance and the Fermi-velocity mismatch between the superconductors and the 2DEG.
85 citations
TL;DR: In this paper, the transport properties of YBa 2 Cu 3 O y with various y and YBa 3 − x M x O y (M = Fe, Co, Mn) with various χ have been analyzed on the basis of the phenomenological theory of electron transport.
Abstract: The transport properties of YBa 2 Cu 3 O y with various y and YBa 2 Cu 3− x M x O y (M = Fe, Co, Mn) with various χ have been studied. The results obtained have been analyzed on the basis of the phenomenological theory of electron transport in the case of a narrow conductive band. The narrow-band model permits one to account for the specific features of the transport coefficients of HTSC and to determine the band-structure parameters from comparison of the experimental and calculated temperature dependences of the thermoelectric power. The dependences of band-structure parameters and superconductive properties on y and χ have been analyzed together. The differences in the influence of different 3d metals on the band structure were revealed. Taking into account the crystallochemical-studies data we drew the conclusion that the oxygen sublattice disordering has a determinant effect on band-structure parameters and superconductive properties of YBa 2 Cu 3 O y .
82 citations
TL;DR: Substitution of Tl for Hg was performed in the Hg based 1223 phase HgBa 2 Ca 2 Cu 3 O 8 + δ (T c = 135 K), resulting in an increase of the superconducting transition temperature to 138 K.
Abstract: Substitution of Tl for Hg was performed in the Hg based 1223 phase HgBa 2 Ca 2 Cu 3 O 8 + δ ( T c = 135 K), resulting in an increase of the superconducting transition temperature to 138 K for samples with a nominal composition of Hg 0.8 Tl 0.2 Ba 2 Ca 2 Cu 3 O 8 + δ . The crystal structure of this solid solution has been investigated by neutron powder diffraction techniques at room temperature and at 10 K. The compound has the same crystal as Hg-1223 with the space group symmetry P4/mmm and lattice parameters a = 3.8489(1), c = 15.816(1) A . Rietveld analysis results indicate that Hg is partially replaced by Tl, and the oxygen content, δ, is 0.33. The lattice-parameter changes resulting from the Tl substitution are too small to account for the T c change by mimicking the effect of pressure. No phase transition occurs down to 10 K.
TL;DR: In this article, the Holstein model in one dimension is examined using a variety of techniques, i.e., exact diagonalization, perturbation theory, and the adiabatic approximation.
Abstract: The Holstein model in one dimension is examined using a variety of techniques, i.e. exact diagonalization, perturbation theory, and the adiabatic approximation. In agreement with other authors we find that the model is polaronic for all coupling strengths. In the case of two electrons we examine the regimes under which binding occurs. We find that binding decreases as the phonon frequency increases, for a given coupling strength. We provide a numerical verification for the effect of retardation, which gives rise to the well-known pseudopotential effect.
TL;DR: In this article, the authors show that the c-axis and the interplanar distances related to the position of Ba and O(2, 3) exhibit a sigmoidal dependence on rR.
Abstract: RBa2Cu3O7 and [R0.5Pr0.5]Ba2Cu3O7 compounds with RY and lanthanides have been prepared and studied at ambient temperature by means of XRD and infrared-reflectance spectroscopy. In Pr free compounds, increasing the Shannon radius of R, rR, leaves the puckering of the Cu(2)−O(2, 3) sheet unaffected. In the Pr containing compounds, the PrO coordination polyhedron is typical of R+3 ions in eight-fold coordination and argues against differentiating Pr in terms of valence or hybridization. Instead, a repositioning of Ba within the cuprate block is observed, leading to the flattening of the sheet and the anomalously high frequency of the Ba and Cu(2)−O(2, 3) B1u modes. In RPr compounds the c-axis and the interplanar distances related to the position of Ba and O(2, 3) are found to exhibit a sigmoidal dependence on rR. It is argued that this sigmoidal dependence is related to the two-mode behavior exhibited by several phonon modes of these compounds.
TL;DR: In this article, the authors have found that the Bi(2212) grains in unreacted Ag/Bi(2223) tapes at the end of the cold-rolling process show already a high degree of texture.
Abstract: We have found that the Bi(2212) grains in unreacted Ag/Bi(2223) tapes at the end of the cold-rolling process show already a high degree of texture. X-ray diffraction analysis and current transport measurements at 77 K in magnetic fields up to 0.5 T have been performed before and after reaction on tapes with thicknesses in the range from 75 to 286 μm (filament thickness between 30 and 115 μm). The critical current density j c increases for smaller tape thicknesses, a maximum being found for a tape thickness of about 90 μm, below which sausaging effects are observed. The mean misalignment angles of Bi(2223) grains in these tapes have been determined by angle-dependent transport critical curent data at 77 K. We have found that the mamin reason for the enhancem,ent of j c for thin tapes in due to the higher average texture of the Bi(2223) grains when compared to thick tapes. This is a direct consequence of the high degree of texture of Bi(2212) grains after the cold-rolling process, which has been observed here on 90 μm thick tapes with j c (77 K, 0 T) reaching between 25 and 30 kA/cm 2 .
TL;DR: In this paper, a minimum critical current density in a BSCCO/Ag polycrystalline sample of 1.1. × 5 A/cm 2 at 77 K in a self-field was reported.
Abstract: We report for the first time a minimum critical current density in a BSCCO/Ag polycrystalline sample of 1.1. × 5 A/cm 2 at 77 K in a self-field. For practical applications of (Bi, Pb)2Sr2Ca2Cu3Oy (BSCCO-2223) high-temperature superconducting material, the critical current density (Jc) is a limiting factor. A requirement for a Jc of 105 A/cm2 in long length wires and tapes, at 77 K and high magnetic field, has been established. The transport Jc were measured on samples that retained a 10 μm thickness of the superconducting core after they were removed from rolled tapes. The thin superconducting layer region next to the silver sheath was shown to be the actual current carrying region in the tape. These results confirm the presence of a high critical current density region adjacent to the silver-sheathed tapes, and provide a basis for further improvements in Jc. Regions of silver were also found to be present in the superconducting core.
TL;DR: In this paper, a new approach towards enhanced flux pinning in high-temperature superconductors is described, with a potential for scale-up to bulk application, and carbon nanotubes can be embedded in Bi 2 Sr 2 CaCu 2 O 8+ x superconductor, thus displacing the superconducting material from a cylindrical volume.
Abstract: This paper describes a new approach towards enhanced flux pinning in high-temperature superconductors, and with a potential for scale-up to bulk application. It is shown that carbon nanotubes can be embedded in Bi 2 Sr 2 CaCu 2 O 8+ x superconductor, thus displacing the superconducting material from a cylindrical volume suitable for strong pinning of vortex lines along its length. The best-case superconductor-nanotube composite thus prepared was found to surpass the pure ceramic superconducting matrix in terms of pinning properties. Although optimum conditions were yet not reached the intragrain critical current density was estimated to have been increased by about one order of magnitude.
TL;DR: In this article, a Ba 2 Cu 3 O 7−δ composite high-T c superconducting material has been fabricated in air using a melt-textured growth (MTG) technique.
Abstract: A (Nd/Y)Ba 2 Cu 3 O 7−δ composite high- T c superconducting material has been fabricated in air using a melt-textured growth (MTG) technique. The material has a very high critical current density, J c ∼ 8.77 × 10 4 A cm −2 at 77 K, B = 0 T, and J c ∼ 7.59 × 10 4 A cm −2 at 77 K, B = 0.5 T, with B || c (using the modified Bean model formula) which is attributed, in part, to the partial substitution of the Nd 3+ in the Ba 2+ site. The presence of Y211 and/or Nd422 precipitates were not detected within the 123 grains indicating that these phases are not important in order to obtain samples with a high J c in this composite material.
TL;DR: In this paper, the influence of hydrostatic pressure up to 1.1 GPa on critical parameters (Tc, Hc1, jc) of YBa2Cu4O8 (Y124) and GdBa2cu4O 8 (Gd124) ceramics has been investigated using a CuBe pressure bomb.
Abstract: The influence of hydrostatic pressure up to 1.1 GPa on critical parameters (Tc, Hc1, jc) of YBa2Cu4O8 (Y124) and GdBa2Cu4O8 (Gd124) ceramics has been investigated using a CuBe pressure bomb. A linear of Tc with the growth of pressure was found, with dTc/dp = 5.5 K/GPa and 4.5 K/GPa for Y124 and Gd124, respectively. The difference in Tc increase on pressure, dTc/dp, between the two compounds is assumed to have the following possible mechanisms: first, the replacement of yttrium by gadolinium constrains the change of interionic distances within and between the CuO2 planes and thus the change of charge transfer integral having an effect on the density of electronic states at CuO2 planes; secondly, this replacement modifies the charge transfer into the CuO2 planes from rare-earth sites. It was found that the critical current density increased a little under pressure; nevertheless a different behaviour on temperature has been observed for the two compounds. Insignificant changes of Hc1 under pressure were found at temperatures up to 40 K. However, at higher temperatures a little growth of Hc1 was observed.
TL;DR: In this article, experimental data of the upper critical field of the heavy-fermion superconductor URu2Si2 are analyzed, in the clean limit, taking account of both orbital and Pauli limits.
Abstract: Experimental data of the upper critical field of the heavy-fermion superconductor URu2Si2 are analysed, in the clean limit, taking account of both orbital and Pauli limits. A discussion on the possible existence of a non-uniform superconducting state expected in clean Pauli-limited superconductors (FFLO state) follows. We also present calculations determining the influence of an AF ground state on the existence of such a non-uniform superconducting state.
TL;DR: In this paper, a simple and efficient synthetic route to high-temperature superconducting oxyfluorides was proposed, where solid phase fluorination of Sr 2 CuO 3 with NH 4 F at relatively low temperatures was used to achieve a transition temperature of 46 K.
Abstract: We report here a simple and efficient synthetic route to high-temperature superconducting oxyfluorides. Solid phase fluorination of Sr 2 CuO 3 with NH 4 F at relatively low temperatures produces superconducting Sr 2 CuO 2 F 2+δ ; this simple preparation route eliminates the need for F 2 gas. Importantly, as-synthesised samples from the NH 4 F route appear to have the optimally doped value of the superconducting transition temperature, T c , of 46 K. Moreover, Ba doping for Sr results in an increase in T c , for both F 2 gas and NH 4 F routes, to a maximum value of 64 K for Sr 1.4 Ba 0.6 CuO 2 F 2+δ ; this represents the highest ever recorded value for a material with the confirmed La 2 CuO 4 structure.
TL;DR: In this article, the O 2 partial pressure dependence of oxygen content in Nd 1+ z Ba 2− z Cu 3 O y were determined over the entire range of z, and the T - z - p [O 2 ] dependence of high-temperature decomposition was also determined.
Abstract: The temperature ( T ) and O 2 partial pressure ( p [O 2 ]) dependence of oxygen content ( y ) in Nd 1+ z Ba 2− z Cu 3 O y were determined over the entire range of z . These data were represented by chemical thermodynamic models. The T - z - p [O 2 ] dependence of high-temperature decomposition was also determined, as were the effects of various T - t - p [O 2 ] treatments on the superconducting transition temperature ( T c ). These results were compared with those from the Y and La homologs.
TL;DR: In this paper, localized bosons in the form of tightly bound electron pairs of polaronic origin and itinerant valence electrons were proposed to induce a BCS-like superconductivity in the fermionic subsystem.
Abstract: On the basis of well established experimental results we propose a picture for high- T c superconductivity in which two types of charge carriers play a role: (1) localized bosons in the form of tightly bound electron pairs of polaronic origin and (2) fermions in the form of itinerant valence electrons. The level of the bosons is supposed to fall inside the band of fermions and due to a boson a-fermion pair-exchange coupling the intrinsically localized bosons can condense and simultaneously induce a BCS-like superconductivity in the fermionic subsystem. We study the doping dependence for this model for such quantities as the critical temperature, the BCS-like gap in the single-particle excitation spectrum of the fermions, the penetration depth and the plasma frequency and compare these results with experiment.
TL;DR: In this article, the microstructure of 45° grain boundaries in epitaxial YBaCuO together with the d x 2 − y 2 symmetry of the superconducting wave function accounts for the measured magnetic field modulation patterns of the critical currents.
Abstract: We show that the microstructure of 45° grain boundaries in epitaxial YBaCuO together with the d x 2 − y 2 symmetry of the superconducting wave function accounts for the measured magnetic field modulation patterns of the critical currents. Our results resolve the conflict between contradictory observations of other authors and unfold a consistent picture of d-wave-symmetry of the YBaCuO superconducting ground state. Evidence for currentless junctions with a phase difference of π is given.
TL;DR: In this article, a superconducting thin films of copper oxycarbonates have been synthesized for the first time using laser ablation, and the resistance measurements show a broad transition with T c onset ranging from 80 to 100 K and zero resistance below 60 K.
Abstract: Superconducting thin films of copper oxycarbonates have been synthesized for the first time using laser ablation. The resistance measurements show a broad transition with T c onset ranging from 80 to 100 K and zero resistance below 60 K. Magnetic measurements indicate diamagnetism below 58 K in the best film. The ED and HREM studies show that these new oxycarbonates are tetragonal with a ∼ a p and c ∼ ( m + 2) × a p , and that the films consist of an intergrowth of serveral members of a family with the general composition (CaCuO 2 ) m (Ba 2 CuO 2 CO 3 ) n . The main member that forms the matrix, Ba 2 Ca 3 Cu 4 CO 3 O 8 ( m = 3; n = 1), consists of double layers of CuO 4 groups sandwiched by pyramidal copper layers and interconnected with layers of CO 3 groups. In a general way, the different members of the series can be described as the intercalation of layers of carbonate groups in the infinite-layer structure CaCuO 2 . The origin of superconductivity and the relationships with bulk “BaCaCu” oxycarbonates that are prepared at high pressure are discussed.
TL;DR: In this article, phase-stability studies of Y1Ba2Cu3O7−x(+Ag) and BaCu2O2 (+Ag) over the temperature range 780°C-950°C and oxygen partial pressure range 1.0 × 10 −6 −2.0× 10 −1 atm.
Abstract: We have undertaken phase-stability studies of Y1Ba2Cu3O7−x(+Ag) and BaCu2O2 (+Ag) over the temperature range 780°C–950°C and oxygen partial pressure range 1.0 × 10 −6 –2.0 × 10 −1 atm. Ag was found to have no effect on the thermodynamic stability of either phase. Partial melting of YBCO was found at temperatures as low as 800°C ± 10°C, with significant peritectic melting at temperatures as low as 900°C ± 10°C. These thermochemical studies have enabled us to determine the optimum conditions for reduced temperature and pressure (RTP) processing of Ag clad Y1Ba2Cu3O7−x pellets and tapes. The classic domain structure with strong texturing over short lengths was produced in samples melt processed by controlling oxygen partial pressure at a fixed temperature. The RTP method has the advantage that the second phase Y2BaCuO5 particle size can be controlled. Moreover, the technique offers the promise of long lengths of flexible Y1Ba2Cu3O7−x tapes with high current carrying capability.
TL;DR: In this article, the authors have successfully fabricated ceramic superconductors of Hg-1223 with substitution of Pb for Hg, i.e. Hg1−xPbxBa2Ca2Cu3Oy, by using encapsulation and short time annealing techniques.
Abstract: We have successfully fabricated ceramic superconductors of Hg-1223 with substitution of Pb for Hg, i.e. Hg1−xPbxBa2Ca2Cu3Oy, by using encapsulation and short time annealing techniques. Samples fabricated in this manner may have nearly single phase structure and some enhancement of Tc can be achieved through this method. The materials of this ceramic with x ranging from 0 to 0.5 exhibit high-temperature superconductivity at temperatures falling in the range from 103 to 135 K. The sample with optimum doping of Pb of x = 0.34 and ° = 0.40 has a highest Tc of 135 K, and its diamagnetic onset temperature Tcdia = 143 K. The composition of this sample has been found experimentally and is specified by a formula of Hg0.66Pb0.34Ba2Ca1.98O8.4. The valence number for the copper constituent is given by V Cu = 2.30 . X-ray diffraction (XRD), transmission electron microscope (TEM) and electron probe microanalyzer (EPMA) analyses have been carried out for detailed investigation. Results indicate that the critical temperature for samples which were post-annealed with either oxygen flow or in high pressure of oxygen gas was increased, while that for those annealed in argon flow was decreased. Furthermore, the as-prepared samples were slightly in the under-doped state with respect to the vacancy. It was shown that Tc parabolically depends on the valence state of copper.
TL;DR: In this paper, X-ray powder diffraction, electron diffraction and iodometric titration and resistance measurements were performed on Bi2Sr2−xLaxCuO6+δ and La substituted Bi-2201 compounds.
Abstract: The Bi2+xSr2−xCuO6+° (Bi-2201) phases and La substituted Bi-2201 compounds with general formulas Bi2Sr2−xLaxCuO6+δ and Bi2.3−xSr1.7LaxCuO6+δ were prepared and characterized by X-ray powder diffraction, electron diffraction, iodometric titration and resistance measurements. The monophasic samples of Bi2+xSr2−xCuO6+δ were obtained in air at 850°C for 0.15 ≤ x ≤ 0.4, while synthesis at 740°C in oxygen flow extended this range to the stoichiometric cation composition (0 ≤ x ≤ 0.4). The superconducting transitions were not detected for these monophasic samples in contrast to the La substituted phases which exhibited superconductivity, and the highest Tc,onset = 33 K was found for the Bi2Sr1.6La0.4Cu6.33 compound. Superconductivity in the Bi2Sr2−xLaxCuO6+δ series exists in overdoped and underdoped regions. Incommensurately modulated structures of nonsuperconducting Bi2.3Sr1.7CuO6.23 phase and superconducting Bi2Sr1.7La0.3CuO6.28 phase (Tc,onset = 26 K) were refined from X-ray powder data by a Rietveld technique using the four-dimensional space group P:A2/a:−11. The maximal Cu displacements from the average position in the first structure was found to be sufficiently larger than in the latter one. The distorted structural arrangement of the (CuO2) layers in the Bi2.3Sr1.7CuO6.23 structure can be a reason for the suppression of superconductivity in this phase, while their less corrugated configuration in the La containing Bi-2201 structure leads to the existence of superconductivity.
TL;DR: In this article, the critical velocity ν ϕ ∗ associated with the electronic instability predicted by Larkin and Ovchinnikov displays a cross-over effect from magnetic-field independent behavior at high fields to the proportionality of ρ ∗ ∼B − 1 2 at low fields.
Abstract: The critical velocity ν ϕ ∗ associated with the electronic instability predicted by Larkin and Ovchinnikov displays a cross-over effect from magnetic-field independent behavior at high fields to the proportionality ν ϕ ∗ ∼B − 1 2 at low fields. This low-field behavior results from the fact that ν ϕ ∗ multiplied with the inelastic quasiparticle scattering time τin must reach at least the intervortex distance, thereby ensuring spatial homogeneity of the nonequilibrium quasiparticle distribution. At high magnetic fields the flux-flow instability becomes unobservable due to the onset of fluctuation effects contributing to the resistivity.
TL;DR: In this paper, a comparative study on thermal treatment conditions, microstructure, and flux pinning properties for doped and undoped samples has been performed in order to evaluate the effect of SrZrO 3 addition.
Abstract: Silver sheathed tapes of Bi-2212 undoped and doped with SrZrO 3 have been prepared, using co-precipitated oxalates as precursors. Thermal treatment conditions have been optimised, varying the maximum processing temperature T max . A comparative study on thermal treatment conditions, microstructure, and flux pinning properties for doped and undoped samples has been performed in order to evaluate the effect of SrZrO 3 addition. The doped samples contain SrZrO 3 submicron particles, appearing as aggregates included between Bi-2212 lamellae with a Cu-free phase and the rest of the liquid, and also distributed inside Bi-2212 grains. The presence of SrZrO 3 surplus leads to higher critical current densities in samples prepared in a wide T max range; in addition, the lowest T max , at which high J c can be obtained, becomes 5 deg lower (875°C). At T = 5 K critical currents for all the samples are of the order of 10 5 A/cm 2 and are independent of doping, while at 60 K doped samples exhibit roughly two times higher J c in zero field and more than an order of magnitude higher J c at H = 40 mT. A magnetisation decay study reveals that samples with SrZrO 3 surplus have a larger fraction of pinning centres with high energy. The enhanced pinning properties can be related to the finely dispersed zirconate inclusions found in the superconductor matrix.
TL;DR: In this paper, scanning force microscopy (SFM) is applied to study the surface morphology of polished single-crystalline substrates such as SrTiO3, LaAlO3 (100), NdGaO3(001), and MgO (100) for thin-film growth of high-temperature superconductors.
Abstract: Scanning force microscopy (SFM) is applied to study the surface morphology of polished single-crystalline substrates such as SrTiO3 (100), LaAlO3 (100), NdGaO3 (001) and MgO (100), which are used for thin-film growth of high-temperature superconductors. SFM reveals the misorientation of the substrate surface showing terraces with steps of a unit cell in height. The polishing leads to irregular shape of the step ledges. Oxygen anneal initiates partial regrowth of the surface. The average terrace width compares well to the surface misorientation. On SrTiO3 substrate surfaces cross-shaped features are observed indicating extended defects in the crystal. LaAlO3 substrates exhibit twin domains which cause severe surface roughening resulting in height variations of over 20 nm. MgO surface reacts with the ambient as demonstrated by comparing a freshly cleaved clean MgO surface to the same sample stored for several weeks under ambient conditions. Micron-sized particles appear on aging.