Showing papers in "Physica C-superconductivity and Its Applications in 1992"
TL;DR: In this paper, an extension to Morelands break junction technique is developed in order to obtain a clean and stable, mechanically adjustable junction, where the coupling of two electrodes can be varied in vacuum.
Abstract: An extension to Morelands break junction technique is developed in order to obtain a clean and stable, mechanically adjustable junction. As a function of an externally applied force the coupling of two electrodes can be varied in vacuum. Experiments are described of a junction with niobium electrodes at 4.2 K which undergo a continuous change in normal resistance R N , from 1 to 10 9 Ω upon applying an increasing force. In this resistance range we discern a transition from a weak link regime to a tunnel regime. The current voltage ( I–V ) curves are reproducible upon adjustment changes in the whole resistance range. In the weak link regime the two electrodes of the junction are in physical contact with each other. The product of the critical current and normal resistance is compared with predictions of Ambegaokar-Baratoff and Kulik-Omelyanchuk. The product of the excess current and normal resistance shows a logarithmic increase for low R N values and decreases for the highest R N values in the weak link regime. Subharmonic gap structure, originating from multiple Andreev reflections is observed over a wide range of R N . In the transition regime the two electrodes are not in contact but there is still a large overlap of the superconducting and quasiparticle wave functions. In this regime a finite slope of the “critical current part” in the current voltage curve is observed. The I–V curves show features characteristics for both a weak link and a tunnel junction. In the tunnel regime there exists a vacuum gap between the electrodes and the Josephson coupling is suppressed. A considerable subgap current is observed, where the product of the subgap current and normal resistance is constant over almost four orders of magnitude of R N . A decreasing conductance near zero bias shows up in this regime. The normal resistane exhibits an exponential behaviour upon variations in the vacuum gap. The absolute stability of the distance between the two junction electrodes is estimated to be better than 0.5 pm over a 100 mV voltage range.
334 citations
TL;DR: In this article, the magnetic flux distribution in high-Tc YBaCuO (YBa2Cu3O7) single crystals with in-plane anisotropy was studied.
Abstract: Indicator ferrimagnetic films with in-plane anisotropy are applied for studying the magnetic flux distribution in high-Tc YBaCuO (YBa2Cu3O7) single crystals. Induction gradients are measured to estimate the critical current density Jc. Temperature dependences of Jc are found to be different for different field directions. For the field along the orthorhombic c-axis Jc∥ab exhibits an exponential temperature decay. In the case of H∥ab, however, Jc∥ab is changed just a little up to 70 K and then drops sharply near Tc.
232 citations
TL;DR: Using a combination of Z-contrast scanning transmission electron microscopy, scanning tunneling microscopy and plan view diffraction contrast imaging, this paper studied the growth and relaxation mechanisms of YBa2Cu3O7−x deposited on MgO and SrTiO3 substrates.
Abstract: Using a combination of Z-contrast scanning transmission electron microscopy, scanning tunneling microscopy, and plan view diffraction contrast imaging, we have studied the growth and relaxation mechanisms of YBa2Cu3O7−x deposited on MgO and SrTiO3 substrates. Two-dimensional island growth occurs on SrTiO3 substrates, with relaxation through the nucleation of dislocation half-loops. The threading dislocation segments then have a screw component and can lead to kinetic roughening through the development of growth pyramids. In contrast, growth on MgO occurs by true three-dimensional island growth (with no wetting layer), most of the interface being incommensurate with the substrate (although crystallographically aligned). Dislocations with both edge and screw components are generated on island coalescence. A highly anisotropic surface energy is shown to be responsible for cell-by-cell c⊥ growth being thermodynamically preferred, although at high supersaturations a transition to a⊥ growth occurs.
149 citations
TL;DR: In this paper, a simple but reliable procedure to evaluate J c (B ) from magnetization measurements has been developed, which applies to the fully penetrated state and to situations where the flux density gradient has reversed completely.
Abstract: A simple but reliable procedure to evaluate J c ( B ) from magnetization measurements has been developed. It applies to the fully penetrated state and to situations where the flux density gradient has reversed completely. In an approach consistent with the Bean model, a uniform current density is assumed to flow throughout the entire sample being correlated to the corresponding average magnetic induction in the sample. In combination with the anisotropic Bean model, we present evaluations of J c ( B ) for a typical single crystalline high-temperature superconductor.
141 citations
TL;DR: In this paper, the transport critical current density anisotropy measurements have been conducted on liquid-phase processed YBa 2 Cu 3 O x superconductors (123) with differing Y 2 BaCuO 5 (211) content and particle size.
Abstract: Transport critical current density anisotropy measurements have been conducted on liquid-phase processed YBa 2 Cu 3 O x superconductors (123) with differing Y 2 BaCuO 5 (211) content and particle size. It is found that for specimens with micron-size 211 inclusions and low 211 content, the critical current density, J c , increases with 211. In contrast, J c is determined to decrease with 211 content when the specimens contain large 211 particles and high volume fraction of 211. Moreover, the ability of 211 to act as pinning sites appears to diminish at moderate magnetic fields, whereas the crystal defects that are associated with the 123–211 interfaces appear to be effective pinning centers at high fields. It is proposed that the variation in J c with 211 content is not unique, but depends on the 211 content and the particle size, as well as the strength of the applied magnetic field.
139 citations
TL;DR: In this article, the authors investigated the composition YSr2Cu3-xMxOy and obtained the 123-structure for M= Li, Al, Ti, V, Cr, Fe, Co, Ga, Ge, Mo, W and Re.
Abstract: We have investigated the composition YSr2Cu3-xMxOy and obtained the 123-structure for M= Li, Al, Ti, V, Cr, Fe, Co, Ga, Ge, Mo, W and Re. Superconductivity is observed for the compounds with M=Ti, V, Fe, Co, Ga, Ge, Mo, W and Re at low levels of substitution context x. The highest Tzeroc is 73 K for the compound YSr2Cu2.85Re0.15O7.12. Tcs are enhanced by decreasing x or increasing y, which draw the rise of hole concentration pave (Cu2+p). Except for the compounds with M=V and Cr, the structures are tetragonal, while those of the compounds YSr2Cu3-xCrxOy are orthorhombic, and YSr2Cu3-x VxOy shows the structural transition from tetragonal to orthorhombic by HIP treatment with the simultaneous transition from superconductivity to metallic transport property. This Sr-based 123-structure is stabilized by doping elements with ionic radii=0.35-0.49 A or 0.525-0.62 A supposing 4- or 6-coordination, respectively, except for M=Li.
130 citations
TL;DR: In this paper, the magnetization curves of rapidly relaxing type-II superconductors are calculated by means of Monte Carlo simulations and numerical solutions of the partial differential equation for thermally activated flux-creep.
Abstract: The magnetization curves of rapidly relaxing type-II superconductors are calculated by means of Monte Carlo simulations and numerical solutions of the partial differential equation for thermally activated flux-creep. Several models for the current dependence of the activation energy ( U ( j )= U c (1- j / j c ), U ( j )=( U c / μ )(( j c / j ) μ -1) and U ( j )= U c ( j c / j )) are considered. The calcu lated curves reproduce all the features exhibited by experimental magnetization curves, even when the critical current is assumed to be field-independent. This remarkable result shows explicitly that strong flux relaxation effects can lead to spurious field-dependent critical currents. The characteristic features of the magnetization curves are related to the relaxation behaviour of the corresponding flux density profiles. The dependence of hysteresis loops on the magnetic field sweep rate is investigated in detail and is shown to contain basically the same information as the time dependence of the magnetization during relaxation.
126 citations
TL;DR: In this paper, high quality YBa2Cu3O7-x (YBCO) single crystals have been grown by a flux method and characterized by low field magnetization, a−b plane resistivity, microwave surface resistance and heat capacity measurements.
Abstract: Large, high quality YBa2Cu3O7-x (YBCO) single crystals have been grown by a flux method and characterized by low field magnetization, a−b plane resistivity, microwave surface resistance and heat capacity measurements. The crystals obtained are nearly free standing after crystal growth and are easily harvested. They have dimensions of up to 5 mm along the a-(b-) axis and up to 1 mm along the c-axis. After oxidation, the crystals show a superconducting transition at Tc=93.2 K with transition width, ΔTc, of < 0.25 K as determined by low field magnetization, a−b plane resistivity and heat capacity data. The heat capacity jump ΔCp of the superconducting transition is 6.9 mJ/(gK).
124 citations
TL;DR: In this paper, a complete set of electrical resistivity data along both the c -axis ( ϱ c ) and ab -plane (π� ab ) have been obtained via the DC four-probe method for the first time.
Abstract: Large single crystals of (La 1 − x Sr x ) 2 CuO 4 , up to 5 mm along the crystal c -axis, have been grown over a composition range of x = 0.03 to 0.15, covering the whole superconducting region in this system. A complete set of electrical resistivity data along both the c -axis ( ϱ c ) and ab -plane ( ϱ ab ) have been obtained via the DC four-probe method for the first time. As x was varied, both resistivities changed monotonically from semiconductive to metallic in character and the critical temperature, T c defined by the mid point of the resistive transition curves, passed through a maximum value of 38.8 K at x = 0.072. The anisotropy ratio ϱ r / g 9 ab at 50 K decreased drastically with increasing x , from nearly 4000 to 160, while it was appoximately 350 at x = 0.072> Magnetization hysteresis curves ( H || c , ± 9 T) revealed the presence of an additional pinning mechanism in the over-doped region ( x ⩾ 0.072) giving rise to a second peak at moderately high fields. This was attributed to enhanced flux pinning by oxygen vacancies nominally present in this composition range.
124 citations
TL;DR: In this paper, various properties of the flux-line lattice (FLL) in high-T c superconductors are compiled, derived, and discussed, and useful general and updated expressions for the energy and magnetic field of arbitrary flux line arrangements in anisotropic and layered supercondors are presented.
Abstract: Various properties of the flux-line lattice (FLL) in high- T c superconductors are compiled, derived, and discussed. Useful general and updated expressions for the energy and magnetic field of arbitrary flux-line arrangements in anisotropic and layered superconductors are presented. From these follow the nonlocal elastic moduli of the FLL which are then applied to the intricate problems of thermal fluctuations and thermal depinning of the FLL and to the linear response of the FLL to applied AC fields. Explicit expressions are derived for the complex AC penetration depth, surface impedance, AC resistivity and for the AC susceptibility of films and cylinders, taking full account of elastic pinning and viscous drag of vortices, and of thermally activated linear flux creep or tunneling. These AC properties are then applied to vibrating superconductors and to thermally activated flux diffusion, which gives a quantitative explanation of the depinning lines observed in various experiments. Finally, the shear-limited critical current density is estimated and FLL decoration experiments are discussed.
123 citations
TL;DR: In this article, the relationship between superconductivity and the crystal structure of La 2− x M x CuO 4 (M Sr, Ba) is studied under high pressures up to 20 kbar.
Abstract: Relations between the superconductivity and the crystal structure of La 2− x M x CuO 4 (M Sr, Ba) are studied under high pressures up to 20 kbar. In the Sr-doped crystal, bulk superconductivity is observed both below and above the critical pressure P c where the high-temperature tetragonal (THT) phase is stabilized down to less than 20 K, the lowest temperature examined, instead of the orthorhombic phase. The superconducting transition temperature T c of the orthorhombic phase below P c is depressed in comparison with that of the THT phase above P c . The depression of T c is caused by a tilt of the CuO 6 octahedra in the orthorhombic phase. It is found for the Ba-doped crystal that the depression of T c for x⋍0.11−0.14 is removed, except for x⋍0.125 when a structure transition at T 1 ∼60 K disappears under high pressure. In the vicinity of x =0.125, there exists another origin for the depression of T c in addition to the structural transition at T c .
TL;DR: In this article, a preliminary phase diagram is given for the system La 2 CuO 4+δ, o ≤ δ ≤ 0.09, prepared electrochemically at room temperature.
Abstract: A preliminary phase diagram is given for the system La 2 CuO 4+δ , o ≤ δ ≤ 0.09, prepared electrochemically at room temperature. Room-temperature X-ray data distinguish two orthorhombic phase fields, O I for δ M for δ > 0.055. The orthorhombicity of O I decreases with increasing δ, that of O M increases with δ. A spinodal phase segregation into an oxygen-rich superconductive phase and an oxygen-poor antiferromagnetic phase occurs in the temperature interval 220K T ≤ T sp ; below 220 K, the mobility of the interstitial oxygen becomes too low for a further static phase segregation. The phase segregation causes an increase in resistivity and a decrease in the Seebeck coefficient with decreasing temperature in the interval 220 T T sp . A similar transport behavior occurs in the range T c T T ϱ ≤ 120 K in samples 0.02 ≤ δ ≤ 0.04; from comparisons with the La 2− x Sr x CuO 4 system, we suggest that at lower temperatures this behavior is the signature for a further dynamic hole segregation, induced by atomic displacements, into domains more rich in mobile holes within the superconductive phase. The diamagnetic AC susceptibility associated with this superconductivity is independent of the magnetic field below 0.01 mT. In the compositional range 0.06 ≤ δ ≤ 0.09, the Seebeck data indicate a nearly constant concentration of itinerant holes in the superconductive CuO 2 planes. We conclude that in the O M phase oxygen-oxygen interactions within the superconductive layers trap out at oxygen-atom clusters the excess mobile holes Δ P = 2 δ −0.12 from the CuO 2 planes. An H c1 = 64 mT was obtained for δ = 0.09. A step in the resistivity drop at T c signals a temperature interval T c T T on within which the superconductive pairs are confined to small domains. Whether these domains represent a second superconductive phase correlated to additional oxygen ordering that fails to develop with increasing δ or should be considered a pair-fluctuation phenomenon is not resolved.
TL;DR: In this article, a small nonconducting and non-magnetic sample weighing 5.48 g was placed over a levitating superconducting disk and the loss of weight was measured with high precision using an electro-optical balance system.
Abstract: Shielding properties of single-phase dense bulk superconducting ceramics of YBa 2 Cu 3 O 7− x against the gravitational force were studied at temperatures below 77 K. A small non-conducting and non-magnetic sample weighing 5.48 g was placed over a levitating superconducting disk and the loss of weight was measured with high precision using an electro-optical balance system. The sample was found to lose from 0.05 to 0.3% of its weight, depending on the rotation speed of the superconducting disk. Partial loss of weight might be the result of a certain state of energy which exists inside the crystal structure of the superconductor at low temperatures. The unusual state of energy might have changed a regular interaction between electromagnetic, nuclear and gravitational forces inside a solid body and is responsible for the gravity shielding effect.
TL;DR: In this paper, a planar tunnel junctions based on high-quality YBa 2 Cu 3 O 7 thin films are studied as a junction of crystallographic orientation. But the results of the tunneling conductance analysis are limited to the case where the CuO 2 planes are parallel to the junction surface.
Abstract: Reproducible planar tunnel junctions based on high-quality YBa 2 Cu 3 O 7 thin films are studied as a junction of crystallographic orientation. Using Pb as a counter-electrode, all junctions exhibit zero bias anomalies, the sign of which depends upon the crystallographic orientation of the film. For c -axis, or (001) films, in which the CuO 2 planes are parallel to the junction surface, a 5 mV wide dip in the tunneling conductance, which is independent of applied magnetic field up to 5 T, is observed. Conversely, for the (103) and (100) orientations, in which the CuO 2 plane points toward the junction surface, a peak in the tunneling conductance, which is strongly magnetic-field dependent, is observed. Analysis is performed in terms of spin-flip and Kondo-type scattering at the interface (Anderson and Applebaum theory).
TL;DR: Sr 2 CuO 2 (CO 3 ) was prepared at 1273 K and 0.01 MPa CO 2 partial pressure in a flowing gas of O 2 CO 2 using a mixture of SrCO 3 and CuO powders as a starting material as mentioned in this paper.
Abstract: Sr 2 CuO 2 (CO 3 ) was prepared at 1273 K and 0.01 MPa CO 2 partial pressure in a flowing gas of O 2 CO 2 using a mixture of SrCO 3 and CuO powders as a starting material. The compound has a tetragonal structure with lattice constants a = 7.8045(1), and c = 14.993(1) A , and its space group is 14. The formula per unit cell is 8 Sr 2 CuO 2 (CO 3 ), and measured and calculated densities are D m = 4.71 g/cm 3 , and D x = 4.81 g/cm 3 , respectively. The crystal structure was refined by Rietveld analysis on X-ray powder diffraction and neutron powder diffraction data. The final residuals ( R F ) were 4.31 and 4.27% for the X-ray and neutron data, respectively. The structure consists of deformed [CuO 6 ] octahedrons and layers of ordered triangular CO 3 groups. Sr atoms having eight near oxygen neighbors are between the [CuO 6 ] octahedrons and the CO 3 layers.
TL;DR: In this article, the authors extended the Iarkin-Clvchinnikov approach to the problem of viscous flux flow instability in dirty superconductors near T, by taking into account quasiparticle heating, which is, in principle, unavoidable in finite magnetic fields in the case of real heat removal.
Abstract: We have extended the Iarkin-Clvchinnikov approach to the problem of viscous flux flow instability in “dirty” superconductors near T, by taking into account quasiparticle heating, which is, in principle, unavoidable in finite magnetic fields in the case of real heat removal. According to the relationship between the magnetic field E value and the parameter Br, introduced by us, there may be two qualitatively different mechanisms of instability: one due to heating of quasiparticles at B z+ Br, and the other (considered by Larkin and Ovchinnikov) of nonthermal nature at B 4: &. The results obtained have allowed us to interpret some peculiarities in the resistive transition, observed experimentally for wide superconducting films. Based on resistive measurements, a new way of finding the quasiparticle energy relaxation time is proposed. In the case of perfect acoustic matching of the fdm and the substrate, the inelastic scattering times of quasiparticles on both the quasiparticles and the phonons can be obtained separately.
TL;DR: In this paper, the Ferrell-Glover-Tinkham sum rule relating the missing area in the low frequency conductivity to the penetration depth can be violated in certain superconductors.
Abstract: It is pointed out that the Ferrell-Glover-Tinkham sum rule relating the “missing area” in the low frequency conductivity to the penetration depth can be violated in certain superconductors. Its breakdown indicates that the effective mass of the carriers changes in entering the superconducting state, and implies a change in the conductivity at frequencies much higher than the superconducting gap, possibly near infrared or visible. The model of hole superconductivity predicts the occurrence of this phenomenon.
TL;DR: The crystal structure of a new compound, (C0.4Cu0.6)Sr2(Y0.86Sr0.14)Cu2O7, is derived from the structure of YBa2Cu3O7.
Abstract: The crystal structure of a new compound, (C0.4Cu0.6)Sr2(Y0.86Sr0.14)Cu2O7, is derived from the structure of YBa2Cu3O7. Forty percent of CuO chains in the YBa2Cu3O7 structure are replaced by CO3 groups. This new compound has a superstructure along the a-axis and the c-axis. Diffuse superlattice reflections having periods of a∗/2-a∗/3 and c∗/2 were observed in electron diffraction patterns. Locally ordered distributions of C and Cu atoms were seen high-resolution images taken by transmission electron microscopy with an incident beam parallel to [010]. The basic structure of this superstructure was determined by neutron powder diffraction, assuming orthorhombic symmetry with the space group, Pmmm (lattice constants: a=3.8278(2), b=3.8506(2) and c=11.1854(5) A ).
TL;DR: In this paper, the microstructure of YBa2Cu3O7 grown on steep steps in (001 pseudocubic LaAlO3 substrates) was studied by high-resolution electron microscopy of cross-sectional and plan-view samples.
Abstract: The microstructure of YBa2Cu3O7 grown on steep steps in (001 pseudocubic LaAlO3 substrates was studied by high-resolution electron microscopy of cross-sectional and plan-view samples. Steps with angles of about 80° were obtained by ion milling. On the substrate plane, the films grew with the c-axis parallel to [001] while on the flank of a step the c-axis was parallel to the [100] direction of the substrate. As a result, two [010] tilt axis grain boundaries were formed at which the YBa2Cu3O7 lattice changed orientation by approximately 90°. In the upper grain boundary, a [100] tilt axis and, on the average, a (013) habit plane alternated with a [010] tilt axis and a (1 0 3) habit plane. This alternating structure was caused by twinning in the orthorhombic film structure. The lower grain boundaries were found to be rather irregular and consisted of a chain of (0 1 3)(0 1 3) and (0 1 0)(0 0 1) type segments exhibiting a tendency to tilt the whole habit plane toward the a-b plane of the flank film. Dislocations, stacking faults and misfit strains were also observed in or close to the boundaries. Grain boundary modeling indicated a good agreement with the experimental image and permitted us to determine the atomic plane of a boundary.
TL;DR: In this paper, two distinct superconducting phases were found for La2CuO4+δ, and the maximum doped-hole concentration from combined Sr-doping and electrochemical oxygen doping for 0≤x≤0.16 holes/formula unit was found.
Abstract: La2CuO4 can be electrochemically oxidized in aqueous base at 295 K to form a superconductor with Tc≈44 K [Wattiaux et al., C.R. Acad. Sci. (Paris) 310 (1990) 1047]. We have confirmed and extended this study. We conclude that the electrochemical reaction forms La2CuO4+δ. Two distinct superconducting phases are found at δ≈0.03 (Tc≈32 K) and δ 0 .08(T c ≈4 K) Bulk superconductivity is found in similarly prepared La2-xSrxCuO4+δ with Tc≈40 K for 0.01≤x≤0.15. Normal stat e magnetic susceptibility data for La2CuO4.11 and La1.92Sr0.08CuO4.07 are nearly identical with those of conventionally prepared La1.85Sr 0.15CuO4, indicating that the hole doping level (p) in the CuO2 planes of the three compounds is nearly the same. Iodometric titration experiments confirmed that the maximum doped-hole concentration from combined Sr-doping and electrochemical oxygen doping for 0≤x≤0.15 is p≈0.16 holes/formula unit. The electrochemical oxidation of the starting materials does not appear to be reversible.
TL;DR: In this article, a selfconsistent calculation of the radial component Br(r), of the magnetic induction as well as of the current density distribution, jc(r) and the volume pinning force, fp, was performed using Bz(r)-data, measured by means of the magneto-optical Faraday effect on c-axis oriented YBa2Cu3O7−δ-films.
Abstract: In thin film superconductors with magnetic fields applied perpendicular to the film plane, demagnetization effects lead to strong deviations of the flux density distribution B ( r ) from the predictions of the conventional critical state model. In particular if the magnetization M is taken as a probe for the critical current density j c of a thin film, the radial stray field component B r parallel to the film plane has to be taken into consideration. Especially in thin circular films with thicknesses lower than the London penetration depth, λ, the circular critical current density, jc=1/μ0(∂Bz/∂B−∂Bz/∂r), is determined by the gradient ∂Br/∂z rather than by ∂Bz/∂r. Using Bz(r)-data, measured by means of the magneto-optical Faraday effect on c-axis oriented YBa2Cu3O7−δ-films, we perform a selfconsistent calculation of the radial component Br(r), of the magnetic induction as well as of the current density distribution, jc(r), and the volume pinning force, fp.
TL;DR: In this paper, temperature dependence of the electrical resistivity of polycrystalline specimens of Y1−xPrxBa2Cu3O6.02 (0.95±0.02) are reported.
Abstract: Measurements of the temperature dependence of the electrical resistivity of polycrystalline specimens of Y1−xPrxBa2Cu3O6.95±0.02 (0⩽x⩽1) are reported. The electrical resistivity exhibits metallic behaviour for 0⩽x⩽0.50 and Tc, where Tc is the superconducting critical temperature, and 0.50 T⪆ 100 K . the resistivity determenined values of Tc decrease monotonically with x ≈ 0.57. The Tc versus x curve does not confrom to the universal relation based on the theory of Abrikosov and Gor'kov. However, for 0⪅x⪅0.3, the Tc versus x curve can be described by an empirical relation which incorporates the filling (or localization) of mobile holes in the CuO2 planes and superconducting electron pair breaking. Methods of sample preparation are discussed.
TL;DR: In this article, the critical current density (Jc) values of variously processed Ag-clad Bi2Sr2Ca1Cu2Ox tapes are related to their microstructures as observed by TEM, SEM, and X-ray pole figure analysis.
Abstract: The critical current density (Jc) values of variously processed Ag-clad Bi2Sr2Ca1Cu2Ox tapes are related to their microstructures as observed by TEM, SEM, and X-ray pole figure analysis. The coupled microstructure-Jc results confirm the general importance of a highly c-axis aligned microstructure for high critical current density. However, TEM experiments revealed a “colony” microstructure in all sample in which groups of several grains with nearly perfect c-axis alignment (colonies) are formed but where the c-axes of adjacent colonies are not parallel. SEM imaging of etched cross-sections showed that important features of this colony misalignment are not accurately reflected by stand alone X-ray pole-figure analysis or TEM imaging. Near-perfect alignment occurs within a few microns of the Ag cladding for all processing methods. However, the processing details control the macroscopic grain alignment within the bulk of the superconductor. In particular, second-phase particles disrupt the alignment of macroscopic colonies. These misalignments may control the transport critical current, yet they are not clearly evidenced by either macroscopic pole-figure analysis or by very local TEM examination.
TL;DR: In this paper, the magnetic hysteresis loops of Bi 2 Sr 2 CaCu 2 O 8 and Tl-2212 (Tl 2 Ba 2 CaCo 2 O8 )-type single crystals have been irradiated by 6.0 GeV Pb ions, resulting in the creation of tubes of amorphized material (latent tracks).
Abstract: Bi-2212 (Bi 2 Sr 2 CaCu 2 O 8 ) and Tl-2212 (Tl 2 Ba 2 CaCu 2 O 8 )-type single crystals have been irradiated by 6.0 GeV Pb ions. The irradiation results in the creation of tubes of amorphized material (latent tracks) which extend throughout the whole thickness of the crystals. Such induced defects act as efficient pinning centers and lead to unusual large changes in the magnetic hysteresis loops. A strong shift of the irreversibility line to high fields and temperatures and a significant charge of its shape are also reported for both compounds, demonstrating that pinning effects have to be taken into account to describe the onset of the FLL motion.
TL;DR: In this article, a new copper oxide based superconductor is reported based on the ordered interleaving of NbO 6 or TaO 6 octahedra between the apices of copper oxide pyramids.
Abstract: A new copper oxide based superconductor is reported based on the ordered interleaving of NbO 6 or TaO 6 octahedra between the apices of copper oxide pyramids. Good bulk superconductivity is observed for only a narrow range of temperatures, annealing times and cooling rates. The relatively low T c of the prototype compound Sr 2 Nd 1.5 Ce 0.5 NbCu 2 O 10−δ , may be due to the presence of the insulating NbO 6 layers or the wide (Nd, Ce) 2 O 2 layers which separate the CuO 2 planes.
TL;DR: In this article, the authors derived multiphase equilibria for the 2-layer superconductor Bi 2 (Sr 1− x Ca x ) 3 Cu 2 O 8, which is a trigonal bipyramid with a length of 13.7 mol% parallel to the SrOCaO edge, and an extension of 9 mol% perpendicular to it.
Abstract: The extension of the single-phase field of the two-layer superconductor Bi 2 Sr 2 CaCu 2 O 8 is clarified for 830°C in air. The single-phase field of “2212” is a trigonal bipyramid with a length of 13.7 mol% parallel to the SrOCaO edge, and an extension of 9 mol% perpendicular to it. The tolerance of the lattice concerning CuO variation is smaller than ± 1.2 mol%. The CaO-rich solid solution is deficient in CuO compared to the ideal 2212 solid solution, Bi 2 (Sr 1− x Ca x ) 3 Cu 2 O 8 , whereas the SrO-rich side has excess CuO. Multiphase equilibria around “2212” as well as the single phase field were derived by combining the XRD and EDX results of 91 samples of different compositions with the previously published results. The intersections of the multiphase equilibria with three different planar sections of the compositional tetrahedron are shown.
TL;DR: The phase diagram of the lead-substituted Bi 2 O 3 -SrO-CaO-CuO system has been investigated in the temperature range 825°C-1100°C, and in static air along the line Bi 1.6 Pb 0.4 Sr 2 Ca n −1 Cu n O 2 n +4+x as mentioned in this paper.
Abstract: The phase diagram of the lead-substituted Bi 2 O 3 -SrO-CaO-CuO system has been investigated in the temperature range 825°C–1100°C, and in static air along the line Bi 1.6 Pb 0.4 Sr 2 Ca n −1 Cu n O 2 n +4+ x . It involves nine distinct solid phases and two liquid ones. The transformation and melting lines of the superconducting n = 1, 2, 3 phases are located below 900°C. Below the solidus (∼855°C), the sequence of stable { n } phases is {1}, {1}+{2}, {2}, {2}+{3}, and {3}. The {3} phase (110K) exists as the dominant phase between ∼835°C and ∼875°C for 3 n t >∼900°C) mainly contains the transformation and melting lines of CuO, (Ca, Sr) 2 CuO 3 , and (Ca, Sr) 14 Cu 24 O y . The liquids involves a eutectic point at (∼930°C, n =2) and a critical end point of miscibility of the two liquids L 1 (bismuth rich) and L 2 . The presence of Pb shifts the transformation lines of the superconducting phases downwards by 10–15°C. The practical implications of this diagram are discussed.
TL;DR: In this article, the critical current density and the flexural strength α max of YBa 2 Cu 3 O 7−δ composite superconductors were measured and the microstructure analysis revealed that the Y 2 BaCuO 5, Ag and BaZrO 3 were finely dispersed in the matrix.
Abstract: The high- T c YBa 2 Cu 3 O 7−δ composite superconductors containing Zr, Ag and Y 2 BaCuO 5 have been fabricated by melt processing. Both the critical current density J M c and the flexural strength α max of these samples were measured. The value of J M c increased initially for all inclusions added. The microstructure analyses revealed that the Y 2 BaCuO 5 , Ag and BaZrO 3 were finely dispersed in the matrix. We believe that the fine dispersion of these inclusions is most likely responsible for an increase in J M c . The largest value of J M c we obtained is 64 800 A/cm 2 on the sample containing 50 mol.% Y 2 BaCuO 5 . In contrast, the value of α max was limited below 60 MPa due to the presence of mechanically weak grain boundaries. The addition of Ag to YBa 2 Cu 3 O 7−δ was effective in increasing both α max and J M c .
TL;DR: In this paper, the fracture toughness of melt-powder-melt-growth (MPMG) processed YBaCuO samples was measured and the effect on the dispersed Y2BaCoCuO5 (211) particles in the 123 superconducting phase was found.
Abstract: The fracture toughness of melt-powder-melt-growth (MPMG) processed YBaCuO samples was measured and the effect on the fracture toughness of the dispersed Y2BaCuO5 (211) particles in the 123 superconducting phase was found. Preferred fracture planes were the (100), (010) and (001) planes, and the fracture toughness of the (001) plane was lower than those of the (100) and (010) planes. The indentation fracture method indicated that K c = 0.88-1.2 MPa m 1 2 in an inert environment for YBaCuO crystals without the 211 particles, when the crack plane is the (100) plane or the (010) plane. The presence of the 211 particles in the 123 matrix increased the fracture toughness in this direction up to 1.6-2.1 MPa m 1 2 .
TL;DR: In this article, the authors considered a model of superconductivity where pairing originates in gain of kinetic rather than potential energy of the carriers, and showed that a change in the frequency-dependent conductivity occurs at frequencies much higher than the scale set by the superconducting energy gap.
Abstract: We consider models of superconductivity where pairing originates in gain of kinetic rather than potential energy of the carriers In such systems, a change in the frequency-dependent conductivity occurs at frequencies much higher than the scale set by the superconducting energy gap This property follows from a general sum-rule argument To clarify the physical origin of this spectral weight transfer we consider several microscopic hamiltonians that give rise to an effective hamiltonian with a kinetic pairing interaction These models describe small polarons with a non-linear interaction with a background degree of freedom, that gives rise to an effective mass enhancement that depends on the local charge occupation Superconductivity in these models can be understood as arising from the partial “undressing” of carriers that occurs upon pairing The same “undressing” occurs in these systems upon doping in the normal state, which causes superconductivity to disappear at high doping Optical conductivity is calculated for one of the model hamiltonians to illustrate the effect It is suggested that some of the observed phenomenology of high- T c oxide superconductors resembles the behavior of the class of superconductors discussed here