Magnetovoltage Measurements and Hysteresis Effects in Polycrystalline Superconducting Y1Ba2Cu3O7−x /Ag in Weak Magnetic Fields

The magnetovoltage measurements (V–H curves) with different sweeping rates (dH/dt) of the external magnetic field in Ag-added polycrystalline YBa₂Cu₃O_7?x sample (YBCO/Ag) were investigated. The measurements of V–H curves were carried out as functions of the transport current (I) and temperature (T). Upon cycling H, all V–H curves measured for different values of I exhibit a clockwise hysteresis effects in forward region. The hysteresis effects in the V–H curve were interpreted in terms of two-level magnetic system, which considers the superposition of the external magnetic field and the local magnetic fields in the intergrain boundaries induced by magnetic dipole moment of neighbor superconducting grains. The analysis of magnetovoltage data showed that the flux trapping in the junction network has a negligible effect on the evolution of the V–H curves and the irreversibilities arise mainly from the flux trapping inside the grains. It is shown that the width of V–H curves shows a universal scaling behavior with respect to the applied current below the critical temperature T _c . The comparison of V–H curves of the YBCO/Ag sample with those of YBCO shows that adding Ag to the superconducting structure weakens the pinning properties of Josephson medium and provokes the instabilities in measured dissipation. The presence of Ag in the superconducting matrix causes marked decrease in hysteresis effects and makes the V–H curves dH/dt dependent. At high values of dH/dt, the instabilities and plateau regions in V–H curves increase significantly as compared to those of observed in YBCO. In addition, the interrelation between the evolution of V–H curves obtained for different values of I and the critical current I _c was demonstrated experimentally.     

Anomalous Behavior in Y1−x Pr x Ba2Cu3O7−δ

It is found that the samples of x=0.25 and 0.5 in the series samples of Y_1?x Pr _x Ba₂Cu₃O_7?δ demonstrate some anomalous behavior. First, the changes of T _c value of them are a bit different from their neighbor samples. Second, under a small magnetic field, i.e. 5–100 Oe, a little rise in the T _c value with the rise of the field is observed. The anomalous behavior is primitively explained by the local structure changes and the magnetic properties. It is attributed to local structural changes, i.e. the chemical bonds in the Cu–O₂ plane.     

Normal and Anomalous Pseudogap of Superconducting Y0.9−x Pr x Ca0.1Ba2[Cu1−y Zn y ]3O7−δ

Electrical resistivity measurements on the superconducting oxides of the compositions Y_0.9−x Pr _x Ca_0.1Ba₂[Cu_1−y Zn _y ]₃O_7−δ (0≤x≤0.20 and 0.0≤y≤0.10) sintered in oxygen atmosphere were carried out to obtain the normal and anomalous pseudogaps in underdoped and overdoped samples. It is observed that pseudogap temperature T ^* decreases with increasing doping level p in the underdoped case. For the overdoped sample with y=0.06, T ^* shows no p dependence.      

X-ray absorption spectroscopy of Pr x Y1−x Ba2Cu3O7−y single crystals

Substituting Y in orthorhombic (Y,RE)Ba₂Cu₃O₇ by any rare-earth element RE has generally little effect on the superconducting properties. For RE = Pr, however, superconductivity is completely suppressed. To elucidate this effect we have studied the unoccupied electronic structure of Pr_xY_1–xBa₂Cu₃O_7–y (x = 0.0,0.4, 0.8) by polarization-dependent O1s x-ray absorption spectroscopy on detwinned single crystals. Along with the comparison of undoped (y 0.9) to the doped materials (y 0.1), this allows a test of the current theoretical explanations for the suppression of superconductivity. While we can rule out models involving hole filling or charge transfer from planes to chains our data is consistent with approaches based on Pr4f–02p hybridization.     

Doping Effect of Y2Ba4CuMoO y Particles on Microstructure and Superconducting Properties in Polycrystalline YBa2Cu3O7−δ

Synthesis of polycrystalline YBa₂Cu₃O_7?δ (YBCO) + x mol.% Y₂Ba₄CuMoO _y (Y-24Mo1) (x=0.0, 0.1, 0.2, 1.0, 2.0 and 20.0) superconductors have been done in air via a standard solid state reaction technique. The effects of Y-24Mo1 particles on the microstructure and the superconducting transition temperature were systematically investigated through XRD and SEM techniques. When Y-24Mo1 was added to the YBCO, the orthorhombic structure maintained even at the highest concentration. Tiny peaks of Y-24Mo1 having (022) orientation and some other impurity were detected in the XRD spectrum when x=20.0. SEM results reveal the presence of some entities, and these entities changed with increase of Y-24Mo1 content. The Y-24Mo1 addition modifies the normal state resistivity of samples from metallic to semiconducting, and is accompanied by a two-step transition. Moreover, low Y-24Mo1 concentration leads to a relatively sharp superconducting transition and a broad transition take place for Y-24Mo1 addition greater than 0.2 mol.%. These results can be induced by the inhomogeneities, by the change of the effective coupling strength, and by the variation of the coupling between grains.     

X-Ray and High-Resolution Neutron Diffraction Studies on Nd x Y1−x Ba2Cu3O7−δ Superconductors

A synthesis of neodymium-substituted YBCO superconductor Nd _x Y_1?x Ba₂Cu₃O_7?δ (x=0,0.25,0.5,0.75,1) has been done using a dissolved method in order to obtain homogeneous crystals and higher critical current density. The effects of the substitutions on the structural and magnetic properties of the superconductors after sintering at 970 ^°C have been examined. Crystallinity of the synthesized powders was confirmed using X-ray and high-resolution neutron diffraction (XRD and HRPD) techniques. Rietveld analyses for both diffraction data sets gave increasing lattice parameters with addition of Nd content and decreasing orthorhombicity. Such addition also caused a decrease in occupancy of the oxygen in the O(4) site. Further investigation using SQUID showed critical temperature of the superconductors between 90.9 and 92.0 K. The critical current density (J _c ) was calculated from the magnetic hysteretic loop at 5 K as 40 kA?cm^?2 for Nd_0.25Y_0.75Ba₂Cu₃O_7?δ sample and 100 kA?cm^?2 for Nd_0.5Y_0.5Ba₂Cu₃O_7?δ sample. We also found that increasing Nd content on the Nd _x Y_1?x Ba₂Cu₃O_7?δ superconductor samples can improve their resilience of superconductivity and critical current density.     

Study of the Y1−x Er x Ba2Cu3O7−δ system: electrical resistivity measurements

In-plane electric conductivity of Y _1–x Er _x Ba ₂ Cu ₃ O _7– single crystals with Er content x ranging from 0 to 0.48 have been measured as a function of temperature from 85 K to 300 K. The temperature dependencies of reduced resistance for all sample are identical suggesting that erbium substitution for yttrium does not produce sizable scattering of charge carriers.     

Stoichiometry Analysis and Normal-State Properties of SmBa2Cu3−x Ru x O7−δ Superconducting Phase

The electrical resistivity of normal and superconducting states for SmBa₂Cu_3?x Ru _x O_7?δ (Sm-123) phase with 0.00 ≤ x ≤ 0.50, prepared by the conventional solid-state reaction technique, was studied. X-ray powder diffraction (XRD), scanning electron microscope (SEM), particle-induced X-ray emission (PIXE), Rutherford backscattering spectroscopy (RBS) and electrical resistivity measurements were performed in order to investigate the effect of Ru⁴⁺ ions substitution in Sm-123 phase. Both the phase formation and superconducting transition temperature T _c enhance up to x = 0.05. For x > 0.05, suppression of both the phase formation and T _c is observed and the superconductivity is completely destroyed around x = 0.50. The normal-state electrical resistivity was analyzed by the two- and three-dimensional variable range hopping (2D-VRH and 3D-VRH) and Coulomb gap CG. The dominant mechanism for Sm-123 phase is CG with x ≤ 0.20 while is 3D-VRH for x ≥ 0.30.     

Terahertz Conductivity of Overdoped Y1−x Ca x B2Cu3O7−δ

In the current work we have investigated the terahertz response of Ca overdoped YBaCuO thin films using both time and frequency domain spectroscopy methods. For both methods a basic data analysis was performed using the two fluid and a variable dielectric function (VDF) models. The imaginary part of the conductivity was proportional to 1/ω, known from the delta-function response. The real part of conductivity showed a well known frequency and temperature dependence, where it increases below T _c and obtains maxima at about 50 K. However, a sharp decrease of the real part of the conductivity was observed at about 10 cm^?1. This decrease occurs below T _c and becomes dominant as temperature decreases. It was observed on the 5% and 10% Ca doped samples but it was more dominant on the 10% case. Moreover, this sharp decrease in σ ₂(ω)at 10 cm^?1 was not observed in optimally doped YBCO samples. We would like to stress at this point that these values are much smaller than those obtained by Microwave and Tunneling measurements, arguing for the existence of a complex order parameter in the overdoped regime with an imaginary component of about 1.8 meV.     

Bond Angle Study of Self-compensating Y1−x Ca x Ba2−x La x Cu3O z System

Single-phase samples of a self-compensating Y_1?x Ca _x Ba_2?x La _x Cu₃O _z system were synthesized through a solid-state reaction method with x<0.4. The structure of all samples were characterized by X-ray diffraction and refined by the Rietveld method. Superconducting properties have been investigated by the DC magnetization measurement. The critical temperature (T _c ) decreases evidently with the increment of x although the carrier concentration remains constant in the samples for different doping level. Careful study of the chemical bonds in the crystalline lattice demonstrates that the T _c is closely correlated to four pairs of bond angles in the unit cell. The analysis indicates that crystalline structure is one of the important factors to high-T _c superconductivity, and its influence is independent of the carrier concentration.     

Suppression of superconductivity in the Er1− x Pr x Ba2Cu3O7−δ

Measurements of electrical resistivity. X-ray diffraction patterns, magnetic susceptibility and thermoelectric power of the Er_1-x Pr _x Ba₂Cu₃O_7- system have been made. The superconducting transition temperature was found to decrease monotonically with praseodymium concentration, x. From the susceptibility data, it was determined that the valence of praseodymium lies between +3 and +4. The thermoelectric power was found to increase with x, and the slopes of dS/dT were negative except for the case x= 0. The. tendency of the thermopower to change with increasing praseodymium concentration has been qualitatively explained using the theory for strongly correlated systems.     

Excess Conductivity Analysis of Tl1−x Y x Ba2Ca2Cu3O10−δ Superconductors

Excess conductivity analyses of resistivity data of Tl_1?x Y _x Ba₂Ca₂Cu₃O_10?δ (x=0, 0.04, 0.06, 0.08, 0.1) samples have been carried out by following Lawrence & Doniach (LD) and Maki-Thompson (MT) models. In the critical regime important superconductivity parameters have been elucidated by employing Ginzburg-Landau number N _G of Ginzburg Landau theory. Our samples have shown a decrease in the T _c (R=0) and magnitude of diamagnetism with increased Y-doping. The cell parameters and volume of the unit cell increase with doping of Y⁺³ in Tl_1?x Y _x Ba₂Ca₂Cu₃O_10?δ (x=0, 0.04, 0.06, 0.08, 0.1) which shows a decrease in the density of charge carriers in the conducting CuO₂ planes. Since the Fermi vectors of the carriers, K _F=[3π ² N/V]^1/3=[3π ² n]^1/3, their coherence length along the c-axis, ξ _c=?K _F/2mΔ, and the Fermi velocity, V _F=?K _F/m depend on density of mobile charge carriers, the doping of Y⁺³ suppresses it and hence the superconductivity parameters. We have confirmed these conjectures with the excess conductivity analyses (FIC) of our conductivity data. The FIC analysis of conductivity data has shown a decrease in the values of ξ _c, V _F, B _c(0), B _c1(0) and J _c(0) with increase doping of Y (except for the samples with x=0.04). The width of two dimensional conductivity regimes is shrunken with increased Y-doping. From these studies it is concluded that presence of Y⁺³ in the unit cell of TlBa₂Ca₂Cu₃O_10?δ impedes the flow of the mobile charge carriers to the conducting CuO₂ planes which induce suppression in the superconductivity parameters. The studies also stress the vital role of mobile charge carriers in the mechanism of high temperature superconductivity.     

Superconducting and Normal-State Properties of (Y1?x Bi x )Ba2Cu3O7?δ (x=0.00–0.30)

The effect of Bi in (Y_1−x Bi _x )Ba₂Cu₃O_7−δ for x=0.00–0.30 has been investigated by powder X-ray diffraction method, dc electrical resistance measurements and scanning electron microscopy. The resistance versus temperature curves showed metallic behavior for samples with x≤0.20 and semiconductor-like behavior for x=0.25 and 0.30. The resistance versus temperature curves showed onset transition temperature (T _{c onset}) between 89 and 93 K and a broadening of the superconducting-transition width with increasing Bi content for all samples. T _{c onset} decreased linearly indicating a pair-breaking mechanism in T _c suppression for x=0 to 0.3. The non-systematic broadening of the transition width may be due to the inhomogeneities in the samples as Bi content was increased. No substitution of Bi for Y was observed.     

Microwave Surface Impedance of Nd-rich Nd 1+x Ba 2−x Cu 3 O 7−δ Thin Films

Surface impedance measurements on highly c-axis epitaxial Nd _1+x Ba _2–x Cu ₃ O ₇ (x=0, 0.09 and 0.12) films grown by d.c. magnetron sputtering on LaAlO ₃ substrates are presented. It is found that the zero temperature London penetration depth correlates well with the critical temperature of the films and with the corresponding number of carriers. The low temperature penetration depth follows a linear T law for optimally doped Nd123 sample and a T ² law in Nd-rich samples. In the case of the heavily underdoped samples (T _c < 60K) the T ² law extends to temperatures higher than T _c/2. The possible role of the Nd/Ba ions substitution on the penetration depth and surface resistance is discussed.     

Permanent Magnet Enhancement of Fully Superconducting MgB2-YBa2Cu3O7−x Bearing

This paper reports the experimental verification and improved concept of the previously reported fully superconducting magnetic bearing using bulks on both the rotor and the stator in a cylindrical geometry. Experimental measurements on pulsed magnetisation and levitation force between a magnetised 25.5 mm diameter YBCO bulk inside an MgB₂ hollow cylinder are reported proving the concept of a bulk–bulk bearing. The maximum force achieved after field cooling of the MgB₂ bulk in the field of a 1.68 T magnetised YBCO bulk was 501 N. The improved concept relies on additional ring shaped permanent magnets placed on the bottom of the MgB₂ cylinder. These rings create additional axial force in the bearing system. Permanent magnet rings can boost the force for the existing bearing design by enhancing the field trapped in the MgB₂ tube and providing a ‘cushion’ of magnetic field for the bottom YBCO bulk by exploiting zero field cooling. In particular the enhancement of the force is largely due to the favourable distortion of the trapped field in the MgB₂ cylinder and is much greater than the direct repulsive force between a magnetised YBCO bulk and permanent magnet only. Various permanent magnet configurations are evaluated by modelling of levitation force using the perfectly trapped flux model. The high force densities of bulk–bulk bearings allow them to support large loads useful for application such as flywheel energy storage.     

The Effect of Cadmium Substitution on the Superconducting Properties of Tl1−x Cd x Ba2Ca2Cu3O9−δ Compound

In order to study how Cd substitution for Tl changes, the superconducting properties of Tl_1?x Cd _x Ba₂Ca₂Cu₃O_9?δ compound with x=0, 0.1, 0.2 and 0.3. We have prepared samples with Cd in place of Tl by the solid state reaction method. It has been found that the increase of the Cd concentrations of all samples produced increases the transition temperature from 113 to 123 K and changes the mass density ρ _m, C/a and volume fraction V _Ph(1223).. XRD shows that the partial substitution of Tl with Cd is helpful to form the superconducting 1223 phase (TlBa₂Ca₂Cu₃O_9?δ ). It has been shown that samples mainly contain tetragonal phase 1223 with a few additional mixed 1212 and 1201 phases.     

Formation of gas folds during pressing of Y1Ba2Cu3O7−δ powder

Conditions for the formation of density discontinuities during pressing of Y₁Ba₂Cu₃O_7−δ powder are described. The experimental results are explained using catastrophe theory. It is hypothesized that the density discontinuities are gas folds formed during pressing. Pis’ma Zh. Tekh. Fiz. 25, 74–78 (August 12, 1999)     

Cation composition variation observed in Y1Ba2Cu3O7−z

The cation concentration ratio in the individual grains (1 to 25 μm) of four Y₁Ba₂Cu₃O_7−z pellets was investiga spectroscopy. A spread in the cation composition was observed even in a sample showing a narrow superconducting transition. The concentrations of Y, Ba and Cu were in the ranges 12–20, 30–35 and 48–53 at%, respectively. It is, therefore, likely that the Y₁Ba₂Cu₃O_7−z structure covers a finite area on the YO_1.5-BaO-CuO phase diagram.