The Role of Ca in Superconducting and Magnetic Properties of Y1?x Ca x Ba2Cu3O7?δ (x=0.0–0.30)

We report the structural, superconducting, magnetic and granular properties of Y_1−x Ca _x Ba₂Cu₃O_7−δ ; x=0.0, 0.1, 0.2 and 0.3. Rietveld fitted X-ray diffraction data confirm the single-phase formation for all the samples. The orthorhombicity of the parent compound (x=0.0) decreases and becomes closer to the tetragonal structure for higher Ca concentration. The superconducting transition temperature (T _c) decreases with increasing Ca content due to both over-doping and ensuing disorder in the superconducting CuO₂ planes with Ca doping. DC susceptibility measurements reveal a reduction of the Meissner fraction with Ca doping, suggesting the flux pining effect. The ac susceptibility measurements reveal the enhancement of grain coupling with increasing Ca content in the system. The average grain size is found to increase with Ca doping. Scanning Electron Microscopy (SEM) observations indicate better grain connectivity in terms of narrow grain boundaries for Ca doped samples. It is concluded that limited Ca doping enhances the superconducting performance of the YBa₂Cu₃O_7−δ system.     

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.     

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.     

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.     

Co-doping effects of Ca and Ce on the Superconducting Properties in Y1?x Ca x (Ba1?y Ce y )2Cu3O7?δ

A series of Y_1?x Ca _x (Ba_1?y Ce _y )₂Cu₃O_7??? (0??x??0.3, 0??y??0.3) polycrystalline superconductor samples were prepared using the solid-state reaction technique. The phase identification, crystal structure, and superconducting transition temperature (T _c ) were studied by means of X-ray diffraction (XRD) and resistivity measurements. The results indicted that the phase of samples changed from orthorhombic phase to tetragonal phase with increasing Ca concentration x and Ce concentration?y, and Ce did not form the superconducting structure. The lattice constants had a little change. The a-axis and c-axis lattice parameters increased. The b-axis lattice parameter decreased. The T _c and resistance had an obvious dropping tendency with increasing Ca and Ce concentrations. The transition width became sharper with the increase of x (=y). We drew a conclusion that the Ce-doping had an effect for strengthening the intergrain connectivity, and it counteracted the weakening effect of Ca-doping which introduced the hole causing a reduction in the interlayer coupling strength.     

The Effects of Chemical Doping and Hydrostatic Pressure on T c of Y1−y Ca y Ba2Cu3O x and NdBa2Cu3O x Single Crystals

Westudied T _c of Y Ba ₂ Cu ₃ O _x (Y123), Y _0.89 Ca _0.11 Ba ₂ Cu ₃ O _x (YCa123) and NdBa ₂ Cu ₃ O _x (Nd123) single crystals with various oxygen contents x. Compared to T _c (x) of Y123 the T _c (x) curves of YCa123 are shifted to lower oxygen contents and the maximum transition temperature T _c,max decreases with increasing Ca content whereas in Nd123 T _c (x) is shifted to higher oxygen contents and T _c,max is increased. According to the universal parabolic T _c (n _h ) behavior the differences in T _c (x) of Y123, YCa123 and Nd123 can be ascribed to different hole concentrations n _h in the CuO ₂ planes caused by doping via changes in chemistry or structure. In order to study the influence of structural changes on T _c we examined the hydrostatic pressure effect dT _c /dp (p 0.6GPa). In the underdoped region, at n _h 0.11, the examined compounds show a peak in dT _c /dp which is very pronounced for systems with well ordered CuO chains. As this peak occurs at the same n _h in all investigated systems it is not caused by oxygen ordering, but its origins might be found in a strong influence of lattice deformations on the electronic structure.     

Doping of Nb to the Ba and Cu Sites in the Y0.6Gd0.4Ba2Cu3O7−δ

Polycrystalline samples of Y_0.6Gd_0.4Ba_2?x Nb _x Cu₃O_7?δ and Y_0.6Gd_0.4Ba₂Cu_3?x Nb _x O_7?δ with different Nb contents (x=0.025, 0.075, 0.125, 0.175 and 0.225) were prepared using the solid state reaction method. Structural and electrical properties of new compounds were investigated with optical microscope, scanning electron microscopy (SEM), X-ray diffraction (XRD), Four Point Probe (FPP). The results indicated that Nb constituted YBa₂NbO₆ structure instead of substituting to the Y_0.6Gd_0.4Ba₂Cu₃O_7?δ structure. YBa₂NbO₆ structures gathered between grains. Transition temperature did not change significantly, but critical current (J _c) values decreased with increase of Nb concentration. The underlying reason is that the oxygen concentration of compounds remained unchanged due to Gd; thus, T _c values did not change significantly.     

Fluctuation Induced Conductivity in Hg-Doped (Cu0.5Tl0.5?x Hg x )Ba2Ca3 Cu 4O12?δ Superconductor

The fluctuation induced conductivity (FIC) analysis on resistivity data of the Hg-doped (Cu_0.5Tl_0.5−x Hg _x )Ba₂Ca₃Cu₄O_12−δ (x=0, 0.15, 0.25, 0.35) superconductor has been carried out in the light of the Aslamazov–Larken (AL) theory. The superconducting properties have been improved with Hg-doping (up to x=0.25) at Tl sites in the Cu_0.5Tl_0.5Ba₂O_4−y charge reservoir layer. These samples are also post-annealed in oxygen to improve intergrain connectivity and to optimize the carriers’ density in CuO₂ planes. The results of FIC analysis have shown three-dimensional (3D) and two-dimensional (2D) fluctuations in the order parameters. It has been observed that there is an increase in the cross-over temperature (T _o) as well as in zero resistivity critical temperature {T _c(0)} with Hg-doping. These properties have been further improved after post-annealing in oxygen, which may be due to improved inter-grain coupling and optimization of mobile carriers in CuO₂ planes. A direct correlation between T _o, T _c(0) as well as zero temperature coherence length ξ _c(0) has been observed.     

Pore Structure and Transport Properties in Bulk YBa2Cu3O7?δ Doped with Sb2O3

In this work we have studied the pore structure and electrical transport properties of superconducting YBa₂Cu₃O_7?y polycrystalline samples doped by the addition of different Sb₂O₃ concentrations, i.e. resulting in (YBa₂Cu₃O_7?y )_1?x (Sb₂O₃) _x . The samples were prepared through the solid-state reaction method. Rietveld analyses of X-ray diffraction data were used to investigate how the lattice parameters are modified by doping. Specific superficial area measurements identified the principal characteristics of the pore structure of the samples and how these properties change with doping. The superconducting properties were studied by using zero field cooling magnetization and transport critical current measurements. The critical temperature of the samples does not depend on the doping level, but their transport critical current density strongly decreases as the Sb₂O₃ concentration is increased. Our experimental results suggest that for the samples studied here there is not a direct correlation between the modification by doping of both, the pore structure and the transport critical current density.     

Magnetic Transport Properties in GdBa2Cu3?x Ru x O7?δ Superconducting Phase

Bulk superconducting samples of type GdBa₂Cu_3?x Ru _x O_7?δ phase, Gd-123, with?x ranging from 0.0 to 0.15 were prepared by the conventional solid-state reaction technique. X-ray powder diffraction (XRD) and the electrical resistivity measurements were performed in order to investigate the effect of Ru⁴⁺ ions substitution on Gd-123 phase. Enhancement of the phase formation and the superconducting transition temperature T _c for GdBa₂Cu_3?x Ru _x O_7?δ phase up to x=0.05 was observed. The effect of magnetic field up to 4.4?kG on the electrical resistivity behavior of the prepared samples was studied to investigate the flux motion of this phase. The derived flux pinning energy?U, based on the thermally activated flux creep TAFC model, decreased with increasing the magnetic field?B. The flux pinning energy followed the exponent behavior as U(B)～B ^?β . The superconducting transition width ΔT increased as the magnetic field increased, showing the scaling relation as ΔT～B ⁿ . Using Ambegaokar and Halperin AH theory, the magnetic field and temperature dependence of U was found to be U(B,T)～ΔTB ^?η , η=β+n. The critical current density J _c (0) enhanced up to x=0.05, beyond which it decreased with further increase in Ru-content.     

Synthesis and Excess Conductivity Analyses of Ce-Doped Tl1−x Ce x Ba2Ca2Cu3O10−δ Superconductors

The role of mobile carriers density supplied by TlBa ₂ O _4?δ charge reservoir layer is investigated by doping Ce in TlBa ₂Ca ₂Cu ₃ O _10?δ superconductors. The T _c(onset) and T _c(R= 0) versus Ce contents have shown almost a parabolic-shaped phase diagram which is interpreted in terms of hole’s doping. The magnetic measurements showed that magnitude of diamagnetism is gradually suppressed with Ce doping at Tl site. Fourier transform infrared (FTIR) revealed that the phonon modes associated with apical oxygen atoms type Cu(2)–O _A –Tl are softened, whereas the planar oxygen modes are hardened with the doping of Ce the unit cell. In the excess conductivity, analyses of conductivity data of Tl _1?x Ce _x Ba ₂Ca ₂Cu ₃ O _10?δ samples have shown a suppression of T _{c m f} (K) and the width of 2D conductivity regimes, whereas the coherence length along the c-axis ξ _c(0) and the interlayer coupling J are enhanced with the doping of Ce ⁺⁴ in the charge reservoir. The increase in ξ _c(0) is interpreted in the form of homogenization of the carrier’s density of carriers the CuO ₂ planes. The process of homogenization is accomplished with the reduction of density of carriers in the outer CuO ₂ that consequently promotes increase in the phase relaxation time, τ _? , of the carriers, their Fermi velocity, and the energy required to break apart the Cooper pairs.     

Superconductivity in Co and Li Substituted Cu0.5Tl0.5Ba2Ca2Cu3?x Co x O10?��

We have tried to explore the possible sources of suppression of superconducting properties in cobalt substituted Cu_0.5Tl_0.5Ba₂Ca₂Cu_3?x Co _x O_10??? (x=0,0.05,0.1,0.5) superconductor. It can be expected from these studies that Co substitution at Cu sites in the CuO₂ planes somehow localizes the carriers and decreases the mobile carriers density. The density of mobile carriers in the CuO₂ planes can be restored by partial substitution of Li at Tl sites in Cu_0.5Tl_0.5Ba₂O_4??? charge reservoir layer of Cu_0.5Tl_0.5Ba₂Ca₂Cu_3?x Co _x O_10??? superconductor; since alkali metals are known to lose their outermost ??s-orbital?? electron easily, which can be supplied to the CuO₂ planes. The post-annealing experiments on these samples have been carried out in flowing oxygen and nitrogen and the motivation behind these extreme post-annealing conditions was to optimize the carriers density in the CuO₂ planes of Cu_0.5Tl_0.5Ba₂Ca₂Cu_3?x Co _x O_10??? superconductor. The optimized free carrier density in the CuO₂ planes enhances the superconducting properties of the final compound.     

Fluctuation Conductivity of RE1−2x Ca x M x Ba2Cu3O7−δ (RE=Nd,Y and M=Pr,Th) Superconductors

Polycrystalline samples of RE_1?2x Ca _x M _x Ba₂Cu₃O_7?δ with RE=Nd, Y and M=Pr, Th (with 0.0≤x≤0.10), superconductors were prepared by the standard solid-state method. Resistivity was measured as a function of temperature and doping concentration x. Excess conductivity was analyzed using the modified Lawrence-Doniach (LD) expressions. The fluctuation regions, crossover temperatures, coherence lengths, and effective layer thickness were obtained and the values were compared for both samples. For both samples, it was found that with increasing doping, the crossover temperatures were reduced, while the coherence length decreased. The upper critical field and critical current density were increased with increasing doping concentration due to the introduction of disorder and the enhancement of flux pinning by charge neutral doping. Furthermore, the coherence lengths of the Nd-based samples are larger than that for the Y-based samples by a factor 2. It was found that the value of critical current density in Nd(CaPr)-123 is higher than Y(CaTh)-123, from which it is suggested that CaPr doping is more effective than CaTh doping.     

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)     

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.     

Magnetically and Thermally Modulated Microwave Absorption in Y1Ba2Cu3O7–x Single Crystal near Tc

The microwave absorption R in the Y ₁ Ba ₂ Cu ₃ O _7–x single crystals was investigated near T _c 92K and in the external magnetic field 0 < H 9kOe. A modified ESR spectrometer was used in the experiment. The method of temperature modulation, along with the usual method of magnetic-field modulation, was first applied in studying of the microwave response of these crystals. Peaks in the temperature dependencies of the signals R/H and R/T observed in the vicinity of T _c were differently shaped and shifted one with respect to another. The evolution of the peaks with variation of the magnetic field and angle between H and the c-axis was traced. It has been shown that the observed difference of the temperature dependencies of the derivatives R/H and R/T occurs due to the field-induced broadening of the superconducting transition, which is inherent in the high-T _c superconductors.     

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.     

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.     

Enhancing Superconductivity in Low-dimensional TlBa2Ca2Cu3O10?δ System

Mg-doped Tl(Ba_2?x Mg _x )Ca₂Cu₃O_10??? (x=0, 0.25, 0.50, 0.75, 1.0, 1.25, 1.5) superconductors are synthesized at the normal pressure and the possible mechanism of superconductivity in these compounds is studied. Tl(Ba_2?x Mg _x )Ca₂Cu₃O_10??? samples have shown an orthorhombic crystal structure and their c-axis length decreases up to Mg-doping of x=0.75 and then increases up to Mg-doping of x=1.50. In these studies we have investigated the role of decreased thickness of charge reservoir layer on the mechanism of superconductivity. The T _c (R=0) in as-prepared Tl(Ba_2?x Mg _x )Ca₂Cu₃O_10??? (x=0, 0.25, 0.5, 0.75, 1.0, 1.25) samples was 100, 98, 101, 102, 100, 96?K and in the oxygen post-annealed samples the T _c (R=0) is observed around 99, 98, 108, 127, 109, 97?K, respectively. The magnitude of the superconductivity after Mg-doping is improved in Tl(Ba_2?x Mg _x )Ca₂Cu₃O_10??? (x=0.25, 0.5, 0.75, 1.0, 1.25) samples. It was observed from the FTIR absorption measurements that the phonon modes related to CuO₂ planar oxygen atoms are hardened with the doping of Mg in the charge reservoir layer. These studies have shown that the thickness of charge reservoir layers decreases with Mg-doping, which most likely makes the charge transfer mechanism more efficient, which increases the magnitude of superconductivity in the final compound.