Cu0.5Tl0.5Ba2Ca3Cu4−y Zn y O12−δ (y=0, 1.0, 2.0, 3.0, 3.5): Superconductor with Four ZnO2 Planes

A new Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−y Zn _y O_12−δ (y=0, 1.0, 2.0, 3.0, 3.5) superconductor with four ZnO₂ planes is reported. The structure of the material remains tetragonal for all Zn doping concentration. The substitution of Zn at CuO₂ planar site was carried out following Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−y Zn _y O_12−δ (y=0, 1.0, 2.0, 3.0, 3.5) formula. Contrary to all previous studies of Zn doping in all copper oxide high temperature superconductors, the zero resistivity critical temperature T _c(R=0), critical current density and quantity of diamagnetism increase with increased Zn concentration. The onset temperature of superconductivity in these samples was observed at 128 K and T _c(R=0) at 122 K for y=3.5. The volume of the unit cell observed through X-ray diffraction scan is found to decrease with increase Zn doping; promoting an increase in Fermi vector K _F and effective density of states which results in enhanced superconductivity parameters. The synthesis of Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−y Zn _y O_12−δ material by this method is highly reproducible.      

Effect of Sn-doping on the sintering process of MgB<Subscript>2</Subscript> and its superconductive properties

Bulk (Mg_1.02B₂)_1−x Sn _x samples (x = 0.0, 0.01, 0.03 and 0.05) were synthesized by in situ sintering at 850 °C for one hour. Based on the phase identification and microstructure observation, the Mg₂Sn and Sn impurities are found as the main impurities in Sn-doped samples. According to the magnetization measurements, the low doping level of Sn was observed to have small influence on the grain connectivity, and thus a high critical current density was maintained at low field. However, the values of the critical current density at high field in the Sn-doped samples show a little decrease.     

Cd-Doped Cu0.5Tl0.5Ba2Ca2Cu3?yCdyO10?δ (y=0,0.5,1.0,1.5,2.0) Superconductors

Superconducting properties of cadmium doped Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y Cd _y O_10−δ (y=0,0.5,1.0,1.5,2.0) samples have been studied using X-ray diffraction, resistivity, ac-susceptibility and FTIR absorption measurements. In X-ray diffraction studies these samples have shown to have tetragonal structure. The zero resistivity critical temperature and magnitude of diamagnetism are suppressed with the increased incorporation of Cd in the final compound. A change in the shape of FTIR absorption spectra, after doping, has shown the incorporation of Cd in the unit cell. A systematic hardening of the apical oxygen modes and softening of the CuO₂ planar modes of vibration with increased Cd doping have shown that it is incorporated in the unit cell of Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y Cd _y O_10−δ (y=0,0.5,1.0,1.5,2.0) superconductors. The FTIR absorption measurements of these samples have shown that hardening of the apical oxygen modes of types Cu(1)–O(2)–Tl and Cu(1)–O(2)–Cu(2)/Cd _y (y=0,0.5,1.0,1.5,2.0) increases with the increase of Cd doping in the samples. A softening of the CuO₂ planar oxygen mode Cu(2)–O–Cu(2) is also observed with the increased Cd doping in the final compound. It is most likely that hardening of the apical oxygen modes and the softening of the planar modes of vibration are associated damped harmonic oscillations produced by heavier Cd atoms in the CuO₂ planes, which suppress the phonon population from a desired level, reducing the magnitude of superconductivity in the final compound.     

Superconductivity and Electron–Phonon Interaction in?Cu0.5Tl0.5Ba2Ca2Cu3?yMyO10?δ (M=0, Si, Ge, Sn, and?y=0,?1)

Superconducting properties of Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y M _y O_10−δ (M=0, Si, Ge, Sn and y=0, 1) samples have been studied by using x-ray diffraction (XRD), resistivity, ac-susceptibility and Fourier Transform Infrared (FTIR) absorption measurements. We have tried to explore the role of the electron-phonon interaction in bringing about superconductivity in the Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y M _y O_10−δ material. A change in the cell parameters from XRD and the change in the shape of FTIR absorption spectra have shown the incorporation of doped elements (M=Si, Ge, Sn) in the unit cell of the final compound. It is most likely that the change in oxygen phonon modes is associated with the oscillations of heavier and lighter atoms in MO₂/CuO₂ planes. These oscillations of heavier and lighter atoms suppress the density of the phonon population from the desired level for optimum superconductivity in the final compound.     

AC Response of Cu0.5Tl0.5Ba2Ca2Cu3?xSnxO10?δ Superconductor

We have studied the ac response of Sn doped Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−x Sn _x O_10−δ superconductor samples from their ac-susceptibility measurements under different magnitudes of ac magnetic fields; H _ac=0.4, 4, 16 A/m. The samples with x=0.5 and 1.0 have shown strong flux pinning and intergrain coupling. However, the sample with Sn doping of x=1.5 has shown very poor flux pinning characteristics.     

Suppression of Superconductivity Due to Enhanced Co Doping in?Cu0.5Tl0.5Ba2Ca2Cu3?yCoyO10?δ Superconductors

Fluctuation Induced Conductivity (FIC) of Co-doped Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y Co _y O_10−δ (y=0, 0.05, 0.1, 0.5) Superconductors have been studied as to find the possible cause of T _c suppression in these compounds. We used Aslamazov–Larkin (AL) and Lawrence–Donaich (LD) equations to analyze the FIC of our samples. In this regard, it is observed that the data seems to fit well with 3D (three-dimensional) AL equations. In addition, crossover temperature from 2D to 3D has been found. The width of the temperature window for 3D conductivity is continuously decreasing with enhanced Co concentration. The shrinking of 3D temperature region with increased Co doping can be correlated with the suppression of superconductivity. It is observed that the coherence length and th interplanar coupling strength have also been decreased with increased Co doping, consistent with the magnetic pair-breaking effect.     

Be-Doped Cu0.5Tl0.5Ba2Ca1(Cu0.5Zn1.5)O8?δ Superconductors

We have enhanced the superconducting properties of newly discovered Cu_0.5Tl_0.5Ba₂Ca₁(Cu_0.5Zn_1.5)O_8−δ superconductor by doping Be at Ca sites. The superconducting properties, such as critical current density, in-field magnetic properties and quantity of diamagnetism, are enhanced by Be doping at the Ca sites. The decreased c-axis length and the volume of the unit cell have shown that the inter-ZnO₂-plane coupling is enhanced. We have not observed any localization of the carriers in the neighborhood of Zn atoms in Cu_0.5Tl_0.5Ba₂Ca_1−y Be _y (Cu_0.5Zn_1.5)O_8−δ (y=0, 0.15, 0.30, 0.45, 0.6) superconductors, as proposed in the previous studies. One must expect such effects if present, through the decreased c-axis length by Be doping. The decreased c-axis length results in enhancement of coherence length and Fermi velocity of the carriers, which in turn result in enhanced superconductivity parameters. The presence of Be at the termination ends of the crystals results in enhanced inter-grain coupling and substantially improved their weak link behavior. The optimization of the carriers in CuO₂/ZnO₂-planes have been found to enhance the T _c (R=0) and the magnitude of diamagnetism in Be-doped samples. Also the softening of phonon modes with the increased Zn doping evidenced the incorporation of Zn at the CuO₂ planar sites.     

Superconducting Properties of Zn-Doped Cu0.5Tl0.5Ba2Ca2Cu3?yZnyO10?δ Superconductors

A reproducible synthesis and characterization of Zn-doped Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y Zn _y O_12−δ (y=0, 0.5, 1.0, 1.5) superconductors at a relatively lower synthesis temperature of 840°C are studied by using X-ray diffraction, resistivity, ac-susceptibility and FTIR absorption measurements. The X-ray diffraction (XRD) studies of these samples have shown a tetragonal structure in which the c-axis length has been found to decrease with increased Zn doping. The critical temperature and magnitude of diamagnetism have not been significantly affected with the doping of Zn at this synthesis temperature. The magnitude of diamagnetism in the as-prepared undoped samples is decreased, whereas it remains stable (unchanged) in oxygen post-annealed samples. The apical oxygen phonon’s modes of type Tl–O_A–M(2) and Cu(1)–O_A–M(2) {where M=Cu/Zn} and the planar oxygen phonon modes of type M(2)–O_P–M(2) are also softened with the increase of Zn doping. We interpreted the softening of these oxygen related phonon modes linked with the decreased c-axis length, reduced John–Teller distortions and increased mass of Zn (65.38 amu) as compared to that of Cu (63.54 amu) (Kaplan et al., Phys. Rev. B 65, 214509, 2002).     

Fluctuation-Induced Conductivity of Cu0.5Tl0.5Ba2Ca2Cu2M1O10−δ (M = Si, Sn, Ge) Superconductors

Fluctuation-induced conductivity of Cu_0.5Tl_0.5Ba₂Ca₂Cu₂M₁O_10−δ (M = Si, Sn, Ge) superconductors has been studied from the resistivity vs. temperature data. The logarithmic plots of excess conductivity (Δσ) vs. reduced temperatures [ε=(T−T _c ^mf )/T _c ^mf ] show two crossover temperatures with three exponents. A distinct crossover from two-dimensional to three-dimensional conductivity in accordance with the 2D, 3D Aslamazov–Larkin equations has been observed in all the samples. Another crossover from 2D to 0D fluctuations has also been witnessed. FIC data analysis has been found to be quite useful in explaining the superconducting properties of the samples. Si and Sn doped samples have shown almost similar characteristics, while in Ge doped sample diverse behavior is observed.     

Effect of Pb Doping on the Superconducting Properties of?(Cu0.5?xPbxTl0.5)Ba2Ca2Cu3O10?δ

The effect of Pb doping on the superconducting properties of (Cu_0.5−x Pb _x Tl_0.5)Ba₂Ca₂Cu₃O_10−δ (x=0.0, 0.15, 0.25, 0.35) samples has been investigated. Lead is doped in Cu_0.5Tl_0.5Ba₂O_4−δ charge reservoir layer and at the CuO₂ planar sites. A multiphase material is achieved with the doping of Pb at the CuO₂ planar sites; however, a predominant single-phase (Cu_0.5−x Pb _x Tl_0.5)Ba₂Ca₂Cu₃O_10−δ (x=0.0, 0.15, 0.25, 0.35) material is synthesized with the doping of Pb at the charge reservoir layers. Formation of multiphase material with the doping of lead at the planar sites showed that its substitution at the planar site is not possible and the formation of PbO₂ planes is less likely. In the samples doped at the charge reservoir layer, the zero critical temperature [T _c (R=0)] is systematically depressed with the increased concentration of lead. The T _c (R=0) and magnitude of the diamagnetism are enhanced after post-annealing the samples in oxygen atmosphere. An apical oxygen mode is observed around 438 cm⁻¹ in undoped samples, which is shifted to 457–461 cm⁻¹ in the Pb-doped samples. This shift in the peak position is most likely associated with the connectivity of apical oxygen atoms with Pb atoms of (Cu_0.5−x Pb _x Tl_0.5)Ba₂O_4−δ (x=0.0, 0.15, 0.25, 0.35) charge reservoir layers. The presence of Pb in the charge reservoir layer and its increased concentration, somehow, stops the flow of mobile carriers to the conducting CuO₂ planes. The decreased density of mobile carriers diminishes the critical temperature and magnitude of diamagnetism in the final compound. The increased oxygen diffusion in the unit cell achieved by post-annealing in oxygen replenishes the concentration of carriers in conducting CuO₂ planes, which increases the T _c (R=0) and the magnitude of diamagnetism. These experiments have shown that the density of mobile carriers plays a vital role in the mechanism of superconductivity and their depressed density suppresses the superconductivity parameters.     

Suppression of Anti-ferromagnetism by K Doping in?(Cu0.5Tl0.5?xKx)Ba2Ca2Cu3O10?δ Superconductors

In the inner CuO₂ planes (IP) of (Cu_0.5Tl_0.5)Ba₂ Ca₂Cu₃O_10−δ superconductors the density of the carriers is in the under-doped state which promotes enhancement in the anti-ferromagnetism. Consequently, the critical temperature of the final compound is suppressed. In the present studies, we have enhanced the density of mobile carriers in the inner CuO₂ planes by doping K at the charge reservoir layer by preparing (Cu_0.5Tl_0.5−x K _x )Ba₂Ca₂Cu₃O_10−δ superconductors. The higher density of mobile carriers increases the Fermi velocity V _F, and hence enhances the superconductivity parameters such as T _c, H _c, and J _c. The main objective of these experiments was to suppress the anti-ferromagnetism in the final compound. We have enhanced the inter-plane coupling by partially substituting Be and Mg at the Ca sites. The melting point of the final compounds is significantly reduced with the incorporation of Be and Mg. The incorporation of these elements has been found to facilitate the formation superconductors with higher numbers of CuO₂ planes, i.e., (Cu_0.5Tl_0.5−x K _x )Ba₂Ca_3−y M _y Cu₄O_12−δ .     

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.      

The Effect of Co Substitution on the Crystal Structure and Electrical Resistivity of (La0.85Ag0.15)MnO3 Compounds

The effect of Co substitution on the crystal structure and electrical transport properties of La_0.85Ag_0.15Mn_1−y Co _y O₃ compounds (0≥y≤0.50) has been studied. Structural transition from rhombohedral to orthorhombic symmetry has been observed with Co doping. The lattice parameters are found to increase with doping up to y=0.20, then it decreases. It is explained on the basis of transition from high spin state to low spin state of Co ions with increase in doping beyond y=0.20. Ferromagnetic (FM) metallic behavior with colossal magneto-resistivity has been observed up to y=0.10. However, for y≥0.15 compounds, the temperature dependence of resistivity ρ(T) follows semiconducting behavior. The electrical resistivity in the metallic region could be explained based on electron–electron and electron–magnon scattering mechanisms. The data in the semiconducting region could be explained based on the variable range hopping model for y=0.2 and adiabatic small polaron hopping model for y≥0.3.      

Effect on Diamagnetism and Phonon Modes due to Mg and Be Doping at Ca Sites in Cu0.5Tl0.5Ba2Ca3?yMyCu4O12?δ (y=0 and 1.5 for M=Mg, Be) High Temperature Superconductors

We synthesized as prepared and oxygen annealed Cu_0.5Tl_0.5Ba₂Ca_3−y M _y Cu₄O_12−δ (y=0 and 1.5 for M=Mg, Be) High Temperature Superconductors (HTSCs) by using the solid state reaction method. It is observed that superconducting properties and the magnitude of the diamagnetism are enhanced by the doping of Mg and Be at the Ca site; this is due to smaller ionic sizes and higher electronegativities of Mg and Be as compared to Ca. Furthermore, interplanar coupling is also improved and shrinking of the axis length is the direct evidence of it. The carrier’s concentration in conducting planes is improved by carrying out post-annealing experiments in the oxygen atmosphere. Moreover, FTIR absorption measurements are also incorporated.     

A Study of the Magnetic Properties of?Bi1.64?xPb0.36CdxSr2Ca2Cu3Oy Superconductor

The flux pinning energy and magnetic properties of Bi_1.64−x Pb_0.36Cd _x Sr₂Ca₂Cu₃O _y (BPCSCCO) with x=0.0, 0.02, 0.04 and 0.06 were studied. A series of Bi-2223 superconductor samples with a nominal composition of BPCSCCO was synthesized and the effect of Cd substitution for Bi was investigated. As a result, Cd addition has been found to improve the superconducting properties of the Bi-Pb-Sr-Ca-Cu-O system. The effects of the annealing time and the amount of Cd doping on the structure, AC magnetic susceptibility, ρ–T curves and flux pinning energy were investigated. Also, for all samples the relation between the current and voltage in the mixed state was found to follow the model relationship V=α I ^β . The maximum value of β is 22.30, which is obtained for the sample with an annealing time of 270 h and a Cd content of 0.04.     

Evaluation of electrical conduction in iodine-doped polypyrrole

Electrical conductivity of polypyrrole has been measured after doping with different iodine concentrations. A thermally activated electrical conductivity was found which was pseudoohmic and increased with doping level. The results can also be fitted by log σ versusT ^−1/2 and log σ versusT ^−1/4 dependences, instead of the Arrhenius log σ versusT ⁻¹ dependence. From these results it was concluded that within the experimental scatter no significant distinction can be made between these different temperature dependence laws. Hence these data can only enable one to speculate about the true underlying transport model, rather than to draw decisive conclusions. Electrical conductivity results predicting the role of iodine dopant concentration on the conduction process of semiconducting polypyrrole are discussed.     

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.     

Synthesis of New Pb-Based 1232 Cuprate Containing Boron in?the?(Pb0.5B0.5) Sr2(Er3?x?yCexSry)Cu2Oz System

New Pb-based layered cuprates containing boron with the 1232 structure have been synthesized in the (Pb_0.5B_0.5)Sr₂(Er_3−x−y Ce _x Sr _y )Cu₂O _z system. Nearly the single 1232 phase samples are obtained for the nominal composition of 1.6≤x≤1.9 and y=0.2. A sample with the composition of (Pb_0.5B_0.5)Sr₂(Er_1.2Ce_1.6Sr_0.2)Cu₂O _z shows resistivity-dropping phenomenon below 10 K and it shows a diamagnetic signal at about 9.0 K, though the superconducting volume fraction is very small. From these results, the sample may well be a new Pb-based superconductor with the 1232 structure.     

Effect of calcium substitution on superconductivity and hole concentration in La1·5Ba1·5Cu3O z

The superconducting properties of single phase La_1·5−x Ca _x+y Ba_1·5−y Cu₃O _z , 0·0≤x≤0·60 (LC) and 0·0≤y≤0·70 (CB), compounds with tetragonal triple-perovskite structure are studied, using X-ray diffraction for their resistivity, a.c. susceptibility, and oxygen-content. La_1·5−x Ca _x Ba_1·5Cu₃O _z (LC) samples, 0·15≤x≤0·60, are superconducting withT _c ^R=0 between 40 and 74 K. With the increase inx, the oxygen content, hole concentration in the CuO₂ layers as well as theT _c increase. It is interesting to find that although the hole concentration and oxygen stoichiometry of the LaCa_0·5+y Ba_1·5−y Cu₃O _z (CB) compounds increase with the increase iny, theT _c ^R=0 remains nearly constant around 74 K fory=0·0−0·70. A correlation exists between theT _c and the hole concentration for LC and CB compounds.     

Influence of doping Nb5+ and Mn2+ on the PTCR effects of Ba0.92Ca0.05(Bi0.5Na0.5)0.03TiO3 ceramics

As a positive temperature coefficient of resistivity (PTCR) material, Ba_0.92Ca_0.05(Bi_0.5Na_0.5)_0.03TiO₃ ceramics with donor doping of Nb⁵⁺ and acceptor doping of Mn²⁺ were prepared by a conventional mixed oxide method. The influence of contents of Nb⁵⁺ and Mn²⁺ on the microstructure and PTCR characteristics of Ba_0.92Ca_0.05(Bi_0.5Na_0.5)_0.03TiO₃ ceramics sintered at 1,360°C for 2 h was investigated. The result showed that the Curie temperature (T _c) was shifted to a lower temperature with increasing of the content of Nb⁵⁺ and the resistance jump (ρ_max/ρ_min) was enhanced with doping of Mn²⁺. The grain size of ceramic sample decreased with increasing of contents of donor Nb⁵⁺ and acceptor Mn²⁺. The Ba_0.92Ca_0.05(Bi_0.5Na_0.5)_0.03TiO₃ ceramic with 0.4 mol%Nb⁵⁺ and 0.04 mol%Mn²⁺ exhibited a low ρ_RT of 5.0 × 102 Ω cm, a typical PTCR effect of ρ_max/ρ_min > 10³, and a T _c of 158°C.