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.     

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).     

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 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.     

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.     

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.     

Improvement of Superconductivity with the Modification of Charge Reservoir Layer in (Cu0.5Tl0.5?xMx)Ba2Ca2Cu3O10?δ

The relatively higher electronegative elements (M = Pd, Nb, Bi, Hg) have been partially doped at Tl sites in Cu_0.5Tl_0.5−x M _x Ba₂O_4−δ (x=0,0.25) charge reservoir layer of Cu_0.5Tl_0.5−x M _x Ba₂Ca₂Cu₃O_10−δ superconductor. These elements may retain more oxygen in the charge reservoir layer due to their higher electronegativity as compared to Tl, and the higher population of oxygen in the charge reservoir layer can optimize the charge carriers’ density in the conducting CuO₂ planes. The optimum density of mobile charge carriers in the conducting CuO₂ increases Fermi wave-vector K _F and Fermi velocity v _F of the carriers, which results in the improvement of superconducting properties of the material.     

Role of Mobile Charge Carriers and Fluctuation Induced Conductivity in (Cu0.5Tl0.5?xKx)Ba2Ca3Cu1Zn3O12?δ Superconductor

The main role played by mobile carriers and fluctuation induced conductivity (FIC) in (Cu_0.5Tl_0.5−x K _x )Ba₂Ca₃Cu₁Zn₃O_12−δ (x=0, 0.25) superconductor samples are analyzed in the light of the Aslamazov–Larken (AL) theory. The enhancement of three-dimensional (3D) conductivity and grain size is achieved with K-doping at Tl sites in the (Cu_0.5Tl_0.5−x K _x )Ba₂O_4−δ charge reservoir layer, which are further improved by post-annealing the samples in oxygen. The improvement of these factors causes the increase in cross-over temperature (T _o), zero resistivity critical temperature {T _c(R=0)} and T _c(onset) of diamagnetism to their higher values. We have also evaluated the exponents (λ _2D and λ _3D), zero temperature coherence lengths ξ _c(0), and interlayer coupling (J) for these samples and tried to correlate them to the superconductivity order parameters.     

Preparation of Y0.5[Yb1?xNdx]0.5Ba2Cu3Oz Superconductors Using Nitrate Routes

The chemical solution nitrate route was used to prepare Y_0.5[Yb_1−x Nd _x ]_0.5Ba₂Cu₃O _z superconducting phase with x=0.1, 0.2 and 0.3. Different calcination temperatures from 870 to 900 °C were used. Both XRD and DSC techniques were utilized in monitoring the Y_0.5[Yb_1−x Nd _x ]_0.5Ba₂Cu₃O _z superconducting phase and the non-superconducting impurity phases such as BaCuO₅, CuO and RE₂BaCuO₅. These two techniques were used to study the effect of impurity phases on the decomposition temperature of Y_0.5[Yb_1−x Nd _x ]_0.5Ba₂Cu₃O _z superconducting phase. The value of x, the Nd amount, and the calcination temperature both influenced the type and number of impurity phases, which resulted in changing the eutectic and peritectic reaction temperatures.     

A new series of mercury-based high-Tc superconductors (Hg, W)Sr2(Y1?x Cax)Cu2

A new series of superconducting mercury-based layered cuprates has been synthesized. X-ray diffraction indicates that the compounds with formula Hg_0.8W_0.2Sr₂Y_1–x Ca _x Cu₂O₆₊ (0.3x0.6) exhibit the 1212 structure with space groupP _4/mmm . The investigation of superconductivity determined by electrical resistance measurement shows that the superconducting transition temperature (T _c , onset) of Hg_0.8W_0.2Sr₂Y_0.4Ca_0.6Cu₂O₆₊ is up to 94 K.     

Structural and dielectric properties of Na1?xBaxNb1?x(Sn0.5Ti0.5)xO3 ceramics

Lead-free (1 − x)NaNbO₃/xBa(Ti_0.5Sn_0.5)O₃ (x = 0.1, 0.125, 0.15, 0.175, 0.2, and 0.3) ceramics were elaborated by the conventional ceramic technique. Sintering has been made at 1523 K for 2 h. The crystal structure was investigated by X-ray diffraction with CuKα radiation at room temperature. As a function of composition, these compounds crystallize with tetragonal or cubic symmetry. Dielectric measurements show that the materials have a classical ferroelectric behavior for compositions in the range 0.10 ≤ x ≤ 0.15 and relaxor one for compositions in the range 0.15 < x ≤ 0.30. Temperatures T _C or T _m decrease as x content increases. The ferroelectric behavior has been confirmed by hysteresis characterization. For x = 0.1, a piezoelectric coefficient d ₃₁ of 42.146 pC N⁻¹ was obtained at room temperature. The evolution of the Raman spectra was studied as a function of temperature for x = 0.1.     

Optimum Synthesis Temperature of (Cu1?xTlx)Ba2Ca3Cu4O12?δ Superconductor

The effect of synthesis temperature on the superconducting properties of (Cu_1−x Tl _x )Ba₂Ca₃Cu₄O_12−δ (CuTl-1234) samples has been explored. Almost all the superconducting parameters studied in this research work are observed to be suppressed with the increase of synthesis temperature beyond 880 °C, which may be due to impurities caused by the volatility of some constituents such as thallium and oxygen deficiencies as well in the final compound. The Fluctuation Induced Conductivity (FIC) analysis has shown a decrease in the cross-over temperature (T ₀) and the shift of three-dimensional (3D) Aslamasov–Larkin (AL) regions to the lower temperature with the increase of synthesis temperature beyond 880 °C. A direct correlation between the cross-over temperature (T ₀), the zero temperature coherence length {ξ _c (0)}, the zero resistivity critical temperature {T _c (R=0)} as well as carrier concentration has also been observed.     

Enhanced Superconductivity in?(Cu0.5Tl0.25M0.25)Ba2Ca2Cu3O10?δ Samples

The elements (M = Li, Na, K) of group I-A of the periodic table having lower electronegativity as compared to that of Tl have been partially doped at Tl sites in the (Cu_0.5Tl_0.25M_0.25)Ba₂O_4−δ charge reservoir layer of (Cu_0.5Tl_0.25M_0.25)Ba₂Ca₂Cu₃O_10−δ superconductor. The objective was to investigate the role of electron–phonon interactions and mobile charge carriers in the oxide high temperature superconductors. It has been observed from these studies that the superconductivity parameters were improved after the doping of lower electronegative elements in the charge reservoir layer. The substitution of lower electronegative elements retains smaller amount of oxygen in (Cu_0.5Tl_0.25M_0.25)Ba₂O_4−δ charge reservoir layer, which in turn facilitates the transfer of mobile charge carriers to the conducting CuO₂ planes.     

New Pb-based 1212 Cuprate Superconductors Containing Sulfur, (Pb0.75S0.25)Sr2 (Y1?xCax)Cu2Oz

New Pb-based 1212 layered cuprates containing sulfur have been synthesized in the (Pb_0.75S_0.25) Sr₂ (Y_1−x Ca _x ) Cu₂O _z system. X-ray diffraction analysis shows that the almost single-phase samples are obtained within a region of 0.0≤x≤0.5. The crystal structure of each sample has a tetragonal symmetry with the typical lattice constants a=0.3837 nm and c=1.186 nm. As Ca content x is increasing, the semiconductor-like behavior is suppressed. But after only annealing under ambient O₂ pressure, none of the samples show any trace of superconductivity. On the other hand, when the samples are annealed under high O₂ pressure of about 13.6 MPa, they show resistivity dropping phenomenon in a region of 0.5≤x≤0.7. Among them, the lowest resistivity sample with x=0.6 has an onset temperature of the resistivity dropping at about 22.5 K. Moreover, this sample shows a diamagnetic signal at about 21.5 K. These phenomena are attributed to superconductivity.     

Synthesis and Superconductivity of New Cuprates (Pb0.8W0.2)Sr2(Nd1 ? xCax)Cu2O7 ? δ

New cuprates with nominal composition (Pb_0.8W_0.2)Sr₂(Nd_{1 – x} Ca _x )Cu₂O_{7 –} (0x1) were synthesized by solid-state reaction in N₂. The crystal structure was characterized by X-ray powder diffraction (XRD) as tetragonal. Direct current electrical resistance measurements were applied to check the existence of superconductivity in these cuprates. Superconductivity with T _c (onset) up to 82 K is observed when x = 0.6. Synthesis in N₂ is necessary in obtaining superconductivity in these cuprates. Preparation in air or post-treatment in flowing oxygen destroys superconductivity. A comparison is made with previous Pb-based 1212 superconducting oxides. The valence of Pb and the possible position of W in the lattice are discussed.     

New Pb-based Superconductor with the 1222 Structure in?the?(Pb0.75P0.25)Sr2(Eu1.9?xCexSr0.1)Cu2Oz System

New Pb-based 1222 cuprates containing phosphorus have been synthesized in the (Pb,P)Sr₂(Eu,Ce,Sr)₂ Cu₂O _z system. From X-ray powder diffraction study, almost the single 1222 phase samples are found to be obtained in a considerable wide composition area of 0.3≤x≤1.2 for the nominal composition of (Pb_0.75P_0.25)Sr₂(Eu_1.9−x Ce _x Sr_0.1) Cu₂O _z . The crystal structure for each sample has a tetragonal symmetry and the typical lattice parameters are a=0.3851 nm and c=2.922 nm. After annealing under 143 atm O₂ atmosphere at 400 °C, a sample with x=0.3 among the almost-single phase samples shows an onset of resistivity-drop at the highest temperature of about 22 K and zero-resistivity at the highest temperature of about 12 K, then this sample also shows an onset of diamagnetic signal at about 20 K. These phenomena are found to originate from superconductivity of the new 1222 phase.     

Sign reversal of the Hall effect in YBa2(Cu1?x)3O7?δ single crystals

We have measured the in-plane Hall effect and in-plane resistivity of cobalt-doped YBa₂Cu₃O_7– (YBCO) single-crystal samples. The concentration of cobalt ranged from 1 to 3%. As the applied field was varied, a sign reversal of the Hall resistivity was seen for 1 and 2% Co-doped samples, but not for the 3% sample. Our results are expressed in terms of conductivities, as recently recommended by the work of Dorseyet al. and Kopninet al. In the mixed state just below the superconducting transition temperatureT _c, the Hall conductivity _xy is the sum of two terms,C ₁/H andC ₂H, whereC ₁ andC ₂ are field-independent but temperature-dependent. As previously observed by Ginsberg and Manson for undoped YBCO, the coefficientC ₁ is approximately proportional to ² andC ₂ is approximately linear in, where = 1–(T/T _c ). The values ofC ₁ andC ₂ are presented for each cobalt concentration.     

Normal state and superconductivity of ErBa2(Cu1?xCrx)O7?δ(x=0–0.1)

The effects of Cr in ErBa₂(Cu_1–x Cr _x )O_7– (x=0–0.1) superconductor have been investigated. The critical temperature, which was determined by DC electrical resistance measurements, showed no suppression of the onset temperature (T _{c onset}) within the substitution range. The transition width (T _c ) broadened as the Cr content is increased. The normal state changes from the metal-like to semimetal/semiconductor-like for x0.03. Micrographs from the scanning electron microscope, X-ray diffraction pattern, and energy dispersive X-ray analysis results are used to describe the superconducting properties of these materials. The orthorhombic structure was preserved throughout the substitution range. Some possible roles of Cr in the system are discussed.     

Production and dielectric properties of lead-free ceramics with the formula [(Na0.5K0.5)1 ? xLix](Nb1 ? y ? zTaySbz)O3

The influence of past history on the possibility of obtaining high-density ferroelectric ceramics of solid solutions [(Na_0.5K_0.5)_{1 − x} Li _x ](Nb_{1 − y − z} Ta _y Sb _z )O at x = 0–0.14, y = 0, 0.1, 0.2, and z = 0–0.1 is identified. The dynamics of behavior of materials under a field in the process of polarization is evaluated. The relative dielectric permeability of samples in a wide range of temperatures (20–700°C) and frequencies (25 Hz-1 MHz) of a variable electric field is studied. On the basis of study of dielectric, piezoelectric, and elastic properties of solid solutions, prospective objects for further technological elaboration and modification of chemical composition with the goal of improving the electrophysical parameters are selected. Original Russian Text ? I.A. Verbenko, O.N. Razumovskaya, L.A. Shilkina, L.A. Reznichenko, K.P. Andryushin, 2009, published in Neorganicheskie Materialy, 2009, Vol. 45, No. 6, pp. 762–768.     

Enhanced Superconductivity of the Pb-Based 1212 Cuprates in the (Pb0.75S0.25)(Sr2?xBax)(Y0.4Ca0.6)Cu2Oz System by High-Pressure O2 Annealing

We have already reported on substitution effects of Ba for Sr in the sulfur contained Pb-based 1212 compound with the composition of (Pb_0.75S_0.25)(Sr_2−x Ba _x ) (Y_0.4Ca_0.6)Cu₂O _z . It was found that each sample with x=0.3–0.6 showed a resistivity drop originating from superconductivity, and the highest onset temperature of the drop was about 32 K for a sample with x=0.5, but the volume fraction was very small. More recently, we have investigated effect of high-pressure O₂ annealing on superconductivity of the Ba-substituted samples with x=0.4–0.6. As a result, the sample with x=0.5 is found to show an onset of the resistivity drop at the highest temperature of about 41.7 K and it shows an onset of the diamagnetic signal at about 42.0 K. These transition temperatures are higher by ∼10 K than those of the previously reported sample with x=0.5. Moreover, the superconducting volume fraction is increasing more by the high-pressure O₂ annealing. These results indicate that the high-pressure O₂ annealing effectively promotes superconductivity of the present (Pb_0.75S_0.25)(Sr_2−x Ba _x )(Y_0.4Ca_0.6)Cu₂O _z system.