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).     

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.     

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.     

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.     

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.     

Neutron structural investigations of Y1?xCaxBa2Cu3?yCoyO7±δ

The results of a systematic powder neutron study on Y_1–x CaxBa₂Cu_3–y Co _y O_7± for A (x=y=0), B (x=0;y=0.2), C (x= 0;y=0.4), D (x=y=0.2), and E (x=y=0.4) are investigated with a view to understanding the relation between the structural parameters and superconductivity. Rietveld refinements of the structures show that: (a) Co substitutes at the chain Cu(1) sites only, except for sample E, where the presence of a minor amount of Co at the planar Cu(2) site cannot be ruled out; (b) Co substitution reduced thec-parameter, which is reduced even further upon substitution of Ca at the Y-sites; (c) the occupancy factors of the chain O(1) site indicate an average coordination of 4.8 and 5.1 for Co for samples B and C, but only 4 for D and E; (d) Cu/Co(1) atoms for B and C display large thermal parameters, suggesting a displacement from their ideal centrosymmetric location; (e) the apical Cu(1)-O(4) bond lengthens upon substitution of Co in samples B and C but undergoes shortening upon substitution of Ca in the case of samples D and E. The apical Cu(2)-O(4) bond, on the other hand, shows just the opposite trend; (f) samples D and E show a reduction in the separation of CuO₂ layers and their oxygen content; and (e) the bond valence of the Cu(2) ion shows the lowest value of 2.127 for the nonsuperconducting sample C.     

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.     

AC-field Frequency Response of?Cu0.5Tl0.5Ba2(Ca2?qMgq)Cu3O10?δ Bulk Superconductor

The dielectric properties of Cu_0.5Tl_0.5Ba₂Ca_2−q Mg _q Cu₃O_10−δ (q=0, 0.5, 1.0 1.5) superconductor samples were studied at two temperatures of 80 and 290 K by capacitance (C) and conductance (G) measurements with the test frequency (f) in the range of 10 KHz to 10 MHz. We have presented the measurements of the dielectric constants (ε′ and ε″), dielectric loss factor (tan δ) and ac-conductivity (σ _ac) as a function of frequency and temperature. A negative capacitance (NC) experience has been observed, which is most likely due to different contact electrodes and superconductor samples’ Fermi levels. Since metals have their Fermi levels higher than ceramics, there is a flow of the carriers from the ceramic samples towards the metal electrodes. The dielectric polarization phenomenon is observed, which is due to dislodgment of mobile charges from their equilibrium position relative to fixed charges of the reservoir layer. The improved inter plane coupling promoted by Mg substitution at Ca site would change the dielectric response of Cu_0.5Tl_0.5Ba₂Ca_2−q Mg _q Cu₃O_10−δ superconductors. To observe such effects in Mg doped Cu_0.5Tl_0.5Ba₂Ca_2−q Mg _q Cu₃O_10−δ superconductors, di- electric measurements were carried out both at room temperature (290 K) and in the superconducting state closer to the boiling point of liquid nitrogen (80 K). The excess conductivity arising due to superconducting state of material has been determined, and its role in the mechanism of superconductivity is suggested. The negative dielectric constant (ε′) and dielectric loss factor (tan δ) show strong dispersion at low frequencies. The lower thermal agitation at 80 K may enhance the polarizability and hence the dielectric constants (ε′ and ε″).     

Synthesis and abrasive properties of nanoparticulate MoO2-modified Al2 ? xFexO3 and Fe2 ? yAlyO3 solid solutions

X-ray diffraction, IR spectroscopy, particle-size analysis, and chemical analysis are used to elucidate the general mechanisms of the formation of nanoparticulate molybdenum-dioxide-modified Al_{2 − x} Fe _x O₃ and Fe_{2 − y} Al _y O₃ solid solutions prepared via heat treatment of ammonium hydroxycarbonate complexes, (NH₄)₂Al₂Fe(OH)₅(CO₃) · nH₂O. The addition of molybdenum dioxide (within 0.005 mol %) is shown to enhance the polishing performance of the oxides for final polishing of nonferrous metals and alloys (copper and brass) by a factor of 6–7 relative to unmodified aluminum iron oxides, which is attributable to the increased chemical activity of the abrasive material. The surface roughness value R _a achieved is below 0.005 μm.     

Superconducting gap in PrxY1?xBa2Cu4O8 and YBa2?ySryCu4O8 probed by infrared phonon self-energies

We report a reflectivity study of thez-polarized TO-phonons of Pr _x Y_1–x Ba₂Cu₄O₈ and YBa_2–y Sr _y Cu₄O₈ alloys in the temperature range 10–300 K. Anomalies of the frequency and linewidth of the plane-oxygen vibration at300 cm^–1 due to the opening of the superconducting gap are found to occur upon crossing the superconducting transition temperatureT _c . Phonon self-energy effects are strongly dependent onT _c , providing evidence for a relative shift of the gap with respect to the energy of phonon.On leave from the Institute for Semiconductor Physics, Ukrainian Academy of Sciences, 252650 Kiev-28, Ukraine.     

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.     

GaxIn1 ? xBiyAszSb1 ? y ? z/InSb and InBiyAszSb1 ? y ? z/InSb heterostructures grown in a temperature gradient

Heterophase equilibria in the Ga _x In_{1 − x} Bi _y As _z Sb_{1 − y − z} -InSb and InBi _y As _z Sb_{1 − y − z} -InSb systems have been analyzed within the simple solution approximation. Ga _x In_{1 − x} Bi _y As _z Sb_{1 − y − z} /InSb and InBi _y As _z Sb_{1 − y − z} /InSb heterostructures have been grown in a temperature gradient, and their composition and structural perfection have been assessed.     

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.     

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.     

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.     

Effect of nonstoichiometry on the structure and microwave dielectric properties of Ba1 ? x(Zn1/2W1/2)O3 ? x and Ba(Zn1/2 ? yW1/2)O3 ? y/2

We have synthesized Ba_{1 − x} (Zn_1/2W_1/2)O_{3 − x} and Ba(Zn_{1/2 − y} W_1/2)O_{3 − y/2} barium tungstates with different deviations from cation stoichiometry (x = 0.01–0.05, y = 0.01–0.05), determined the phase composition of ceramics fabricated from the tungstates, and investigated their electrical properties. Even slight deviations from cation stoichiometry in Ba(Zn_1/2W_1/2)O₃ lead to the formation of the scheelite phase BaWO₄, and its content increases with heat-treatment temperature. Barium or zinc deficiency in the systems studied improves the sintering behavior of Ba(Zn_1/2W_1/2)O₃ and increases the degree of 1: 1 B-site cation ordering, which in turn ensures an increase in microwave quality factor, Q.     

Electrical properties and microstructure of CdxHg1 ? x ? yCrySe crystals

Crystals of the Cd _x Hg_{1 − x − y} Cr _y Se (x = 0.4, y = 0.1) quaternary solid solution have been grown by the Bridgman method, and their microstructure and electrical properties have been studied. The crystals are shown to contain various types of inclusions in the form of filaments and triangles.     

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.     

Microstructure and Young’s modulus of high-pressure LixNa1 ? xTayNb1 ? yO3 ceramics

The microstructure and elastic properties of ceramic samples of high-pressure Li _x Na_{1 − x} Ta _y Nb_{1 − y} O₃ perovskite solid solutions have been studied for the first time in relation to their composition and the synthesis temperature. The results demonstrate that, with increasing sintering temperature, the Young’s modulus of the Li_0.17Na_0.83Ta _y Nb_{1 − y} O₃ solid solutions decreases, which can be understood in terms of the recrystallization behavior of the disordered solid solutions under high-pressure synthesis conditions.     

Band gap modified Al-doped Zn1?xMgxO and Zn1?yCdyO transparent conducting thin films

Al-doped Zn_1−x Mg _x O and Zn_1−y Cd _y O thin films were prepared on glass substrates by sol–gel method. The codoping thin films showed preferential c-axis orientation, and the lattice constant c evaluated from the shift of the position of (002) peak displayed an increasing evolution from x = 8 at.% to y = 8 at.%, indicating a roughly statistical substitution of Mg²⁺ and Cd²⁺ for Zn²⁺ in their solid solution. The effects of narrowing and widening band gap (E _g) on conductivity of (Cd, Al) and (Mg, Al) codoped ZnO thin films were simultaneously investigated using transmission spectra and electrical measurements. The transmittances of these films are obviously decreased by vacuum annealing to 50–60%. However, the carrier concentration and Hall mobility both increase, and resistivity decreases with narrowing band gap in 1 at.% Al-doped Zn_1−x Mg _x O and Zn_1−y Cd _y O thin films from x = 8 at.% to y = 8 at.%. It is revealed that the conductivity of Al-doped ZnO thin films could be enhanced by this simple band gap modification.