Magnetization Processes of (La0.7Pb0.3MnO3)1?x(SiO2)x Composites

The ceramic composites, (La_0.7Pb_0.3MnO₃)_1−x (SiO₂) _x , with diluted magnetic properties are prepared using solid-sate sintering route. Magnetization processes of (La_0.7Pb_0.3MnO₃)_1−x (SiO₂) _x composites are explored in this study. Ferromagnetism is gradually attenuated due to the magnetic dilution induced by the increase of SiO₂ content. Clearly, irreversible behavior is observed in the zero-field cooling and the field cooling (ZFC–FC) curves at a low field of 100 Oe. Saturation magnetization decreases as x increases while ferromagnetic transition temperature remains around 346 K for all composites. All the composites exhibit ferromagnetic hysteresis behavior which can be modeled by the law of the approach to saturation in the form M=M _S(1−a/H). The term a/H expresses the deviation of magnetization from saturation. The smaller factor a for La_0.7Pb_0.3MnO₃-rich samples results in sharper square curve which should be associated with the long-range spin order of ferromagnetic coupling.     

The Effects of Substitution of Sn for Cu in Bi1.75Pb0.25Sr2CaCu2.3?xSnxSnxOy

The influence of Sn doping on superconductivity in the Bi-based 2212 phase is studied in this paper. For the samples R–T relations and magnetic hysteresis loops were measured. X-ray powder diffraction analysis was also performed. For Bi_1.75Pb_0.25Sr₂CaCu_2.3–x Sn _x O _y , the experimental results show that by adding the proper amount of Sn the superconductivity of the samples can be improved. As x = 0.15, the critical temperature T _c, the critical current density J _c, and the magnetic pinning force density F reach a maximum. At T = 11 K, the critical state parameters H _c1, H _c2, , , and are calculated and compared with the results reported by other researchers. The experimental results also show that the Sn doping is able to speed up the growth of the 2223 phase. In brief, Sn doping is an effective way of improving the superconductivity in Bi-based superconductors.     

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.     

Electrical properties of Zn2(TiaSnb)1 ? xZrxO4 solid solutions

The electrical conductivity, band gap, dielectric permittivity, and molar polarizability of Zn₂(Ti _a Sn _b )_{1 − x} Zr _x O₄ solid solutions have been determined. All of the synthesized samples are dielectrics with semiconducting behavior of conductivity. The phase diagram of the Zn₂TiO₄-Zn₂SnO₄-Zn₂ZrO₄ system is presented.     

The Effect of Silver Substitution in Bi1.7Pb0.3Sr2Ca2?xAgxCu3Oy

The effect of Ag substitution on the properties of high-temperature superconductor Bi_1.7Pb_0.3Sr₂Ca_2−x Ag _x Cu₃O _y system have been investigated. The electrical and structural properties of the samples, prepared by the conventional solid-state reaction method, have been characterized by X-ray diffraction (XRD), electrical resistance and scanning electron microscopy (SEM) studies. XRD analysis reveals a multiphase structure of the samples, whereas SEM micrographs indicate some morphological changes induced by silver addition. It was found that an increase of the amount of Ag₂O addition leads to an enhancement of the critical temperature and the percentage of Bi-2223 phase in the phase mixture.     

Superconductivity of YBa2?xNdxCu3Oy Solid Solution

The solubility of Nd at the Ba sites and the superconductivity of YBa_2–x Nd _x Cu₃O _y were investigated by X-ray powder diffraction and measurements of the electrical resistance and ac susceptibility. The single Re123 phase was obtained for x0.30. The onset transition temperature is insensitive to the Nd content x in the region of x0.40. All are higher than 95 K. The zero resistance transition temperatures , however, exhibits two-step variation with the increase of x. For x0.25, are all above 92 K. The highest of 94 K was obtained for x=0.25. For x0.3 drops sharply to about 84 K. Finally falls to 30 K and is below 10 K for x=0.5. The two-step variation of T _c might be an indication of the existence of two trap levels for holes.     

Extraordinary behaviour of the Y1?xPrxBa2Cu3O7?δ system

Investigations of Y_1–x M _x Ba₂Cu₃O_7– (M=Ce, Th)c-axis oriented thin film specimens show that the rate of depression ofT _c withx is larger for M=Th, than for M=Ce and Pr, and suggest that Ce, like Th, is tetravalent in this compound. Hall effect measurements on Y_1–x Pr _x Ba₂Cu₃O_7– single crystals reveal aT ² dependence of the cotangent of the Hall angle in the normal state and a negative Hall anomaly belowT _c in the superconducting state, in agreement with recent reports. Our research shows that the depth, , of the negative Hall signal scales withT/T _c and that the maximum value of decreases linearly withx and vanishes atx0.24. Magnetoresistance measurements on Y_1–x Pr _x Ba₂Cu₃O_7– single crystals indicate that the irreversibility lineH(T ^*) obeys a universal scaling relation characterized by anm=3/2 power law nearT _c, with a crossover to a more rapid temperature dependence of belowT/T _c 0.6, similar to that observed for polycrystalline specimens.     

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.     

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.     

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).     

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.     

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.     

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.     

Relation of Structure and Superconductivity in Self-compensated Y1?xCaxBa2Cu3?xAlxOz System

The co-doped compounds of Y_1−x Ca _x Ba₂Cu_3−x Al _x O _z , with x from 0.1 to 0.4, were synthesized through a solid-state reaction method. Structural and superconducting properties have been investigated by X-ray diffraction, Rietveld refinement, and DC magnetization measurement. The lattice constant a decreases while b increases with the addition of x. The difference between a and b diminishes gradually. Careful study of the crystalline structure shows that the critical temperature (T _c ) changes monotonically with some local structural parameters, such as the difference between Ba and Cu(2) atoms’ Z coordinates, the bond length of Cu(2)–O(4), and the bond angle of Cu(2)–O(2)–Cu(2), which are all closely related to the interaction between the perovskite block and the rock salt block in the unit cell. The results indicated that the influence of the crystalline structure on superconductivity is important and independent of the carrier concentration.     

Normal-state Conduction Mechanisms in GdBa2Cu3?xRuxO7?δ Superconducting Phase

Bulk superconducting samples of type GdBa₂Cu_3−x Ru _x O_7−δ , Gd-123, with x=0.0–0.3 were prepared by the conventional solid-state reaction technique. X-ray powder diffraction (XRD), scanning electron microscope (SEM), electron dispersive X-ray (EDX) and electrical-resistivity measurements were performed in order to investigate the effect of Ru⁴⁺ ions substitution on Gd-123 phase. Enhancement of both 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 from XRD and electrical-resistivity measurement, respectively. This enhancement was confirmed with the calculated relative volume fraction. For x>0.05, suppression of both phase formation and T _c was obtained and the superconductivity was completely destroyed around x=0.3. 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 results showed that the dominant mechanism was CG for x≤0.075, while was 3D-VRH for x≥0.15.     

Fluctuation Conductivity and Phase Separation in Polycrystalline Y1?xCexBa2Cu3O7?δ Superconductors

Fluctuations on the electrical conductivity of polycrystalline Y_1−x Ce _x Ba₂Cu₃O_7−δ superconductors were investigated with Ce contents from x=0.01 up to 0.10. The results showed that the resistive transition proceeded in two stages, intragranular and intergranular, as indicated by the temperature derivative of the resistivity and verified by critical exponent analysis. Also, from the critical exponent analysis, we have observed a splitting of the pairing transition in all investigated samples. Our results strongly suggest that this pairing transition splitting is related to the occurrence of a phase separation associated with Ce doping. In the regime of near-zero resistance state, the temperature dependence of our data is consistent with a power-law behavior which is indicative of a phase transition from a paracoherent to a coherent state of the granular array.     

Room-temperature oxidation of La2?xSrxCuO4

A series of oxidized La_2–x Sr _x CuO_4+y compounds has been prepared by treating the starting materials with a solution of Br₂ in NaOH at room temperature. The structural modifications due to the oxidation of the materials have been studied by X-ray diffraction. Some of the observed changes are: (i) a large increase in the long parameter of the unit cell for samples with Sr content and (ii) a slight decrease in a along the whole range ofx. Interesting features have been observed regarding the critical temperatures of these materials: transition temperatures are higher for those containing lower Sr amounts (some of them were even nonsuperconducting before the oxidation treatment) in contrast to materials with x0.15, whoseT _c's do not change very much. The influence of both oxygen and strontium contents on the structural modifications and the superconducting properties of the oxidized materials will be discussed.     

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.     

Critical and Gaussian Conductivity Fluctuations in Granular Ho1?xPrxBa2Cu3O7?δ Superconductor

Fluctuations in the electrical conductivity of polycrystalline Ho_1−x Pr _x Ba₂Cu₃O_7−δ superconductors were investigated with Ho contents from x=0.01 up to 0.10. Samples were prepared by the standard solid-state reaction technique. The method of analysis is based on the determination of the quantity c_s^-1=-\fracddTlnDs\chi_{\sigma}^{-1}=-\frac{d}{dT}\ln\Delta\sigma, where Δσ=σ−σ _R is the fluctuation conductivity. The results show that the resistive transition proceeds in two stages as seen by the temperature derivative of the resistivity near T _C . In the normal phase, Gaussian and critical fluctuation conductivity regimes were identified. The pairing transition splitting, associated with Pr doping and related to the occurrence of a phase separation, as observed in studies with other rare earth elements, was not clearly observed. On approaching the zero-resistance state, our results show a power-law behavior that corresponds to a phase transition from a paracoherent to a coherent state of the granular array. This behavior was not affected by Pr doping.     

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.