Structure, Phonon Vibration, and Spin Correlation of LaBa2Cu3?xCoxOy

Polycrystalline samples LaBa₂Cu_3–xCo _x O _y (0 x 1.0) were synthesized by solid state reaction method. The structure, phonon vibration, conduction, and spin correlation were investigated by means of X-ray diffraction, infrared spectra, resistivity, and electron spin resonance. It is found that there are orthorhombic–tetragonal and tetragonal–orthorhombic structural transitions with Co doping, and the conduction behavior changes from metallic to semiconducting. With the increase of Co content, the Cu(1)—O(1) phonon mode around 531 cm^–1 softens, the Cu(2)—O(2) phonon mode around 657 cm^–1 hardens, and the Cu(1)—O(4) mode around 583 cm^–1 is nearly unchanged. The Cu²⁺ spins tend to localize with Co doping. The changes in structure, phonon vibration, and spin correlation with Co doping are analyzed and discussed.     

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.     

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.     

IR Spectra of La1.85Sr0.15Cu1?xMxO4?δ(M = Zn, Ni, Mg)

Polycrystalline samples La_1.85Sr_0.15Cu_1–xZn_xO_4– (0 x 0.3), La_1.85Sr_0.15Cu_1–xNi_xO_4– (0 x 0.3), and La_1.85Sr_0.15Cu_1–xMg_xO_4– (0 x 0.3) were synthesized by the solid-state reaction method. The crystal structure and phonon vibration were investigated by means of X-ray diffraction (XRD) and infrared spectrum. Zn, Ni, and Mg doping results in the lattice parameter c decreasing and a (b) increasing. The change of the phonon modes around 504 cm^–1 and 681 cm^–1 can be satisfactorily interpreted in terms of the change of the crystal structure and the itinerant nature of charge carrier in CuO₂ sheet. The relation between the structure and phonon vibration is discussed.     

Synthesis, and study of magnetic properties, of Bi1?xCdxFeO3

Ceramic Bi_1−x Cd _x FeO₃ (x = 0.0, 0.05, and 0.1) samples were prepared by a citrate-gel method. The as-prepared compounds were calcined at 600 °C for 3 h to obtain nearly single-phase materials. The crystal structure, examined by X-ray powder diffraction (XRD) and Rietveld analysis, confirmed that the samples crystallize in a rhombohedral (space group, R-3c no. 161) structure. Magnetic measurements were carried out on the resultant powders from 300 to ~2.5 K. Magnetic hysteresis loops showed a significant increase in magnetization as a result of Cd doping in BiFeO₃.     

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.     

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.     

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).     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Preparation, characterization and conductivity studies of Li3?2xAl2?xSbx(PO4)3

New NASICON type materials of composition, Li_3−2x Al_2−x Sb _x (PO₄)₃ (x = 0·6 to 1·4), have been prepared and characterized by powder XRD and IR. D.C. conductivities were measured in the temperature range 300–573 K by a two-probe method. Impedance studies were carried out in the frequency region 10²−10⁶ Hz as a function of temperature (300–573 K). An Arrhenius behaviour is observed for all compositions by d.c. conductivity and the Cole-Cole plots obtained from impedance data do not show any spikes on the lower frequency side indicating negligible electrode effects. A maximum conductivity of 4·5 × 10⁻⁶ S cm⁻¹ at 573 K was obtained for x = 0·8 of the Li_3−2x Al_2−x Sb _x (PO₄)₃ system.     

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.     

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.