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Polycrystalline samples of (CaxLa1-x)(Ba1.75-xLa0.25+x)Cu3O7+δ tetragonal superconductor for Ca content(x)from 0.0 to 0.6 were synthesized by a simple aqueous solution technique using citric acid. The combined results of X-ray diffraction and Raman scattering analysis indicated that the polycrystalline samples for the entire Ca content (x) were of single-phase character. The Tc onset values for (x) from 0.0 to 0.6, determined precisely from the complex magnetic susceptibility measurements, increased from 45.0 K for x=0.0 to 78.7 K for x = 0.4, remained almost constant at 80 K between x=0.5 and x=0.6. It was also found that sintering condition affected rather dramatically superconducting properties in this system.

We measured the temperature dependence of the 2nd, 3rd and 5th higher-harmonic susceptibilities of(La1-xCax)(Ba1.75-xLa0.25+x)Cu3O7+δ, x=0.2, 0.4, 0.5, prepared by the amorphous metal-complex method. We attempted to fit the calculations based on the Kim-Anderson critical-state model to the experimental results of χn. The analysis shows that the pinning force density is enhanced with increasingχ.

Raman spectra of Sr14-xCaxCu24O41 in single-crystal and polycrystalline samples were investigated. At first, from the polarization and a comparison with previous reports on isostructural compounds, we identify the (4Ag+4B1g+3B2g+B3g) modes that are Raman active within the simple Fmmm structure. Second, it is clear that hole carriers transfer from chain to ladder with substituting Sr for Ca in Sr14Cu24O41, because on the one hand the intensity for modes associated with oxygen vibrations for chain increases and on the other hand the one associated with ladder decreases with Ca content x increasing. Third, we observe new additional modes at low temperature in this system. Especially, for both Sr14Cu24O41 and Sr7.2Ca6.8Cu24O41, the temperature, at which the new mode near 500 cm-1 shows up, is nearly identical to the temperature, at which the electrical and magnetic anomalies have been observed. This result is consistent with the picture that holes in the ladder are paired and localized below the temperature.

To test the utility of the modified Kim-Anderson critical-state model for study of the magnetic response of ceramic superconductors, we measured the initial magnetization, hysteresis loops and ac complex susceptibilities Xn=X'n-iX"n(n =1, 2, 3, 5) of a single-crystal Bi2Sr2CaCu2O8+d bulk sample. The results were simulated in the framework of the model. Analyses of Xn have shown that generation of the higher harmonics can be qualitatively reproduced, where the material parameters k and B0, surface barriers ΔH and lower critical fields Hcl, involved were used as fitting parameters. Comparison of the observed data with the calculated results has revealed that nonzero values of Hcl, play an essential role in the magnetic characteristics of the sample.

We studied, both theoretically and experimentally, the ac magnetic response of type-II superconductors superimposed on a weak Hdc field. Theoretical simulations were made within the construct of the Kim-Anderson critical-state model, where the critical current density Jc is assumed to be given by Jc=k/(Bo+|Bi|). k and Bo are temperature-dependent material parameters, and Bi is the local flux density in the sample. We also measured complex susceptibilities Xn=X′n-iX″n (n=1, 2, 3, 5, 7) of a melt-processed YBa2Cu3Oy specimen and analyzed the observed Xn by applying the magnetization equations derived from the Kim-Anderson model. We found that the experimental results were qualitatively well reproduced. Present results indicate that, for a specimen of which Jc sensitively varies with Bi, the effect of a weak Hdc should be explicitly taken into consideration for studies of the ac magnetization.

Two types of Ca-substituted Y123 phase, (A) Y1-0.75xBa2-0.25xCaxCu3O7-d and (B) Y1-xCaxBa2Cu3O7-d have been prepared by a polymerized complex method X-ray powder diffraction and complex magnetic susceptibility measurements have shown that only type (A) is virtually of single phase without any impurities such as BaCuO2 and Y2BaCuO5. The superconducting transition temperature Tc decreases from 92.2K (x=0) to 83.2K (x=0.2) with increasing x in (A). Furthermore, we investigated the effect of Ca-substitution on phonon Raman spectra of type (A) by using a micro-Raman spectrometer. Ba and 0(4) phonons are significantly shifted towards lower frequencies by 3cm-1 and 4cm-1, respectively, when x is increased from 0 to 0.2. A possible origin of the decrease in both Tc and Ba-0(4) layers phonon energies are discussed in terms of both hole doping in the CuO2 planes and internal pressure induced by the Ca-substitution.

We studied the ac-magnetic response of Co-doped YBa2Cu3O7-d oxide superconductors, YBa2Cu3-xCoxO7-d (x=0.03-0.3), in terms of complex susceptibility X=X'-iX". The samples were synthesized by the modified polymerized complex method. For all the samples, the superconducting transition was observed. As expected, the onset temperature decreased with increasing x. We found that the magnetic shielding due to the weak-link coupling is completed within the temperature region smaller than that for the samples prepared by the original polymerized complex method. This indicates that the modified method presented here is more useful for fabrication of sintered ceramic superconductors with better homogeneity, i.e., with larger critical current density.

Phonon Raman spectra of the modulation-free Bi1.2 Pb0.8 Sr2 Ca1-y, YyCu2Oz(0.2≤y≤0.8) have been investigated both in polycrystalline and single crystal forms. Polarized Raman spectroscopic measurements of single crystals made it possible to clarify some controversies on the Raman mode assignments for Bi2 Sr2 CaCu2 O8+d (Bi2212), most notably in high-frequency three prominent oxygen modes observed at 290, 460 and 630cm-1, which we assigned to AgOBi+B1gOCu, Ag OBi and Ag Oap _??_ x, respectively. The energies of the three oxygen modes show drastic change with a decrease of the hole concentration: (1)Ag OBi+B1g OCu. mode is shifted towards higher frequencies, (2)AgOBi mode hardly varies for y 0.6, and Ag Oap _??_ x mode is shifted towards lower frequencies. The observed changes in these oxygen modes are similar to those observed for the modulated Bi2 Sr2 Ca1-y Yy Cu2Oz. We propose that the Y/Ca substitution both in the modulated and modulation-free Bi2212 systems induced a hole filling on the CuO2 plane, which can explain the behaviour of the oxygen modes observed in their Raman spectra.