Superconductivity of Bi1.6Pb0.4Sr2Ca3Cu4O12

The superconducting ceramics Bi1.6Pb0.4Sr2Ca3Cu4O12 have been prepared by the melt-casting method.A zero resistance temperature at 60K has been observed.It has been found that the superconducting phase temperature Tc increases with increasing sintering temperature,The effect of Pb content on the superconductivity of the ceramic has been studied.The microstructure of the Sample has been investigated by scanning electron microscopy.Phase analysis has been carried out by x-ray diffraction patterns and energy dispersive analysis through x-ray spectroscopy.     

Superconductivity of LaMCuO system (MBa,Sr and Ca

Superconductivity of the compounds of LaMCuO system with K₂NiF₄ type structure has been studied. For the sintered compounds with MBa, Sr and Ca, the highest onset transition temperatures, T_ci, estimated from resistivity measurement are about 36.9K, 43.0K and 23.5K, respectively and the highest temperatures where the resistivity vanishes, T_cf, are about 27.8K, 34.0K and 15.5K, respectively. Almost complete Meissner effect has been confirmed by low frequency ac susceptibility measurement. Crystals of the compounds with MBa and Sr have been prepared. As the result of the preliminary experiment for a crystal with Ba, it is found that T_cf of the crystal is higher than any one of the sintered specimens with MBa, while the T_ci of the crystal is nearly the same as the highest one of the sintered specimens.     

Superconductivity and magnetism in Y1−x Ca x Sr2GaCu2O7−δ

Samples of Y_1–x Ca _x Sr₂GaCu₂O_7– (x=0, 0.4) doped with⁵⁷Fe, prepared under various oxygen pressures, have been studied by magnetometry and Mössbauer spectroscopy. Most of the iron ions (Fe³⁺) enter the Ga(Cu(1)) site. For thex=0 sample, the Mössbauer spectra of the iron nuclei in the Cu(2) sublattice display magnetic order of Cu,T _N=370 K. The iron ions in the Ga site display magnetic order only at low temperatures. At temperatures above 90 K, these iron ions display a pure quadrupole doublet Mössbauer spectrum. The samplex=0.4 also displays magnetic order of the Cu(2) site,T _N370 K. A sharp drop in the hyperfine field is observed atT _N, probably associated with a first-order phase transition or two-dimensional ordering. The iron nuclei in the Ga site display paramagnetic long spin relaxation time phenomena at 4.2 K. Thex=0.4 sample prepared under 110 atm oxygen pressure, displays superconductivity,T _c50 K. The Mössbauer spectra give evidence of the presence of two phases. One displays magnetic order, the other is paramagnetic, the last is probably associated with the superconducting phase.     

Superlattice structures in solid solution of high-Tc YBa2Cu3O7−δ and (Pb,Cu)Sr2(Ca,Y)Cu2O7−δ superconductors

A complete solid solution range exists between the systems YBa₂Cu₃O_7−δ and (Pb,Cu)Sr₂(Ca,Y)Cu₂O_7−δ has been found with general stoichiometry (Pb_0.75xCu_1−0.75x)(Sr_2xBa_2−2x)(Ca_0.5xY_1−0.5x)Cu₂O_7−δ. Energy dispersive spectrometry and X-ray diffraction identified that a true solid solution exists. Superlattice structures observed by electron diffraction across the solid solution range have a modulation range have a modulation periods along a* which can be varied by altering both the compositional parameter x and the overall oxygen content. The existence of these superlattices infers that the solid solution is non-random and therefore thermodynamically non-ideal. The superconducting transition temperatures, T_c, across the solid solution range are also strongly dependent on the composition, x, but no direct relationship with the modulation period has been established. From these studies it may be concluded that the solid solution between known superconductors is possible, although involving some partial ordering of the lattice, but ordering of cations in the rock-salt to charge reservoir layer is not a significant factor in determining the superconducting properities, which depend more closely on the overall composition and hence on the ability of the charge reservoir layer to transfer charge to the superconducting layers.     

Superconductivity above 100 K in multi-phase Y−Ba−Cu−O

We report the synthesis, characterization and superconducting temperature of high-temperature superconducting Y_0.3Ba_0.7CuO₃, Y_0.4Ba_0.6CuO₃ and Y_1.2Ba_0.8CuO₄. The volume fraction of the superconducting phase is estimated to be of the order of 10%.HighT _c superconductivity withT _c -onsets up to 125 K (midpoint 102 K, zero resistance: 90 K) is observed in multi-phase Y–Ba–Cu–O oxygen defect compounds with the nominal composition of Y_1.2Ba_0.8CuO_4–y (0y0.4).     

Lattice dynamics of the superconducting Bi2Sr2Ca(Cu,Sn)2O8−y

Phonon anomaly was observed below 140 K for the absorption area (lnA) and the isomer shift () plotted against the temperature (T) in the¹¹⁹Sn Mössbauer study of the Bi₂Sr₂Ca(Cu_0.995Sn_0.005)₂O_8–y superconductor (glass-ceramic,T _c(on)=86 K), which was prepared by heat treatment of the glassy sample. The anomaly suggests that the softening (quenching) of the lattice vibration causes the superconducting transition, as was observed in several YBa₂(Cu, Sn)₃O_7–y superconductors. Fitting of the theoretical curve for the normal phonon of the combined Debye and Einstein model to the lnA versusT and versusT plots yielded _D and _E of 240 and 140 K, respectively. These results suggest that the probe (Sn⁴⁺) was located at the interstitial site close to the Cu site, and the softening of the Cu site vibration could be detected before the superconducting transition took place. Disordered structure seems to be favorable for the detection of phonon anomaly in the Bi system.     

Copper Defects and Conductivity in Bi—Pb—Sr—Ca—Cu—O Glasses

Glass ceramics of the composition (Bi_0.8Pb_0.2)₄Sr₃Ca₃Cu₄O₈ prepared by the melt quenching technique and the crystalline phases produced by the rapid thermal annealing have been studied by electrical resistivity and electron paramagnetic resonance (EPR) measurements in the temperature range from liquid helium up to room temperature. The concentration of the EPR active Cu^2? paramagnetic centers decreases as conductivity increases for the glass ceramics and disappears after crystallization and the growth of superconducting phases, similar to bulk high-T_c superconductors. The KPR spectra of both glass and crystallized ceramics after short-time annealing indicate the coexistence of Cu^2? paramagnetic ions and the exchange coupled clusters.     

Seebeck-effect in the mixed state of Y−Ba−Cu−O

We report on measurements of the Seebeck-effect, the Nernst-effect, and the magnetoresistance in the mixed state of ac-axis oriented expitaxial film of Y–Ba–Cu–O. In contrast to conventional superconductors we find a large Seebeck-coefficientS, which is comparable in magnitude to the Nernst-effect. The broadening of the super-conducting transitions of magnetoresistance and Seebeck-effect are rather similar with respect to (1) the temperature dependence, (2) the dependence on the direction between magnetic field and crystal axis and (3) the dependence on the direction between magnetic field and driving forces. The large Seebeck-effect has to be attributed to dissipation due to normal quasiparticle-excitations, since the vortex-contribution to the Seebeck-effect is by far too small to account for the observed magnitude ofS. It is argued that such a quasiparticle contribution to the dissipation is large in the high-T _c superconductors because of the small coherence lengths and thus the small vortex cores. Another possibility is that granularity leads to dissipation proportional to the normal state transport properties. The Seebeck-voltage depends on all dissipative processes other than vortex motion, whereas the Nernst-effect depends only on the vortex motion. Therefore by measurements of thermomagnetic effects the various dissipative properties may be separated.     

Superconductivity in the LnBaCuO system (Ln = Y,Ho, Er)

Superconductivity around 90K is observed in mixed and single phase system of LnBaCuO (Ln = Y, Ho, Er). Resistivity, X-ray powder diffraction and AC susceptibility measurements were carried out on samples of the mixed phase systems Ln_2−xBa_xCuO_4−y (Ln = Ho, Er), Y_1.2Ba_0.8Cu_1+xO_4+x−y(0<x<5) and single phase system Ba₂LnCu₃O₇ (Ln = Y, Ho, Er). Ln_2−xBa_xCuO_4−y consists of Ba₂LnCu₃O₇, BaCuLn₂O₅, Cu₂Ln₂O₅ and starting materials and shows a small diamagnetic signal below T_c in the temperature dependent AC susceptibility. Y_1.2Ba_0.8Cu_1+xO_4+x−y is a mixture of Ba₂YCu₃O₇, BaCuY₂O₅ and CuO and also indicates a small diamagnetic signal at 92K. Interesting resistivity anomalies occur in the Y_1.2Ba_0.8Cu₄O_7−y and Ho_0.8Ba_1.2CuO_4−y composites including large drops in the resistivity at ∼220 and ∼120K, respectively. The pure compounds, Ba₂LnCu₃O₇ (Ln = Y, Ho, Er) show sharp resistive superconducting transitions and bulk Meissner effects of 90%.     

Superconductivity of the Sr2Ca12Cu24O41 spin-ladder system: are the superconducting pairing and the spin-gap formation of the same origin?

Pressure-induced superconductivity in a spin-ladder cuprate Sr2Ca12Cu24O41 has not been studied on a microscopic level thus far although the superconductivity was already discovered in 1996. We have improved the high-pressure technique using a large high-quality crystal, and succeeded in studying the superconductivity using 63Cu nuclear magnetic resonance. We found that the anomalous metallic state reflecting the spin-ladder structure is realized and the superconductivity possesses an s-wave-like character in the meaning that a finite gap exists in the quasiparticle excitation: At a pressure of 3.5 GPa, we observed two excitation modes in the normal state from the relaxation rate T-11. One gives rise to an activation-type component in T-11, and the other T-linear component linking directly with the superconductivity. This gapless mode likely arises from free motion of holon-spinon bound states appearing by hole doping, and the pairing of them likely causes the superconductivity.     

μSr and vortex pinning in Bi2Sr2Ca1−x Gd x )Cu2O y

We present transverse fieldSR and bulk magnetisation studies on powders of Bi₂Sr₂(Ca_1{itxGd _x )Cu₂O _y for 0x<0.5. We have found that the effective pinning decreases rapidly withx even for values ofx for whichT _c remains unchanged. We have interpreted this decrease in terms of changes in _ab (obtained fromSR) which cause a decrease in vortex-vortex interactions. The field distribution spectra in these superconductors have been obtained using a maximum entropy analysis and are found to agree with the calculations by Harshmanet al.     

Nonlinear properties of multiphase high-temperature superconductors of the Bi–Sr–Ca–Cu–O system in the temperature range of the superconducting transition

The nonlinear characteristics of high-temperature superconductors of the Bi–Sr–Ca–Cu–O system have been experimentally investigated in the temperature range of the superconducting transition under the influence of a harmonic alternating magnetic field. The effect of the generation of odd harmonics in the signal of response to a harmonic alternating magnetic field for multiphase high-temperature superconductors containing regions with different values of the critical temperature in their bulk has been observed for the first time. The mechanism of harmonic generation in a superconductor in the resistive state, which is associated with the switch effect, i.e., with the redistribution of eddy current density between the local regions of the superconductor, has been considered.     

Changes in the phonon spectra of Bi 2212 superconductors connected with the metal-semiconductor transition in the series of Bi2Sr2(Ca1−xYx)Cu2O8 compounds

By means of inelastic neutron scattering the phonon density of states (PDOS) has been measured for compounds Bi₂Sr₂(Ca_1–xY_x)Cu₂O₈₊ for different values ofx. The transition from a superconducting to a semiconducting material is paralleled by characteristic changes in the PDOS as it was observed before for other HTC ceramics. The results are discussed on the basis of a model calculation and it can be shown that high frequency oxygen vibrations in the Cu–O sublattice are stongly renormalized in the superconducting compound.     

Superconducting energy gap in Bi2Sr2Ca2Cu3O10+δ (Bi2223) single crystals

Current-voltage characteristics of S-I-S tunnel break junctions fabricated from pure undoped Bi2223 single crystals (T _c =110 K) were measured. High quality of the crystals enabled production of good tunnel junctions with a low or almost zero leakage current and well developed gap structure in the tunneling spectra. The peak-to-peak energy gap values 2Δ_p-p in different crystals and the tunnel junctions ranged from 80 to 105 meV. The tunneling conductance in the superconducting state was normalized to that in the normal state and compared to a smeared BCS density of states. A simple fit of the data gave the average value of Δ=38.5 meV and reduced gap 2Δ/kT _c ?8, consistent with a very strong coupling mechanism.     

Formation of the (Bi, Pb)2Sr2Ca2Cu3O10+δphase: reaction mechanism and kinetics

In order to elucidate the formation mechanism of the Bi, Pb(2223) phase, extensive investigations have been performed by means of combined differential thermal analysis and thermo-gravimetric measurements, X-ray diffraction as well as scanning electron microscopy and energy dispersive X-ray microanalysis.Starting from co-precipitated oxalate powders, the transformations leading to the formation of the Bi, Pb(2223) phase were studied. Based on these investigations the Bi,  Pb(2223) phase formation process, involving the Bi, Pb(2212), Ca₂PbO₄and alkaline-earth cuprate phases as precursors has been separated into elemental steps. A new model for the formation mechanism of the Bi, Pb(2223) phase will be presented.The Bi, Pb(2223) phase formation kinetics has been studied using the Avrami relation for isothermal phase transformations. The analysis of the Bi, Pb(2223) phase evolution revealed a marked change of the Avrami exponent during the course of the Bi, Pb(2223) phase formation after about 12 hours sintering at 857 ° C.The activation energy for the formation of the Bi, Pb(2223) phase has been determined under various experimental conditions. The nominal composition of the precursor powders, the temperature at which they were calcined as well as the size of their constituents were found to have a significant influence on the value of the activation energy.     

Superconductivity of Y–Ba–Cu–O and substituted systems

Superconductivity of Y-Ba-Cu-O system is studied in the composition 2:2:3 and 1:2:3 of Y:Ba:Cu. The effect of replacement of Y or Ba by divalent Sr and Ca, trivalent Ce and tetravalent Zr is studied. X-ray diffraction, SEM and TEM techniques are used for materials characterization. Superconducting transition temperatures are measured resistively. Rapid resistance drop observed above 230 K in Y-Ba-Sr-Cu-O and Y-Ba-Ca-Cu-O systems indicate the possible existence of superconductivity above 230 K. Substitution of Ce in place of Y is found to reduce the onset Tc from 95 K to 80 K. For the first time, replacement of Cu by Zr in Y-Ba-Cu-O has yielded the onset Tc of about 105 K.     

In-plane London penetration depths near the critical temperature of Tl2Ba2Ca n−1Cu n O2n+4 and (Bi,Pb)2Sr2Ca n−1 Cu n O2n+4 (n=2,3)

From isothermalM(H) curves nearT _c , measured on polycrystalline Tl₂Ba₂Ca _n–1Cu _n O_2n +4 and (Bi, Pb)₂Sr₂Ca _n–1Cu _n O_2n+4 (n=2,3) samples, we deduce the in-plane penetration depths _ab as functions of temperature. An estimate according to the BCS weak-coupling clean-limit fit, which produces the data nearT _c very well, yields _ab(0)=3100 Å, 2320 Å 2210 Å, and 1960 Å for Bi₂Sr₂CaCu₂O₈, Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀, Tl₂Ba₂CaCu₂O₈, and Tl₂Ba₂Ca₂Cu₃O₁₀, respectively. A comparison between strong-coupling and weak coupling fitting curves clearly favours the weak-coupling temperature dependence of _ab nearT _c .