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.     

Single-phase Bi2+x(Ca,Sr) n+1 Cu n O2n+4+δ, n = 0, 1, 2, 3 AND ∞, and some substitutional DERIVATIVES

The preparation, structure and properties of five members of the homologous series Bi_2+x(Ca,Sr) _n+1 Cu _n O_2n+4+δ are discussed, namely for n = 0, 1, 2, 3 and ∞. The two end members are insulating phases with a fixed oxygen stoichiometry but the members n = 1, 2 and 3 are superconducting phases with T_c depending on the calcium to strontium ratio and on oxygen stoichiometry as determined by annealing temperature and oxygen partial pressure. Maximum zero resistance T_c 's obtained are n = 1: 76 K, n = 2: 91 K and n = 3: 106 K. The effects of Pb-substitution in n = 2 and n = 3 are discussed with particular reference to the stabilisation of the latter phase. Rare-earth substitution is shown to raise T_c for n = 2 to at least 101 K without the introduction of the n = 3 phase.     

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.     

High-resolution electron microscopy and electron diffraction on Bi2Sr2-x La x Ca n Cu1+n O6+2n+δ

Solid solutions, Bi₂Sr_2-x La _x CuO₆₊ withx< 1.3 and Bi₂Sr_2-x La _x CaCu₂O₈₊ with probablyx<1.3 are reported. Both solid solutions show considerable nonlinear changes in the unit cell dimensions and a change in the modulation vector from 4.7b to 3.5b forx=0 andx=1.2, respectively. It is shown that Bi₂Sr_2-x La _x CaCu₂O₈₊ shows sometimes the absence of the 0.54 nm period alonga, whereas the modulation remains, suggesting both features not to be correlated. The investigation of the solid solution Bi₂Sr_2-x La _x CaCu₂O₈₊ with probablyx<1.3 are reported. Both solid solutions show considerable nonlinear changes in the unit cell dimensions and a change in the modulation vector from 4.7b to 3.5b forx=0 andx=1.2, respectively. It is shown that Bi₂Sr_2-x La _x CaCu₂O₈₊ is severly hampered by the presence of the n=0 structure in the reaction product when one starts with stoichiometric mixture. This can be overcome by adding extra Ca and heating directly just below the melting point. However addition of Ca to the mixture leads to changes in the solid solution range, because Ca can replace Sr.     

Dielectric response of Cu0.5Tl0.5Ba2(Ca2−yMgy)(Cu0.5Zn2.5)O10−δ bulk superconductor to frequency and temperature

The dielectric properties of Cu_0.5Tl_0.5Ba₂Ca_2?yMg_yCu_0.5Zn_2.5O_10?δ (y = 0, 0.5, 1.0, 1.5) superconductor samples were studied at 79 and 290 K by means of capacitance (C) and conductance (G) measurements with the test frequency (f) in the range of 10 KHz to 10 MHz. A negative capacitance (NC) phenomenon has been observed, which is most likely arising due to higher Fermi level of ceramic superconductor samples than metal electrodes. Also the NC may be due to the space charge located at the multiple insulator–superconductor interfaces (grain boundaries) in the materials. The negative dielectric constant (ε′) and loss factor (tan δ) show strong dispersion at low frequencies. The lower thermal agitation at 79 K may enhance the polarizability and hence the dielectric constants (ε′ and ε″).     

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.     

PHASE TRANSITIONS IN Bi2Sr2(Cu1-zFez)O6+δ

We have investigated, by X-ray diffraction, a series of single crystals of Bi-based oxides with the nominal composition Bi₂Sr₂(Cu_1-zFe_z)O_6+δ(0≤z≤0.55). In this system we observed two structural phase transitions with the increase of the doping content. The first transition, from an incommensurate monoclinic phase to an incommensurate orthorhombic phase, occurs at a doping content of iron z_Fe=0.027. The second one corresponds to a phase transition from an incommensurate orthorhombic phase to a commensurate orthorhombic phase at z_Fe=0.34. The comparison of these results with those for more limited substitutions of Zn and Ni indicates the significant role of the insertion of the extra oxygen in the (Bi-O)_∞ double layers.     

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.     

Superconductivity and processing of single Bi−O layered cuprates in the (Bi,Pb)−Sr−(Ca,Y)−Cu−O system

The effects of improved materials processing on single Bi–O layered cuprates in the (Bi, Pb)–Sr–(Ca, Y)–Cu–O system have been investigatged. For Bi-1212 we have improvedT _c to 102 K. The bulk nature of superconductivity is confirmed by the presence of superconducting volume fractions ( ZFC) around 30–40%. The critical current density is 2×10⁶ Acm^–2 at 5 K and 0 T. Moreover, indications for the presence of a second phase probably Bi-1223 with a transition to superconductivity in the range of 115–150 K have been found.     

Peculiarities of magnetic flux penetration and trapping in monocrystalline (YBa2Cu3O7−x and Bi2Sr2CaCu2O8+x ) and polycrystalline (YBa2Cu3O7−x ) samples

A comparative analysis of the magnetic flux trapping in monocrystalline and polycrystalline HTSC samples is carried out, and the possibility of employing the dependence of the trapped magnetic flux on the external magnetic field for obtaining comparative estimates of the effect of pinning centers is determined.     

Growth of Cu0.5Tl0.5Ba2Ca3Cu4−yZnyO12−δ superconductor with optimum carriers

We have tried to vary the carriers concentration in Cu_0.5Tl_0.5Ba₂Ca₃Cu_4?yZn_yO_12?δ (y = 0, 1, 1.5, 2, 2.5) superconductor with the help of post-annealing experiments carried out in nitrogen, oxygen and air and to investigate its effects on the superconductivity parameters. The zero resistivity critical temperature [T_c(R = 0)], the magnitude of diamagnetism and critical current [I_c(H = 0)] are found to increase in Zn free samples after post-annealing in oxygen and air, while these superconducting properties have been suppressed after post-annealing in nitrogen at 550 °C for 6 h. The post-annealing of Zn-doped samples in air has marginally increased the superconducting properties, while these properties have been suppressed after post-annealing in nitrogen and oxygen. These studies have led us to the definite conclusion that the Zn-doped material has grown with optimum carriers concentration.     

Effect of K-doping to the weak link behaviour of (Cu0.5Tl0.5−xKx)Ba2Ca2Cu3O10−δ superconductors

A systematic study of the weak link behaviour for (Cu_0.5Tl_0.5?xK_x)Ba₂Ca₂Cu₃O_10?δ (x = 0, 0.25) superconductors samples has been carried out using electrical resistivity and AC-susceptibility techniques. The K-doped (Cu_0.5Tl_0.5?xK_x)Ba₂Ca₂Cu₃O_10?δ samples were synthesized by solid-state reaction method. In magnetic susceptibility measurements, the real (χ′) and imaginary (χ″) parts of the magnetic susceptibility of (Cu_0.5Tl_0.5?xK_x)Ba₂Ca₂Cu₃O_10?δ (x = 0, 0.25) samples were measured as a function of temperature under various DC-magnetic fields up to 172 Oe. It is observed from these studies that the magnitude of the diamagnetism is substantially enhanced by K-doping. The possible reasons for the enhanced magnitude of diamagnetism have been investigated. It is observed from in-field magnetic measurements that the inter-grain coupling is improved with the K-doping. It is concluded from these studies that potassium atoms appearing at the crystal boundaries enhance inter-grain coupling and pinning mechanism in K-doped (Cu_0.5Tl_0.25K_0.25)Ba₂Ca₂Cu₃O_10?δ superconductors.     

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.     

Critical temperature and orthorhombicity of Y2Ba4Cu7O15−δ

Superconducting properties have been systematically studied for oxygen content controlled Y₂Ba₄O₇O_15?δ (Y247) polycrystalline bulks sintered under various conditions, 940–975 °C and P(O₂) = 4–16 atm. T_c of oxygen annealed Y247 samples at 400 °C varied largely depending on the sintering conditions from ～60 to ～90 K. Samples sintered under higher P(O₂) exhibited lower T_c, whereas they were confirmed to be of Y247 single phase by powder X-ray analyses. Strong relationship between orthorhombicity defined as 1000(b ? a)/(a + b) and T_c was found in Y247 as in the cases of REBa₂Cu₃O_7?δ with light rare-earth elements, such as La, Nd, Sm, Eu and Gd. In addition, T_c‘s of samples with nominal compositions of Y_2+xBa_4?xO₇O_15?δ systematically decreased with an increase of x. These results indicated that partial substitution of Y for Ba occurred in the Y247 compounds possibly due to high occupancy ratio of oxygen at the Cu–O_1-δ chain during the sintering process.     

Atomic surface structures on multi-layered cuprate superconductor Ba2Ca4Cu5O10(O1−xFx)2 observed by STM

We investigate the scanning tunneling microscopy (STM) on the multi-layered cuprate superconductor Ba₂Ca₄Cu₅O₁₀(O_1?xF_x)₂ (F0245, T_c = 79 K, x = 0.72).The STM images show clear atomic lattice structures and large random spot structures. Among the regular square-lattice atomic corrugation with the period of the lattice constant a ～ 0.38 nm, another kind of atomic spots arranged into the fourfold cross shaped clusters is clearly observed along the diagonal direction with the period of 0.26 nm. These clusters are being distributed inhomogeneously, which are due to the charge imbalance associated with the apical O/F rate. The apical O and F sites are also identified from the positions of such clusters in the STM topographic images.     

Bi2Sr2Ca(Cu1-xMnx)2Oy单晶的结构与超导电性

用自助熔剂法制备了Ｂｉ２Ｓｒ２Ｃａ（Ｃｕ１－ｘＭｎｘ）２Ｏｙ（ｘ＝０．０１,０．０５,０．０８,０．１０）单晶。单晶的ｃ轴长度随Ｍｎ含量的增加而减小。Ｒ－Ｔ曲线测量表明,零电阻温度Ｔｃ随ｘ的增大面逐渐下降。对Ｍｎ掺杂量较高的一些单晶,发现其Ｒ－Ｔ曲线在１０５Ｋ左右有一陡降,表明Ｍｎ掺杂量较高的单晶中可能有微量的Ｂｉ２２２３相成分存在。