Superconductivity in Bi2.2Ca2.8−x Sr x Cu2O y system and effect of rare earth substitution

The oxide system Bi_2.2Ca_2.8−x Sr _x Cu₂O _y has been investigated for superconductivity as a function of Ca/Sr ratio withx=0.7, 1.0, 1.4 and 1.8. All these compositions are found to be superconducting with onset temperature in the range of 80–90 K implying a large homogeneity range between Sr and Ca for the superconductivity to occur in this system. Effect of partial replacement of Bi by rare earths has been studied and it is observed that 20% replacement of Bi by Nd and Eu does not significantly affect the superconducting transition temperature.     

Peltier-effect in the mixed state of (Bi,Pb)2Sr2Ca2Cu3Oδ

We present measurements of the Peltier-effect in the mixed state of Bi_1.76Pb_0.24Sr₂Ca₂Cu₃O_δ. The Peltier-coefficient broadens in a magnetic field quite similar to the resistivity and the thermopower. Comparison with the thermopower shows that Onsagers relation holds well. The occurrence of the Peltier-heat in the mixed state well below T_c implies that the electric current is accompanied by a large heat current. We show that the vortex contribution to the Peltier-heat is negligibly small. Therefore the heat current has to be attributed to normal quasiparticle excitations. Our results indicate that this quasiparticle contribution to the heat current remains large even at temperatures far below T_c.     

Anisotropic fluctuation magnetoconductivity in Bi2Sr2Ca2Cu3Ox thin films

Measurements of the fluctuation magnetoconductivity as a function of both the magnetic field up to 13 T and the temperature in thin films of Bi₂Sr₂Ca₂Cu₃O_x are presented. The variation of the magnetoconductivity with the magnetic field strength is quadratic at temperatures distantly above the critical temperatureT _c and changes to an entirely negative curvature nearT _c . The latter behavior is attributed to an inhomogeneous critical temperature in the films. The results are analyzed in terms of recent theoretical models for fluctuation-enhanced magnetoconductivity in layered superconductors. The magnetoconductivity in a magnetic field oriented normal to the copper-oxide layers is dominated by the orbital contribution of the Aslamazov-Larkin effect. The magnetoconductivity in the parallel orientation is distinctly smaller and provides evidence for the corresponding Zeeman contribution. The latter measurement also indicates that the Maki-Thompson process may be insignificant in Bi₂Sr₂Ca₂Cu₃O_x.     

Crystallization kinetics of Bi1.7V0.3Sr2Ca2Cu3Ox glass-ceramic

Crystallization kinetics of Bi_1.7V_0.3Sr₂Ca₂Cu₃O_x glass-ceramic was investigated using the differential scanning calorimetry (DSC) technique. Three characteristic phenomena were observed in the studied temperature range. The activation energies for glass transition temperature and crystallization phenomena were determined by different theoretical models. Applying the modified Johnson–Mehl–Avrami (JMA) equation reasonably suggests that crystallization process of the Bi_1.7V_0.3Sr₂Ca₂Cu₃O_x glass-ceramic is carried out by a bulk growth in three dimensions.     

Volume dependence of the raman frequencies in Y Ba2Cu3Ox single crystals with different oxygen content

Abstract

The Raman spectra of Y B _a2C_u3O_x, single crystals (x=6.25; 6.75; 7.0) were measured at pressures up to 22 GPa (35 GPa for x=6.25) at room temperature in nearly hydrostatic conditions. The frequency-volume curves for most of the Raman-active fundamental vibrations were derived from the present data making use of a previous high-pressure study of the equations of state of Y Ba₂ Cu ₃ O _x compounds.     

Structure and oxygen content in the high-temperature superconductor YBa2Cu3O7−δ

The effects of oxygen deficiency on the processes of ordering the oxygen vacancies and of the formation of superstructures are discussed in the layered perovskite structure of YBa₂Cu₃O_7−δ. Samples subjected to different heat and gas treatments are studied with X-ray diffraction techniques to get this information.     

Superconductivity in an intermetallic oxide Hf_3Pt_4Ge_2O

Discovery of a new superconductor with distinct crystal structure and chemistry often provides great opportunity for further expanding superconductor material base, and also leads to better understanding of superconductivity mechanisms. Here, we report the discovery of superconductivity in a new intermetallic oxide Hf_3Pt_ Ge_2O synthesized through a solid-state reaction. The Hf_3Pt_ Ge_2O crystallizes in a cubic structure(space group Fm-3 m) with a lattice constant of a = 1.241 nm, whose stoichiometry and atomic structure are determined by electron microscopy and x-ray diffraction techniques. The superconductivity at 4.1 K and type-II superconducting nature are evidenced by the electrical resistivity, magnetic susceptibility, and specific heat measurements. The intermetallic oxide Hf_3Pt_ Ge_2O system demonstrates an intriguing structural feature that foreign oxygen atoms can be accommodated in the interstitial sites of the ternary intermetallic framework. We also successfully synthesized a series of Hf_3Pt_ Ge_2O1+δ(-0.25 ≤δ≤ 0.5), and found theδ-dependent superconducting transition temperature Tc. The atomic structure and the electronic structure are also substantiated by first-principles calculations. Our results present an entirely new family of superconductors with distinct structural and chemical characteristics, and could attract research interest in further finding new superconductors and exploring novel physics pertaining to the 5 d-electron in these intermetallic compound systems.     

Weak link problem and intragrain current density in polycrystalline Bi1Ca1Sr1Cu2Ox and Tl2Ca2Ba2Cu3O10

Polycrystalline sintered specimen were investigated by means of ac susceptibility and inductive critical current measurements. Main objects are whether the new highT _c oxides have a weakly coupled intergranular structure and to get information about the intragrain critical current density. The Tl₂Ca₂Ba₂Cu₃O₁₀ specimen shows a connective nature similar to YBa₂Cu₃O₇ with low intergrain currents (100 A/cm² at 100K,B=0 T) and high intragrain ones (4.5·10⁵ A/cm² at 100K,B=0 T). The investigation of the Bi₁Ca₁Sr₁Cu₂O_x specimen becomes complex due to the presence of two superconducting phases and the low intragrain critical current density of the lowerT _c phase.     

Effect of vapors of the simplest alcohols on the structure and properties of YBa2Cu3Oy compounds

With low-temperature treatments (200–300°C), the effect of alcohol vapors on the structural and electrophysical properties of YBa₂Cu₃O_y (123) having different oxygen content has been studied. It has been established that CuO and YBa₂Cu₃O_y are catalysts for the dehydration of alcohols to form water and aldehydes. Similar to the action of water vapors, the treatment in vapors of the simplest alcohols leads to hydration of 123 and its transformation into the pseudo-124 phase. In this case, planar stacking faults are formed to be centres of pinning and can improve the critical characteristics of high-temperature superconductors (HTSС) in magnetic fields. The influence of the alcohol vapors is more ‘mild’ as compared to the treatment in water vapors. At t?=?300°C, the alcohols are reducing agents of copper, followed by the decomposition of 123 into simple compounds.     

Stages of thermally induced gas-exchange in the high-temperature superconductor YBa2Cu3O7−δ

The oxygen stoichiometry in the YBa₂Cu₃O_7−δ high temperature superconductors is studied, using DSC/DTA techniques on samples subjected to different heat and gas treatments. Characteristic temperature regions of high temperature superconductivity exchange with oxygen in gas phase are identified and the resulting structural changes are discussed.     

Influence of experimental conditions on the electric field effect in the ceramic (BiPb)2Sr2Ca2Cu3Ox

An experimental study is reported of the influence of temperature (T), electric field polarity (±E), as well as of changes in the electrode/insulator/superconductor (E/I/S) measuring system on the field effect in the ceramic (BiPb)₂Sr₂Ca₂Cu₃O_x. It has been established that at 77 K and for E⩾60 MV/m the critical current I _c and conductivity of the sample increase for I>I _c, irrespective of the field polarity. For lower fields and a negative electrode potential the conductivity in an electric field may decrease. The field effect decreases with increasing temperature, to practically vanish near T _c where the sample is still in superconducting state. Experiments carried out with more complex systems E/I/M/I/S and E/I/M/S (M stands for a metallic foil) support the conclusion that it is the external electric field that is responsible for the observed effects. Fiz. Tverd. Tela (St. Petersburg) 39, 1967–1970 (December 1997)     

对靶溅射技术制备的La2/3Ca1/3MnO3/YBa2Cu4O8/La2/3Ca1/3MnO3薄膜中Tc的降低

本文用对靶溅射技术制备了La2/3Ca1/3MnO3/YBa2Cu4O8/La2/3Ca1/3MnO3薄膜.与YBCO单层薄膜相比,由于超导/铁磁系统中的磁性邻近效应,三层薄膜表现出较低的超导转变温度.薄膜的R～T测量曲线显示出超磁阻(CMR)效应和超导转变,预示着超导和铁磁特性共存于LCMO/YBCO/LCMO三文治结构.     

Effect of chemical substitution and oxide additives on the Tc behaviour of the oxide superconductor,Bi4Sr3Ca3Cu4O16

Studies on the superconductivity behaviour of pure and doped (Pb,TI) Bi₄Sr₃Ca₃Cu₄O₁₆ phases are reported. Effect of oxide additives PbO, BaPbO₃ and BaBiO₃ has also been examined. Results show that an oxygen treatment at 600°C for 12 h during the final stages of the synthesis has the beneficial effect of sharpening the T_c ^zero of the 4334 Bi-composition. Pb and TI substitution has only a marginally beneficial effect on the T_c , though it is not definitely known whether substitution is actually occurring in the system. PbO as an additive raises the T_c ^zero to 89 K, whereas other additives do not basically affect the properties of the 4334 system.     

Commensurability oscillations in NdBa2Cu3Oy single crystals

Commensurability between inter-vortex distance and crystal lattice constant is investigated by angular dependent magnetization in very pure twinned and twin-free NdBa₂ Cu₃ O_y single crystals. With increasing temperature the incommensurate states split up and become finally commensurate with half the vortex distance. These new commensurate states are related to a substructure of the intrinsic pinning potential within the unit cell and discussed with respect to temperature, field, anisotropy, and twin structure.     

Effect of Sb substitution on the superconductivity of Bi2(Ca,Sr) n+1Cu n O4n+2 (n=1, 2 and 3) systems

Effect of incorporation of Sb in place of Bi in the bismuth cuprate superconductors has been examined. The nominal compositions studied are MCa_1·5Sr_1·5Cu₂O_8+δ and MCa₂Sr₂Cu₃O_10+δ, whereM=Bi_2−x Sb _x or Bi_1·5−x Sb _x Pb_0·5. Different preparative routes such as the ceramic method, the matrix route as well as the melt route were employed to prepare the materials. No indication of either Sb entering the lattice or enhancement ofT _c is noted from resistivity, magnetic susceptibility and microwave absorption measurements. Communication No. 162 from Materials Research Centre.