Temperature dependence of the critical current in YBa2Cu3O7−δ and Bi2Sr2Ca1Cu2O8 Josephson junctions

We have studied the stationary Josephson effect on YBa₂Cu₃O_7−δ (T_c=90 K) and Bi₂Sr₂Ca₁Cu₂ O₈ (T_c=80 Kand 87 Kfortwosamplesofdifferentorigin) ceramic based junctions. The temperature dependence of the critical current near T_c has been found as I_c≈(T_c-T) for the Y-Ba-Cu-O samples indicating that they should be classified as S-N-I-N-S type junctions. The I-V curves of the Bi-Sr-Ca-Cu samples show the typical behaviour of S-I-S structures. Using Ambegaokar-Baratoff's theory for Bi₂Sr₂Ca₁Cu₂O₈, the temperature dependence of the superconducting state gap Δ(T) was calculated and it was evaluated that 1.452Δ(0)/k_BT_c3.5.     

Increase in the diamagnetic response from low-density Bi1.8Pb0.3Sr1.9Ca2Cu3Ox high-temperature superconductors and Bi1.8Pb0.3Sr1.9Ca2Cu3Ox + Ag composites

Low-density polycrystalline Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x high-temperature superconductors with a foamlike microstructure and composites consisting of this superconductor and silver in an amount of 20, 25, and 30 vol % are synthesized. The microstructure, as well as the temperature and field dependences of the magnetization, M(T) and M(H), are studied. It is found that, in Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x high-temperature superconductors and Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x + Ag composites, the diamagnetic response is enhanced and the screening properties are improved compared with high-temperature polycrystalline superconductors with the same composition that are prepared by the standard technology. The observed effect is explained by the features of magnetic flux penetration into a porous medium.     

The effect of Sb concentration on the superconducting properties and phase composition of Bi-Pb-Sb-Sr-Ca-Cu oxide materials

The effect of Sb superconducting materials with nominal composition Bi_1.6Pb_0.4–x Sb _x Sr₂Ca₂Cu₃O _y (x=0.01–0.14) and Bi_1.5Pb_0.5–x Sb _x Sr₂Ca₂Cu₃O _y (x=0.025–0.12) has been investigated. These materials were subjected to DTA, ICP emission spectrometry, SEM, XRD and electric resistance measurements. It was found that the introduction of Sb raises the melting temperature of the material, it also raisesT _c (up to 110K) at several defined stoichiometric ratios. A certain lack of correspondence between the 2223 phase volume and theT _c value was noticed and a possible explanation has been proposed.     

Critical addition of Ti to maintain the Tc of (Bi1-zTiz)1.6Pb0.4Sr2Ca2Cu3Ox above 100 K

Influence of Ti addition (substitution for Bi) on T_c is studied for high T_c (Bi_1-zTi_z)_1.6Pb_0.4Sr₂Ca₂Cu₃O_x. We found a critical Ti addition to keep high T_c (above 100 K) of the (Bi_1-zTi_z)_1.6Pb_0.4Sr₂Ca₂Cu₃O_x. The high-T_c phase can be found up to 0.5 of z. It is explained by an abnormal T_c change which is found from 0.3 to 0.5 of z. Based on results of the electrical resistivity and x-ray diffraction, an abnormal T_c change is discussed.     

Submillimeter transmission spectra of Sr2Ca2Cu4Oy

Transmission spectra of Sr₂Ca₂Cu₄O_y, which is a part of high T_c superconducting system Bi₂Sr₂Ca_n–1Cu_nO_z, and their temperature evolutions were investigated by monochromatic quasioptical submillimeter spectroscopy. Strong increase of high frequency conductivity in the range 80 to 350K was revealed. The obtained results were analyzed on basis of thermally activated carriers model.     

Electrical and infrared study of Bi2Sr2Ca1Cu2O8 in semiconducting, superconducting ceramic and superconducting glass ceramic state

We have studied the electrical and infrared properties of Bi₂Sr₂Ca₁Cu₂O₈ compound in three states. Electrical and IR measurements show that the pure powder state sample is a semiconductor, the ceramic Bi₂Sr₂Ca₁Cu₂O₈ sample prepared after annealing at 820°C for 240 hours shows a T_c of 85 K, whereas Bi₂Sr₂Ca₁Cu₂O₈ sample prepared through glassy route, i.e. melting at 1250°C and annealing at 820°C for 240 hours shows a drop of T_c by 5 K. The infrared spectra of superconducting ceramic and glass ceramic states in the available frequency range of measurement reveals the presence of three phonons. Since the vibrational mode around 595 cm^–1 is due to CuO₂ layers and as the CuO₂ layers are responsible for T_c in the ceramic superconductors, any change in these layers will affect the T_c. The shifting of the 595 cm^–1 mode towards lower frequencies in the glass ceramic due to different preparation process, indicates that there is a change in CuO₂ layers resulting in a change of T_c, which is confirmed by Four probe dc measurements.     

Superconducting screen printed thick films of YBa2Cu3O7 and Bi1.6Pb0.4Sr1.6Ca2.4Cu3O10 on polycrystalline substrates

Thick films of YBa₂Cu₃O₇ and Bi_1.6Pb_0.4Sr_1.6Ca_2.4Cu₂O₁₀ have been prepared using a screen printing method on polycrystalline substrates. The films have been characterized by X-ray diffraction, electrical measurements and SEM observations. YBa₂Cu₃O₇ films exhibited T_c ranging from 80 to 87 K on Y₂BaCuO₅ substrates while Bi_1.6Pb_0.5Sr_1.6Ca_2.4Cu₃O₁₀ films on polycrystalline MgO reached the zero resistance state at 104 K with critical current densities of up to 10 A/cm² at 77 K.     

Synthesis,Mössbauer studies and superconductivity of substituted Bi 2201 phase

We report X-ray diffraction, resistivity and Mössbauer studies on several substituted 2201 Bi cuprates, Bi₂Sr₂Cu_1?xFe_xO_y(O≤x≤1) and Bi₂Sr_1.6La_0.4Cu_1?xFe_xO_y and Bi_1.6Pb_0.4Sr_1.6La_0.4Cu_1?xFe_xO_y (O≤x≤0.04). The first series is not superconducting and a new phase appears for x=1. For La substituted samples, T_c increases up to 35K and decreases strongly with x. All Mössbauer spectra were fitted well with four quadrupole doublets, with relative population depending on samples.     

Influence of heat treatment on the superconducting properties of Bi1.6Pb0.3Sb0.1Sr2Ca2Cu3O y

Differently heat-treated samples with the nominal composition Bi_1.6Pb_0.3Sb_0.1Sr₂Ca₂Cu₃O _y were studied by resistance and ac susceptibility measurements in the temperature range 77 K<T<200 K. The X-ray diffraction patterns at room temperature were also investigated. The experimental results show that repeated quenchings can depress the formation of the 80 K phase, and enhance the formation of the 110 K phase.     

Modulated structure of the superconducting compounds Bi2Sr2Ca n-1Cu n O y withn=1 and 2

The incommensurate modulated structures of high-T _c superconducting phases Bi₂Sr₂Ca _n–1Cu _n O _y (n=1 and 2) have been studied using symmetry properties of four-dimensional super-spacegroups and electron diffraction as well as high resolution electron microscopy are used to describe the properties of the incommensurate modulated structure in the compounds. The main results are planar monoclinic symmetryP ^B _{2/b 11} orP B _{b 1} for Bi₂Sr₂CuO _y (n=1) and orthorhombic symmetryN ₁₁₁ ^Bbmb orN ₁₁₁ ^Bb2b for Bi₂Sr₂CaCu₂O _y (n=2). The temperature dependence of the modulated structure of Bi₂Sr₂CaCu₂O _y has been investigated from –190°C to 800°C. Some structure properties of Bi₂Sr₂Ca₂Cu₃O _y (n=3) are presented for comparison.     

ELECTRONIC STRUCTURE OF THE T*-TYPE SUPERCONDUCTORS (Bi0.5Sr1.5)(Y, Ce)2Cu2Oy STUDIED BY PHOTOELECTRON SPECTROSCOPY

The electronic structure of T^*-type compounds (Bi_0.5Sr_1.5)(Y_2-xCe_x)Cu₂O_y has been studied by X-ray photoelectron spectroscopy. From analysis of the results, it was found that cation deficiency in Ce⁴⁺ introduced holes to conduction planes in the T^* phase as in the p-type infinite layer compounds. Some Cu ions in the sample without cation-deficiency contained two-oxygen coordination, which resulted in the incompleteness of the CuO₂ sheets. Bi ions in the T^* phase cuprates take exact valence of 3_{+, Pb ions cannot replace them due to the too large an ionic radius of Pb²⁺/Pb⁴⁺.}     

Phase composition and properties of superconducting ceramics based on Bi1.7Pb0.3Sr2Ca2Cu3Oy precursors fabricated by melt quenching in a solar furnace

Production of superconducting ceramics based on precursors with rated composition Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O _y is studied. The precursors are synthesized in a solar furnace by melt rapid quenching. The phase composition of the samples is examined by microstructural and X-ray analyses. The temperature dependences of the resistance and magnetic susceptibility are measured. The influence of the composition and crystal structure of the substrate on texturing in the Bi-Sr-Ca-Cu-O system is studied. It is found that the type of quenching plays a significant role, while the type of substrate is of minor significance.     

新型超导体MgB2和MgCNi3热、电输运性质研究

报道了两种新型超导体MgB₂,MgCNi₃和氧化物高温超导体Bi_{2< /sub>Sr2Ca0.9Ce0.1Cu2O8+y 的热导率-温度关系和电阻率-温度关系.实验发现氧化物高温超导 体在进入超导态后热导有所上升，出现极大值后再下降，而MgB2和MgCNi3 则单调下降. 由Wiedemann-Fra 关键词： 热导率 超导体 2}')" href="#">MgB₂ 3')" href="#">MgCNi₃     

Effective atomic charges and charge transport in superconductor lattices

Effective atomic charges in Rba₂Cu₃O₇ compounds (here R stands for yttrium or a rare-earth metal), Tl₂Ba₂Ca₂Cu₃O₁₀, and Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀ have been determined. In Rba₂Cu₃O₇, the holes are shown to be delocalized primarily on the oxygen-chain sublattice. Two models of charge distribution within the lattices of Tl₂Ba₂Ca₂Cu₃O₁₀ and Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀ are proposed, which assume the presence of holes at oxygen sites in the Cu(2)-O plane but differ in the occupation of the 3d shell of Cu atoms at the nodes of Cu(1). The atomic charges of Tl, Bi, and Cu(1) on these two latter lattices are possibly controllable. The results obtained are shown to be qualitatively consistent with Hall measurements of the hole concentration and of its temperature dependence in the compounds studied. Fiz. Tverd. Tela (St. Petersburg) 39, 2118–2122 (December 1997)     

Semiconducting TlSr2RCu2O7 (R=rare earth) and its superconducting derivatives

Semiconducting TlSr₂RCu₂O₇ (R=Pr or Er) with a 1212-type structure has been synthesized in the single-phase form. Partial substitution of Sr²⁺ for R³⁺ converts this semiconductor to a 90 K superconductor TlSr₂(R_1–y Sr _y )Cu₂O₇. A combination substitution, Sr²⁺ for R³⁺ and Pb⁴⁺ for Tl³⁺, leads to the Ca-free 100 K superconductor (Tl, Pb)Sr₂(R, Sr)Cu₂O₇. The results are explained in the framework of the mixed Cu²⁺/Cu³⁺ valence.     

Magnetotransport and magnetotunneling in single-layer, two-layer, and three-layer Bi2Sr2Can−1CunOz (n=1,2,3) single crystals

In continuous magnetic fields H up to 28 T, we have studied the out-of-plane transport properties and tunneling characteristics of high-quality nondoped single crystals of the Bi-cuprate family: Bi₂Sr₂CuO_6+δ (Bi2201), Bi₂Sr₂CaCu₂O_8+δ (Bi2212) and Bi₂Sr₂Ca₂Cu₃O_10+δ (Bi2223) grown by an identical method. For all compounds the out-of-plane magnetotransport ρ_c(H) is negative in the temperature region where ρ_c(T) shows in the normal state a semiconducting-like temperature dependence. The negative magnetoresistance of ρ_c corresponds to the suppression of the semiconducting temperature dependence of ρ_c(T) which is found to be isotropic. For the Bi2201 compound, where the normal state can be reached in the available magnetic fields (28 T), a nearly complete suppression of the low-temperature upturn in ρ_c(T) is observed in the highest magnetic fields with a tendency towards a metallic behavior down to the lowest temperatures (0.4 K). Using the break-junction technique, especially for the Bi2212 and Bi2232 compounds, a clear superconducting gap structure can be observed. Both for temperatures above the critical temperature and for magnetic fields above the upper critical field, a pseudogap structure remains present in the tunneling spectra. The applied magnetic fields yield a stronger suppression of the superconducting state compared to that of the normal-state gap structures as manifested in ρ_c(T) transport and tunneling.     

Transport properties of a and c axis oriented (Y1?xCax)Ba2Cu3Oy thin films

Anisotropy and Hall effect measurements have been performed in calcium-doped, i.e., overdoped YBa₂Cu₃O_y ((Y_1−xCa_x)Ba₂Cu₃O_y) thin films witha andc axis orientations. In highly overdoped films (x=0.4), the anisotropy of the normal resistivity decreases and a drastic change in Hall conductivity in the mixed state is observed. The change in Hall conductivity in the overdoped region is consistent with recent experimental results for La_2−xSr_xCuO₄ films and seems to be common in highT _c superconductors.     

Bi系高温超导体的119Sn的穆斯堡尔谱研究

本文采用¹¹⁹Sn作为探针原子,对Bi_1.68Pb_0.32Sr_1.75Ca_1.75Cu_2.65-xSn_xO_y(0.2≤x≤0.8)样品进行室温下穆斯堡尔谱测量,结合X射线衍射与电性能测量结果,分析Sn在样品中的占位情况与电性能之间的关系,并得出四配位Cu位对高T_c相的超导电性起着较关键作用的结论。 关键词：     

T*相超导体(Bi,Sr)2(Y,Ce)2Cu2Oy的性质研究

研究了T^*相超导体（Bi，Sr）₂（Y，Ce）₂Cu₂O_y的成相规律，发现缓冷对T^*相单相样品的制备是有利的．淬火样品的主相物质为Ce₂Y₂O₇，完全不含T^*相结构．在室温到熔化温度的范围，热分析未观察到结构相变．在240—460℃，相对每个Cu原子超导体失氧约0.1个氧．氧气中烧结并缓冷到室温的样品在30K出现零电阻现象 关键词：     

Magnetic field dependence of vortex activation energy: A comparison between MgB2, NbSe2 and Bi2Sr2Ca2Cu3O10 superconductors

The dissipative mechanism at low current density is compared in three different classes of superconductors. This is achieved by measuring the resistance as a function of temperature and magnetic field in clean polycrystalline samples of NbSe₂, MgB₂ and Bi₂Sr₂Ca₂Cu₃O₁₀ (BSCCO) superconductors. Thermally activated flux flow behaviour is seen in all the three systems and clearly identified in bulk MgB₂. While the activation energy at low fields for MgB₂ is comparable to Bi₂Sr₂Ca₂Cu₃O₁₀, its field dependence follows a parabolic behaviour unlike a power-law dependence seen in Bi₂Sr₂Ca₂Cu₃O₁₀. We analyse our results based on Kramer’s scaling for grain boundary pinning in MgB₂ and NbSe₂.