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.      

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.     

Specific heat anomaly in Bi1.6Pb0.4Sr2Ca2Cu3Oy superconductor

The specific heat anomaly ΔC of Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_y was observed in the temperature range 80–100 K. It is estimated from ΔC that γ15 mJ/mol·K² and H_{c2(T=0)100 T} in this sample. The specific heat anomaly confirmed the occurrence of bulk superconductivity of the high-T_c phase in Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_y superconductor.     

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.     

Analysis of anisotropy of high Tc superconducting oxides using multi-CuO2-layer structure model

The anisotropy of high Tc superconducting oxides such as Bi₂Sr₂Ca₁Cu₂O₈ is analized by use of the multi-CuO₂-layer structure model. It is found that the anisotropy decreases when Tc increases. Under high pressure, the anisotropy also decreases.     

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     

STUDY OF DOPING EFFECT ON THE Bi2Sr2Ca2Cu3Oy SUPERCONDUCTOR

We have systematically investigated the doping effect on the Bi₂Sr₂Ca₂Cu₃O_y superconductive material. After entering into the crystal lattice, Pb and/or Sb atoms cause not only variations of the temperature condition during the sample preparation process, but also variations of structures and superconductivity. By analyzing the experimental results, we have come to the conclusion that the Pb and/or Sb atoms enter into the Bi-O planes, preferably occupy the sites of Bi ions, regulate the crystal structures and the distribution of oxygen ions in Bi-O planes and nearby lattice sites, thus benefiting the formation of the high-T_c phase and improving the superconductivity.     

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.     

Effects of divalent impurities on the fluctuation conductivity of YBa2Cu3O7 single crystals

We have studied experimentally the in-plane fluctuation conductivity near the superconducting transition in single crystal samples of YBa₂Cu₃O₇, Y_0.98Ca_0.02Ba₂Cu₃O₇, YBa_1.9Sr_0.1Cu₃O₇ and YBa₂Cu_2.97Zn_0.03O₇. In order to test the stability of the observed fluctuation regimes, low magnetic fields were applied perpendicular to the Cu-O₂ atomic planes. When the transition is approached from above we first observe a three-dimensional (3D) Gaussian regime then a crossover to a genuine critical region where the exponent is consistent with the predictions of the 3D-XY-E universality class. Decreasing further the temperature towards T_c, our results systematically reveal the occurrence of a regime beyond 3D-XY characterized by a very small critical exponent. We propose that this regime is precursory to a weak first-order superconducting transition driven by antiferromagnetic excitations related to the pseudogap phenomenon. The dilution of divalent impurities in YBa₂Cu₃O₇ does not affect the stability of the fluctuation regime beyond 3D-XY and in the case of Ca doping a further approach towards the first-order behaviour is observed.     

Ce4+离子在Ca2SnO4一维结构基质中的电荷迁移光谱研究

利用高温固相反应法制备了Ca₂Sn_1-xCe_xO₄和Ca_2-ySr_ySn_1-xCe_xO₄一维结构发光体. XPS结果显示 Ca₂SnO₄拥有两种结合能分别为5277 eV和5293 关键词： 2Sn_1-xCe_xO₄')" href="#">Ca₂Sn_1-xCe_xO₄ 2-ySr_ySn_1-xCe_xO₄')" href="#">Ca_2-ySr_ySn_1-xCe_xO₄ 一维结构 电荷迁移光谱     

在H2气氛中Bi2Sr2CaCu2O8+δ的热处理效应

对高品质单相多晶Bi₂Sr₂CaCu₂O_8+δ样品作真空或H₂气氛中不同温度下的热处理。由X射线结构分析,其正常态和超导态性质(特别是载流子浓度)的系统测量结果表明:Bi₂Sr₂CaCu₂0_8+δ样品在H₂气氛中热处理,其氧的释放可分为两个阶段:一是Bi-O层之间的过量氧原子的逸出,降低系统的载流子浓度,从而调节T_c;二是CuO₂面内氧原子的迁移,产生氧空位,使CuO₂面内Cu²⁺的短程二维反铁磁(2D-AFM)有序背景遭到破坏。可以认为:以往文献中报道的Bi₂Sr₂CaCu₂O_8+δ超导体的吸氢效应导致T_c的变化,很可能主要的并不是氢原子的电子掺杂行为,而是与吸氢过程中氧含量的变化相联系。 关键词：     

Dielectric properties for the ordered-perovskite cuprate Sr2Cu(Re0.69Ca0.31)O6

Dielectric properties are reported on polycrystalline cubic ordered-perovskite cuprate Sr₂Cu(Re_0.69Ca_0.31)O₆ in the frequency range 10 Hz-100 kHz at temperature from 300 to 500 K. Both the dielectric permittivity and dielectric loss factor are found to be frequency and temperature dependent. The enhanced value of the low frequency dielectric permittivity is associated to ionic polarization and interfacial phenomena. The material is found to possess significantly high dielectric permittivity. The calculated ac conductivity suggests semiconducting behaviour for the Sr₂Cu(Re_0.69Ca_0.31)O₆.     

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.     

(Bi,Pb)4Ca3Sr3Cu4Oy系统的超导电性与相结构的关联

本文研究了两组单相样品Bi,PbCa₃Sr₃Cu₄O_y和Bi₄Ca₃sr₃Cu₄O_y的物相与超导电性之间的关系.发现两组样品同属4334相材料,但它们的超导电性存在很大差异.引起这种差别的原因可能来自于晶胞内部的原子排列,氧缺位的数目及其分布,以及电子结构等精细结构的不同. 关键词：     

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.     

Vortex Excitations Above Tc in the Cuprate Superconductor Bi2Sr2Ca2Cu3O10 as Revealed by ESR

Using electron spin resonance (ESR) technique we have obtained data evidencing the existence of magnetic vortices in high-temperature superconductors at temperatures above the critical one T _c. We have studied magnetic excitations in Bi₂Sr₂Ca₂Cu₃O₁₀ single crystals above T _c with the method of surface spin decoration. The surface layer of diphenylpicrylhydrazyl was used as a sensitive probe of magnetic field distortions. The temperature dependence of the ESR signal parameters has indicated that far above T _c the magnetic flux of a sample is affected by the superconducting order parameter fluctuations while close to T _c its changes are due to vortex-type excitations.     

A low temperature internal friction study of Y0.85Ca0.15Ba2Cu3O7−δ with different oxygen content

The ceramic sample of Y_0.85Ca_0.15Ba₂Cu₃O_7−δ was prepared by standard solid-state reaction method, and samples with different oxygen concentration were obtained by quenching from high temperature. The internal friction was measured using the vibrating reed method from liquid-nitrogen temperature to room temperature at kilohertz frequency. An internal friction peak was observed around 250 K in Y_0.85Ca_0.15Ba₂Cu₃O_7−δ quenched from 1023 K. The peak is related to the one observed around 220 K (labeled as P3 peak) in undoped YBa₂Cu₃O_7−δ (Y123). This result shows the dependence of P3 peak on carriers density and P3 peak has a strong correlation to the abnormal behavior of Y123 in the underdoped range. The variation of two low temperature thermal activated relaxation peaks (P1 and P2) on oxygen content were also investigated. And consistent explanations were given based on all recent researches.     

新块材超导体Pr1-xCaxBa2Cu3O7-δ(0.4≤x≤0.6)的高压合成与超导电性

采用高温高压合成方法，合成出了Pr_1-xCa_xBa₂Cu₃O_7-δ(0.4≤x≤0.6) 系列块材超导体，在缺氧的Pr_0.5Ca_0.5Ba₂Cu₃O_7-δ四方123结构样品中得到了T_c为98K.实验结果表明Pr在123结构中的价态为大于3+的混合价态，因而空穴填充和杂化导致的载流子局域化是Pr抑制123相超导电性的关键因素. 关键词：     

Temperature dependence of the spin susceptibility of layered cuprates in the pseudogap phase: Comparison with the knight shift data for the Cu(2) site in YBa2Cu4O8 and Bi2Sr2CaCu2O8 + x

The temperature dependences of the Knight shift at the Cu(2) site in YBa₂Cu₄O₈ and Bi₂Sr₂CaCu₂O_{8 + x} compounds were analyzed based on the band structure parameter extracted from photoemission data. Different scenarios for the pseudogap state are discussed, and the densities of states of quasiparticle excitations are calculated.     

(Bi,Pb)2Sr2Ca2Cu3Oy单相样品的超导电性与载流子浓度

对(Bi,Pb)₂Sr₂Ca₂Cu₃O_y单相样品进行不同条件下的热处理,通过X射线衍射、电阻-温度关系、交流磁化率,以及Hall系数等测量,发现样品均具有较好的单相性,随着热处理条件的变化,其超导转变中点温度(T_c)有规律地分布在100—110K之间,T_c随载流子浓度(n_H)的增加而升高。实验结果表明,热处理条件对样品的相结构、超导 关键词：