Transport properties in single phase superconductor Ba2YCu3O9-δ

The electrical resistivity and the thermopower are measured on the single phase superconductor Ba₂YCu₃O₉-δ (δ=2.1). The results indicate that the temperature dependences of the resistance and thermopower exhibit typical metallic behaviour, and the sample conducts via electrons at high temperatures. The behaviour of the thermopower can be described with Mott's semi-classical model. The specific heat of electrons in normal state has been estimated 780mJ/K·mole at 200K, i.e. γ=3.9mJ/K²·mole. Unusual phonon-drag effect is observed above the superconducting transition temperature T_c. Below T_c, the electrical resistivity and the thermopower all drop to zero corresponding to a superconducting ground state.     

Structures of superconducting Ba2YCu3O7-ϖ and semiconducting Ba2YCu3O6 between 25°C and 750°C

The structure and copper valence states of the 100K superconductor, Ba₂YCu₃O₇ have recently been determined by neutron powder diffraction between 5K and 300K¹, and at room temperature². This ′oxygen deficient orthorhombic perovskite structure′, now independently confirmed^3,4, is well ordered and stochiometric, and contains one Cu⁺⁺⁺ and two Cu⁺⁺ atoms distributed over two square planar oxygen co-ordinated sites. In the a-b plane CuO₄ squares are linked by their oxygen corners to form infinite sheets, while along the b-axis CuO₄ squares form infinite chains on the second Cu-site. In this paper we show that when Ba₂YCu₃O₇ is heated above room temperature, it progressively loses all oxygen from the O4 sites on the b-axis chains, while the remaining oxygen sites remain fully occupied. Near 700°C it becomes tetragonal⁵, with the P4/mmm Ba₂YCu₃O₆ structure obtained by X-ray diffraction at room temperature⁶. This material is well ordered, stochiometric and stable when cooled under vacuum. The valence of Cu2 in the a-b sheets remains close to 2, while that of Cu1 in the b-axis chains falls to near 1. The valence of copper in these chains can perhaps fluctuate, since the vibration of O4 out of the chain, which we observed even at 5K¹, implies a reduction of the valence charge on this copper site. We have observed similar large amplitude oxygen vibration at low temperature in La₂CuO₄.     

Structural and superconducting properties of Eu1−xPrxBaSrCu3O7−δ

We have investigated the structural and superconducting properties of Eu_1−xPr_xBaSrCu₃O_7−δ by X-ray diffraction analysis, AC and DC magnetization and DC resistivity measurements. The parent compound crystallizes in the orthorhombic symmetry which transforms to tetragonal symmetry at x=0.2. Superconducting transition temperature T_c decreases monotonically with increasing x which approaches zero at x=0.6. The enhancement in critical concentration x_cr observed in Eu_1−xPr_xBa₂Cu₃O_7−δ on replacing 50 at.% Ba by Sr is attributed to the resulting local structural changes.     

Instabilities in the superconducting system Bax2YCu3O9−δ driven by a pulsed electric current

We have examined the effect of a pulsed electric current on the resistive properties of the high temperature superconductor Ba₂YCu₃O_9−δ. In the range of pulsed current densities 3–5 A/mm² with rise time 0.6 μs, at room temperature, the semiconducting phase is converted to the stable metallic state.     

Electron microscopy and neutron diffraction studies of Ba2YCu3O7−δ compounds

An electron microscopy study of Ba₂YCu₃O_7−δ samples gives evidence of several defects occuring in the stacking sequence along the c axis. It is shown that the oxygen intercalation process is reversible and does not change these defects of the cation lattices.A neutron diffraction study of powdered samples indicates that the oxygen-rich compounds Ba₂YCu₃O_6.92 and Ba₂YCu₃O_6.64 have the same orthorhombic structure, the only difference consisting in the occupancy of the oxygen site around the copper atoms forming linear Cu-O chains. The reduced compound Ba₂YCu₃O₆ has a tetragonal structure corresponding to an empty oxygen site in the plane of these copper atoms. The contraction of the c parameter and of the volume of the unit cell when the oxygen content increases can be explained by the electrostatic attraction between Ba²⁺ and O²⁻ ions introduced in the lattice.     

Infrared spectra of superconducting La2−xSrxCuO4 and YBa2Cu3O7−δ

Abstract

Abstract Infrared spectra of La_2?xSr_xCuO₄ and YBa₂Cu₃O_7?δ are discussed, specially with respect to the disappearance of the high-frequency (～650 cm) band of the superconducting compositions at 300K. Some of the bands persist at 300K eventhough the materials are fairly conducting. YBa₂Cu₃O_7?δ does not show evidence in the far IR spectrum for the presence of an optical gap.     

Elastic properties of superconducting and non-superconducting DyBaSrCu3O7−δ

Ultrasonic longitudinal and shear wave velocities were measured for superconducting and non-superconducting DyBaSrCu₃O_7−δ (Dy-1113) at 10 MHz between 80 and 270 K. The superconducting sample exhibits a step-like elastic anomaly between 190 and 230 K. No elastic anomaly was observed in the non-superconducting Dy-1113. The anomaly observed in the superconducting sample is attributed to structural phase transition caused by oxygen orderings.     

Critical properties of superconducting Ba1−xKxFe2As2

Magnetisation and magneto-resistance measurements have been carried out on superconducting Ba_1?xK_xFe₂As₂ samples with x = 0.40 and 0.50. From high field magnetization hysteresis measurements carried out in fields up to 16 T at 4.2 K and 20 K, the critical current density has been evaluated using the Bean critical state model. The J_C determined from the high field data is >10⁴ A/cm² at 4.2 K and 5 T. The superconducting transitions were also measured resistively in increasing applied magnetic fields up to 12 T. From the variation of the T_C onset with applied field, dH_C2/dT at T_C was obtained to be ?7.708 T/K and ?5.57 T/K in the samples with x = 0.40 and 0.50.     

Crystal structure of the superconducting infinite-chain cuprate Ba2Cu2.89O6−y

The orthorhombic structure of Ba₂Cu_2.89O_6−y (sp. gr. Pccm, a=13.065(15), b=20.654(21) and c=11.431(8) Å) contains two nonintersecting symmetrically equivalent sets of parallel CuO₂-chains running along [110] and [1̄10] directions. Along the c-axis the Cu–O sheets are separated by identical layers of Ba-atoms. The Cu–O chains are distorted within the xy plane producing zigzag. Three of 14 positions of copper atoms were found to be partly vacant that caused the significant shifts of the neighbouring Ba atoms in the plane. The determined structure of Ba₂Cu_2.89O_6−y is compared to that of the monoclinic Ba₂Cu₃O₆ modification, of another infinite-chain compound Sr_0.73CuO₂, and to one of the substructures of ladder compounds (Sr,Ca)₁₄Cu₂₄O_41+y. The superconductivity in Ba₂Cu_2.89O_6−y is associated with the CuO₂-chains consisting of CuO₄-squares sharing edges. The attention is paid to the coincidence of the temperatures of the superconducting transitions in Ba₂Cu_2.89O_6−y and in ladder compound with the temperature of the sharp magnetic transition in another infinite-chain compound Sr_0.73CuO₂.     

Magnetic properties of superconducting EuBa2Cu3O7−x

The magnetic properties of superconducting EuBa₂Cu₃O_7–x with resistiveT _c 96.5 K are measured. The normal state magnetic susceptibility is analyzed within the framework of the Van Vleck-Frank theory, leading to the conclusion that the strong moments of the Eu³⁺ ions are uncorrelated, and do not affect the superconducting state.     

NMR of 89Y in normal and superconducting YBa2 Cu3 O9-δ

Pulsed NMR data for ⁸⁹Y in the high-T_c superconductor YBa₂Cu₃O_9-δ (δ ≈ 2.1) in both the normal and mixed states are presented. Spin-lattice relaxation exhibits a Korringa temperature dependence in the normal state, with T₁T = (4.6 ± 0.1) × 10³ s-K. Below T_c, T₁ increases much more quickly than for a conventional superconductor. From linewidth measurements, the zero-field penetration depth is estimated to be λ ≈ 3800 Å. The Knight shift in the normal state is small, K ≈ 0.02%. Spin-spin relaxation times, determined by Y-Cu dipolar interactions, decrease below T_c due to the detuning of Cu-Cu interactions by fluxoid field gradients.     

Effect of process parameters on the properties of plasma-sprayed superconducting Y1Ba2Cu3O7−x coatings

A study on the effect of powder, spray and coating parameters on superconducting properties of plasma-sprayed Y₁Ba₂Cu₃O_7–x coatings has been carried out. Optimised spray parameters to yield strong, adherent and dense coatings on many substrates including AISI 304 SS and alumina have been obtained. Coatings show superconductivity after oxygen annealing at 950°C for an hour. Coatings thinner than 100 m do not show zero resistivity down to 77 K due to weak links between granular superconductors. Coatings prepared with fine powder have superior superconducting properties, viz. higherT _c andJ _c values than coarse powder coatings. The choice of substrate material strongly affects theJ _c values.     

Influence of Zn concentration on electrical resistance of SmBa2(ZnxCu1−x 3O9−δ

It is known that perovskite type structures (ReBa₂Cu₃O_9–) exhibit superconductivity at temperatures below 90 K /1–3/. The substitution of other ions for Cu is interesting because the Cu-O interactions are probably responsible for the supercoductivity in those systems /4/. In the system Y-Ba-(Ag,Cu)-O Saito found that the gradual decrease of T_c with the increase of Ag concentration is due to the presence of the different phases /5/.In this letter we are reporting the experimental measurements of the electrical resistance of SmBa₂Cu₃O_9– oxygen defect perovskites with Zn substituted for Cu.     

Observation of RF superconductivity in Y1Ba2Cu3O9−δ AT 3 GHz

Two samples of Y₁Ba₂Cu₃O_9−δ, one fabricated at Los Alamos National Laboratory and the other at the University of Wuppertal, were tested in the rf field of a niobium cavity at 3 GHz and in a temperature range from 4.2 to 300 K. The data show a sharp reduction of the Joule losses in the samples beginning at an onset temperature of 98 and 92 K, respectively. The residual losses of the sample were measured at 4.2 K where the host cavity was superconducting. The ratio of these losses to the ones observed slightly above T_c was less than 1.1 × 10⁻². This indicates that only a small fraction of the conduction electrons of the Y₁Ba₂Cu₃O_9-δ sample remain unpaired. The dependence of the rf losses on the rf magnetic field allows the determination of a maximum surface current density which is found to be about 5 × 10⁴ A/cm².     

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.     

循环伏安法研究Ba2YCu3O7—y超导体

本文利用电化学循环伏安法(CV 法),以饱和 Ba(OH)_2水溶液为电解液,对单相Ba_2YCu_3O_(7-y)正交和四方结构超导体进行了研究.结果发现:在典型的正交相中确实存在Cu~(3+)→Cu~(2+)的还原峰,而四方相则无此峰出现.从而首次直接检测到 Ba_2YCu_3O_(7-y)烧结体中 Cu~(3+)离子.     

Influence of Y2Cu2O5 nanoparticles doping on superconducting properties of YBa2Cu3O7-δ

ABSTRACT

The blue phase of YBa₂Cu₃O_{7- δ} (YBCO) family, Y₂Cu₂O₅ (Y202) nanoparticles were prepared and doped into (YBCO) superconductor and the effect of doping on critical current density and critical temperature was investigated. Y202 nanoparticles with particle sizes of 47, 107 and 206?nm were prepared by a sol–gel combustion method and added into the YBCO superconductor by 0.5–2?wt.%. XRD and scanning electron microscope measurements were used to characterize the samples. The measurement of critical current density at 77?K revealed that the doped superconductors had larger critical current density compared to the undoped superconductors. For a fixed dopant concentration, by increasing the size of nanoparticles, the J_c was increased. For the samples including 0.5?wt.% of nanoadditives, J_c was higher. The highest critical current density of 137?A/cm² was measured for the superconductors containing 0.5?wt.% of 206?nm Y202 nanoparticles. Also, by increasing the nanoparticles concentration, the T_c was reduced.     

Phonon transport in superconducting EuBa2Cu3O7−x

We have measured the thermal conductivity, Young modulus sound velocity and internal friction of a polycrystalline sample of the ceramic superconductor EuBa₂Cu₃O_7-x. The low temperature data can be quantitavely understood assuming the interaction of phonons with tunneling systems.     

Anisotropic optical properties of single-crystal GdBa2Cu3O7−δ

The optical spectrum of reduced-T _c GdBa₂Cu₃O_7– has been measured for polarizations parallel and perpendicular to theab plane. The sample was an oxygen-deficient single crystal with a large face containing thec axis. The polarized reflectance from this face was measured from 20–300 K in the spectral region from 30–3000 cm^–1, with 300 K data to 30000 cm^–1. Kramers-Kronig analysis was used to determine the spectral dependence of theab and thec components of the dielectric tensor. The optical properties are strongly anisotropic. Theab-plane response resembles that of other reduced-T _c materials whereas thec axis, in contrast, shows only the presence of several phonons. There is a complete absence of charge carrier response alongc aboveand belowT _c. This observation allows us to set an upper limit to the free-carrier spectral weight for transport perpendicular to the CuO₂ planes.Permanent address: Institute of Physics, CSAV, Prague, Czechoslovakia