Photoemission on the highT c superconductors Y−Ba−Cu−O

XPS and UPS photoemission experiments on the highT _c superconductors (T _c ≈90 K) with nominal composition YBa₂Cu₃O_9-y (y≈2) show the following:

1. The density of electronic states at the Fermi energy is very small, much smaller than in pure Cu.
2. The Cu 2p spectra show only a Cu²⁺ contribution.
3. The Ba core levels show a structure with two components of nearly equal magnitude, which leads to the suggestion that these compounds have large O^2? vacancies coordinated to Ba²⁺ sites.
4. Annealing at 400°C under UHV conditions leads possibly to a partial reduction of Cu²⁺ to lower Cu valence states and to a small increase of the O^2? vacancy component of the Ba²⁺ line.

     

The microregion compositional variation in Y1Ba2Cu3O7−x material

We have investigated the microstructure and compositional distribution of the Y₁Ba₂Cu₃O_7−x superconductor with scanning electron microscopy, energy dispersive X-ray spectrometry and X-ray diffraction. It has been found that the microregion compositions are far from the ideal composition. This is especially due to the yttrium content which is higher than the expected amount in the interfacial region between crystalline grains.     

Positron annihilation anomaly in the high-temperature superconductor YBa2Cu3O7−x

We have measured the ac susceptibility of a wire with a Nb core (1.27 mm diam.) and a Cu cladding (0.37 mm thickness) atT50 K andB0.1 mG. Due to its proximity to Nb, the Cu becomes fully superconducting. From the data we find a breakdown fieldH _b =1.2 (mG) and a coherence length =2.2T ^–1/2 (m) for the Cu, as well as a field penetration depth -34T ^1/2 (m) at the Cu/Nb interface.     

Microwave absorption in a Y1Ba2Cu3O7−x single crystal near the superconducting transition

Y₁Ba₂Cu₃O_7−x single crystals are investigated near T _c ≈92 K in fields 0<H⩽9 kOe using a modified ESR spectrometer. The temperature modulation method is used for the first time, together with the traditional magnetic modulation method, to detect microwave responses in single crystals. Superconducting-transition peaks shifted relative to one another in temperature and differing in shape are observed in the temperature dependence of the corresponding signals ∂R/∂H and ∂R/∂T (R — microwave absorption). The evolution of these peaks as a function of the field and the angle π between H and the c axis of the single crystal is traced. It is shown that the difference in the temperature dependences of the derivatives ∂R/∂H and ∂R/∂T is due to the broadening of the superconducting transition characteristic of HTSCs. Fiz. Tverd. Tela (St. Petersburg) 41, 14–17 (January 1999)     

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.     

Introduction of bismuth into the highT c superconductor La2−x Sr x CuO4−y

Novel high-T _c superconductors La_2–x Bi _x Sr _x–x CuO_4–y have been isolated and studied. The influence of the experimental conditions of synthesis on the superconductivity has been especially studied for the composition La_1.7Bi_0.1Sr_0.2CuO_4–y ; it is shown thatT _c decreases as oxygen is introduced into the matrix, likely due to the partial oxidation of Bi(III) to Bi(V). The highT _c value observed for one sample (T _c 42 K), as compared withT _c 38 K of La_1.8Sr_0.2CuO_4–y suggests that bismuth is also a potential candidate for improving the superconducting properties in these oxides.     

Mechanical characterization of melt-textured Y0.95Er0.05Ba2Cu3O7−δ superconductor prepared in air

The effect of Er substitution on the mechanical properties of ab- and a(b)c-planes of melt-textured YBa₂Cu₃O_7?δ is reported in the present work. The non-oxygenated samples were characterized by scanning electron microscopy, X-ray diffraction and mechanical properties by instrumented indentation and conventional Vickers indentation whereas the superconducting properties were determined by resistivity measurements on oxygenated samples. The X-ray pattern and EDS analysis revealed the presence of Y-123, Y-211 and BaCeO₃ phases. Er substitution up to 5 wt.% did not affect the superconducting properties. No difference in hardness was observed for the ab- and a(b)c-planes. Elastic modulus of the a(b)c-plane was 10% higher than of the ab-plane. Differences in indentation fracture toughness obtained by conventional Vickers indentation of the ab- and a(b)c-planes was observed. The addition of 5 wt.% of Er did not affect significantly the mechanical properties of melt-textured samples when compared with pure melt-textured YBa₂Cu₃O_7?δ.     

Specific heat studies of high-Tc superconductor YBa2Cu3O7−x

Specific heat studies of the high-T_c superconducting compound YBa₂Cu₃O_7−x with bulk transition temperature at 92K are reported. A distinct anomaly of electronic origin in the specific heat is observed with granular-like behavior corresponding to a Sommerfeld constant γ = 7±2mJ(moleCuK²)^±1 Debye temperature (φo ≈ 400K) is obtained by fitting the experimental data with the theoretical Debye specific heat.     

Temperature independent leakage current in Au/TbMnO3/Y Ba2Cu3O7−x capacitors

Au/TbMnO₃/Y Ba₂Cu₃O_7?x capacitors were fabricated on LaAlO₃ substrates and electrical characteristics were studied in the temperature range from 300 to 25 K. When temperature is higher than 225 K, Ohmic conduction is observed in the capacitors. Non-linear current–voltage characteristics are exhibited at low temperatures. Moreover, temperature-independent leakage currents are found at temperatures lower than 150 K. These electrical characteristics can be explained in terms of conduction transition originated from a thermal activation between 225 and 150 K, and the temperature-independent leakage currents are governed by the Poole–Frenkel emission and minor trap ionization energies.     

Seebeck-effect in the mixed state of Y−Ba−Cu−O

We report on measurements of the Seebeck-effect, the Nernst-effect, and the magnetoresistance in the mixed state of ac-axis oriented expitaxial film of Y–Ba–Cu–O. In contrast to conventional superconductors we find a large Seebeck-coefficientS, which is comparable in magnitude to the Nernst-effect. The broadening of the super-conducting transitions of magnetoresistance and Seebeck-effect are rather similar with respect to (1) the temperature dependence, (2) the dependence on the direction between magnetic field and crystal axis and (3) the dependence on the direction between magnetic field and driving forces. The large Seebeck-effect has to be attributed to dissipation due to normal quasiparticle-excitations, since the vortex-contribution to the Seebeck-effect is by far too small to account for the observed magnitude ofS. It is argued that such a quasiparticle contribution to the dissipation is large in the high-T _c superconductors because of the small coherence lengths and thus the small vortex cores. Another possibility is that granularity leads to dissipation proportional to the normal state transport properties. The Seebeck-voltage depends on all dissipative processes other than vortex motion, whereas the Nernst-effect depends only on the vortex motion. Therefore by measurements of thermomagnetic effects the various dissipative properties may be separated.     

Dynamics of atoms in the high-temperature superconductor YBa2Cu3O7−δ

In order to obtain insight into the role played by the phonons in the superconducting phase transition, we have performed infrared reflectivity and inelastic neutron scattering experiments on single phase YBa₂Cu₃O_7–(0.1,T _c =94 K, T _10–90=1 K). These experimental results, together with published Raman scattering and specific heat data are used to determine the parameters of a rigid-ion model; the model is able to reproduce qualitatively the experimental data. On the basis of our model we derive and discuss the phonon density of states and the pattern of atomic displacements of some important vibrational modes. In particular, the model reveals the existence of high-frequency oxygen breathing modes and of low-frequency copper dimerization modes in both the CuO₃-chains and the CuO₂-planes. Furthermore, Jahn-Teller like oxygen modes exist in the chains. The possible role of these modes with respect to the pairing mechanism is discussed. In addition there exist low-frequency chain-degrading modes which favour jumps of oxygen atoms out of the CuO₃-chains into neighbouring vacant sites in the basal plane.     

Effects of Y2O3 additions on the oxygen diffusion in top-seeded melt growth processed YBa2Cu3O7−y superconductors

To understand the effect of Y₂BaCuO₅ (Y211)/YBa₂Cu₃O_7?y (Y123) interfaces on the oxygen diffusion in single grain YBa₂Cu₃O_7?y superconductors, single grain Y123 superconductors with 0.05 and 0.3 moles of Y₂O₃ additions were fabricated by a top-seeded melt growth (TSMG) process. Y123 compacts with Y₂O₃ additions were subjected to melt growth heating cycles with a cooling rate of 1 °C/h through a peritectic temperature (1015 °C) and then annealed at 450 °C for 200 h in flowing oxygen. The superconducting temperature (T_c) and critical current density (J_c) were estimated for the three different regions (top surface (s), intermediate (i) and center (c)) of samples. The amount of Y211/Y123 interface area in single grain Y123 superconductors was successfully controlled by Y₂O₃ additions. The T_c values of s regions were higher than those of i and c regions, which indicates the presence of more oxygen at the sample surfaces. In addition, the T_c values of i and c regions of the Y123 sample with 0.3 mole Y₂O₃ addition were higher than those of the same regions of the Y123 sample with 0.05 mole Y₂O₃ addition due to the promoted oxygen diffusion through Y211/Y123 interfaces and other related defects. In spite of the promoted oxygen diffusion by Y₂O₃ addition, the large T_c difference among the regions still existed, which suggests sluggish oxygen diffusion into single Y123 grains.     

Magnetic ordering on Yb3+ in YbBa2Cu3O7−x

The high critical temperature superconductor YbBa₂Cu₃O_7−x has been studied over the range 0.05 to 95K using ¹⁷⁰Yb Mössbauer absorption spectroscopy. Magnetic ordering occurs within the Yb³⁺ sublattice at 0.35K. The saturated magnetic moments are 1.7μ_B. At all temperatures the hyperfine parameters show the presence of distributions attributed to some disordering of the oxygen vacancies.     

Phase transformation and microtwinning in crystals of the high-T c superconductor YBa2Cu3O8−x,x≅1.0

We employed transmission electron microscopy (TEM) and single crystal X-ray diffraction techniques, supplemented by differential thermal and thermo-gravimetric analysis (DTA, TG) and high temperature powder X-ray diffraction, to study microtwinning in orthorhombic crystals of the high-T _c superconductor YBa₂Cu₃O_8–x (x1.0). This twinning is associated with a structural phase transition at 750°C from a tetragonal high temperature phase (s.g.P4/mmm) to the orthorhombic ambient temperature phase (s.g.Pmmm) and seems to be inherent to virtually all orthorhombic crystals of YBa₂Cu₃O_8–x . The domain size ranges from typically 100 Å to 1000 Å. All our observations are compatible with a twin law where the tetragonal (110)-mirror plane is the twin element.     

Effect of oxygen deficiency on the magnetic field-dependent entropy in YBa2Cu3O7−δ

Roulin et al (1988), in one of their experimental papers, have presented a study of field-dependent entropy of high-purity YBa₂Cu₃O_7???δ (YBCO) as a function of oxygen deficiency. In order to explain their experimental results, we have used phenomenological GL-theory of anisotropic HTSCs in the London limit in line with of our earlier paper (Pattanaik et al, Physica B405, 3234 (2010)). Moreover, to account for the applicability of the theory at high field, we have incorporated the effect of vortex overlapping in the London theory done by Nanda (1995). Here, we have presented the variation of change in entropy (ΔS) with magnetic field for different oxygen deficiencies δ?=?0, 0.04, and 0.06. On comparison, we found that our results are in good agreement with the experimental data of Roulin et al (1988). The variation of penetration depth (λ) and anisotropic ratio of effective masses (γ) with concentration is also presented.