Superplastic behaviour of YBa2Cu3O7−x /Ag superconductors

High-temperature deformation characteristics of YBa₂Cu₃O_7–x oxide (YBCO) and YBa₂Cu₃O_7–x /Ag composite (YBCO/Ag) in uniaxial compression have been investigated. A compression test was carried out at temperatures from 780–930°C at initial strain rates between 10^–6 and 10^–4 s^–1. YBCO/Ag composites with fine, dense and equiaxed grains were compressed over 120% with no indication of failure at higher temperatures, and the strain-rate sensitivity exponent, m, was found to be about 0.42–0.46 between 890 and 930°C. They are considered to be one indication of superplasticity. The activation energy for deformation was 500–580 KJ mol^–1. The specimens suffered grain growth slightly during the deformation at 930°C and the majority of growth might be a function of exposure time, temperature and silver content, but each grain maintained the equiaxed shape after extensive superplastic deformation. This is consistent with a grain-boundary sliding mechanism. The silver at grain boundaries acts to decrease the activation energy for deformation and promote the grain-boundary sliding.     

Characterization of YBa2Cu3O7/PrBa2Cu3O7 superlattices

We investigated the structural and superconducting properties ofc-axis oriented (YBa₂Cu₃O₇) _nY /(PrBa₂Cu₃O₇) _npr superlattices with thicknesses of the individual layers down to one unit cell (10≥nY≥1; 18>nPr≥ 1). By transmission electron microscopy and X-ray diffraction we find an excellent structural quality of the samples, though the quantitative analysis shows the existence of defects. In superlattices with decoupled YBa₂Cu₃O₇ layers of two unit cell thickness we find a highT _c value of 75 K. We probed the flux line structure in the superlattices by measurements of the critical current density in magnetic fields. The experiments show that the flux-line dynamics is dominated by the movement of pancake vortices.     

Microstructures of high-J c melt-textured YBa2Cu3O7−x /Ag superconductors

Silver has been previously added to the melt-textured YBa₂Cu₃O_7?x in order to increase the critical current density (J _c ) of these materials. However, the effect of this addition on theJ _c is presently unclear. The purpose of this study is to investigate the effect of silver on both critical current density and the microstructure of the melt-textured YBa₂Cu₃O_7?x superconductors by means of X-ray diffraction, optical polarized microscopy, and transmission electron microscopy (TEM). TheJ _c of the MTG YBCO/Ag samples is more than 10⁴A/cm² under the 5 kOe magnetic field. It has been shown that as the concentration of silver increases, the fraction of the 211 phase dispersed within the 123 matrix decreases. Therefore, theJ _c slightly decreases. These results, together with the effect of the 211 phase, dislocations, and other structure defects on flux pinning, are described in this paper.     

Raman scattering studies of ultrathin-layer YBa2Cu3O7/PrBa2Cu3O7 superlattices

We present Raman scattering studies ofc-oriented ultrathin-layer superconducting (YBa₂Cu₃O₇) _m /(PrBa₂Cu₃O₇) _n superlattices. For the superlattice with (m=2,n=1) sequence, Raman spectra reveal a new line in the spectral region around 320 cm^?1. It is interpreted as a mode representing a combination of IR optical phonons of the Y-sublayers with an admixture of aB _1g type Raman active vibration in the Pr sublayers. This new line, which is similar to those from the interior of the Brillouin zone of the original lattice, does not exhibit superconductivity-induced self-energy effects, although its counterpart in the pure substance does. No additional line is found in the (m=1,n=2) superlattice in the same region, supporting our interpretation for the (m=2,n=1) sample.     

Field modulated microwave absorption in YBa2Cu3O7/PrBa2Cu3O7 multilayers

We describe field modulated microwave absorption measurements on YBa₂Cu₃0₇ / PrBa₂Cu₃O₇ multilayers with PrBa₂Cu₃O₇ thickness in the range 2.5 to 10 nm and with a fixed YBa₂Cu₃O₇ thickness of 10 nm. The Fourier spectrum of the reflected microwave power reveals one fundamental frequency which appears at T_c and even harmonics of the modulation field frequency at lower temperatures. The determination of the irreversibility line near T_c for different values of the PrBa₂Cu₃O₇ thickness and inferred from the vanishing of the nonlinear response signal at 2 in the presence of superimposed dc and modulation fields is reportedit.     

Specific heat of YBa2Cu3O7

Specific heat measurements, including measurements in magnetic fields and at both low temperatures and nearT _c , on a number of YBa₂Cu₃O₇ samples have revealed several correlations among strongly sample-dependent parameters. These correlations suggest that the sample dependence of the parameters reflects a sample dependence of the volume fraction of superconductivity, which is in turn correlated with a low concentration of Cu²⁺ moments. The correlations give a criterion for recognizing the values of the parameters characteristic of the fully superconducting material. Preliminary results on the effects of sample heat treatment are reported. New data on the “linear term” is presented and discussed.     

Low-temperature specific heat of YBa2Cu3O7, YBa2Cu3O6, Y2BaCuO5, YBa3Cu2O7, BaCuO2, and CuO

We have measured the low-temperature specific heat (1.3T20 K) and the dc magnetic susceptibility (100T250 K) of eight samples of the high-T _c superconductor Y _x Ba_3–x Cu₃O_7– (x=0.9, 1.0, 1.1) and of two samples of nonsuperconducting YBa₂Cu₃O₆₊. We have also performed specific heat measurements on the possible impurity phases: YBa₃Cu₂O₇, Y₂BaCuO₅, CuO, and BaCuO_2+x . The superconducting samples all have a nonzero, sample-dependent linear term * and an upturn inC/T at very low temperature. We show that this anomalous behavior is at least partly due to the presence of a small amount (1%) of BaCuO_2+x impurity phase in the measured samples. This is evidenced by the correlation between * and the Curie component of the susceptibility, which is proportional to the amount of paramagnetic impurities.     

Glassy behavior of low-temperature energy relaxation in YBa2Cu3O7 and YBa2Cu3O6

Measuring the power release after rapid cooling a YBa₂Cu₃O₇ sample (m=42.85 g, T_c=91 K) from the equilibrium temperature T₁ (2.35 KT₁15.1 K) to T₀=1.5 K, we observed a time dependence typical of a glass: is proportional to t^–1. The results allow us to determine the linear term of the heat capacity (0.8 mJ/mole · K²) due to the two-level systems. While the low-temperature heat capacity anomaly noticeably decreases, the power release is essentially unchanged after oxygen reduction of the sample.     

Electronic structure of YBa2Cu3O7 and YBa2Cu3O6—charge-self-consistent band structure calculations

Charge self-consistent LCAO band structure (CSCBS) calculations are reported for orthorhombic YBa₂Cu₃O₇ and tetragonal YBa₂Cu₃O₆ assuming ordered vacancy models. The effective atomic charges are used to study the charge transfer. In YBa₂Cu(1)₁Cu(2)₂O₇, the two types of copper atoms have their energy bands almost overlapping with effective valency of each copper as 7/3 (or effective valency of each oxygen as approximately — 13/7), so that electron hopping can take place without any loss or gain of energy while in YBa₂Cu(1)₁Cu(2)₂O₆, Cu(1)₁ is monovalent and Cu(2)₂ are divalent with significant difference in their bands. Therefore, YBa₂Cu₃O₇ should conduct much better compared to YBa₂Cu₃O₆. This corroborates the experimental observations that YBa₂Cu₃O₇ is a (super)conductor while YBa₂Cu₃O₆ is not. The calculated effective charges and DOS support the above view.     

Effect of columnar defects on theJ c anisotropy of YBa2Cu3O7−δ thin films and YBa2Cu3O7−δ /PrBa2Cu3O7−δ multilayers

Epitaxial YBa₂Cu₃O_7?δ thin films (YBCO) and YBa₂Cu₃O_7?δ /PrBa₂Cu₃O_7?δ multilayers (Y/Pr) were irradiated with high-energy heavy ions (770 Mev²⁰⁸Pb) under various directionsφ relative to thec-axis. The irradiation resulted in columnar defects tilted byφ from thec-axis. The angular dependence of their pinning activity was studied by measuring the anisotropy of the critical current density. TheJ _c (B, T,?) behavior of the irradiated YBCO thin films showed an additional peak, which exceeds the intrinsic pinning peak, exactly at the irradiation direction. The Y/Pr multilayers, however, showed an isotropicT _c -enhancement by a factor of 5, without any additional structure in theJ _c(B, T,?) curve.     

Nanostructured phase formation in YBa2Cu3O7/LaAlO3 and YBa2Cu3O7/CeO2/(Ni-W) prepared by deposition from metalorganic solutions

Continuous, phase-pure, oriented yttrium barium cuprate (YBa₂Cu₃O₇) films have been grown by deposition from metalorganic solutions onto lanthanum aluminate substrates and a nickel-tungsten tape with a barrier ceria layer. A “through” process has been used in which a ceria barrier layer and yttrium barium cuprate film grow from chemical solutions. In all steps of the process, the elemental composition of the deposits (Y: Ba: Cu molar ratio) was close to 1: 2: 3. The nanometer-scale structural evolution and phase formation in the YBa₂Cu₃O₇ films have been studied in detail. Depending on synthesis conditions, yttrium barium cuprate films may consist of three-dimensional, platelike, or flaky micro- and nanocrystallites. The degree of orientation of the atomic lattice of the targeted film may vary from partial ordering to a planar texture with an 〈001〉 axis normal to the substrate and with a pronounced azimuthal orientation of the crystallites. The degree of orientation was higher when trifluoroacetate solutions were used.     

Critical current in YBa2Cu3O7 ceramics

Measurements of critical currents using a four-point direct current (dc) method and an alternating magnetic field method have been performed on several superconducting YBa₂Cu₃O₇ ceramics at 77 K. In the presence of a constant magnetic field, the critical currents obtained with the alternating field method are several orders of magnitude larger than the critical currents measured by the dc method. Also, we observed a minimum in the dc critical current as a function of applied transverse magnetic field. Several authors have suggested that these ceramics behave as individual superconducting grains coupled by Josephson junctions. In this paper, we explain the two observations above using that model.     

Superconducting properties of YBa2Cu3O7-Au composites

By X-ray diffraction and photoelectron spectroscopy, we have demonstrated that Au is incorporated in the YBCO lattice up to a concentration 0.1. An XPS study gave evidence for an electronic valence state Au¹⁺. Its replacement in the Cu(1) site appears questionable due to the large difference in ionic radii (Cu²⁺0.073 nm; Au¹⁺0.137 nm). The so called irreversibility lines determined by AC susceptibility measurements are strongly affected by the doping. Up to the limit of solubility in the grains, an improvement in intra and intergrain coupling and flux pinning energy is observed, showing the beneficial interest of YBCO-Au composites.     

Oxygen Phonon Branches in Detwinned YBa2Cu3O7

We report results of inelastic neutron scattering measurements of phonon dispersions on a detwinned sample of YBa₂Cu₃O₇ and compare them with model calculations. Plane oxygen bond stretching phonon branches disperse steeply downwards from the zone center in both the a and the b direction indicating a strong electron-phonon coupling. Half way to the zone boundary, the phonon peaks become ill-defined but we see no need to invoke unit cell doubling or charge stripe formation: lattice dynamical shell model calculations predict such behavior as a result of branch anticrossings. There were no observable superconductivity-related temperature effects on selected plane oxygen bond stretching modes measured on a twinned sample.