Transport properties of oxygen-deficient YBa2Cu3O7?x thin films

We report on measurements of the in-plane resistivity and Hall coefficientR _H (Bc) of various oxygen-deficient epitaxial films of YBa₂Cu₃O_7–x in the normal state. The superconducting transition temperaturesT _c of the samples vary from 14 to 90 K. Both the resistivity and the Hall coefficient exhibit a strong dependence on the oxygen content and the temperature. Asx increases,T _c decreases continuously, while andR _H gradually increase in magnitude. Furthermore, also the characteristic linear dependences ofT andR _H T ^–1 of the highly doped compounds changes to a nonlinear behavior for the samples withT _c lower than 60 K. The unusual doping and temperature dependence ofR _H will be compared to the predictions of our calculations, based on a two-dimensional tight-binding model using the relaxation-time approximation. The model considers also the next-nearest-neighbor hopping, which strongly influences the predicted Hall coefficient. Additionally, the cotangent of the Hall angle cot( _H ) is discussed in the framework of the two-dimensional Luttinger liquid theory.We gratefully acknowledge financial support from the Bundesministerium fiir Forschung und Technologic.     

Dependence of microwave surface resistance on the structure of laser-deposited YBa2Cu3O7?x thin films

The effect of the structure YBa₂Cu₃O_7-x superconducting thin films on microwave surface resistance was investigated. The electon channeling patterns (ECPs) and X-ray experimental results showed that the microwave surface resistanceR _s is strongly correlated with the perfection of the thin films. The films were deposited on LaAlO₃(100) and YSZ(100) substrates. For thin film withR _s of 280, the crystallinity of the thin film shown byw-scanning and-scanning was excellent and the ECPs were very sharp. For thin film with highR _s of 98 m, only bands from the major zone were visible in the ECPs, which suggested poor crystallinity of the film. From this investigation it was shown that the more perfect the thin films, the lower theR _s.     

Measurement of the surface resistance of YBa2Cu3O7?x thin films using stripline resonators

We review methods of measuring surface resistance (R _s ) of thin films using stripline resonators, and present our measurements of theR _s of YBa₂Cu₃O_7–x films as a function of frequency, temperature, and r.f. magnetic field. The films were deposited on LaAlO₃ substrates by two methods: (1) electron-beam coevaporation of Y, BaF, and Cu followed by annealing in O₂, and (2) single-targetin situ sputtering. The measurements were obtained at frequencies from 0.4 to 10 GHz, temperatures from 4 to 90 K, and an r.f. magnetic field range from 0 to 30 Oe. At low temperature and low r.f. field at 0.4 GHz, theR _s values obtained for the two deposition methods are approximately 7×10^–6 and 4×10^–6 , respectively.     

Large-Area YBa2Cu3O7 ? x Films on Sapphire with Excellent Microwave Power Handling Capability

Crack-free thick YBa₂Cu₃O_{7 – x} films are prepared on CeO₂ buffered r-cut sapphire (2 inch in diameter) with thickness up to 700 nm, smooth surfaces (peak-to-valley roughness <10 nm), high critical currents (J _C > 2 MA/cm² at 77 K and 0 T), and low microwave surface resistances (R _s(77K) .4m and R _s(4.2K) .110 at 19.15GHz) comparable to the best values reported in the literature for YBCO films on structurally better matched substrates. These thick YBCO films were able to handle high microwave power corresponding to magnetic field amplitudes (B _HF) up to 54, 37, and 17.4 mT at 4.2, 50, and 77 K, respectively, which for the lower temperatures were limited by the available power of the 25-W HF amplifier. The high-power performance, which to our knowledge belongs to the best reported so far for unpatterned YBa₂Cu₃O_{7 – x} films, was achieved without any degradation of the samples despite frequent thermal cycling.     

Fabrication and Characterization of YBa2Cu3O7 ? x Thin Film Step Junctions on MgO Substrates

We have investigated the tunneling properties of proximity-effect superconductor-normal-superconductor junctions based on high-temperature superconductor YBCO thin films and normal Ag layer. The systematic analysis of the properties of the films and the fabrication process which leads to a good-quality junctions is discussed in detail. The properties of the tunneling barrier are investigated by dc and rf measurements: the dc properties reveal that our junctions fully behave as expected from the de Gennes model. Regular Shapiro steps in the current–voltage characteristics are obtained as a result of microwave irradiation, indicating reliable rf properties of the junctions.     

Nanoindentation of multilayered epitaxial YBa2Cu3O7-δ thin films and coated conductors

Nanoindenter tests were carried out on YBa₂Cu₃O_7-δ (YBCO) coatings with different buffer layers, in order to obtain the Hardness (H) tendency, Young's modulus (E), and the fracture mechanism activated during the indentation process. Different pop-ins were observed in the load-displacement curves, and correlated with their residual nanoindentation imprints visualized by Atomic Force Microscopy. A trench was made by Focused Ion Beam in order to better understand the plastic behaviour activated under the residual imprint at 650 mN of applied load. During the first steps of nanoindentation experiments an elastic regimen takes place and the Hertzian equations can be applied to obtain the E for each YBCO coating. All YBCO coatings present similar E values and H tendencies. However, the YBCO/CeO₂/Yttrium Stabilized Zirconia system exhibits a better mechanical stability probably due to the absence of microcracks under the indentation. In addition, an experimental process using nanoindentation technique is obtained in order to isolate the kind of buffer layer employed (single crystal substrate or metallic substrate).     

Superconductivity of YBa2?xNdxCu3Oy Solid Solution

The solubility of Nd at the Ba sites and the superconductivity of YBa_2–x Nd _x Cu₃O _y were investigated by X-ray powder diffraction and measurements of the electrical resistance and ac susceptibility. The single Re123 phase was obtained for x0.30. The onset transition temperature is insensitive to the Nd content x in the region of x0.40. All are higher than 95 K. The zero resistance transition temperatures , however, exhibits two-step variation with the increase of x. For x0.25, are all above 92 K. The highest of 94 K was obtained for x=0.25. For x0.3 drops sharply to about 84 K. Finally falls to 30 K and is below 10 K for x=0.5. The two-step variation of T _c might be an indication of the existence of two trap levels for holes.     

Investigation of two-well potential of the apex oxygen in YBa2Cu2O7?x crystals by Raman spectroscopy

The dependence of oxygen modes intensity on pump power and temperature in YBa₂Cu₃O_7–x single crystals with different oxygen contents (0<x<1) has been measured. When YBa₂Cu₃O_6.5 crystals are heated, the peak intensity of the 500-cm^–1 band (apex oxygen) decreases, that of the 590-cm^–1 band (defect mode) increases, and the peak intensity of the 330-cm^–1 band (plain oxygen) remains unchanged. The resonant dependence of the apex oxygen, and the intensity of the defect modes have also been measured. In order to explain the experimental results we suppose that the apex oxygen ion is in a double-well potential.     

Investigation of Oxidation Kinetics of YBa2Cu3O7?x Ceramics by the Acoustic Emission Method

The oxidation kinetics of YBa₂Cu₃O_7–x ceramics has been investigated by the acoustic emission method. It is observed that the oxidation follows an exponential law. For YBCO the exponential index is a specific parameter and is equal to 1.7 s±10%. It is confirmed that the related narrow surface layer is first oxidized, and oxygen dissolution in the material is controlled by the speed of volume diffusion.     

Determination of the Glassy Exponent of Melt Textured YBa2Cu3O7?x by Magnetic Relaxation

By magnetic relaxation method we obtained the glassy exponent (T, H) of the melt textured Yba₂Cu₃O_7–x sample. The results have a similar behavior compared with ones using the method of magnetic field sweep, and are compatible with the model of collective pinning theory.     

Anisotropic uniaxial pressure effects in YBa2Cu3O7?δ

We present measurements of the uniaxial pressure dependence ofT _c of untwinned YBa₂Cu₃O_7– crystals with various oxygen stoichiometries. For all samples investigated,T _c decreases for pressure alonga, increases for pressure alongb, and, in oxygen deficient samples, increases strongly for pressure alongc. These results are compared to the behavior found in the La_2–x Sr _x CuO₄ and YBa₂Cu₄O₈ systems. Neither the model of pressure-induced charge transfer nor coupling to orthorhombic distortions can explain all the data. However, the presence of singularities in the electronic density of states close to the Fermi energy is a possible origin of the observed behavior. Our preliminary data on the pressure dependence of thec-axis and in-plane resistivities in twinned crystals are consistent with this view.     

Investigation of the quasi-ternary system LaMnO3–LaCoO3–“LaCuO3”. II: The series LaMn0.25?xCo0.75?xCu2xO3?δ and LaMn0.75?xCo0.25?xCu2xO3?δ

This paper investigates the crystal structure, thermal expansion, and electrical conductivity of two series of perovskites (LaMn_0.25−x Co_0.75−x Cu_2x O_3−δ and LaMn_0.75−x Co_0.25−x Cu_2x O_3−δ with x = 0, 0.025, 0.05, 0.1, 0.15, 0.2, and 0.25) in the quasi-ternary system LaMnO₃–LaCoO₃–“LaCuO₃”. The Mn/Co ratio was found to have a stronger influence on these properties than the Cu content. In comparison to the Co-rich series (LaMn_0.25−x Co_0.75−x Cu_2x O_3−δ), the Mn-rich series (LaMn_0.75−x Co_0.25−x Cu_2x O_3−δ) showed a much higher Cu solubility. All compositions in this series were single-phase materials after calcination at 1100 °C. The Co-rich series showed higher thermal expansion coefficients (α_max = 19.6 × 10⁻⁶ K⁻¹) and electrical conductivity (σ_max = 730 S/cm at 800 °C) than the Mn-rich series (α_max = 10.6 × 10⁻⁶ K⁻¹, σ_max = 94 S/cm at 800 °C). Irregularities in the thermal expansion curves indicated phase transitions at 150–350 °C for the Mn-rich series, while partial melting occurred at 980–1000 °C for the Co-rich series with x > 0.15. I. Arul Raj—on leave from Central Electrochemical Research Institute, Karaikudi, 630006 India.     

New Thorium-Doped YBa2(1 ? x)SrxThxCu3O7 ? δ High-Tc Superconductors

Thorium-doped YBa_{2(1 – x)}Sr _x Th _x Cu₃O_{7 –} high-T _c ceramic superconductors (with x = 0.1, 0.15, 0.2, and 0.3) have been carefully prepared by the usual ceramic procedure and sintering technique, with intermediate grindings and annealing under a stream of compressed-air atmosphere. The structural parameters of the prepared samples are calculated and interpreted using X-ray diffraction analysis. The electrical properties are also carefully investigated in the present work using the four-probe technique. Results obtained were compared, explained, interpreted, and discussed in detail; meanwhile, the major goal of the present work and promising deductions are attained at.     

YBa2Cu3O7 ? x 240 K Phase Transition

YBa₂Cu₃O_{7 – x} ceramic samples have been investigated simultaneously by the thermal expansion and the acoustic emission methods during thermocycling through 200–300 K into liquid nitrogen steam. No jumps in the dilatation curves but change in the slope of one has been found at 228 and 234 K on heating and cooling respectively. These points are accompanied by the acoustic emission signals, a small value testifying for oxygen atoms displacement into the crystal lattice. On the basis of these measured experimental data, a thermal expansion coefficient and a specific capacity c _m have been calculated. By this c _m value an oxygen atom activation energy E = 0.15 eV has been calculated. It is shown that an isostructural oxygen-displacement second-order phase transition takes place between certain orthorhombic phases in YBa₂Cu₃O_{7 – x} near 240 K.     

The Influence of Lithium Halides on the Superconducting Properties of YB2Cu3O7?x

The effects of addition to YBa₂Cu₃O_7–x of lithium halides (YBa₂Cu₃O_7–x (LiF) _y , and YBa₂Cu₃O_7–x (LiCl) _y ) on the structural, electric, magnetic, and transport properties are analyzed. Both structural and superconducting properties depend weakly on the lithium content up to y= 0.10. The critical temperature keeps on a value well above 91 K for a small amount of lithium halide (reaching 93.48K. for y= 0.02 in YBa₂Cu₃O_7–x (LiF) _y and 91.30 K in YBa₂Cu₃O_7–x (LiCl) _y ), but for higher concentration of Li it rapidly decreases (81.68K for y= 0.20). The same behavior is exhibited in the lower intragranular critical field. The intragranular critical current density depends on the magnetic field as a power law:j _cB ^–, with a lithium-concentration-dependent . The voltage–current characteristics follow a law typical for granular superconductors, V(I–I _c(B,T)) ^n(B,T). The dependence of the intergranular critical current, I _c, and of the exponent, n, on temperature, magnetic field, and concentration is analyzed.     

Normal-state Conduction Mechanisms in GdBa2Cu3?xRuxO7?δ Superconducting Phase

Bulk superconducting samples of type GdBa₂Cu_3−x Ru _x O_7−δ , Gd-123, with x=0.0–0.3 were prepared by the conventional solid-state reaction technique. X-ray powder diffraction (XRD), scanning electron microscope (SEM), electron dispersive X-ray (EDX) and electrical-resistivity measurements were performed in order to investigate the effect of Ru⁴⁺ ions substitution on Gd-123 phase. Enhancement of both phase formation and the superconducting transition temperature T _c for GdBa₂Cu_3−x Ru _x O_7−δ phase up to x=0.05 was observed from XRD and electrical-resistivity measurement, respectively. This enhancement was confirmed with the calculated relative volume fraction. For x>0.05, suppression of both phase formation and T _c was obtained and the superconductivity was completely destroyed around x=0.3. The normal-state electrical resistivity was analyzed by the two- and three-dimensional variable range hopping (2D-VRH and 3D-VRH) and Coulomb gap CG. The results showed that the dominant mechanism was CG for x≤0.075, while was 3D-VRH for x≥0.15.     

Microwave properties of highly oriented YBa2Cu3O7-δ superconducting thin films

We have performed millimeter-wave frequency (94 GHz) measurements on high-quality YBa₂Cu₃O₇- superconducting films on yttrium-stabilized (100) ZrO₂ and MgO substrates. The 0.2m thin films fabricated by magnetron sputteringin situ with the YBa₂Cu₃O₇- powders as target exhibit superconducting transition temperatures up to 88 K. The critical current density of 6×10⁵ A/cm² at 77 K and the X-ray diffraction spectrum as well as scanning electron microscope photographs indicate these thin films are fullyc-axis oriented, extremely high in density, and universally homogeneous. Millimeter-wave surface resistances have been measured on a hemisphere open resonator in the temperature range of 20 K toT _c and beyond. The surface resistance at 94 GHz and 77 K for these films is found to be about 30 m, nearly 1/4 that for copper, and a drop of two orders in the surface resistance within 4 K is observed, which indicates that these films are good materials for applications in the millimeter-wave range, especially for fabricating microwave devices. We observed such low surface resistance in these thin films due to the near absence of grain and phase boundaries coupled with a high degree of crystalline orientation.     

High-Quality YBa2Cu3O7?x Films with CeO2/YSZ Buffer Layers on 2-Inch Si Wafers Deposited by Pulsed Laser

The pulsed laser deposition (PLD) process is shown for in situ reproducibly fabricating YBa₂Cu₃O_7–x (YBCO) superconducting films with yttrium-stabilized zirconia (YSZ) and CeO₂ buffer layers, nonsuperconducting crystalline YBa₂Cu₃O_7–x (YBCO*) passivation layer, and silver contact film on 2-inch silicon wafers. Variations of less than ±7% in film thickness have been obtained for this multilayer growth over the whole wafer. The YBCO films on 2-inch silicon wafers have homogeneous superconducting properties with zero resistance temperature T _c0 from 88.4 K to 88.9 K. and critical current density J _c at 77 K and zero field from 2.5 × 10⁶ to 7× 10⁶ A/cm². The YSZ, CeO₂ and YBCO layers grow epitaxially on silicon wafers. Full widths at half maximum (FWHMs) of (113) reflections of 40 nm thick YBCO layer from -scan patterns are only 1.71° and 1.85° corresponding to the center and edge of the wafer, respectively. These results are very promising for developing high-quality high-T _c superconducting devices on large-area silicon wafers.     

RF plasma synthesis of YBa2Cu3O7?x powders

Fine powders of YBa₂Cu₃O_7–x have been synthesized by injecting mixed nitrate solutions of yttrium, barium, and copper into an argon rf thermal plasma. In general, the as-produced powders were dark brown and nonconducting. To obtain superconductivity, the as-produced powders were annealed either in a flowing oxygen tube furnace (at 900C) or in a lowpressure oxygen rf plasma. X-ray powder diffraction, scanning electron microscopy, and centrifugal sedimentation were used for powder characterization. For resistance measurements, bulk samples were prepared by isostatic pressing and tube furnace sintering of the annealed powders. The superconducting transition temperature (at 50% drop of resistivity) was 86 K.     

Investigation of the Off-Diagonal Thermoelectric Effect on Textured YBa2Cu3O7?δ

Both the off-diagonal Seebeck effect and the off-diagonal Peltier effect were investigated on the same textured sample YBa₂Cu₃O_7– and with the use of the same experimental setup. The effectiveness of several kinds of heat-conductive media was studied for the measurement. The flatness of both the sample and the heat-conducting block, and the matching between them were found very important for the reduction of the heat resistance of the interfaces. A reasonable agreement was found between the off-diagonal Seebeck coefficient measured by the off-diagonal Seebeck effect and that by the off-diagonal Peltier effect. The steplike feature in the relation between the off-diagonal Seebeck coefficient and the annealing temperature may imply a nonmonotonous change of the Seebeck coefficient along the c axis (S _c) with oxygen content.