Superconducting behavior of high-Tc YBa2Cu3O7 thin films with BaO impurity produced by pulsed laser deposition

We have investigated the superconducting behavior of high-T _c YBa₂Cu₃O₇ (YBCO) thin films containing BaO impure phase produced by pulsed laser deposition. The thin films were characterized by the standard four-probe method, X-ray diffraction (XRD), and scanning electron microscopy (SEM). XRD showed that all these thin films contained BaO impurity, with thec-axis normal to the surface of the substrates. The presence of impurity existed from substrate temperatureT _s of 727 to 796°C. When these thin films with BaO impurity were measured under the magnetic fields, it was found that the critical current densityJ _c increased slightly with increase in magnetic fieldB within the range ofB500 G, in the case ofB perpendicular to thec-axis of the film.     

Critical current density and microstructure of YBa2Cu3O7 thin films post-annealed at a low oxygen partial pressure

Post-annealing of thin films of YBa₂Cu₃O₇ (YBCO) has been performed at 29 Pa and 750°C. For films 0.6 m thick, a critical current density >1 MA cm^–2 is obtained at 77 K, with a sharp eddy current response at 25 MHz. Microstructural investigation of these films by crosssectional and planar transmission electron microscopy reveals that the YBCO film has thec-axis normal to the plane of the substrate in a continuous sheet of varying thickness, frequently covering the entire thickness of the film. Mutually perpendicular rods with thec-axis in the plane of the LaAlO₃ substrate are also seen. The microstructure and critical current density of these films are compared with those of previously reported films post-annealed in atmosphericpressure oxygen.     

Process monitoring during post-annealing of YBa2Cu3O7 thin films containing fluorine

The post-annealing method of producing thin films of YBa₂Cu₃O₇ (YBCO) has taken on a new impetus due to the recent work showing that films of the highest quality can be made by using low partial pressures of oxygen during the annealing cycle. Here it is shown that for films produced by using BaF₂ as a source material, the post-annealing procedure can be closely controlled by monitoring the F that evolves due to the water vapor reaction with BaF₂. The use of an ion-sensitive electrode allowed small F evolution rates (about 1 ng s^–1) to be detected above background, sufficient to measure the F evolution rate from even the smallest samples used. The time interval during which F evolves was found to increase with increasing YBCO film area being annealed.     

Optimization of annealing temperature for YBa2Cu3O7 thin films post-annealed at a low oxygen partial pressure

Post-annealing of YBa₂Cu₃O₇ (YBCO) thin films is usually performed at 850–900°C in atmospheric-pressure oxygen. In this study, coevaporated YBCO films on LaAlO₃ were post-annealed in an oxygen partial pressure of 29 Pa at temperatures in the range 700–825°C. Zero resistance transition temperatures were 89–90 K. Both d.c. (room-temperature resistance and critical-current density) and a.c. parameters (extracted from eddy-current response measurements at 25 MHz) were monitored. The optimum temperature is close to 750°C, which is on the YBCO thermodynamic stability line at this low oxygen partial pressure.     

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.     

Direct Peritectic Growth of c-Axis Textured YBa2Cu3Ox on a Flexible Metallic Substrate

Previous work in the development of YBa₂Cu₃O _x (YBCO) superconducting wires and tapes has been focused on the deposition of YBCO on buffered metallic substrates. Although such an approach has proved successful in terms of achieving grain texturing and high transport current density, critical issues involving continuous processing of long-length conductors and stabilization of the superconductor have not yet been entirely settled. We have developed a novel process, the so-called direct peritectic growth (DPG), in which textured YBCO thick films have been successfully deposited directly onto a silver alloy substrate. No buffer layer is employed in the film deposition process. The textured YBCO grains have been obtained through peritectic solidification over a wide range of temperatures and times. The substrate materials have not demonstrated any observable reaction with the YBCO melt at the maximum processing temperature near 1010°C. The transport J _c has reached a respectable value of 10⁴ A/cm² at 77 K and zero magnetic field. Based on the experimental results in this work, we show that the DPG method offers an effective alternative for the fabrication of long-length YBCO conductors. Also reported is a physical explanation of the texturing mechanism on the metal substrate.     

Film thickness dependence of critical current density for YBa2Cu3O7 films post-annealed at a low oxygen partial pressure

The variation of critical current density at 77 K as a function of film thickness was studied for YBa₂Cu₃O₇ films on (100) LaAlO₃ substrates. Film thicknesses were in the range 0.2–1.6m. The films were deposited by co-evaporation and post-annealed under conditions which have previously resulted in high-quality films (750°C and an oxygen partial pressure of 29 Pa). The critical current density at 77 K exceeds 1 MA cm^–2 for the thinner films, and decreases with increasing film thickness in excess of about 0.4m. The decrease is in rough agreement with a switch fromc-axis toa-axis growth at about this critical thickness. A good anticorrelation was found between room temperature resistivity and critical current density at 77 K. The results are compared to those obtained before by post-annealing at 850°C in 1 atm of oxygen.     

Chemical solution deposition of YBa2Cu3O7−x coated conductors

At present, the development of superconducting YBa₂Cu₃O_7−x coated conductors attracts much attention due to their enormous application potential in electric power systems. Worldwide research is focused on the investigation and improvement of buffer materials and YBa₂Cu₃O_7−x superconducting properties as well as low-cost manufacturing processes in cooperation with industrial companies. Accordingly, chemical solution deposition has emerged as a highly competitive, versatile, and cost-effective technique for fabricating coated conductors of high performance. New chemical solution approaches are under development for buffer layer deposition. In order to achieve high critical current carrying YBa₂Cu₃O_7−x layers, the established trifluoroacetate route is favored. This paper reviews the most recent work on chemical solution deposition within the IFW Dresden while also considering achievements on this specific research topic worldwide.     

Degradation of thin films of YBa2Cu3O7 by annealing in air and in vacuum

The degradation of epitaxial thin films of YBa₂Cu₃O₇ has been studied as a function of annealing temperature in air and in vacuum; some samples had an evaporated overlayer of CaF₂. Degradation was monitored by the measurement of electrical properties after consecutive 30-min annealing treatments. The room-temperature resistance registered significant increases for all samples after annealing at temperatures above about 200°C; the critical current density at 77 K was degraded for annealing temperatures 400°C in air, and 200–250°C in vacuum. By annealing in oxygen at 550°C, electrical properties were restored in degraded bare YBCO samples annealed in vacuum, but not for those annealed in air.     

Effect of PrBa2Cu3O7?x buffer layer thickness on the properties of YBa2Cu3O7?x thin films grown on sapphire by laser ablation

Superconducting YBa₂Cu₃O_7–x (YBCO) thin films were deposited onr-plane A1₂O₃ substrates with PrBa₂Cu₃O_7–x (PBCO) buffer layer by XeCl excimer laser ablation. The thickness of PBCO buffer layer was systematically changed to investigate the superconducting properties of YBCO thin films on sapphire. The structure and surface morphology of the films were characterized by X-ray diffraction and scanning electron microscopy (SEM). Superconducting transition temperatures were varied depending on the buffer layer thickness. Interdiffusion between laser-ablated YBCO thin films and A1₂O₃ substrates had been studied by Auger electron spectroscopy (AES). The results of this study show that diffusion does not occur between the YBCO thin film and the substrate even with 20 Å thick PBCO buffer layer.     

High-quality YBa2Cu3O7 films on 4-in. Wafers of sapphire, gallium arsenide, and silicon

Our technique of reactive thermal co-evaporation has been extended to fabricate large films (up to 4 in.) of YBa₂Cu₃O₇ with high quality. A rotating substrate holder is used to separate the deposition and oxidation processes. This allows free access of the metal vapors. As large substrate wafers we use Al₂O₃, Si, and GaAs with buffer layers of CeO₂, YSZ, and MgO, respectively. On all substrates, the uniformity of thickness and composition was better than 2%. Inductively measuredT _c andj _c (77 K) were 87.5±0.2 K and >1×10⁶ A/cm², respectively, across the full wafer area. This holds also for GaAs substrates due to a new procedure of capping by Si₃N₄.This work was supported by the German Bundesminister für Forschung und Technologie.     

Charge-transfer in YBa2Cu3Ox

The charge-transfer hypothesis is shown to be inconsistent with data for YBa₂Cu₃O_x: (i) The two-step behavior ofT _c(x) (with jumps from zero to 60 K and then to 90 K) is not reflected as a similar, statistically significant two-step behavior in the bond-valence-sum charge of cuprate-plane Cu ions (as once believed), (ii) as a consequence of the law of conservation of charge, the derivatives of the layer charges with respect to oxygen contentx for both the Ba-O layers and the charge-reservoir Cu-O chains have the opposite signs to those predicted, and (iii) the charge-transfer observed for superconducting YBa₂Cu₃O_x is not sufficient to produce superconductivity, as demonstrated by insulating PrBa₂Cu₃O_x, which has virtually the same layer charges.     

Development of random access memory in Nd0.7Ca0.3MnO3/YBa2Cu3O7 heterostructure

The effects of electrode on the resistive switching in Nd_0.7Ca_0.3MnO₃(NCMO)/YBa₂Cu₃O₇(YBCO) heterostructure are investigated at room temperature. For Cu/NCMO/YBCO, resistance can be switched on-and-off from a high- to low-resistance state at a steady ratio of 25% with a pulsed-voltage of ± 3 V. On the other hand, a giant resistance-change as large as 1350% is observed with ± 5 V for Ag/NCMO/YBCO with a fast decay down to 550%. Our experimental results show clear evidences that the nature of interfaces can be modified by the electric field and it dictates the resistive switching behavior of these heterostructure devices, which are the potential candidates for the random access memory.     

Magnetization and Susceptibility Studies on BaZrO3-Doped YBa2Cu3O7 ? x Bulk Superconductors

We studied the YBa₂Cu₃O_{7 – x} bulk superconductor doped with BaZrO₃ up to 50 wt.%, obtained by solid-state reaction powder technology. From DC magnetization loops and low frequency AC susceptibility measurements we determined the influence of the BaZrO₃ doping level on the critical temperature, critical current density, field for full penetration, and intergrain lower critical field. The results show that even high content of BaZrO₃ does not lead to degradation of the superconducting properties of bulk YBa₂Cu₃O_{7 – x} .     

Reproducibility of transition temperature and critical current density of thin films of YBa2Cu3O7 on (100) LaAlO3

Submicrometer epitaxial films of YBa₂Cu₃O₇(YBCO) on (100) LaAlO₃ were made by coevaporation and furnace annealing. Samples from more than a dozen runs are used in this study. The zero resistance transition temperature (T _c) is high (89 or 90 K) if the film composition is phase pure (Ba/Y=2, Cu/Y=3) or if it is enriched in Ba and Cu. For these compositions the critical current density (J _c) at 77 K has an average value of 2×10⁵ A cm^–2, with a tendency for decreasingJ _c with increasing film thickness (0.2 to 0.8m). Variations inJ _c are not correlated with deviations from ideal stoichiometry. Steeper slopes of the resistance-temperature curves above 100 K and lower values of the room-temperature resistivity are associated with high values ofJ _c. If the film composition is enriched in Y relative to Ba and Cu,T _c decreases by several degrees.     

Crystal-chemical anomalies in nonsuperconducting PrBa2Cu3O7

Studies of the crystal chemistry of nonsuperconducting PrBa₂Cu₃O₇ indicate that this compound is strictly isostructural with its superconducting RBa₂Cu₃O₇ (R = Y, rare earth) analogs. Crystallographically, Pr is present in the trivalent state according to the structural trends exhibited by the RBa₂Cu₃O₇ series as a function of R³⁺ ionic radius. The sole structural anomaly attributable to the presence of Pr³⁺ in the YBa₂Cu₃O₇ structure is a next-next-nearest neighbor effect and consists of an unusually short axial Cu-O distance, i.e., a short bond length between the in-plane copper and the chain oxygen. The correlation of this anomaly with the nonmetallic/nonsuperconducting properties of PrBa₂Cu₃O₇ supports a variety of literature reports, both theoretical and experimental, suggesting that the apical oxygen in the YBa₂Cu₃O₇ structure plays a critical role in mediating the appearance of superconductivity. The mechanism by which the f-electrons in Pr³⁺ (f ²) interact with the Cu-O manifold to produce the nonmetallic behavior of PrBa₂Cu₃O₇ remains unknown; however, superconductivity is turned back on for Nd³⁺ (f ³), immediately next to Pr and just slightly smaller. Careful comparative studies of superconducting NdBa₂Cu₃O₇ and nonmetallic PrBa₂Cu₃O₇ are needed to elucidate the critical difference in the behavior of the f-electrons and may shed light on the fundamental mechanism of high-temperature superconductivity in copper oxides.     

Charge Density Variations or Stripes in YBa2Cu3O6+y

For many years it was believed that NMR on the YBa₂Cu₃O_6+y family of superconductors does not support charge density variations or stripes. We discuss the NMR data of YBa₂Cu₃O_6+y (y > 0.63) and show that large charge density variations are actually necessary in order to explain the data.     

Fluctuation conductivity in Li-doped YBa2Cu3O7?x

The paraconductivity of Li-doped YBa₂Cu₃O_7–x was measured. We have found that the character of the fluctuation changes as lithium content increases: instead of a crossover from 2D to 3D behavior (D is the dimensionality), as we have observed for a low doping level, a double crossover to the 2D percolative and 3D percolative regimes becomes manifest by increasing the lithium content.     

Phonon structures in tunneling conductance of superconducting YBa2Cu3O7

We report on measurements of the tunneling conductance structures above the superconducting gap energy using YBa₂Cu₃O₇ polycrystalline junctions. The measured second derivative data are reproducible among the junctions, and the intensities of the common structures at the biases of 37–38, 47–53, 67–77, and 94–95 mV are strong enough to be assigned. These structures are in agreement with those in the neutron phonon density of states in whole energy regions when the energies are measured from the gap edge of 26±1 meV. This correspondence indicates that the electron-phonon interaction contributes to the pairing mechanism of this superconductor.     

Temperature dependence of the infrared optical properties of YBa2Cu3O7: A plasmon damping model

The temperature dependence observed in the mid- and near-infrared optical properties of YBa₂Cu₃O₇ is explained in terms of the Drude model for free charge carriers. In the Drude model, the linear temperature dependence of the dc resistivity arises from the free charge carriers having a temperature-dependent mean free path. This temperature dependence results in the plasmon contribution to the dielectric constant having a damping coefficient which also varies linearly with temperature. We find that the temperature dependence which is observed in the absorption and reflection spectra of YBa₂Cu₃O₇ is consistent with this simple model.