High Quality YBa2Cu3O7 Research Articles

Variation of grain sizes in sintered YBa 2Cu 3O 7−δ by different sintering conditions

The mean grain size of sintered YBaCuO samples is varied by means of different reaction temperatures. By this procedure, samples having similar densities and mean grain sizes ranging from 4 μm up to 14 μm are obtained. X-ray diffraction patterns show high-quality YBaCuO samples having the same oxygen stoichiometry. The effects of high reaction temperatures are discussed. The influence of different grain sizes on the attainable maximal critical current densities is presented in transport current measurements. It is shown that a maximum current carrying capacity is achieved if the mean grain size, 〈 r g 〉, approximately equals the Josephson penetration depth, λ j (T).

On-axis dc magnetron sputtering of large area high quality YBa2Cu3O7 superconducting thin films

It is demonstrated by our experiment that using the in situ dc magnetron sputtering method, it is possible to prepare high quality YBa2Cu3O7 (YBCO) thin films with substrates facing the region of plasma ring. The effect of oxygen resputtering can be greatly reduced with a high argon-oxygen working gas ratio and a large target to substrate distance. It is shown that high quality YBCO thin films of more than 60 mm in diameter, that is about the same size of the diameter of the YBCO target, can be prepared with this process. The thin films were characterized by x-ray diffractometry, and the critical temperatures were determined by both resistive and ac susceptibility techniques.

Pulsed Laser Deposition and Atomic Scale Characterization of Perovskite Oxide Films

ABSTRACTPerovskite oxide films inclusive of high Tc cuprates were fabricated by pulsedlaser deposition (PLD) under both conventional Po2(0.01∼1Torr) and high vacuum(≤10−6 Torr) conditions and their surface morphology, crystal quality, and conductivity were evaluated. Under optimized conventional PLD conditions, we obtained high quality YBa2Cu3O7−δ (YBCO) films that showed clear energy gaps and atomically flat surface image in the small area (4x4nm) STM/STS measurements. AFM and SEM ana!yscs on wider film surfaces (∼1x1μm), however, revealed granular structure of about 250nm grains with their root mean square roughness of 5nm.Under high vacuum PLD, i.e. laser MBE conditions, not only epitaxial but also two dimensional growth has been verified for such films as SrTiO3−x, SrVO3−y, and (AE)CuO2−z(AE=alkaline earth metal) by the RHEED observation of fine streak patterns and intensity oscillations. AFM demonstrated atomically flat surfaces of these films. Oxgen or nitric oxide pressure at the laser MBE growth gave crucial effects on the electrical conductivity of SrTiO3−x. and predominant crystal phase of SrCuO2−z.Insulating BaTiO3 films with atomically flat surfaces could be fabricated by both conventional and high vacuum PLD conditions. RBS channeling measurements revealed very high epitaxial quality (χmin−2%) of our SrTiO3 and BaTiO3 films. The factors controlling the surface morphology and electrical properties of perovskite oxide epitaxial films,are discussed.

High-resolution and analytical transmission electron microscopy of epitaxial thin films and YBa2Cu3O7/PrBa2Cu3O7 superlattices

Transmission electron microscopy (TEM) studies of epitaxial YBa2Cu3O7−x thin films and YBa2Cu3O7/PrBa2Cu3O7 superlattices are summarized. High-resolution imaging of cross-sections and plan views and energy-dispersive X-ray microanalysis and electron energy loss spectroscopy in the transmission electron microscope were the methods applied. In the first section results on YBa2Cu3O7−x thin films With varying oxygen stoichiometry deposited onto SrTiO3 are discussed. Then, YBa2Cu3O7/PrBa2Cu3O7 superlattices deposited onto SrTiO3 and MgO are investigated. Finally, an interface analysis of high-quality YBa2Cu3O7−x thin films deposited onto sapphire with yttrium-stabilized zirconia buffer layers is presented.

Fluctuation-enhanced conductivity and magnetoconductivity of high-quality YBa2Cu3O7?? crystals

We present measurements of the in-plane resistivityρ ab of YBa2Cu3O7−δ single crystals withT c ≃92 K andρ ab (100 K)≃50 μω cm. The temperature dependence of the fluctuation conductivity and of the magnetoconductivity aboveT c is analyzed in terms of direct and indirect fluctuation contributions for layered superconductors. The combination of fluctuation conductivity and magnetoconductivity allows to determine both coherence lengthsξ ab (0) andξ c (0) as well as the phase-relaxation timeτ ϕ of the pairs in an unequivocal manner. Evidence for clean limit type-II superconductivity in our crystals is given by large values of the mean free pathl ab ≫ξ ab (0).

High-quality YBa2Cu3O7−x thin films by plasma-enhanced metalorganic chemical vapor deposition at low temperature

Single-crystalline epitaxial YBa2Cu3O7-x thin films with a sharp superconducting transition temperature of 90 K and a critical current density of 3.3×106 A/cm2 at 77 K were prepared by a plasma-enhanced metalorganic chemical vapor deposition (PE-MOCVD) process. The films were formed in situ on (100) LaAlO3 substrates at a temperature of 670 °C in 2 Torr partial pressure of N2O. X-ray analysis indicated that films grew epitaxially with the c-axis perpendicular to the substrate and the a and b axes uniformly aligned along the LaAlO3 [100] directions. High-resolution transmission electron microscopy along with electron diffraction revealed that the films grew epitaxially with atomically abrupt film-substrate interfaces. The high degree of epitaxial crystallinity of the films was also confirmed by Rutherford backscattering spectroscopy which gave a minimum channeling yield of 9%.

All high T c edge-geometry weak links utilizing Y-Ba-Cu-O barrier layers

High quality YBa2Cu3O7−x/normal-metal/YBa2Cu3O7−x edge-geometry weak links have been fabricated using nonsuperconducting Y-Ba-Cu-O barrier layers deposited by laser ablation at reduced growth temperatures. Devices incorporating 25–100 Å thick barrier layers exhibit current-voltage characteristics consistent with the resistively shunted junction model, with strong microwave and magnetic field response at temperatures up to 85 K. The critical currents vary exponentially with barrier thickness, and the resistances scale linearly with Y-Ba-Cu-O interlayer thickness and device area, indicating good barrier uniformity, with an effective normal metal coherence length of 20 Å.

Deposition of high quality YBa2Cu3O7−δ thin films over large areas by pulsed laser ablation with substrate scanning

We describe the transport and structural properties of YBa2Cu3O7−δ thin films deposited by pulsed laser ablation with computer-controlled substrate scanning. Films were deposited on LaAlO3 and SrTiO3 substrates covering a 2×3 cm area with thicknesses of 90 and 160 nm. The 90-nm thick films exhibited a thickness variation of ±8%, Tco = 90.7 ± 0.5 K, Jc = 4.8 ± 0.2 × 106 A/cm2 at 77 K, and a surface resistance (corrected for finite thickness) at 75 GHz of 10–12 mΩ at 77 K. For the 160-nm thick films, the thickness variation was <5%, Tco = 91.0 ± 0.3 K, Jc=5.4±0.4×106 A/cm2, and corrected surface resistance was 6–10 mΩ. X-ray diffraction showed that the c-axis mosaic in the films is closely related to that of the substrates and that the only in-plane defects are due to the expected twinning in the a-b plane of the film. The c-axis lattice constants were 1.1688±0.0004 nm. The above properties showed a high degree of uniformity across the substrate area and between films from different deposition cycles. The surface resistance values add significantly to the body of results which show that the temperature-scaled values for niobium can be equaled and perhaps surpassed by YBa2Cu3O7−δ.

Evidence for individual vortex pinning in ceramic YBa2Cu3O7−δ

We measured the pinning force density Fp in high quality YBa2Cu3 O7−δ ceramics as a function of the temperature and magnetic field up to 5 T. The results indicate that near 60 K the magnetic vortices are pinned individually. This implies that at this temperature the main pinning mechanism involves localized pinning centers separated by short distances rather than twin boundaries.

A versatile substrate heater for use in highly oxidizing atmospheres

We report the design, construction and performance of a versatile substrate heater that meets the severe design constraints of heating substrates to high temperatures (≤900 °C) in relatively high pressures (hundreds of Torr) of oxidizing gases. The heater has been used to heat substrates by both thermal conduction via a conductive high temperature cement and by direct radiation. In particular, the production of high-quality YBa2Cu3O7−δ thin films grown on radiatively heated LaAlO3 substrates is demonstrated using the pulsed laser deposition (PLD) technique.

Far IR transmission spectra of an YBa 2Cu 3O 7-δ thin film sputtered on an MgO single crystal

Far IR transmission spectra of a high quality YBaCuO thin film deposited on an MgO crystal plate are obtained at 300, 80, 60 and 7 K., Transmission decreases as temperature is decreased, both in the normal and in the superconductive state. The temperature derivative of the observed transmission shows a maximum whose position gives the transition temperature. In the normal state the decreasing transmission is due to a decrease of the observed d.c. resistivity. In the supraconductive state for low temperatures and low frequencies, we assume a null value for the real part of the conductivity, and a nearly A ω law for the imaginary part. One plasma frequency (e.g. ω p = 4300 cm −1 is used for the normal state at all temperatures, and collision frequency appears as a linear function of T. That same plasma frequency, with ω c = 0, can be used to give an acceptable fit to the transmission spectrum at T = 7 K.

Critical current enhancement in multilayered Y1Ba2Cu3O7−y/Y1Ba2(Cu1−xNix)O7−y structures

Multilayers of YBaCuO superconductors and Ni-doped YBaCuO superconductors have been grown by the laser ablation technique to study the flux pinning properties of the nonmagnetic/magnetic superconducting heterostructures. Even though Ni doping depressed the superconducting properties of the single layer films, when sandwiched between high quality YBaCuO films, the composite showed high critical temperatures. Three-layer and five-layer structures of the YBaCuO/YBaCuNiO system become superconducting around 87 K when the doped layer thickness was below 100 Å. Three-layer structures grown on ZrO2(100) showed critical current densities of 3×106 A/cm2 at 77 K, which is about three times larger than the reported results in the literature. Epitaxial layer growth between the two superconductors was indicated by x-ray diffraction measurements, which showed complete c-axis orientation with full width at half maximum of 0.45° in a (006) line scan. Behavior of the critical current up to 0.85 T and temperatures down to 66 K has been studied. Critical current densities close to 4.5×106 A/cm2 in 0.85 T fields perpendicular to the films have been obtained at 67 K.

Growth of high quality YBa2Cu3O7 films on various substrate materials and influence of Zn-doping on superconductivity

Abstract By DC-sputtering from a single stoichiometric target we prepared high quality YBa2Cu3O7 thin films on SrTiO3, LaGaO3, ZrO2, MgO and NdAlCaO4. All films sputtered on these substrates (except NdAlCaO4) showed narrow inductive transitions (ΔTc 86 K), but only on LaGaO3 the transition was as sharp (width dT c dx = −10.5 K (1% Zn) as observed in bulk samples.

Evidence for a kosterlitz-thouless transition in high quality YBaCuO ceramics

A detailed study of the electrical transport properties in good quality ceramic samples of Y 0.5 Sm 0.5 Ba 2 Cu 3 O 7 has revealed all the signatures of a 2D Kosterlitz-Thouless transition. A full consistency of data is obtained from current induced and field induced vortex-antivortex depairing experiments. Paraconductivity effects above T co are analized in terms of the 2D Aslamazov-Larkin and Lawrence-Doniach theories.

An experimental investigation of YBa2Cu3O7 films at millimeter-wave frequencies

We have measured the millimeter-wave (100 GHz) surface resistance of high-quality laserdeposited YBa2Cu3O7 films on SrTiO3 and LaA103 substrates. Due to finite film thickness, radiation losses are important in the normal state and in the superconducting state nearT c. These effects are calculated andR s characteristic of the ohmics losses in the film are extracted from the data. The surface resistanceR s drops rapidly atT c, and a detailed comparison with calculations which include finite mean free path effects suggests a gap which exceeds the weak coupling BCS limit.

YBa2Cu3O7−δ superconducting films with low microwave surface resistance over large areas

We demonstrate that in situ off-axis sputtering from a composite target can reproducibly fabricate very high quality YBa2Cu3O7−δ films over large areas of deposition. A significant reduction in the microwave surface resistance, Rs, compared to many previously reported values is found. Rs values at 10 GHz were found to be as low as 20 μΩ at 4.2 K and 450 μΩ at 77 K. A systematic variation of several properties including the transition temperature, critical current density at high magnetic fields, and the c-axis lattice parameter across the 3.8 cm×3.8 cm (14.5 cm2) deposition area was found.

Magnetic Penetration Depth Measurements and Inhomogeneity in YBa2Cu3O7−δ Superconducting Thin Films

ABSTRACTThe microstrip resonator technique has been applied to study the temperature dependence of the magnetic penetration depth in high quality YBa2Cu3O7−δ thin films. The temperature dependence at low temperatures comes out directly from measured data, with no assumptions about transmission line geometry, dielectric properties, or a model for the temperature dependence of the penetration depth. One can interpret the data in terms of either an exponential decay of λ(T) at low temperatures or as a power law decay. The energy gaps obtained from the exponential decay at low temperature are found to be significantly smaller than weak coupled BCS theory and power-law exponents are in the range of 1.3 to 3.2. These results will be discussed in terms of microscopic theories and the possibility that materials properties dominate the measurement.

Ac susceptibility apparatus for measuring the transition temperature of high-T c crystals, sintered samples, and films

We describe a simple ac magnetic susceptibility system which can be used to observe the superconducting transition temperature in the range 4.2–300 K. The apparatus is extremely flexible and has been tested using high-quality YBa2Cu3O7−x single crystals with volume as small as 2×10−5 cm3, much larger sintered polycrystalline samples of the same compound, and finally thin films deposited over a surface area of 1 cm2. The system measures resonant frequency variations of a parallel LC circuit with the superconductor close to the inductor. Typical frequencies and fields are in the ranges 0.2–10 MHz and 1–2 mG ac in the inductor.

Critical currents in high-quality YBaCuO ceramics investigated by magnetization, a.c. susceptibility and electrical transport

Magnetization measurements are used to separate the intra- and intergrain critical currents of two dense, low-resistivity YBa 2Cu 3O 7 samples above 70 K. Transport (intergrain) critical currents, measured at 77 K, agree well with those deduced from magnetization. Estimates of critical currents from the magnetic field dependence of the imaginary susceptibility generally agree with those determined from the magnetization and electrical transport. At higher temperatures not all of the sample's volume is superconducting, which is reflected in the strong variation of intergrain critical current with temperature. The variation of intergrain (transport) critical current with magnetic field indicates weak-link intergrain coupling. Lower critical fields for bulk and grain respectively were determined.

Effect of stoichiometry and post-annealing conditions on magnetron sputtered films of YBa2Cu3O7 on cubic zirconia

High-quality YBa2Cu3O7 films supported on yttrium stabilized cubic zirconia were made by simultaneous reactive magnetron sputtering using yttrium, barium, and copper metal targets. Superconductivity onsets of 92 K with transition widths of 1.5 K (10%–90%) have been achieved on yttrium stabilized cubic zirconia substrates. Various aspects of the deposition processes are described and the effect of deposition parameters and post-annealing on thin-film stoichiometry, structure, and superconducting properties is discussed.