Dip-Coating of YBa2Cu3O7?δ and YBa2Cu3O7?δ-Ag Films on Polycrystalline Rare-Earth Barium Niobate Substrates

Dip-coating and partial melting technique have been used to fabricate high quality superconducting YBa₂Cu₃O_7– and YBa₂Cu₃O_7–-Ag thick films with T _c(0)=92 K on polycrystalline REBa₂NbO₆ (RE=Pr, Nd, Sm, and Eu) substrates. The superconducting films showed excellent adhesion to the REBa₂NbO₆ substrate. The effect of Ag addition in YBa₂Cu₃O_7– on the current density, microstructure, and crystal orientation of the superconducting films developed on the above substrates have been discussed in detail. Dip-coating technique was found to be one of the easiest method for obtaining good quality superconducting YBa₂Cu₃O_7– thick films with thickness as low as 3 m even on polycrystalline substrates.     

Oxygen mobility and charge transport in YBa2Cu3O7-δ

The out-diffusion of oxygen during heat treatment of fully oxygenated c-axis oriented epitaxial films of YBa₂Cu₃O_7- leads to large electric fields in the films. During heat treatment under oxygen pressure a transient electric resistance peak has been observed also.     

Magnetic and transport properties of PrBa2Cu4O8-δ prepared at 1 atm oxygen pressure

Measurements of physical properties were performed on the PrBa₂Cu₄O_8- sample prepared by the sol-gel method and sintered in I atm oxygen pressure. Instead of exhibiting superconductivity as for R Y or Gd in the RBa₂Cu₄O_8- compound. This compound is unstable, insulating but metallic-like at low temperatures. No superconductivity was observed down to 2 K. Since PrBa₂Cu₄O_8- can be re-written as Cu₂Ba₂PrCu₂O_8- which is a compound of two CuO₂-layers and two CuO-chains, the possibility of anomalous Pr antiferromagnetic ordering around 18 K observed from preliminary magnetic data and the effect of oxygen stoichiotmetry parameter on the metal-insulator transition are explored.     

Normal State Resistivity of YBa2Cu3O7-δ /PrBa2Cu2.8Ga0.2O7-δ c-axis Superlattices : Evidence for an Underdoping Effect of YBa2Cu3O7-δ

We measured the ab-plane resistivity of c-axis YBa ₂ Cu ₃ O _7- /PrBa ₂ Cu _2.8 Ga _0.2 O ₇ superlattices grown by pulsed laser deposition, from T _C to 325 K. The normal-state resistivity and the fluctuation regime above T _C were studied for different thickness of YBa ₂ Cu ₃ O _7- and PrBa ₂ Cu _2.8 Ga _0.2 O _7- in the superlattice period. As the concentration of PrBa ₂ Cu _2.8 Ga _0.2 O _7- is raised, the T _C decreases and the resistivity increases. Evidence is given for an underdoping effect in the YBa ₂ Cu ₃ O _7- layers and our measurements are compared to similar measurements on oxygen-deficient YBa ₂ Cu ₃ O _7- thin films, grown on the same equipment. They are also compared to measurements on YBa ₂ Cu _3-x Co _x O _7- single crystals made by other groups. We present here the different measurements and investigate the mechanisms that could be held responsible for this depletion in charge carriers.     

Cation interdiffusion between thick film of YBa2Cu3O7-δ and ceramic substrate

Thick films of superconducting oxides, YBa₂Cu₃O_7-, were successfully made by conventional screen-printing technology on Al₂O₃, MgO, and ZrO₂ substrates. Interdiffusion between the superconductive film and substrate was investigated using analytical electron microscopy. The results indicate that MgO and ZrO₂ are superior to Al₂O₃ for substrate materials.     

The flexural strength and microhardness of YBa2Cu3O6+δ

The flexural strengths of rectangular YBa₂Cu₃O₆₊ bars, prepared from mixed oxides and carbonates or spray-dried precursors, have been measured at room temperature and at 77 K. Strengths ranged from 17.8 to 57.6 M Pa at room temperature, depending on processing history, and were 20% greater when measured at 77 K. Corrosion of YBa₂Cu₃O₆₊ in humid air at 38° C created two layers of corrosion products, but did not weaken the uncorroded core when failure loads were corrected for the decreased sample dimensions. The Knoop hardness of polycrystalline YBa₂Cu₃O₆₊ ranged from 436 to 447 KHN while the hardness of individual grains of YBa₂Cu₃O₆₊ was 498 KHN. Variations in flexural strength with microstructure were observed and are discussed.     

Influence of micro-structure in the low temperature critical currents of YBa2Cu3O7−δ thin films

Epitaxial YBa₂Cu₃O_7– films nucleate in c-axis oriented single-crystalline islands. The surface of the single-crystalline SrTiO₃ substrates exhibit steps of one third of the YBa₂Cu₃O_7– c-axis. These steps generate a mismatch in the island boundaries between the CuO₂ superconducting blocks. We show that these defect regions are strong candidates for being the pinning centers responsible for the large critical currents observed in Laser Ablated and Sputtered thin films.     

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.     

Magnetic penetration depth measurements with the microstrip resonator technique

The microstrip resonator technique is a convenient way to sensitively measure the temperature dependence of the magnetic penetration depth (T) in superconducting thin films. Because the method relies on measuring the resonant frequency of a high-Q transmission line resonator at microwave frequencies, one can very precisely measure small changes in (T). This technique is applied to studying the low-temperature dependence of (T), since that is in principle a measure of the low-lying pair-breaking excitations of the superconductor. We find that the penetration depth in niobium films is consistent with the predictions of weak coupled BCS theory. The low-temperature dependence of (T) inc-axis YBa₂Cu₃O_7– films can be interpreted as either a weak exponential or as a power law. In addition, the measured value of (0) is found to be strongly dependent on the form of the temperature dependence for (T) used in fitting the data. Best fits over the entire temperature range are obtained with a BCS temperature dependence having values for 2(0)/k _BT_c strictly less than 3.5, consistent with our measurements of the temperature dependence of (T) at low temperatures in YBa₂Cu₃O_7– .