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.     

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– .     

Effects of junction resistance and counterelectrode material on point-contact tunneling into single- and polycrystalline YBa2Cu3O7−y

The effects of junction resistance and Counterelectrode material on the results of point-contact tunneling studies into single- and polycrystalline YBa₂Cu₃O_7–y are illustrated. Although reasonably symmetric I(V) curves predominantly indicate energy gap values ~ 20 meV, large asymmetries are often found for high-resistance junctions. Very low-resistance junctions show the expected behavior of a pure metallic bridge and indicate ~ 2530 meV.     

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.     

Current Noise in HgBa2CuO4+δ High Tc Superconductor

Experimental results of current noise in magnetic fields up to 1 kG after zero field cooling and field cooling are reported in bulk HgBa₂CuO₄₊ prepared with vapor–solid reaction in a two-temperature zone reactor. Noise measurements performed in Hg-based superconductor materials are compared with previously studied superconductors, in particular, YBa₂Cu₃O_7–. A maximum in the noise is observed at the same resistance, approximately one order of magnitude lower than the normal state value and independently of the applied field. This result is consistent with experiments in bulk YBa₂Cu₃O_7– samples and with percolative model simulations of lattice junctions in which the maximum is obtained at the same value of the concentration of superconducting junctions and, thus, at the same macroscopic resistance. Maximum intensity is shown to increase with magnetic field up to 1 kG, with noise values slightly higher in the field cooling procedure. Our results attest that weak-link effects, as in YBa₂Cu₃O_7–, dominate superconducting properties of bulk HgBa₂CuO₄₊. With respect to YBa₂Cu₃O_7–, no saturation at high magnetic field in both noise and magnetoresistance measurements is observed.     

Correlation Between the Morphology of Ag and the Contact Resistivity of the Ag/YBa2Cu3O7?δ Thin Film Contact

The morphology of the silver films deposited and annealed on laser ablated YBa₂Cu₃O_7– thin films and the corresponding contact resistivity have been systematically investigated. A minimum contact resistivity of 6 × 10^–8 cm² was reached at 77 K by annealing Ag/YBa₂Cu₃O_7– contact at the optimum temperature. The effect of the annealing temperature on the contact resistivity was explained by considering the morphology of the silver films and the diffusion of silver into YBa₂Cu₃O_7– film, etc. The difference of the contact resistivity for Ag contact to polycrystalline, single crystal and thin film of YBa₂Cu₃O_7– were also explained.     

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.