Jason Peter Sydow

Basal plane oxygen vapor pressure of Co-doped YBa2Cu3O7−δ

Transport data for YBa2Cu3−xCoxOz thin films, with x=0.3, 0.5, 0.75, and 1.0, are reported as a function of annealing history in O2 and O2/O3 at temperatures ranging from 200 to 500 °C. For x=0.3 and 0.5, O2/O3 anneals resulted in maximal Tc’s significantly higher than any previously reported, while in all cases the annealing substantially increased the film’s conductivity. Subsequent anneals in O2 resulted in oxygen loss, demonstrating that these materials have a much higher oxygen vapor pressure in the well-oxygenated state than YBa2Cu3O7−δ.

Effects of oxygen content on YBCO Josephson junction structures

The high degree of crystal stress and strain present at, and in the vicinity of, high angle grain boundary (GBJ), ramp edge Co doped SNS (Co-SNS), or interface engineered junctions (IEJ) can lead to localized and highly non-uniform regions of basal plane oxygen loss in YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO). These oxygen inhomogeneities will, to a greater or lesser degree, affect or cause the superconducting weak link behaviour demonstrated by these types of junctions. In order to examine the impact of the localized oxygen micro/nanostructure on the weak link behaviour of these three junction technologies, we have utilized ozone anneals to provide a partial pressure of atomic oxygen far in excess of that produced by standard O/sub 2/ anneals.

On the characteristic voltage of highly oxygenated YBCO grain boundary junctions

Early in the study of cuprate grain boundary junctions (GBJs) it was observed that the junctions characteristic voltage (I i R n ) tended to scale, at least approximately, as J To re-examine the cause of this effect we have measured I c R n as a function of the more complete GBJ oxygenation that can be achieved by ozone annealing and electromigration. While these enhanced oxidation techniques can increase J c by up to a factor of 10. I c R n remains saturated at ∼1.2 mV (at 4.2 K) for 24 GBJs on strontium titanate bicrystals. Only with oxygen removal through inert gas annealing is scaling observed. These results can be explained by a model where the GBJ consist of a thin barrier layer, created by the localized cation disorder, sandwiched between two interface layers of non-uniform YBCO of highly variable oxygen deficiency. The barrier layer is largely unaffected by the enhanced oxidation techniques leading to the plateau in I c R n . Inhomogeneities in oxygen content and order within the interface layers are dependent on the quality of the grain boundary growth process and annealing history and in turn are responsible for the variations in I c R n leading to the previously observed scaling. Ozone anneals and electromigration reduces the oxygen inhomogeneities and electromigration reduces the oxygen inhomogeneities within the interface region, facilitating the measurement of the intrinsic I c R n of the grain boundary barrier layer.

Micro-Raman spectroscopy studies of Co doped Y-Ba-Cu-O thin films

We report on the micro-Raman spectroscopy study of thin films of YBa/sub 2/Cu/sub 3-x/Co/sub x/O/sub 7-/spl delta// grown by laser ablation. Depending on the growth pressure, the films are found to be either homogenous or inhomogeneous on a scale /spl Gt/2 /spl mu/m. In either case the films are, on average, oxygen deficient, but this deficit can be substantially improved, yielding /spl delta/ less than zero in thin film microstructures treated by oxygen electromigration. This indicates that the Co addition to such films impedes, but does not forbid the attainment of a relatively optimal degree of occupancy of the dopant, chain oxygen sites.

Effect of ozone anneals on YBa/sub 2/Cu/sub 3-x/Co/sub x/O/sub 2/ thin films

We report on the effect of 200-500C ozone anneals on the resistivity versus temperature (RT) and Raman spectra characteristics of YBa/sub 2/Cu/sub 3-x/Co/sub x/O/sub 2/ (Co-YBCO) thin films, with x=0.3, 0.5, 0.75, and 1.0. Cold wall anneals are conducted at /spl sim/1 aim. of a flowing O/sub 2//O/sub 3/ mixture with /spl sim/2% O/sub 3/ by weight. The enhanced partial pressure of atomic oxygen, relative to canonical O/sub 2/ anneals, provided by ozone to the surface of Co-YBCO films, leads to enhanced oxygenation. This is demonstrated by the observation of an increase in T/sub c/ (25 K to 75 K for x=0.3, 0 to 38 K for x=0.5, and 0 to a T/sub c/(onset) of 19 K for x=0.75) and improvement in Raman spectra data following ozone anneals. This high degree of oxygenation is unstable under O/sub 2/ anneals at temperatures as low as 200C. We attribute this to the fact that the partial pressure of oxygen above well oxygenated Co-YBCO is greater than 1 atm. at /spl sim/200C. We will discuss the implication of these results for the stability and uniformity of SNS ramp edge Josephson junctions which employ Co-YBCO N-layers.

Growth and properties of YSr/sub 2/Cu/sub 2.75/Mo/sub 0.25/O/sub 7-/spl delta// thin films

We report on the epitaxial growth of YSr/sub 2/Cu/sub 2.75/Mo/sub 0.25/O/sub 7-/spl delta// (YSCMO) thin films by pulsed laser ablation, and discuss the superconducting find normal state transport properties of microbridges patterned from such films. To investigate the dopant oxygen mobility of this material, we have used electrical biases applied at near room temperature to induce long range displacement of chain oxygen vacancies in these microbridges. We find that with electromigration, the T/sub c/ of the microbridges can be raised to |60 K. This transition temperature is higher than any previously achieved for this compound, including the T/sub c/s achieved. As result of extended very high pressure oxygen anneals, or by high pressure in situ measurements. Micro-Raman spectroscopy measurements have been used to examine and characterize the oxygen order in the film before and After electromigration. These measurements indicate that, as suggested by the final T/sub c/ achieved, a very high degree of oxygenation and chain oxygen order can be induced in the microbridge by this process. Thus the as-grown properties of YSCMO appear to be dominated by the low level of oxygenation and the low degree of oxygen homogeneity obtained by normal annealing processes.

Optimal oxygenation of YSr2Cu2.75Mo0.25Oz thin films

Abstract Results are presented on the optimization of oxygen content and order in YSr 2 Cu 2.75 Mo 0.25 O z (YSCMO) thin films. It is shown that due to the high vapor pressure of the chain oxygen of this doped cuprate, optimal oxygenation cannot be achieved through annealing in O 2 at 1 atm. Optimization of oxygen content can be accomplished globally by ozone anneals at 1 atm, and locally, in thin film microbridges, by electromigration of chain oxygen vacancies out of the microbridges. Both techniques lead to a maximal T c of 75 K, substantially higher than has been previously achieved in this material. The strong correlation between the enhancement in superconducting and normal state properties, and improvements in oxygen content has been verified by micro-Raman spectroscopy. These results suggest that YSr 2 Cu 3− x M x O z materials in general may be substantially under-oxidized if annealed in O 2 at less than extremely high pressure, or without some other means that can overcome a comparatively high chain-oxygen vapor pressure at typical annealing temperatures. This could affect the interpretation of previous experiments with doped cuprate superconductors.