D. W. Capone

Structural phase transition in YBa2Cu3O7−δ: the role of dimensionality for high temperature superconductivity

Abstract We have performed detailed high temperature X-ray diffraction to study the nature of the structural phase of YBa 2 Cu 3 O 7− δ . The results indicate the existence of a reversible orthorhombic to tetragonal phase transition at a temperature close to 750°C. If the high temperature tetragonal phase is quenched-in at low temperatures the critical superconducting temperature is considerably reduced from 92.5 K. This suggests that the one dimensional Cuue5f8O chains present in the orthorhombic structure are necessary for high temperature superconductivity.

Mechanical and superconducting properties of sintered composite YBa2Cu3O7−δ tape on a silver substrate

Flexible sintered composite YBa2Cu3O7−δ tapes on a silver substrate have been fabricated by the tape casting technique followed by appropriate sintering. The sintered composite tape can be bent into an arc of a 27‐mm‐diam circle without breaking. This represents approximately 2% strain, as compared with a typical YBa2Cu3O7−δ superconductor material without silver, which fractures at a strain of ∼0.1–0.2%. The silver addition does not adversely affect the superconducting properties of the composite.

Upper critical fields and high superconducting transition temperatures of La1.85Sr0.15CuO4 and La1.85Ba0.15CuO4

The superconducting transition temperature Tc of La1.85Sr0.15CuO4 is measured to be 34.7 K (midpoint) with a transition width (10–90%) of 2.6 K. The critical field slope at Tc is 2.13 T/K. Using the Werthamer, Helfand, and Hohenberg expression in the dirty limit we estimate the upper critical field Hc2(0) to be at least 50 T for this system. The measured transition width broadens initially with increasing field to 4 K, then saturates at this value for fields above 2 T. The critical field slope for the lower Tc material La1.85Ba0.15CuO4 is measured to be 1.78 T/K, which yields an Hc2(0) of at least 36 T.

60–90 K superconducting phase transition in YBa2Cu3O7−δ

The kinetic process between the 60 and 90 K superconducting phase transition has been explored to study the oxygen ordering mechanisms. An abrupt change in Tc from 60 to 90 K is observed as the sample’s oxygen content is rapidly varied from around 6.6 to 6.9. No intermediate Tc values are obtained, which suggests an oxygen ordering instability, leading to a phase transition between the 60 and 90 K superconducting phases.

Nucleation and growth kinetics of the tetragonal to orthorhombic transition in YBa2Cu3O7

The nucleation and growth geometry of the tetragonal to orthorhombic transformation in YBa2Cu3Ox is determined to be grain‐boundary nucleation after saturation. In this geometry the orthorhombic phase rapidly nucleates along the grain boundaries, then propagates into the grains as a planar front governed by the much slower volume diffusion of oxygen. This conclusion is reached by fitting the volume fraction of the superconducting, and hence orthorhombic, phase as a function of annealing time at 400u2009°C to the Avrami relation. The volume fraction of the superconducting phase is obtained using magnetization versus field data.

New scaling relation for sputtered NbN films

A scaling relationship has been discovered between the sputtering rate and the partial pressure of argon PAr, which leaves the superconducting properties of NbN films invariant. As PAr increases for a given sputtering rate, there is a linear reduction of Tc together with a rapid increase, followed eventually by saturation, in Hc2 at 4.2 K. These produce a maximum in Jc (measured at 19 T and 4.2 K) at a particular value of PAr. Films exhibiting nearly the same set of superconducting properties and microstructures can be made at a higher fixed sputtering rate if the values of PAr are multiplied by a constant scale factor. This scaling relation is found to hold for two separate sputtering systems, one of which uses a standard diffusion pump while the other uses an oilless turbo pump. Additional comparisons with the results of others reinforces the conclusion that the scaling relationship is universal and thus may provide some insight into those factors in the sputtering process which are important for determ...

High Field Properties of NbN Ribbon Conductors

In this paper we report the first high field measurements on ribbon conductors composed of NbN and Cu deposited onto Hastelloy ribbons. These ribbons are 5 to 8 cm long, 1/8″ wide, and have several microns of NbN, covered with several microns of Cu, deposited onto one side. Such samples have a Tc ~ 1.4.5 K, Hc2 (2.0 K) ~ 26 T in the parallel direction and Jc (20 T, 2.0 K) up to 3 × 104 A/cm2.

Experimental study of the ultimate limit of flux pinning and critical currents

A flux pinning model, which predicts the maximum critical current density attainable in superconductors, is reviewed and some additional conclusions and considerations, particularly in regard to the model, are explored. That such a maximum must exist comes from the realization that flux pinning is strongest in regions of weak superconductivity, but these regions cannot carry a large supercurrent. Since the same regions within the superconductor cannot be used for both pinning and supercurrent conduction, there must be an optimum mix, leading to a maximum Jc. Measurements on films and multilayers of NbN have verified many details of the model including anisotropy effects and a strong reduction in Jc for defect spacings smaller than the flux core diameter. In an optimized multilayer the pinning force reached ∼ 22% of the theoretical maximum. The implications of these results on the maximum critical current density in the new high temperature superconductors are also discussed.