2 Boeing Phantom Works, Seattle, Washington 98124 the generalized BTK theory, with b as the tunnel-cone width, Z the barrier strength, D 0 the d-wave gap maximum, and 2D 0 ͞k B T c the reduced-gap ratio.

Scanning tunneling spectroscopy studies reveal long-range spatial homogeneity and predominantly d x 2 2y 2 -pairing spectral characteristics in under-and optimally doped YBa 2 Cu 3 O 72d superconductors, whereas STS on YBa 2 ͑Cu 0.9934 Zn 0.0026 Mg 0.004 ͒ 3 O 6.9 exhibits microscopic spatial modulations and strong scattering near the Zn or Mg impurity sites, together with global suppression of the pairing potential. In contrast, in overdoped ͑Y 0.7 Ca 0.3 ͒Ba 2 Cu 3 O 72d , ͑d x 2 2y 2 1 s͒-pairing symmetry is found, suggesting significant changes in the superconducting ground state at a critical doping value. A possible consequence of a QCP is the dopingdependent pseudogap phenomenon [13], which may represent a precursor for superconductivity in the cuprates [13]. Early experiments on the Bi-2212 system reported a measured energy gap D ‫ء‬ ͑p͒ that increased monotonically with decreasing p and was nearly independent of temperature [14][15][16][17][18][19]. However, the low-temperature spectra of the optimally doped and underdoped Bi-2212 appeared to consist of a sharp peak feature on top of a broad "hump." Recent bulk measurements on Bi-2212 mesas [20] demonstrated strong temperature dependence associated with the sharp peak, which vanished at the superconducting transition temperature T c , while the hump feature persisted well above T c . The coexistence of these two gaplike features in the superconducting state has been attributed to a different physical origin associated with each gap [20,21].In this work, we address some of these issues via studies of the directional and spatially resolved quasiparticle tunneling spectra on the YBa 2 Cu 3 O 72d (YBCO) with a range of doping levels. The doping dependence of the pairing symmetry, pairing potential, and spatial homogeneity is derived from these studies.The samples used in this investigation included three optimally doped YBCO single crystals with T c 92.9 6 0.1 K, three underdoped YBCO single crystals with T c 60.0 6 1.5 K, one underdoped YBCO c-axis film with T c 85.0 6 1.0 K, two overdoped ͑Y 0.7 Ca 0.3 ͒Ba 2 Cu 3 O 72d (Ca-YBCO) c-axis films [22] with T c 78.0 6 2.0 K, and one optimally doped single crystal containing small concentrations of nonmagnetic impurities, YBa 2 ͑Cu 0.9934 Zn 0.0026 Mg 0.004 ͒ 3 O 6.9 [(Zn,Mg)-YBCO], with T c 82.0 6 1.5 K [8,22]. The spectra of YBCO single crystals were taken primarily with the quasiparticles tunneling along three axes: the antinode axes ͕100͖ or ͕010͖, the nodal axis ͕110͖, and the c axis ͕001͖; while those of the pure and Ca-YBCO films were taken along the c axis. All samples except (Zn,Mg)-YBCO are twinned. The surface was prepared by chemical etching [23,24], and samples were kept either in high-purity helium gas or under high vacuum at all times. Our surface preparation has the advantage of terminating the YBCO top surface at the CuO 2 plane by chemically passivating the layer while retaining the bulk properties of the constituent elements [23,24], thus yielding reproducible spectra for samples of the same 0...

Flux-trap magnets based on high-temperature superconductors were studied for their potential as electronbeam control elements. Applications could include accelerators, Superconducting Rings Solenoid 1 klystrons, and mm-wave amplifiers. We have constructed compact magnets using bulk YBCO crystals that exhibit axial fields of 1.0 -1.2 T at operating temperatures from 65K to 77K.Transverse fields and field stability have also been measured. Finally, methods and mechanisms involved in pulse-charging the superconducting rings will be discussed.