A. E. White

Electron field emission from ion‐implanted diamond

Diamond films and islands grown by chemical vapor deposition were implanted with boron, sodium, and carbon ions at doses of 1014–1015/cm2. This structural modification at the subsurface resulted in a significant reduction of the electric field required for electron emission. The threshold field for producing a current density of 10 mA/cm2 can be as low as 42 V/μm for the as‐implanted diamond compared to 164 V/μm for the high quality p‐type diamond. When the ion‐implanted samples were annealed at high temperatures in order to anneal out the implantation‐induced defects, the low‐field electron emission capability of diamond disappeared. These results further confirm our earlier findings about the role of defects in the electron emission from undoped or p‐type doped diamond and indicate that the improved emission characteristics of as‐implanted diamond is due to the defects created by the ion implantation process.

Critical currents in proton‐irradiated single‐crystal Ba2YCu3O7−δ

Irradiation with 3.5 MeV protons creates defects in Ba2YCu3O7−δ, which can act as strong pinning sites, comparable to those produced by neutron irradiation. Indeed protons as well as neutrons undergo momentum‐transferring collisions that result in atomic displacements; the additional electronic proton‐solid interaction does not appear to yield defects which contribute to pinning. Under optimized conditions we have obtained Jc(T→0, H=1 T)∼2×107 A/cm2 in single crystals. At 77 K the value is still Jc∼1.7×105 A/cm2, decreasing only threefold in a field of 6 T. The fluence and temperature dependence of Jc suggest a model, in which spatially uncorrelated defects pin vortex cores.

Sharp angular sensitivity of pinning due to twin boundaries in Ba2YCu3O7

We have measured the pinning force due to twin boundaries by measuring the magnetic torque produced when a field is applied perpendicular to the c axis and rotated with respect to the ab axes. At 76 K, the torque increases roughly fourfold, when the field is within 2° of the parallel to the twin boundaries. This peak is absent at 27 K. Irradiation with 5×1016 cm−2 3.5 MeV protons increases the overall critical current and eliminates the peaks observed at 76 K.

Enhancement of flux pinning by H+ and Xe+ irradiation in epitaxial thin films of Ba2YCu3O7−δ

Epitaxial Ba2YCu3O7−δ (BYCO) films grown by the ex situ BaF2 process are comparable to single crystals both in crystalline quality and the value and temperature dependence of the critical current (Jc) in an applied magnetic field in the BYCO (001) direction of Ha=0.9 T. With the appropriate dose of either 2 MeV H+ or 135Xe+, we can enhance Jc by a factor of 2 in Ha=0.9 T with little effect on Tc. This is significantly greater than the ∼25% enhancement previously reported for epitaxial BYCO films grown by in situ techniques [Roas, Hensel, and Saemann‐Ischenko, Appl. Phys. Lett. 54, 1051 (1989)]. This provides the opportunity to isolate the induced defects and study their properties.

High critical currents in c-axis textured Bi-Pb-Sr-Ca-Cu-O superconductor ribbons

Abstract A highly oriented layer structure was achieved in Bi-Pb-Sr-Ca-Cu-O superconductor ribbons by a combination processing of spray coating on silver foil, cold rolling and partial melting. Excellent transport J c values as high as 230 000 A/cm 2 at 4.2 K, H ⊥ ab ∼8 T have been obtained. Preliminary attempts to further improve the critical currents of the superconductor-on-silver ribbons by proton irradiation have not been successful as both the transport and the apparent magnetization J c s are found to be monotonically reduced with increased fluence of irradiation. This is in contrast to the results of neutron irradiation on bulk Bi-Pb-Sr-Ca-Cu-O on which a significant increase in J c (magn) is observed. Further studies are needed for better understanding of the irradiation effects in the ribbon samples.

Increased pinning energies and critical current densities in heavy‐ion‐irradiated Bi2Sr2CaCu2O8 single crystals

We report a significant increase in the pinning energy of vortices in single‐crystal Bi2Sr2CaCu2O8 when irradiated with heavy ions such as Ar+. This is in contrast with the results of light ion (H+, He+) irradiations which give pinning energies comparable with those of unirradiated crystals. The stronger pinning is attributed to defects larger than point defects, e.g., clusters or amorphized regions. As a result of higher pinning energies, critical currents persist at markedly higher temperatures and fields.

Critical Current Enhancement in Single-Crystal Ba 2 Ycu 3 O 7

The critical current density, J c , of single crystals of Ba 2 YCu 3 O 7 is comparable to that of melt-textured-growth materials as well as to the intragrain J c of bulk polycrystalline Ba 2 YCu 3 O 7 . Typical values at 77 K and 9 kOe are near 5x 10 3 A/cm 2 , and are presumably limited by weak pinning. We have obtained a significant enhancement of this pinning-limited J c by three techniques: 1) a short reannealing procedure, 2) fast neutron irradiation, and 3) proton irradiation. Interestingly, the combination of the first technique followed by either irradiation is not as effective as the iradiation alone. That is, the best candidates for irradiation are not necessarily those with the highest initial J c .

Enhancement of Flux Pinning Force by Ion Beam Irradiation of Epitaxial Ba 2 Cu 3 O 7-δ Films

Epitaxial Ba 2 YCu 3 O 7-δ (BYCO) films grown by the ex situ BaF 2 process are comparable to single crystals both in crystalline integrity (RBS/ion channeling χ min c ) in an applied magnetic field in the BYCO (001) direction of H a = 0.9T. With the appropriate dose of either 2 MeV H + or 135 Xe + , we are able to enhance J c by a factor of 2 in H = 0.9T with little effect on T c . The ability to change J c by such a large factor in these films is a prerequisite for isolation and study of the induced defects and study their properties.

Pinning by Oxygen Point Defects in Proton-Irradiated Ba2YCu3O7−δ

Proton irradiation of single-crystal Ba2YCu3O7−δ reproducibly increases the critical current tenfold, but does not alter the activation energy for flux creep. This can be accounted for in terms of a conventional flux pinning model. Based on this interpretation, as well as circumstantial structural evidence from TEM and X-ray studies, we propose that the chief effect of low-fluence proton irradiation is to create a higher density of oxygen point defects (which exist even in undamaged crystals). This conclusion is supported by observations of the effect of irradiation on electronic parameters.