A. D. Marwick

Enhanced current density Jc and extended irreversibility in single‐crystal Bi2Sr2Ca1Cu2O8 via linear defects from heavy ion irradiation

Large enhancements in the critical current density Jc were produced in single crystals of the high‐temperature superconductor Bi2Sr2Ca1Cu2O8 by irradiation with high energy Sn ions. In addition, the irreversibility line was moved to considerably higher magnetic fields. In contrast with analogous studies on Y1Ba2Cu3O7, there was little, if any, selective pinning when the magnetizing field was applied parallel to the linear, ion‐damage‐produced tracks.

Thermal stability and electrical properties of hydrogenated amorphous carbon film

The thermal stability of a plasma‐deposited amorphous carbon film was enhanced by using acetylene heavily diluted with He. The film preserved its hardness even after annealing at ≂590u2009 °C in Ar/H2, while a film deposited in similar conditions in an acetylene/Ar mixture softened significantly. The I‐V characteristics of n‐ and p‐type Si/amorphous carbon heterojunctions showed a 0.2 eV discrepancy. This is attributed to an offset in the conduction band of the amorphous carbon with respect to Si.

Hot-Electron Induced Hydrogen Redistribution in SiO2

Hydrogen redistribution within hot-electron injected metal-oxide-silicon capacitors has been studied following electron injection by internal photoemission using hydrogen profiling by nuclear reaction analysis. Measurable hydrogen buildup (~ 1014 H/cm2) at the Si/SiO2 interface occurs for injection fluences of order 1 C/cm2. The amount of buildup is larger for negative-gate injections than for positive gate, whereas the number of interface states, which appear for much lower fluences, is greater in the positive bias case. Since it is known that hydrogen is involved in the generation of interface states, these observations suggest that different mechanisms operate in the low and high-fluence regimes. Large amounts of hydrogen (~ 1015 H/cm2) are found to be released from the Al/SiO2 interface during the charge injection. Most of this hydrogen leaves the samples, but for positive gate bias there is evidence that some enters the SiO2 as a species with a relatively low diffusion coefficient. In addition, some hydrogen enters the Si substrate and is detected by its passivation of boron atoms. The amount of passivation shows a better correlation with the interface state density than does the amount of hydrogen at the Si/SiO2 interface.

Orientation‐dependent critical currents in Y1Ba2Cu3O7−x epitaxial thin films: Evidence for intrinsic flux pinning?

For YBCO epitaxial thin films the basal plane transport critical current density Jc, flowing perpendicular to an applied magnetic field H, depends sensitively on the orientation of the crystal with respect to H. In particular, Jc is sharply peaked and greatly enhanced when H is precisely parallel to the copper‐oxygen planes. Experiments on a series of epitaxial monolithic and superconductor‐insulator multilayer thin films provide clear evidence that the enhancement is a bulk, rather than surface or thin sample, phenomenon. Measurements of the orientation dependence are presented and compared with a model of ‘‘intrinsic flux pinning’’ by the layered crystal structure.

Hydrogen segregation at the Al/Si(1 1 1) interface

Abstract The density depth profile of hydrogen trapped at an epitaxial solid–solid interface has been measured by neutron reflection. The interface was between a Si(1xa01xa01) substrate and an epitaxial film of Al 1500xa0A thick, into which hydrogen was introduced by low-energy implantion. The measurements, carried out at room temperature, indicated that a total of 0.7×10 16 xa0H atoms/cm 2 were trapped at the interface. This number corresponds approximately to one atomic layer of liquid hydrogen; however, hydrogen was found to be spread over a thickness of 60xa0A. Cooling the sample from room temperature to 220xa0K did not significantly alter the distribution of hydrogen trapped at the interface.

Limits to critical currents in high‐temperature superconductors: What we can learn from tailored defects

We consider factors that limit the loss‐free conduction of electric current in the vortex state of high‐temperature superconductors (HTS). Simple arguments lead to an estimate of the maximum possible critical current densities arising from optimally‐sized linear defects that pin each vortex core over its entire length. In a series of model experiments, such linear defect structures have been tailored at controlled densities in several HTS materials by irradiation with high‐energy heavy ions. Results from these studies are compared with the estimated zero‐temperature theoretical limits. At higher temperatures, the irradiated materials provide unique systems for evaluation of the current theories of flux motion that account for both the intrinsic, anisotropic material properties and the unique defect structures. In particular, from measurements that relate the pinning to the angle between the vortex and defect lines, direct information can be obtained about the influence of material anisotropy on the vortex...

Magnetic studies of current conduction and flux pinning in high‐Tc cuprates: Virgin, irradiated, and oxygen deficient materials

To increase the current density and pinning of magnetic flux in high temperature superconductors, defects with point‐like and line‐like geometries were created in controlled numbers using ion irradiation methods. Single crystals of Y1Ba2Cu3O7 and Bi2Sr2Ca1Cu2O8 superconductors were studied using dc magnetic methods. These studies showed greatly increased irreversibility in the vortex state magnetization and enhanced intergrain current density Jc following irradiation. Linear defects, created by irradiation with energetic heavy ions, are particularly effective in pinning vortices at higher temperatures and magnetic fields. Further investigations of ‘‘flux creep’’ (the time dependence of magnetization) are well described by recent vortex glass and collective pinning theories. Complementary investigations have delineated the role of oxygen deficiency δ on pinning in aligned Y1Ba2Cu3O7−δ materials.

Enhanced vortex-pinning strength and magnetic-irreversibility via columnar defects in single crystal Bi2Sr2CaCu2O8

Abstract To enhance vortex pinning in the high-Tc superconductor Bi2Sr2CaCu2O8, single crystals were irradiated with energetic heavy ions to create columnar defects. Irradiations were made at various angles from the c-axis. The pinning energy U0 was investigated by ac magnetometry near B(Tirr), the limit of quasistatic magnetic irreversibility. Irradiation increased U0 by a factor of ∼3, relatively independent of the ion-beam exposure, which increased both Jc and the irreversible Bue5f8T regime. Independence of Jc and U0 from the magnetic-field orientation suggests the predominance of strong pins interacting with 2-D pancake vortices.