Implantation, damage, and regrowth of high Tc superconductors
Abstract We have observed superconductivity in thin films of (La1−xSrx)2CuOy which are fabricated by implanting evaporated La/Cu multilayer films with Sr and annealing in oxygen. The films are insulating and partially transparent as-implanted, but they darken and become conducting at annealing temperatures as low as 500°C. The resistive behavior is very sensitive to the annealing conditions and the superconducting layer appears to be buried beneath the surface. Similarly, implantation of F, O, and Ne into single crystals and high quality (χmin
Irradiation effects in Ba2YCu3O7 superconductor
Superconducting property changes induced by electron, neutron and ion irradiations in Ba2YCu3O7 pellets and thin films were reviewed in consideration of the critical current density enhancement with radiation-produced defects. The transition temperature decreases almost proportionally to the fluence concurrently with a linear increase in normal state resistivity at an early stage of the irradiations. The ratio of the degradation rate for the transition temperature to that for the resistivity is in the range from 0.4 to 0.8 K/μΩ·m in the pellets irradiated with MeV-electrons and reactor-neutrons and 0.1 to 2 K/μΩ·m for high quality thin film irradiated with 600 keV Ar-ions. The critical magnetization current density in sintered pellets is increased by irradiation with electrons, neutrons and high-energy Xe ions to relatively low fluences where the transition temperature is decreased by about 1 K or less. The enhancement is significant in higher magnetic fields for the electron and neutron irradiated samples. Furthermore, the critical transport current density is increased in magnetic fields more than 1 T in an oriented thin film irradiated with 25 MeV 16O ions to a fluence that decreases the transition temperature by about 3 K.
O2 constraints from Paleoproterozoic detrital pyrite and uraninite
Redox-sensitive detrital grains such as pyrite and uraninite in sedimentary successions provide one of the most conspicuous geological clues to a different composition of the Archean and early Paleoproterozoic atmosphere. Today, these minerals are rapidly chemically weathered within short transport distances. Prior to the rise of oxygen, low O2 concentrations allowed their survival in siliciclastic deposits with grain erosion tied only to physical transport processes. After the rise of oxygen, redox-sensitive detrital grains effectively vanish from the sedimentary record. To get a better understanding of the timing of this transition, we examined sandstones recorded in a scientifi c drill core from the South African 2.415 Ga Koegas Subgroup, a mixed siliciclastic and iron formation–bearing unit deposited on the western deltaic margin of the Kaapvaal craton in early Paleoproterozoic time. We observed detrital pyrite and uraninite grains throughout all investigated sandstone beds in the section, indicating the rise of oxygen is younger than 2.415 Ga. To better understand how observations of detrital pyrite and uraninite in sedimentary rocks can quantitatively constrain Earth surface redox conditions, we constructed a model of grain erosion from chemical weathering and physical abrasion to place an upper limit on ancient environmental O2 concentrations. Even conservative model calculations for deltaic depositional systems with suffi cient transport distances (approximately hundreds of kilometers) show that redox-sensitive detrital grains are remarkably sensitive to environmental O2 concentrations, and they constrain the Archean and early Paleoproterozoic atmosphere to have <3.2 × 10 –5 atm of molecular O2. These levels are lower than previously hypothesized for redox-sensitive detrital grains, but they are consistent with estimates made from other redox proxy data, including the anomalous fractionation of sulfur isotopes. The binary loss of detrital pyrite and uraninite from the sedimentary record coincident with the rise of oxygen indicates that atmospheric O2 concentrations rose substantially at this time and were never again suffi ciently low (<0.01 atm) to enable survival and preservation of these grains in short transport systems.