Tae Won Noh

A new experimental approach to two-dimensional percolation

Abstract By using a computer to generate random patterns and then reproducing these patterns by photolithographic techniques, we have prepared a random stick system and a bond percolation system for experimental studies of 2D percolation. The agreement between the measured conductivity exponents for these two systems supports the universality hypothesis.

Magnetic behavior of the high Tc composite compound of Y1Ba2Cu3O7-δ in the superconducting phase

Abstract Detailed measurements of the magnetic properties in the superconducting state of a composite sample with approximately 35% Y1Ba2Cu3O7-δ and a Tc of 87K are compared with results on a corresponding single phase sample. The susceptibility is approximately linear in temperature below and near Tc indicating large magnetic field penetration near the transition. At low temperatures, a volume flux expulsion of 5% is found in lowest magnetic field, with Hc1 indicated to be 1.5kG. Magnetization loops are presented, showing hysteretic behavior even at 4.2K and 100G. At higher temperatures, the magnetization curves bear a striking resemblance to those for a large κ type II superconductor. The Curie susceptibility associated with spin in the non-superconducting phase remains unscreened below Tc in sharp contrast to the behavior of the rare earth 1-2-3 compounds, indicating that the magnetic flux percolates easily through the composite.

Enhancement mechanisms for the far-infrared absorption of metal-smoke-insulator composites based on the clustering of metal smoke

The strong far-infrared absorption observed by many workers in metal-smoke-insulator composites has been re-investigated thoroughly using well characterised samples of Ag-smoke-Teflon composites. The absorption coefficient of well separated Ag smoke can be explained quantitatively by the Maxwell-Garnett theory modified to include the size distribution of the Ag particles and the oxide coating effects. Particle clustering is shown to have an enormous impact on the far-infrared absorption of these composites. Comparisons between the authors experimental data and certain clustering modes are made. These comparisons show that most of the experimental results are consistent with the predictions of the cluster percolation model.