Conductor to insulator transition and insulator to conductor transition, studied by gas discharges and breakjunctions

摘要

This thesis is divided in two parts: part I deals with experiments in which a gas is transformed in a good conductor by application of a high electric field, part II deals with measuring the conductance of a small gold contact during the moment it breaks. The main goal of part I is to measure the formation and propagation of streamers, by using an imaging technique utilizing an ICCD camera. The starting images reveal that the emission pattern exhibits many, point-like, intense spikes, which implies that the ionization process starts at many places at once, in contrast to the finger-like streamers found in computer-simulations. Furthermore it is shown that for both polarities the propagation speed is more or less constant, in spite of the fact that the applied electric field strongly varies, due to the point-plate geometry. The measured images in magnetic field reveal that the discharge channels are curved and that the curvature increases with field strength. This proves that photoionization does not play a major role in the evolution of the discharge. It is found that up to 7 mm from the point-electrode the curvature of the discharge can be described by a constant Hall angle. At larger distances this is not possible, which shows that at those distances the local field of the streamer tip dominates the applied field, this also explains the constant propagation speed of the streamer. In part II we study the disappearance of conductivity when a metal contact is interrupted. We demonstrate that just before the contact is broken, the conductance disappears in quantized steps. This quantization improves when a magnetic field is applied parallel to the probing current. This result can be explained by a selection of point contacts with better quantization, through the magnetic field