SPATIAL MEASUREMENTS NEAR THE INSTABILITY THRESHOLD IN ULTRAPURE GE

Spatially and temporally resolved measurements are presented near the threshold for a spontaneous current instability in liquid-He-cooled ultrapure Ge. Periodic current oscillations occur above a sharp threshold in applied dc voltage; each cycle is due to the passage of a single space-charge domain through the sample. For sufficiently large applied dc voltage below this threshold, domain formation can be triggered by a small superimposed voltage pulse. As the dc voltage is increased, the triggered domain grows larger and travels farther into the sample. Nearer threshold, current pulses due to single domains spontaneously occur at irregular intervals; as the threshold for spontaneous oscillation is approached, the pulses occur in increasingly larger groups, and the number of pulses per group diverges at the threshold. The statistics of group length and the exponent associated with this divergence agree well with the predictions of Pomeau-Manneville type-III intermittency.