Low-frequency voltage noise in mesa-shaped stacks of intrinsic high- Tc Josephson junctions

Abstract

We investigated the low-frequency noise behaviour in high temperature superconductor (HTS) Josephson junction stacks made of thin film TBCCO-2212 layers on sapphire substrates. The measured stacks possess a top gold electrode and they are 3 μm × 3 μm in size as well as about 70 nm in height. They are prepared by means of photolithography and Ar ion milling. The voltage noise density showed clearly visible Lorentz-like bumps. We observe random telegraph noise in the sub-mV range as well as random switching in the mV-range. Furthermore the telegraph noise shows a clearly discernible two level fluctuation behaviour with one level dominating in time. With increasing bias current the absolute noise amplitude slightly grows and the lifetimes of both levels decrease considerably. However, the lifetimes of the upper and lower level do not show a simple Poisson distribution. We assume that the observed low-frequency behaviour is due to the quasiparticle injection through the normal conducting top electrode into the stacks.