Tunnelling barriers for high temperature superconductor devices

In this contribution we will discuss the requirements to be imposed on the barriers of HTS Josephson junctions as well as quasi particle injection devices. Factors that play a role are noise immunity, operating margins, injection efficiency etc., leading to the need for barriers in which the charge transport is dominated by direct tunnelling. So far, it is unclear if this has ever been observed in any practical device. We will demonstrate that localised states, that are probably ubiquitous in the materials used, are of prime importance in understanding device properties. In the model system we investigated, i.e. DyBCO/PrBCO/DyBCO, gallium-doping on the copper chain sites reduces the density of localised states appreciably without affecting the barrier height. Furthermore, it seems that due to an on-site Coulomb repulsion, pair transport proceeds via direct tunnelling, at least in the temperature region T<Tc/2. Quasi particles, in contrast, tunnel resonantly via one or more localised states. At higher temperatures and bias voltages inelastic processes may dominate. These resonant tunnelling processes reduce the normal state resistance, and hence the IcRn-product. The consequences of these findings will be discussed and directions for future work set.