Material aspects for preparing HTS quasiparticle injection devices

Quasiparticle (QP) injection devices based on HTS could play an important role in future superconducting applications if material aspects can be better controlled. One reason why this kind of device received little attention in the past is the lack of an appropriate barrier for QP tunnelling. In a series of experiments, we used different barriers to test if they are suitable, i.e. if a current and possibly a voltage gain can be achieved. We improved the performance of planar YBCO/natural barrier/Au devices and a current gain of more than 6 at 40 K was observed. Most devices, however, showed signs of heating effects. Another barrier material was SrTiO/sub 3/ with layers of 5-6 nm thickness. Current-voltage characteristics showed that the barriers were continuous and we observed current gains of up to 3 at 60 K. PrBa/sub 2/Cu/sub 3/O/sub 7-x/ is an interesting candidate if one could overcome the problem of resonant inelastic tunnelling for QP. In a series of experiments we demonstrated that, even for 3 Mn thin PBCO barriers on a- and c-axis oriented YBa/sub 2/Cu/sub 3/O/sub 7-x/, most devices showed at best a current gain of 1. However, we have indications that a current gain of 10 could be possible with unity voltage gain.