Device-quality tunnel junctions on the high Tc superconductor HgBa2CuO7+d

SIN and SIS tunnel junction device (e.g. photon detectors, logic elements) require quasiparticle characteristics that exhibit sharp current onsets at the gap voltage and very lop sub-gap conductances. Progress is reported on the development of such junctions on high-Tc cuprates using mechanical point contacts. In general, these contacts display the optimum characteristics that can be obtained from HTS native-surface tunnel barriers. Most cuprates display a sub-gap conductance which monotonically increases pith voltage about the minimum value at zero bias. However, tunneling data of unusually high quality have been obtained for the recently discovered Hg-based cuprate, HgBa2CuO4 (Tc = 96 K). SIS′ tunneling data using a Nb tip are presented which exhibit very low and flat sub-gap conductances and sharp conductance peaks as expected from a BCS density of states. These results are slightl improved over earlier published results pith SIN junctions. Use of the experimental data to simulate the performance of a quasiparticle mixer demonstrates that noise temperatures approaching the quantum limit are possible for SIS and SIN mixers in the range 1–5 THz.