High-frequency currents and heterodyne mixing in superconducting tunnel junctions

Both Nb/AlO;A slotted-line technique and the Nb devices were used to measure the admittance of the tunnel junction at 87.1 GHz. The admittance is composed of the quantum conductance and quantum susceptance. This work reports the first direct observation of the singularity in the quantum susceptance, which is related to the singularity in the reactive quasiparticle currents.