X-RAY-CHARACTERISTICS OF A NIOBIUM SUPERCONDUCTING TUNNEL JUNCTION WITH A HIGHLY TRANSMISSIVE TUNNEL BARRIER

The results of an investigation into the x-ray properties of a superco nducting tunnel junction (STJ) are presented. The photoabsorption of a n x-ray photon by one of the thin superconducting films of the junctio n results in the production of quasiparticles, which may subsequently tunnel through the thin oxide barrier into the second superconducting film. The transfer of charge across the barrier is detected, and gives a measure of both the x-ray photon energy and the effective energy ga p epsilon of the superconducting film in which the photoabsorption occ urred. A charge output of 55% of the theoretical maximum has been obta ined for a niobium-based STJ. Such a charge output indicates a mean en ergy epsilon of congruent-to 4.7 meV is required to create a single ch arge carrier in the junction such that epsilon/DELTA congruent-to 3, w here 2DELTA is the junction energy gap. This is the lowest value of ep silon/DELTA obtained to date for x-ray photoabsorption in STJs. The en ergy resolution of the device is, however, still poor, with a full wid th half maximum of congruent-to 200 eV for 6 keV x rays, compared with the theoretical Fano limited resolution of congruent-to 4 eV. The pri nciple mechanisms which are believed to degrade the resolution are dis cussed. These are principally quasiparticle recombination, phonon leak age out of the junction, quasiparticle diffusion into and out of the t unneling region, in addition to possible local variations in the energ y gap. A Monte Carlo simulation of the nonequilibrium system has been performed. The results assist in the identification of the major charg e loss mechanisms, and indicate various means by which Fano limited re solution may be obtained.