Strong Quasiparticle Trapping In A 6x6 Array Of Vanadium-Aluminum Superconducting Tunnel Junctions

摘要

A 6x6 array of symmetrical V/Al/AlOx/Al/V Superconducting Tunnel Junctions(STJs) was fabricated. The base electrode is a high quality epitaxial film witha residual resistance ratio (RRR) of ~30. The top film is polycrystalline withan RRR of ~10. The leakage currents of the 25x25 mm^2 junctions are of theorder of 0.5 pA/mm^2 at a bias voltage of 100 mV, which corresponds to adynamical resistance of ~ 3 10^5 ohms. When the array was illuminated by 6 keVX-ray photons from a 55Fe radioactive source the single photon charge outputwas found to be low and strongly dependent on the temperature of the devices.This temperature dependence at X-ray energies can be explained by the existenceof a very large number of quasiparticle (QP) traps in the Vanadium. QPs areconfined in these traps, having a lower energy gap than the surroundingmaterial, and are therefore not available for tunneling. The number of trapscan be derived from the energy dependence of the responsivity of the devices(charge output per electron volt of photon input energy).