Research activities of Saitama University on N[lc]b-based superconducting tunnel junctions for photon detectors

Abstract

Two-dimensional shapes of Nb-based superconducting tunnel junctions (STJs) for suppressing Josephson critical current with small applied magnetic field have been studied. Normal-distribution-function-shaped STJ is proposed for reducing the applied magnetic field. STJS with a normal-distribution-function shape showed magnetic field dependence of the critical current without side lobes. Almost one-thirtieth smaller magnetic field was required to suppress critical current. Ta trapping layers were newly introduced to achieve better energy resolution for photon detection. It was attempted to use Ta trapping layers with a low critical temperature of 0.5 K and short coherence length. The properties of beta-Ta enable production of a photon detector with better energy resolution as well as high sensitivity. Nb-based superconducting junctions with beta-Ta trapping layers showed responses as large as those with Al trapping layers and high energy resolution for 5.9 keV X-ray irradiation.