Quasiparticle diffusion in tantalum using superconducting tunnel junctions

Abstract

A specially designed device based on superconducting tunnel junctions was used to measure the diffusion of excess electronic quasiparticles generated in a superconducting film of tantalum by the absorption of x-ray photons. The device was made out of a thin film of epitaxial tantalum. At both ends, a microfabricated $\mathrm{Al}\ensuremath{-}{\mathrm{AlO}}_{x}\ensuremath{-}\mathrm{Al}\ensuremath{-}\mathrm{Nb}$ tunneling junction was placed onto the film. Both tunneling junctions were operated in a current-biased mode in a small magnetic field and were used to monitor the diffusion of excess quasiparticles generated in the Ta. For the data analysis, the Ta absorber has been modeled as a chain of cells, and the time evolution and diffusion of the distribution of energy carried by quasiparticles has been calculated using the Chang-Scalapino equations. In this way, we determined the values of the quasiparticle diffusion length $\ensuremath{\Lambda}=\sqrt{{D}_{\mathrm{abs}}{\ensuremath{\tau}}_{\mathrm{eff}}},$ the diffusion rate ${D}_{\mathrm{abs}},$ and the effective lifetime ${\ensuremath{\tau}}_{\mathrm{eff}}.$