Quasiparticle diffusion in tantalum using superconducting tunnel junctions

A specially designed device based on superconducting tunnel junctions was u sed to measure the diffusion of excess electronic quasiparticles generated in a superconducting film of tantalum by the absorption of x-ray photons. T he device was made out of a thin film of epitaxial tantalum. At both ends, a microfabricated Al-AlOx-Al-Nb, tunneling junction was placed onto the fil m. Both tunneling junctions were operated in a current-biased mode in a sma ll magnetic field and were used to monitor the diffusion of excess quasipar ticles 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 d istribution of energy carried by quasiparticles has been calculated using t he Chang-Scalapino equations. In this way, we determined the values of the quasiparticle diffusion length Lambda = root D(abs)tau(eff), the diffusion rate D-abs, and the effective lifetime tau(eff).