QUASI-PARTICLE TRAPPING AND THE DENSITY-OF-STATES IN SUPERCONDUCTING PROXIMITY STRUCTURES

An experimental study of quasiparticle trapping in epitaxial and polyc rystalline Ta films on epitaxial Nb is presented using three-terminal double tunnel junction devices. It is shown that polycrystalline Ta is a more effective trap than epitaxial Ta. The experimentally measured tunneling density of states is used to calculate the inelastic quasipa rticle scattering rates in the two types of Ta using the standard theo ry of Kaplan et al. [Phys. Rev. B 14, 4854 (1976)]. The agreement of t his calculation with the experimental results shows that the tunneling density of states may be used to determine scattering rates in proxim itized superconducting films whose thickness is greater than the coher ence length. This result is important since no existing theory satisfa ctorily describes the density of states in such proximity structures, which are currently being developed for use in high-resolution particl e spectrometers.