Superconducting quasiparticle behavior: Trapping, propagation, and loss

Detectors operating at low temperatures provide superior energy resolution and sensitivity to small energy depositions. Many of these detectors use superconducting materials to absorb incident particles or photons. An absorption event creates electronic excitations, called quasiparticles, whose number and energy contain information about the original interaction. We describe a series of experiments designed to probe quasiparticle behavior. Quasiparticles which enter an adjoining material with lower energy gap and scatter inelastically are spatially confined or trapped. We have studied quasiparticle trapping between a superconductor, aluminum, and a normal metal, silver, at temperatures near 100 mK. We have measured that over 80% of the excitation energy of a trapped quasiparticle is converted to thermal electronic excitations in the silver. This fraction is almost constant for electron temperatures in the trap between 85 and 380 mK and phonon temperatures between 100 and 250 mK. Our results demonstrate that normal traps are a useful means of concentrating quasiparticle energy for measurement. We explain our results with a calculation describing quasiparticle thermalization. Quasiparticle energy is transferred to electrons in the trap because the electron-electron scattering rate is a factor of 10 larger than the electron-phonon rate for a 0.22 meV excitation. We have also studied quasiparticle propagation and loss in superconducting aluminum in order to guide the design of detectors and microrefrigerators. We have made the first observation of an important prediction of BCS theory: the energy dependence of the quasiparticle group velocity. In addition, we have measured the rates at which quasiparticles are lost to recombination and trapping in Abrikosov vortices. We fit our measurements of propagation and loss using finite difference solutions to the diffusion equation. This computational technique is also used to calculate gas desorption rates from interstellar silicates. Finally, we propose and demonstrate a superconducting transistor based on quasiparticle trapping.