Probability of quasiparticle self-trapping due to localized energy deposition in nonequilbrium tunnel-junction detectors

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Abstract
Voltage-biased, superconducting tunnel junctions are investigated as x-ray detectors for applications requiring both high quantum efficiency and better than 1% energy resolution. The nonequilibrium quasiparticles, produced as the energy deposited degrades to the few-meV-per-excitation level, tunnel and are detected before they are lost to recombination. Previous event modeling ignored the energy cascade under the assumption that the equilibrium of the electrodes is minimally perturbed by the deposited energy. In this paper we demonstrate that that assumption is invalid. We calculate the local energy density as the average quasiparticle energy becomes of the proper magnitude to suppress the gap. The fraction of the nonequilibrium quasiparticles that become spatially trapped (never to tunnel) in the order-parameter well that their existence creates may vary between events. If so, the source of the observed non-Poisson-limited energy resolution of this class of detectors would be identified. The input parameters used were evaluated in equilibrium. Thus our conclusions need to be confirmed via a fully nonequilibrium calculation of the cascade.