Quasiparticle diffusion and the energy resolution of superconducting tunneling junctions as photon detectors. I. Theory

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Abstract
One of the factors that degrades the energy resolution of superconducting tunnel junctions (STJ’s) as photon detectors at energies above a few keV is the spatial dependence of the response on the photoabsorption site. To assess the role of spatial inhomogeneities we have analyzed quasiparticle diffusion processes in detail and developed a general analytical theory to describe the evolution of the quasiparticle density in an STJ. This theory underlies an analytical model of the STJ response surface, and the corresponding spectral line shape. It accounts for effects of quasiparticle recombination, multiple quasiparticle tunneling, phonon coupling between the electrodes, asymmetry between base and counter electrode, losses at edges and localized traps, and diffusive losses into electrical connections, and is structured such that the effect of these features are added independently, so that the influence of each process can be assessed independently and possible new processes can easily be included.