Response of superconductive films to localized energy deposition

Abstract

Superconductive tunneling junctions are promising candidates as detectors for low-energy x-ray spectroscopy in particle, solid-state, and astrophysics. However, the energy resolution achieved so far deviates from the ultimate resolution calculated on statistical grounds by about one order of magnitude. This discrepancy may be due to the local variation of the relaxation processes following the absorption of the quanta. In this paper we present temperature- and energy-dependent calculations of these processes. It is shown that during the relaxation a substantial amount of the initial energy is converted into phonons with energies less than $2{\Delta }_b$, where ${\Delta }_b$ is the energy gap of the superconductor in thermal equilibrium at the temperature $T_b$. This energy is lost for further production of excess quasiparticles needed for tunneling, and the loss process depends on the quasiparticle and phonon diffusion properties. Its local variation over the junction area limits the energy resolution. In addition the experimentally observed nonlinear energy response can also be reproduced by the calculations. The nonlinearity stems from the strong self-recombination of quasiparticles in the local hot spot. Various superconducting materials were studied in order to select the most suitable one for future superconducting tunneling detectors.