The cooldown process of superconducting tin films in contact with cold sapphire

Momentary current voltage characteristics of tin-tunnel junctions evaporated on an a-cut sapphire were measured during the cooldown process which starts immediately after stopping the steady-state heating from a constantan heater which was evaporated at the top of the tin diode with a SiO_x-layer in between. The measured momentary current-voltage characteristics cannot be distinguished within experimental error from thermal characteristics taken at a higher substrate temperature. Hence, at the tunnel junction, the electron system can be described at each time during the cooldown process by an electron temperatureT₁^el(t). It is shown that the decay time of the electron system is far too long to be in accordance with a one-temperature model combined with the acoustic mismatch model, which has been verified in pulse-heating experiments during the heatup and steady-state time interval in a preceeding paper.¹ We observed a pure exponential decay of the electron temperature starting at the onset of superconductivity atT_C=3.72 K and ending at the bath temperature which is usually aboutT_C/2. The measured decay constant is within a 10% maximum deviation equal to the effective recombination time _eff of the injected quasiparticles, well-known from other investigations. We found the same dependence of on bath temperature, on the thickness of the tin film, and on oxygen content as was observed for _eff in earlier experiments. Simple models for the behavior of the electron and phonon system during the cooldown process are investigated to explain this unexpected decay behavior at temperatures far away from the bath temperature.