A. Anthore, J. Flyktman, F. Giazotto, T. Heikkilä, F. Hekking, J. Pekola, A. Savin
We have recently demonstrated state-of-the-art superconductor based micro-refrigerators. The performance of these NIS coolers is, however, still far from ideal especially at the lowest temperatures, where the most interesting physics could be found. In general low electron temperature is always a challenge and cooling electrons directly by this method seems most appropriate to achieve this. Recently we also observed that the electron energy distribution in the cooled metal is not always thermal, i.e., it does not obey Fermi-Dirac distribution and the system does not thus have a temperature in a strict sense. This deviation is due to the slow inelastic electron-electron relaxation in a mesoscopic conductor, and it can be exploited in interesting new devices, e.g., in a “cold electron Josephson transistor”, where the cooler controls the electronic energy distribution of the normal electrode N of a SNS Josephson junction. Recently we demonstrated the operation of such a transistor in the thermal regime, i.e., when the temperature of N was controlled by the electronic cooler.
A very challenging objective is to develop an easy to use, miniaturized refrigerator from room temperature down to millikelvin temperatures. Development of the solid state microcooler and extending its temperature range up using the NIS technique is not a viable route at least above liquid helium temperature (4.2 K). We have started collaboration with dir. Sami Franssila at the Microelectronics Centre of HUT to exploit micromachining techniques on silicon to develop fluidic microcoolers in the temperature range above the present range of NIS refrigerators.