Applic ation of Bragg superlattice filters in low temperature microrefrigerators

摘要

We propose to use the Bragg interference filter technology for fabrication of microrefrigerators. The idea of using superconductor-insulator-superconductor (SIS) or normal metal-insulator-superconductor (SIN) tunnel junctions as cooling elements in icro-refrigerators is attractive because of the absence of (micro-) regrigerators operating below 150 K. There are corresponding experiments (1) on SIN tunnel junctions where an attached to the SIN tunnel junction membrane was cooled down. Theoretical approaches (both phenomenological (1) as well as microscopic (2) show that the cooling effect exists also in SIS tunnel junction. However this was not observed experimentally because of inefficient thermal contact between SIS tunnel junction and the membrane that must be cooled. The microscopic approach to cooling is based on the "phonon deficit effect" (2) in nonequilibrium regime of tunnel junctions. In some circumstances, when the applied voltage does not exceed the superconducting energy gap (DELTA) the probability of phonon absorption from the heat-bath is higher than its emission in the nonequilibrium regime of tunnel junctions. There is an appropriate absorption window in the phonon emission spectra (2, 3) and by absorbing these phonons from the heat bath the SIS or SIN tunnel junction can refrigerate its environment. This effect can be improved by use of phonon filters placed between the tunnel jucntion and the heath-bath (4). Such a filter can be the Bragg interference superlattice (Bragg's grating) which is well studied for problems of optical communications. Bragg interference filters are used also for detection of phonons emitted by tunnel junctions (5). Usually such filters cut low and high frequencies, and the used detector may detect well separated frequencies. In contrast, to enhance the refrigeration process one needs filters with very broad spectral transmission properties or a large transmission band with one or two narrow stop bands. The type of the needed filter will depend on the types of the used tunnel junction. Corresponding discussion is presented.