SUPERCONDUCTING TUNNELLING AND PHONONSSUPERCONDUCTING TUNNEL JUNCTIONS AS PHONON SOURCES AND DETECTORS

Experimental results pertaining to the behavior of superconducting diodes when used in phonon generation and detection are discussed in some detail for a generator voltage V ⩽ 6 Ɗ. We show that many aspects of the behavior can be fully explained by an average recombination lifetime and the quasiparticle density of states of a superconductor. We also show how this average recombination time can be derived from experimental dependence of the signal on the detector bias which, in addition, clearly shows the influence of the ac Josephson effect. Further analysis shows that this experimental behavior is consistent with the Riedel Singularity in the ac Josephson current at a bias voltage = 2 Ɗ. Finally, we discuss one aspect which can only be interpreted in terms of the energy dependence of the recombination and relaxation rates and the detailed spectral distribution of the emitted phonons. This aspect is the deviation of the signal from its linear dependence on the generator current for V < 4 Ɗ. This deviation has the form of a Lorentzian with a peak at voltage Vm. By simple arguments, we show that the spectral width of the recombination phonon peak is given by Vm – 2 Ɗ. By calculating the spectrum of the generated phonons and hence the detector signal we show that the above predictions are essentially correct