The quantitative determination of phonon energy spectra using superconducting heterojunctions

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Abstract
A technique is described for converting the non-equilibrium currents observed at different bias values in a superconducting tunnel junction, formed between superconductors of different energy gaps and subject to a high-energy incident phonon flux, into an energy distribution for the incoming phonons. The effects both of the theoretical approximations made in deriving the numerical results used in the analysis and of the non-ideal characteristics, observed in real tunnel junctions on the accuracy of the technique are discussed. Results from the application of the technique to various broad-band phonon sources are then described. The thin film cupro-nickel heater is found to emit largely in accord with the acoustic mismatch model, whereas the superconducting tin phonon fluorescer shows enhanced emission above the tin gap energy.