Tunneling Measurements of Phonon Spectra and Density of States in Superconductors

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Abstract
Using the technique of electron tunneling between superconducting films, we have measured the density-of-states variation with energy in a number of soft superconductors. In lead we show that the density-of-states variation suggests the form of the phonon spectrum which is effective in the coupling of the Cooper pairs. The spectrum has transverse and longitudinal peaks located at 4.4 and 8.5 meV, respectively. A solution of the Eliashberg gap equation using this phonon spectrum and reasonable values for coupling constants gives good agreement between theoretical and experimental density-of-states plots, as shown by Schrieffer et al. In addition to the density-of-states variation due to the two main peaks in phonon density, we resolve fine structure which is the effect of the Van Hove critical points in the phonon spectrum. Reasonable agreement is found between the energy and type of structure observed in the density-of-states variation and the occurrence of critical points in the dispersion curves obtained by neutron scattering. From similar experiments we suggest that the tin phonon spectrum extends to 17.7 meV with critical points as low as 3.4 meV. In indium the end of the spectrum appears to be 14.8 meV with a transverse peak between 3 and 7 meV and a longitudinal peak from 11 to 14 meV.

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