Point-contact spectra for sodium and potassium

Using pseudopotential theory, with two different pseudopotentials and a Monte Carlo technique, we have calculated the point-contact spectral functions of Kulik, Omel'yanchuk, and Shekhter and of van Gelder for realistic models of sodium and potassium. We have also calculated ${\ensuremath{\alpha}}^{2}F(\ensuremath{\omega})$, the electron-phonon interaction squared times the phonon density of states. We find that the two point-contact spectral functions are almost identical to each other but differ from ${\ensuremath{\alpha}}^{2}F$. We have compared these functions with the experimental point-contact data on sodium and potassium of Jansen et al. There is quanitative agreement between the calculated functions and experiment for the case of Na using the Taylor pseudopotential. For potassium there is reasonably good agreement between all the calculated point-contact spectral functions and experiment with features in common that do not show up in ${\ensuremath{\alpha}}^{2}F$. For both sodium and potassium, the main longitudinal peak is broader for experiment than for any of our calculated functions. We suggest that this broadening may be due to impurity scattering that is not taken into account in our present calculations.

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