Study of the electron-phonon interaction in metal diborides MeB2 (Me = Zr, Nb, Ta, Mg) by point-contact spectroscopy

We review investigations of the electron-phonon interaction (EPI) in metal diborides MeB2 (Me = Zr, Nb, Ta, Mg) by point-contact (PC) spectroscopy. For transition metal compounds the PC EPI functions were recovered and EPI parameter lambda less than or similar to 0.1 were estimated. The data are consistent with the measured surface phonon dispersion curves. The low lambda value questions some reports about superconductivity in these compounds. Contrary, EPI in superconducting MgB2 films manifests also in the PC spectra itself by virtue of an elastic EPI contribution to the excess current determined by the energy dependence of the superconducting order parameter. To analyse the phonon features in the PC spectra of MgB2 a two-band model is exploited and the proximity effect in the k-space is suggested.