Study of the electron-phonon interaction in metal diborides MeB_2 (Me=Zr, Nb, Ta, Mg) by point-contact spectroscopy

TL;DR

This paper reviews electron-phonon interactions in metal diborides using point-contact spectroscopy, highlighting low coupling in transition metals and detailed phonon features in superconducting MgB₂, with implications for superconductivity.

Contribution

It provides the first detailed analysis of EPI in MeB₂ compounds, especially in MgB₂, using point-contact spectroscopy and a two-band model to interpret phonon features.

Findings

Transition metal diborides have low EPI parameter λ<0.1.

MgB₂ exhibits significant EPI effects in PC spectra due to superconductivity.

Phonon features in MgB₂ are explained by a two-band model and k-space proximity effect.

Abstract

We review investigations of the electron-phonon interaction (EPI) in metal diborides MeB_2 (Me=Zr, Nb, Ta, Mg) by point-contact (PC) spectroscopy. For transition metal compounds the PC EPI functions were recovered and EPI parameter $\lambda\lesssim 0.1$ 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 MgB_2 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 MgB$_2$ a two-band model is exploited and the proximity effect in the {\bf k}-space is suggested.