Advances in point-contact spectroscopy: two-band superconductor MgB2 (A review)

Analysis of the point-contact spectroscopy (PCS) data on the new dramatic high-T${}_c$ superconductor MgB${}_2$ reveals quite different behavior of two disconnected $\sigma$ and $\pi$ electronic bands, deriving from their anisotropy, different dimensionality, and electron-phonon interaction. PCS allows direct registration of both the superconducting gaps and electron-phonon-interaction spectral function of the two-dimensional $\sigma$ and three-dimensional $\pi$ band, establishing correlation between the gap value and intensity of the high-T${}_c$ driving force – the $E_{2g}$ boron vibration mode. PCS data on some nonsuperconducting transition-metal diborides are surveyed for comparison.