Common features in high Tc cuprates and diborides

The normal state of magnesium diboride, the simplest metallic system showing the macroscopic quantum coherence at high temperature (high Tc superconductivity––HTcS) shows some features that are common with the cuprates, shedding light on the pairing mechanism for Tc amplification in HTcS: (1) two or more electronic components associated with multiple Fermi surface portions; (2) crossing of electronic topological transitions (ETT) where the Fermi surface changes its topology and (3) phonon softening in extended Kohn anomalies (EKA) over a large wave-vector range.