Multiband superconductivity 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) the system is made of two or more electronic components associated with multiple Fermi surface portions; (2) the chemical potential is tuned where the Fermi level in one Fermi surface portion is near the crossing of an electronic topological transition (ETT) where the Fermi surface changes its topology; and (3) an optical phonon branch shows softening due to an extended Khon anomaly (EKA) over a large wave vector range.