  The Fermi energy, Ef of newly discovered high-Tc superconductor MgB2 and
cuprates is estimated from the measured London penetration depth using a
parameter-free expression. Ef of MgB2 and more than 30 lanthanum, yttrium and
mercury-based cuprates appears to be about or below 100 meV, depending on
doping. There is every evidence that the remarkable low value of Ef and the
strong coupling of carriers with high-frequency phonons is the cause of high Tc
in all newly discovered superconductors. Taking into account that the carriers
mainly reside on oxygen in cuprates, on boron in magnesium diborade, and on
carbon in doped fullerenes, these superconductors form what is essentially
$nonadiabatic$ 'metallic' oxygen, boron and carbon, respectively. The boron
isotope effect on the carrier mass in magnesium-diboride similar to the oxygen
isotope effect on the supercarrier mass observed by Guo-meng Zhao et al. in
cuprates is predicted.
