Nonadiabatic polaronic superconductivity in MgB2 and cuprates
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.