Giant Anharmonicity and Nonlinear Electron-Phonon Coupling in MgB2 : A Combined First-Principles Calculation and Neutron Scattering Study

First-principles calculations of the electronic band structure and lattice dynamics for the new superconductor MgB2 are carried out and found to be in excellent agreement with our inelastic neutron scattering measurements. The numerical results reveal that the E2g in-plane boron phonons near the zone center are very anharmonic and strongly coupled to the planar B σ bands near the Fermi level. This giant anharmonicity and nonlinear electron-phonon coupling is key to quantitatively explaining the observed high Tc and boron isotope effect in MgB2