Nonadiabatic sudden increase of the cooperative kinetic effect at lattice energy stabilization -: Microscopic mechanism of superconducting state transition:: Model study of MgB2

The theory of the nonadiabatic electron-vibration interactions has been applied to the study of MgB2 superconducting state transition. It has been shown that at nonadiabatic conditions in which the Born-Oppenheimer approximation is not valid and electronic motion is dependent not only on the nuclear coordinates but also on the nuclear momenta, the fermionic ground-state energy of the studied system can be stabilized by nonadiabatic electron-phonon interactions at broken translation symmetry. Moreover, the new arising state is geometrically degenerate; i.e., there are an infinite number of different nuclear configurations with the same fermionic ground-state energy. The model study of MgB2 yields results that are in a good agreement with the experimental data. For distorted lattice, with 0.016 Angstrom/atom of in-plane out-of-phase B-B atoms displacements out of the equilibrium (E-2g phonon mode) when the nonadiabatic interactions are most effective, it has been calculated that the new arising state is 87 meV/unit cell more stable than the equilibrium-high symmetry clumped nuclear structure at the level of the Born-Oppenheimer approximation. The calculated T-c is 39.5 K. The resulting density of states exhibits two-peak character, in full agreement with the tunneling spectra. The peaks are at +/-4 meV, corresponding to the change of the pi band density of states, and at +/-7.6 meV, corresponding to the sigma band. The superconducting state transition can be characterized as a nonadiabatic sudden increase of the cooperative kinetic effect at lattice energy stabilization (NASICKELES). (C) 2004 Wiley Periodicals, Inc.