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

Abstract

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 Å/atom of in-plane out-of-phase BB atoms displacements out of the equilibrium (E2g 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 Tc 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 π band density of states, and at ±7.6 meV, corresponding to the σ band. The superconducting state transition can be characterized as a nonadiabatic sudden increase of the cooperative kinetic effect at lattice energy stabilization (NASICKELES). © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005