SYMMETRY BREAKING, NON-ADIABATIC ELECTRON-PHONON COUPLING AND NUCLEAR KINETIC EFFECT ON SUPERCONDUCTIVITY OF MgB_2

摘要

Theory of non-adiabatic electron-vibration interactions has been applied to the study of MgB_2 superconductivity. It has been shown that at the non-adiabatic conditions when 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 some systems can be stabilised by electron-phonon interactions at broken translation symmetry and an energy gap in one-particle metalic spectrum can be opened. More over, the new arising state is geometricaly degenerate - i.e., there is an infinite number of different nuclear configurations with the same fermionic ground state enegy. The superconducting state transition can be characterised as a Non-Adiabatic Sudden Increase of the Cooperative Kinetic Effect at Lattice Energy Stabilization (NASICKELES). Comparing to the experiments, the model study of MgB_2 yields very good results. Calculated T_c is 39.5 K, and density of states exhibits two-gap character in full agreement with the tunneling spectra. The peaks are at +/-4 meV that is connected to π band and at +/-7.6 meV for σ band.