The Property of Phonon Gap in Iron-Based Superconductors FeSe, LiFeAs and SrFeAsF

Abstract

First-principles calculations are performed for iron-based superconductors FeSe, LiFeAs and SrFeAsF, and the reasons of generation and loss of the phonon gaps in them are analyzed by analogy with electronic energy band theory. In iron-based superconductors FeSe and LiFeAs, lattice vibrations are affected by so strong changes in the periodic potential that the phonon spectra open a full gap. After considering electron–spin interactions, the enhancement of the periodic potential results in a broadening of the gap. From binary FeSe to ternary LiFeAs to quaternary SrFeAsF, the full phonon gap undergoes the process from decrease to disappearance, which is closely related to the frequencies overlapping of different atoms by vibrations. The fewer the material components and the larger the atomic number ratio, the more possible the production of the full phonon gap or the appearance of wider gap. In addition, the phonon gaps of FeSe and LiFeAs locate around the frequency of \(10^{12}\) Hz, whose filtering properties for elastic waves are hopefully to be applied in quantum acoustics.