First-principles study on phonon properties of iron-based fluoride superconductors SrFe1-xCoxAsF (x=0, 0.125)

Abstract

Using plane-wave pseudopotential method based on first-principles, we calculate the phonon spectra (including phonon dispersion curves and phonon density of states) and electron-phonon coupling constants of SrFe1-xCoxAsF (x=0, 0.125) in tetragonal nonmagnetic (NM) and orthorhombic striped antiferromagnetic (SAF) states. Results show that under striped antiferromagetic interaction the spin-phonon coupling is stronger than the electron-phonon coupling, leading to the decrease of phonon spectra width; and the increased effective phonon quality due to spin effects makes the frequencies of coupling vibration between Fe and As atoms reduced. In addition, doping and spin effects are two effective methods to enhance the electron-phonon coupling, however, the calculated superconducting transition temperature is far lower than the experimental measurement, which rules out the simple electron-coupling superconducting pairing mechanism in SrFe1-xCoxAsF.