Electron-phonon Coupling Calculation Of Bulk FeSe In Heavily Electron-doped And High Pressure

The first-principles calculation of density functional theory?DFT?and the Wannier interpolation technique were used to study the electron-phonon coupling?EPC?of the simplest structure in iron-based superconductors----FeSe,under heavily electron doped and high pressure,respectively.Firstly,we investigate the EPC in FeSe under three electron doping levels,i.e.,0.0 e/cell,0.4e/cell and 0.8 e/cell.Our calculation results show that FeSe doped with 0.4 e/cell have the largest EPC constant?.It is found that the EPC constant?can be improved by 31%at a doping level of 0.4e/cell,with respect to the pristine case.Through analysis,we think that the shifting of Fermi surface nesting function and the aggrandizement of EPC matrix around M point account for the strengthened?at 0.4 e/cell doping.Nevertheless,the results calculated by McMillan-Alan-Days formula show that the superconducting transition temperature₍?8? of FeSe doped with 0.4 e/cell is only 0.01 mK.Therefore,our findings clarify that electron doping alone can not rise the₍?8? to the experimental value.Subsequently,we studied the EPC superconductivity of bulk FeSe under different pressures.Because iron-based superconductors are layered compounds,and the interaction of layer and layer is Van der Waals force.In order to describe the Van der Waals interactions of iron-based superconductors under different pressure conditions,our calculations adopted the Van der Waals amendment with semiempirical method.We study the EPC of bulk FeSe under different pressures,based on the first-principles calculation and Wannier interpolation.Our calculation results show that FeSe at 25 GPa have the largest EPC constant?.It is found that the EPC constant?can be improved by 50%at 25 GPa,with respect to the pristine case.Through analysis,we think the aggrandizement of EPC matrix around M and the phonons softening along the?-M direction,and the aggrandizement of EPC matrix around A point account for the strengthened?at 25 GPa.Nevertheless,the results calculated by McMillan-Alan-Days formula show that the superconducting transition temperature₍?8? of FeSe at 25 GPa is only 0.027 mK.Therefore,our findings clarify that under pressure alone can not rise the₍?8? to the experimental value.