Factors affecting the electron-phonon coupling in FeSe under pressure

Hydrostatic pressure on superconductor FeSe increases the Wyckoff position, z_Se, and decreases lattice constants. However, previously only the increasing of z_Se was emphasized to be important in determining the electron-phonon coupling. We explicitly study each of the two factors individually to understand its influence on the electron-phonon coupling, band structure and Raman frequencies. We find that the increasing of z_Se enhances the states around the Fermi level more, while the decreasing of the lattice constants enhances the phonon frequencies more, which together increase the electron-phonon coupling under pressure. Based on the above facts, we predict and prove that the in-plane biaxial strain on FeSe increases the electron-phonon coupling due to the increasing z_Se value and the decreasing in-plane lattice constant. Our results uncover the factors affecting the increase of the electron-phonon coupling and provide information on how to enhance the electron-phonon coupling in FeSe.