Factors affecting the electron-phonon coupling in FeSe under pressure.

The hydrostatic pressure on superconductor FeSe increases Wyckoff position zSe and decreases lattice constants. However, previously only the increasing zSe 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 the zSe 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 zSe 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.