FeSe is an important prototype material to study the superconducting related properties, and the electron-phonon coupling (EPC) plays an important role in FeSe. Pressure changes the structure of the FeSe, which results in the changes of the EPC.
In recent research, a team led by professor ZENG Zhi, from Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences studied the influence of the structural change factors, namely the zSe (Wyckoff position of Se) and lattice constants, individually, on the electron-phonon coupling, electronic structure, and the Raman frequencies under pressure. They predicted that the in-plane biaxial strain on FeSe increased the electron-phonon coupling due to the increasing zSe and the decreasing in-plane lattice constants.
The result was published in Physical Chemistry Chemical Physics.
Previous studies mainly focused on the increase of the EPC related to the A1g mode and zSe only, but pressure also changed lattice constants. Studying the effect of the two individual factors on the electronic structure and the phonon frequencies under pressure would provide a clear understanding of the EPC in the FeSe.
In this research, scientists took turns to keep one factor fixed to the ground state and change another by hand to simulate the increased pressure effect. Under the considered conditions, the EPC constantly increased.
By discussing the electron and phonon properties under the consider conditions, it could be seen that the increase of the zSe enhances the EPC by increasing the states around the Fermi level, whereas the decrease of the lattice constants enhanced the EPC by increasing the Se-Se (A1g) and Fe-Fe (B1g) phonon frequencies.
Besides, the decrease of the lattice constants and the increase of the zSe could also be realized by applying an in-plane biaxial strain. With the increase of biaxial strain, the states at the Fermi level and the Raman frequencies increase. As the result, the in-plane biaxial compressive strain is predicted to increase the EPC constant.
This work explicitly explored two factors individually for the understanding of the influence of each factor on the electronic and phonon properties, proved the in-plane biaxial strain is a good way to increase the EPC, and provide strategy for experimentalist to increase the EPC.
The research was supported by the financial support from the National Natural Science Foundation of China and NSF DMREF. The calculations were performed in the Computational Science of HFIPS, the ScGrid of Supercomputing Center, Computer Network Information Center of Chinese Academy of Science and AM-HPC. Numerical computations were performed at the Hefei advanced computing center.
Fig. 1 The DFT results of the calculated EPC constants, the band structures and Raman frequencies. (Image by LI Tingting)
Fig. 2 The Fermi surface and Raman frequencies of FeSe under the in-plane biaxial strain compared with that under the ground state. (Image by LI Tingting)