Spin-phonon coupling and effect of pressure in the superconductor LiFeAs: Lattice dynamics from first-principles calculations

The lattice dynamics and the effect of pressure on superconducting LiFeAs in both nonmagnetic (NM) and striped antiferromagnetic (SAF) phases are investigated using the plane-wave pseudopotential, density-functional-based method. While the obtained electron-phonon coupling $\lambda$ is very small for the NM calculation, the softening of phonon in the SAF phase may lead to a large increase in $\lambda$. In the SAF phase, strong anisotropy of the phonon softening in the Fe plane is found to arise from different spin orders in the $x$ and $y$ directions, indicating that the phonon softening is of spin-phonon coupling origin. For the SAF structure, the calculated variation trend of the electronic density of states and the phonon frequencies under pressure can explain a large negative pressure coefficient of $T_c$ in the LiFeAs compound.

Figure 1

The calculated phonon-dispersion curves for LiFeAs.

Figure 2

(Color) Energy curves as a function of atom displacement. (a) For $\text{As}{A}_g$ mode and (b) for $\text{Fe}{E}_g$ mode. The calculated Fe-magnetic moment vs As displacement is shown in the inset of (a), negative As displacement corresponds to As atoms moving toward the Fe plane. Phonon displacement patterns for the in-plane $\text{Fe}{E}_g$ mode are shown in the inset of (b).