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 λ is very small for the NM calculation, the softening of phonon in the SAF phase may lead to a large increase in λ. 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 Tc in the LiFeAs compound.