Lattice Dynamics of LiFeAs studied by Inelastic Neutron Scattering and Density Functional Theory calculations

TL;DR

This study combines inelastic neutron scattering and DFT calculations to map the phonon dispersion in LiFeAs, revealing limited electron-phonon coupling and no nematic instability, with temperature-dependent phonon hardening.

Contribution

It provides a comprehensive experimental and theoretical analysis of phonon modes in LiFeAs, clarifying the role of electron-phonon interactions in its superconductivity.

Findings

Good agreement between experiment and DFT results.

Small average electron-phonon coupling constant.

Absence of nematic instability in phonon dispersion.

Abstract

We investigated the lattice dynamics of the unconventional superconductor LiFeAs using inelastic neutron scattering experiments and density-functional theory (DFT) calculations. By comparing the neutron scattering intensities with lattice-dynamics simulations we can identify the polarization symmetry of all modes along the main-symmetry directions yielding a complete experimental picture of the phonon dispersion. Overall there is good agreement between the experimental and DFT results, which renders an overlooked strong electron phonon coupling unlikely. Our DFT calculations reveal only a small averaged electron-phonon coupling constant. The transversal acoustic in-plane branches exhibit a normal dispersion for small propagation vectors indicating the absence of a nematic instability. Several modes exhibit considerable hardening upon cooling that can be attributed to the anisotropic shrinking of the LiFeAs lattice.