Anomalous Phonons in CaFe2As2

TL;DR

This study combines neutron scattering experiments and DFT calculations to explore phonon behaviors in CaFe2As2, revealing discrepancies and temperature-dependent anomalies that suggest strong electron-phonon interactions relevant to superconductivity.

Contribution

It provides a comprehensive experimental and theoretical analysis of phonons in CaFe2As2, highlighting discrepancies with calculations and identifying anomalous phonon behaviors near the structural transition.

Findings

Certain phonon modes are broader and softer than DFT predictions.

Phonon frequencies show strong temperature dependence near 173K.

Discrepancies suggest significant electron-phonon coupling or anharmonicity.

Abstract

Extensive inelastic neutron scattering measurements of phonons on a single crystal of CaFe2As2 allowed us to establish a fairly complete picture of phonon dispersions in the main symmetry directions. The phonon spectra were also calculated by density functional theory (DFT) in the local density approximation (LDA). There are serious discrepancies between calculations done for the optimized structure and experiment, because the optimised structure is not the ambient pressure structure but is very close to the collapsed structure reached at p = 3.5 kbar. However, if the experimental crystal structure is used the calculation gives correct frequencies of most phonons. The most important new result is that linewidths/frequencies of certain modes are larger/softer than predicted by DFT-LDA. We also observed strong temperature dependence of some phonons near the structural phase transition near 173K. This behavior may indicate anomalously strong electron phonon coupling and/or anharmonicity, which may be important to the mechanism of superconductivity.