Coupling of the A_{1g} As-phonon to magnetism in iron pnictides

TL;DR

This paper investigates how the A_{1g} As-phonon resonance in iron pnictides couples to magnetism, explaining experimental Raman spectroscopy observations through a detailed electron-phonon interaction model involving magnetic anisotropy.

Contribution

It introduces a five orbital tight-binding model that identifies two distinct electron-phonon coupling channels affecting phonon behavior in magnetically ordered iron pnictides.

Findings

Different Raman polarization responses are explained by two coupling mechanisms.

The model reproduces the qualitative behavior of phonon intensities observed experimentally.

Two distinct electron-phonon interaction channels are identified and characterized.

Abstract

Charge, spin and lattice degrees of freedom are strongly entangled in iron superconductors. A neat consequence of this entanglement is the behavior of the A_{1g} As-phonon resonance in the different polarization symmetries of Raman spectroscopy when undergoing the magneto-structural transition. In this work we show that the observed behavior could be a direct consequence of the coupling of the phonons with the electronic excitations in the anisotropic magnetic state. We discuss this scenario within a five orbital tight-binding model coupled to phonons via the dependence of the Slater-Koster parameters on the As position. We identify two qualitatively different channels of the electron-phonon interaction: a geometrical one related to the Fe-As-Fe angle and another one associated with the modification upon As displacement of the Fe-As energy integrals pdsigma and pdpi. While both mechanisms result in a finite B_{1g} response, the behavior of the phonon intensity in the A_{1g} and B_{1g} Raman polarization geometries is qualitatively different when the coupling is driven by the angle or by the energy integral dependence. We discuss our results in view of the experimental reports.