Nematic response revealed by coherent phonon oscillations in BaFe$_2$As$_2$

TL;DR

This study uses optical pump-probe spectroscopy to reveal nematic electronic order in BaFe$_2$As$_2$ through phase analysis of coherent phonon oscillations, highlighting a strong coupling between nematicity and lattice vibrations.

Contribution

It demonstrates that the phase of coherent phonon oscillations can serve as a sensitive probe for nematic order in iron-based superconductors.

Findings

Phase of coherent phonons varies systematically with temperature.

Nematic response persists even in the tetragonal phase.

Coherent phonon phase analysis reveals strong nematic-lattice coupling.

Abstract

We investigate coherent phonon oscillations of BaFe$_2$As$_2$ using optical pump-probe spectroscopy. Time-resolved optical reflectivity shows periodic modulations due to $A_{1g}$ coherent phonon of $c$-axis arsenic vibrations. Optical probe beams polarized along the orthorhombic $a$- and $b$-axes reveal that the initial phase of coherent oscillations shows a systematic deviation as a function of temperature, although these oscillations arise from the same $c$-axis arsenic vibrations. The oscillation-phase remains anisotropic even in the tetragonal structure, reflecting a nematic response of BaFe$_2$As$_2$. Our study suggests that investigation on the phase of coherent phonon oscillations in optical reflectivity can offer unique evidence of a nematic order strongly coupled to a lattice instability.