Measurement of the c-axis optical reflectance of AFe$_2$As$_2$ (A=Ba, Sr) single crystals: Evidence of different mechanisms for the formation of two energy gaps

TL;DR

This study measures the c-axis optical reflectance of BaFe$_2$As$_2$ and SrFe$_2$As$_2$ single crystals, revealing different mechanisms for the formation of two energy gaps compared to the ab-plane response.

Contribution

It provides new insights into the anisotropic optical properties and proposes a novel band structure evolution model across the SDW transition.

Findings

Only the smaller energy gap is visible along the c-axis.

The high-energy gap structure seen in the ab-plane is nearly invisible along the c-axis.

Different mechanisms are suggested for the formation of the two energy gaps.

Abstract

We present the c-axis optical reflectance measurement on single crystals of BaFe$_2$As$_2$ and SrFe$_2$As$_2$, the parent compounds of FeAs based superconductors. Different from the ab-plane optical response where two distinct energy gaps were observed in the SDW state, only the smaller energy gap could be seen clearly for \textbf{E}$\parallel$c-axis. The very pronounced energy gap structure seen at a higher energy scale for \textbf{E}$\parallel$ab-plane is almost invisible. We propose a novel picture for the band structure evolution across the SDW transition and suggest different driving mechanisms for the formation of the two energy gaps.