Origin of the spin density wave instability in AFe$_2$As$_2$ (A=Ba, Sr) as revealed by optical spectroscopy

TL;DR

This study uses optical spectroscopy to investigate the SDW transition in BaFe$_2$As$_2$ and SrFe$_2$As$_2$, revealing partial energy gaps and supporting an itinerant electron nesting mechanism.

Contribution

It provides experimental evidence that the SDW instability in AFe$_2$As$_2$ compounds is driven by Fermi surface nesting rather than local exchange interactions.

Findings

Observation of partial energy gaps with two energy scales.

Significant reduction in Drude spectral weight and scattering rate.

SDW instability likely caused by Fermi surface nesting.

Abstract

We performed optical spectroscopy measurement on single crystals of BaFe$_2$As$_2$ and SrFe$_2$As$_2$, the parent compounds of FeAs based superconductors. Both are found to be quite metallic with fairly large plasma frequencies at high temperature. Upon entering the spin-density-wave (SDW) state, formation of partial energy gaps was clearly observed with the presence of surprisingly two different energy scales. A large part of the Drude component was removed by the gapping of Fermi surfaces (FS). Meanwhile, the carrier scattering rate was even more dramatically reduced. We elaborate that the SDW instability is more likely to be driven by the FS nesting of itinerant electrons rather than a local-exchange mechanism.