Ultralow-dissipative conductivity by Dirac fermions in BaFe$_2$As$_2$

TL;DR

This study reveals ultralow-dissipative conductivity in BaFe$_2$As$_2$ caused by Dirac fermions, observed through terahertz spectroscopy, highlighting the unique transport properties of Dirac cone quasiparticles in the SDW state.

Contribution

The paper provides direct spectroscopic evidence of Dirac fermions contributing to ultralow-dissipative conductivity in BaFe$_2$As$_2$, a novel observation in iron-based superconductors.

Findings

Conductivity spectra follow the Drude model above the SDW transition.

In the SDW state, $\sigma_2$ is suppressed while $\sigma_1$ remains nearly Drude-like.

Extra conductivity from Dirac cone quasiparticles exhibits ultralow dissipation.

Abstract

We report on the anomalous behavior of the complex conductivity of BaFe$_{2}$As$_{2}$, which is related to the Dirac cone, in the terahertz (THz)-frequency region. Above the spin-density-wave (SDW) transition temperature, the conductivity spectra follow the Drude model. In the SDW state, the imaginary part of the complex conductivity, $\sigma_2$, is suppressed in comparison to that expected according to the Drude model. The real part, $\sigma_1$, exhibits nearly Drude-like behavior. This behavior (i.e., almost no changes in $\sigma_1$ and the depression of $\sigma_2$) can be regarded as the addition of extra conductivity without any dissipations in the Drude-type conductivity. The origin of this ultralow-dissipative conductivity is found to be due to conductivity contribution from quasiparticles within the Dirac cone. In other words, we are able to observe the dynamics of Dirac fermions through the conductivity spectra of BaFe$_2$As$_2$, clearly and directly.