Ultralow-Dissipative Conductivity by Dirac Fermions in BaFe2As2
We report on the anomalous behavior of the complex conductivity of BaFe2As2, which is considered to be 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 formula. In the SDW state, the imaginary part of the conductivity, σ2, is suppressed in comparison with that expected according to the Drude model, whereas the real part, σ1, exhibits nearly Drude-like behavior. This behavior (i.e., almost no changes in σ1 and the depression of σ2) can be regarded as reflecting the addition of extra conductivity without any dissipations in the Drude-type conductivity. We argue that the origin of this ultralow-dissipative conductivity is found to be the contribution from quasiparticles within the Dirac cone.