Optical study on the spin-density wave properties in single crystalline Na$_{1-\delta}$FeAs

We report an optical investigation on the in-plane charge dynamics for Na$_{1-\delta}$FeAs single crystal. A clear optical evidence for the spin-density wave (SDW) gap is observed. As the structural/magnetic transitions are separated in the Na$_{1-\delta}$FeAs case, we find the SDW gap opens in accordance with the magnetic transition. Comparing with the optical response of other FeAs-based parent compounds, both the gap value 2$\Delta$ and the energy scale for the gap-induced spectral weight redistribution are smaller in Na$_{1-\delta}$FeAs. Our findings support the itinerant origin of the antiferromagnetic transition in the FeAs-based system.