Optical study of Dirac fermions and related phonon anomalies in the antiferromagnetic compound CaFeAsF

摘要

We performed optical studies on CaFeAsF single crystals, a parent compound of the 1111 -type iron-based superconductors that undergoes a structural phase transition from tetragonal to orthorhombic at T_s= 121 K and a magnetic one to a spin density wave (SDW| state at T_N = 110 K. In the low-temperature optical conductivity spectrum, after the subtraction of a narrow Drude peak, we observe a pronounced singularity around 300 cm~(-1) that separates two regions of quasilinear conductivity. We outline that these characteristic absorption features are signatures of Dirac fermions. similar to what was previously reported for the BaFe_2As_2 system [Z.-G. Chen et al., Phys. Rev. Lett. 119, 096401 (2017)]. In support of this interpretation, we show that for the latter system this singular feature disappears rapidly upon electron and hole doping, as expected if it arises from a van Hove singularity in between two Dirac cones. Finally, we show that one of the infrared-active phonon modes (the Fe-As mode at 250 cm~(-1)) develops a strongly asymmetric line shape in the SDW state and note that this behavior can be explained in terms of a strong coupling with the Dirac fermions.