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

TL;DR

This study investigates the optical properties of CaFeAsF, revealing signatures of Dirac fermions and phonon anomalies associated with magnetic and structural phase transitions, advancing understanding of electronic excitations in iron-based superconductors.

Contribution

It provides the first optical evidence of Dirac fermions in CaFeAsF and links phonon anomalies to strong coupling with these fermionic states.

Findings

Identification of a singularity at 300 cm$^{-1}$ indicating Dirac fermions.

Observation of asymmetric phonon line shape in the SDW state.

Disappearance of the singularity upon doping, consistent with Dirac fermion behavior.

Abstract

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$_2$As$_2$ system [1]. 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 behaviour can be explained in terms of a strong coupling with the Dirac fermions.