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

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.

arXiv comment4 pages, 4figures

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Cited by in corpus (10)

• Universality of Energy Equipartition in One-dimensional Lattices
• Optical Signature of a Crossover from Mott- to Slater-type Gap in Sr${}_2$Ir${}_{1-x}$Rh${}_x$O${}_4$
• Scaling of the Fano effect of the in-plane Fe-As phonon and the superconducting critical temperature in Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$
• Two-Gap Superconductivity in CaFe_{0.88}Co_{0.12}AsF Revealed by Temperature Dependence of the Lower Critical Field H_{c1}^c(T)
• Large CaFeAsF single crystals of high quality grown by the oscillating temperature technique
• Linear scaling relation between two-dimensional massless Dirac fermion Fermi velocity and Fe-As bond length in iron arsenide superconductor systems
• Multiple gaps revealed by low temperature specific heat in the 1111-type CaFe_{0.88}Co_{0.12}AsF single crystals
• In-plane electronic anisotropy revealed by interlayer resistivity measurements on the iron-based superconductor parent compound CaFeAsF
• Observation of the Dirac Dispersions in Co-doped CaFe2As2
• Superconductivity above 30 K due to the introduction of oxygen in CaFeAsF