Spin-density-wave-induced anomalies in the optical conductivity ofAFe2As2, (

Abstract

We report the complex dielectric function of high-quality AFe2As2, (A=Ca, Sr, Ba) single crystals with TN ≈ 150 K, 200 K, and 138 K, respectively, determined by broadband spectroscopic ellipsometry at temperatures 10 ≤ T ≤ 300 K and wavenumbers from 100 cm −1 to 52000 cm −1 . In CaFe2As2 we identify the optical spin-density-wave gap 2∆SDW ≈ 1250 cm −1 . The 2∆SDW/(kBTN) ratio, characterizing the strength of the electron-electron coupling in the spin-density-wave state, amounts to ≈ 12 in CaFe2As2, significantly larger than the corresponding values for the SrFe2As2 and BaFe2As2 compounds: 8.7 and 5.3, respectively. We further show that, similarly to the Ba-based compound, two characteristic SDW energy gaps can be identified in the infrared-conductivity spectra of both SrFe2As2 and CaFe2As2 and investigate their detailed temperature dependence in all three materials. This analysis reveals the existence of an anomaly in CaFe2As2 at a temperature T * ≈ 80 K, well below the Néel temperature of this compound, which implies weak coupling between the two SDW subsystems. The coupling between the two subsystems evolves to intermediate in the Sr-based and strong in the Ba-based material. The temperature dependence of the infrared phonons reveals clear anomalies at the corresponding Néel temperatures of the investigated compounds. In CaFe2As2, the phonons exhibit signatures of SDW fluctuations above TN and some evidence for anomalies at T * . Investigation of all three materials in the visible spectral range reveals a spin-density-wave-induced suppression of two absorption bands systematically enhanced with decreasing atomic number of the intercalant. A dispersion analysis of the data in the entire spectral range clearly shows that CaFe2As2 is significantly more metallic than the other two compounds. Our results single out CaFe2As2 in the class of ThCr2Si2-type iron-based materials by demonstrating the existence of two weakly coupled and extremely metallic electronic subsystems.

Spectrally resolved femtosecond reflectivity relaxation dynamics in undoped spin-density wave 122-structure iron-based pnictides

“…The bottleneck model therefore consistently describes the data including the divergent-like increase 14 of τ B at the magnetostructural transition and the obtained gap parameters are consistent with results obtained by means of the steady state optical spectroscopy 30,36,37 .…”

“…8, 9, 10). The relative gap magnitudes are consistent (see Table I) with previously reported values 13,23 and the highest of the two gaps observed in the optical conductivity 30,36 , with a larger discrepancy for Eu-122 37 .…”

“…The spectral dependence of the most intensive component SW is very similar to the static reflectivity change when crossing the magneto-structural transition 30 in Ba-122. In Sr-122 however, the similarity is only in the peak position at ∼ 1.7 eV with a large difference between the positions of the zero crossings.…”

“…(3) and the indirect coupling through the order parameter modulation of ǫ(k) and matrix elements, as suggested by the similarity of the spectral dispersions between the transient photoinduced and the static phase-transition induced 30 reflectivity changes.…”

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

“…Although optical investigations into the parent compounds of IBSs have been conducted by many groups [25][26][27][28][29][30][31][32][33], the optical signatures of the Dirac fermions have so far been only discussed for the 122 compound [13]. The evolution of these Dirac fermions with electron and hole doping has also not yet been reported.…”