Spin-density-wave-induced anomalies in the optical conductivity of AFe2As2, (A=Ca, Sr, Ba) single-crystalline iron pnictides

Abstract

We report the complex dielectric function of high-quality $A{\text{Fe}}_2{\text{As}}_2$, ($A$=Ca, Sr, Ba) single crystals with $T_{\mathrm{N}}\approx 150\text{K}$, $200\text{K}$, and $138\text{K}$, respectively, determined by broadband spectroscopic ellipsometry at temperatures $10\le T\le 300\text{K}$ and wavenumbers from $100{\text{cm}}^{-1}$ to $52000{\text{cm}}^{-1}$. In ${\text{CaFe}}_2{\text{As}}_2$ we identify the optical spin-density--wave gap $2{\Delta }_{\mathrm{SDW}}\approx 1250{\text{cm}}^{-1}$. The $2{\Delta }_{\mathrm{SDW}}/(k_{\mathrm{B}}{T}_{\mathrm{N}})$ ratio, characterizing the strength of the electron-electron coupling in the spin-density--wave state, amounts to $\approx 12$ in ${\text{CaFe}}_2{\text{As}}_2$, significantly larger than the corresponding values for the ${\text{SrFe}}_2{\text{As}}_2$ and ${\text{BaFe}}_2{\text{As}}_2$ 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 ${\text{SrFe}}_2{\text{As}}_2$ and ${\text{CaFe}}_2{\text{As}}_2$ and investigate their detailed temperature dependence in all three materials. This analysis reveals the existence of an anomaly in ${\text{CaFe}}_2{\text{As}}_2$ at a temperature $T^{\ast }\approx 80\text{K}$, 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 ${\text{CaFe}}_2{\text{As}}_2$, the phonons exhibit signatures of SDW fluctuations above $T_N$ and some evidence for anomalies at $T^{\ast }$. (full abstract in PDF)

Revised version published in PRB