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Spin-density-wave-induced anomalies in the optical conductivity of A Fe 2 As 2 , (<mml:math xmlns:mml="http://www.w3.org/1998/Math/…

2013; American Physical Society; Volume: 88; Issue: 18 Linguagem: Inglês

10.1103/physrevb.88.184511

1550-235X

Aliaksei Charnukha, D. Pröpper, T. I. Larkin, Dunlu Sun, Zhiwei Li, C. T. Lin, Thomas Wolf, B. Keimer, A. V. Boris,

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We report the complex dielectric function of high-quality $A{\text{Fe}}_{2}{\text{As}}_{2}$, ($A=\text{Ca}$, Sr, Ba) single crystals with ${T}_{\mathrm{N}}\ensuremath{\approx}150\phantom{\rule{4pt}{0ex}}\text{K}$, $200\phantom{\rule{4pt}{0ex}}\text{K}$, and $138\phantom{\rule{4pt}{0ex}}\text{K}$, respectively, determined by broadband spectroscopic ellipsometry at temperatures $10\ensuremath{\le}T\ensuremath{\le}300\phantom{\rule{4pt}{0ex}}\text{K}$ and wave numbers from $100\phantom{\rule{4pt}{0ex}}{\text{cm}}^{\ensuremath{-}1}$ to $52000\phantom{\rule{4pt}{0ex}}{\text{cm}}^{\ensuremath{-}1}$. In ${\text{CaFe}}_{2}{\text{As}}_{2}$ we identify the optical spin-density-wave gap $2{\ensuremath{\Delta}}_{\mathrm{SDW}}\ensuremath{\approx}1250\phantom{\rule{4pt}{0ex}}{\text{cm}}^{\ensuremath{-}1}$. The $2{\ensuremath{\Delta}}_{\mathrm{SDW}}/({k}_{\mathrm{B}}{T}_{\mathrm{N}})$ ratio, characterizing the strength of the electron-electron coupling in the spin-density-wave state, amounts to $\ensuremath{\approx}12$ 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 ${T}^{*}\ensuremath{\approx}80\phantom{\rule{4pt}{0ex}}\text{K}$, well below the N\'eel 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 N\'eel 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 ${T}^{*}$. 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 ${\text{CaFe}}_{2}{\text{As}}_{2}$ is significantly more metallic than the other two compounds. Our results single out ${\text{CaFe}}_{2}{\text{As}}_{2}$ in the class of ${\text{ThCr}}_{2}{\text{Si}}_{2}$-type iron-based materials by demonstrating the existence of two weakly coupled and extremely metallic electronic subsystems.