We studied the infrared response of polycrystalline samples of the iron arsenide superconductor $(\mathrm{Rb},\mathrm{Cs}){\mathrm{Ca}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2$ (Rb,Cs-12442), which has a bilayer structure similar to the high-$T_c$ cuprates ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ (YBCO) and ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$. The $c$-axis reflectivity spectra $R_c$ have been derived from the reflectivity spectra of the polycrystalline samples $R_{\mathrm{poly}}$ and the in-plane spectrum of a corresponding Cs-12442 crystal $R_{ab}$ using a geometrical averaging approach with $R_c=3R_{\mathrm{poly}}-2R_{ab}$. In analogy to the $c$-axis response of the cuprates, we observe a superconductivity-induced transverse plasma mode and a phonon anomaly that are both signatures of local electric field effects that arise from a large difference between the local conductivities in the intra- and interbilayer regions. Using a multilayer model developed for the cuprates, we obtain a good description of the $c$-axis response and derive the local conductivities at $T\simeq T_c$ of ${\sigma }_1^{\mathrm{bl}}(\omega \to 0)\simeq 1000{\mathrm{\Omega }}^{-1}{\mathrm{cm}}^{-1}$ and ${\sigma }_1^{\mathrm{int}}(\omega \to 0)\simeq 15{\mathrm{\Omega }}^{-1}{\mathrm{cm}}^{-1}$, respectively, that are similar to the ones previously found in underdoped YBCO. Different from the cuprates, we find no evidence of a normal-state pseudogap in terms of a partial suppression of the low-energy electronic states that sets in already well above $T_c$. There is also no clear sign of an onset of precursor superconducting pairing correlations well above $T_c\simeq$ 30 K. This highlights that the pseudogap and the precursor superconducting pairing well above $T_c$ are unique features of the cuprates with their strong electronic correlations and, for example, not just the result of a strongly anisotropic electronic response due to the layered crystal structure.

• Received 18 April 2020
• Accepted 28 May 2020

DOI:https://doi.org/10.1103/PhysRevB.101.214512