Superconductivity-Induced Transverse Plasma Mode And Phonon Anomaly In The C-Axis Response Of The Bilayer Compound Rbca2fe4as4f2

We studied the infrared response of polycrystalline samples of the iron arsenide superconductor (Rb, Cs)Ca2Fe4As4F2 (Rb,Cs-12442), which has a bilayer structure similar to the high-T-c cuprates YBa2Cu3O7 (YBCO) and Bi2Sr2CaCu2O8. The c-axis reflectivity spectra R-c have been derived from the reflectivity spectra of the polycrystalline samples R-poly and the in-plane spectrum of a corresponding Cs-12442 crystal R-ab using a geometrical averaging approach with R-c = 3R(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 similar or equal to T-c of sigma(bl)(1) (omega -> 0) similar or equal to 1000 Omega(-1)cm(-1) and sigma(int)(1) (omega -> 0) similar or equal to 15 Omega(-1)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 similar or equal to 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.