Superconductivity-induced transverse plasma mode and phonon anomaly in the c-axis response of the bilayer compound RbCa2Fe4As4F2

Abstract

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-Tc cuprates YBa2Cu3O7 (YBCO) and Bi2Sr2CaCu2O8. The c-axis reflectivity spectra Rc have been derived from the reflectivity spectra of the polycrystalline samples Rpoly and the in-plane spectrum of a corresponding Cs-12442 crystal Rab using a geometrical averaging approach with Rc=3Rpoly−2Rab. 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≃Tc of σ1bl(ω→0)≃1000Ω−1cm−1 and σ1int(ω→0)≃15Ω−1cm−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 Tc. There is also no clear sign of an onset of precursor superconducting pairing correlations well above Tc≃ 30 K. This highlights that the pseudogap and the precursor superconducting pairing well above Tc 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.

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