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Recently, a new family of iron-based superconductors called 12442 was discovered and the muon spin relaxation (µSR) measurements on KCa2Fe4As4F2 and CsCa2Fe4As4F2 polycrystals, two members of the family, indicated that both have a nodal superconducting gap structure with s + d pairing symmetry. Here we report the ultralow-temperature thermal conductivity measurements on CsCa2Fe4As4F2 single crystals (Tc = 29.3 K). A negligible residual linear term κ0/T in zero field and the field dependence of κ0/T suggest multiple nodeless superconducting gaps in CsCa2Fe4As4F2. This gap structure is similar to CaKFe4As4 and moderately doped Ba1−xKxFe2As2, but contrasts to the nodal gap structure indicated by the µSR measurements on CsCa2Fe4As4F2 polycrystals. arXiv:1811.06379v1 [cond-mat.supr-con]

We studied the infrared response of polycrystalline samples of the iron arsenide superconductor (Rb, Cs)Ca 2 Fe 4 As 4 F 2 (Rb,Cs-12442), which has a bilayer structure similar to the high-T c cuprates YBa 2 Cu 3 O 7 (YBCO) and Bi 2 Sr 2 CaCu 2 O 8 . 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 intraand 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 T c of σ bl 1 (ω → 0) 1000 −1 cm −1 and σ int 1 (ω → 0) 15 −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 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.