Superconductivity-induced transverse plasma mode and phonon anomaly in the c-axis response of the bilayer compound RbCa2Fe4As4F2
Abstract
With infrared spectroscopy we studied the out-of-plane (c-axis) response of the iron arsenide superconductor ACa2Fe4As4F2 (A = Rb,Cs) which has a bilayer structure similar to the high Tc cuprates YBa2Cu3O7 (YBCO) and Bi2Sr2CaCu2O8 (Bi2212). In analogy to the cuprates, we observe a superconductivity-induced transverse plasma mode (tPM) 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 inter-bilayer 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) 1\,000 -1cm-1 and σ1int(ω → 0) 15 -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.