19F NMR Study of the Bilayer Iron-Based Superconductor KCa2Fe4As4F2 *

Abstract

We report a 19F nuclear magnetic resonance (NMR) study on single-crystal KCa2Fe4As4F2 (T c ∼ 33.3 K). The 19F NMR spectral shape of KCa2Fe4As4F2 is weakly dependent on temperature and the Knight shift is small, which implies weak coupling between the CaF layer and the FeAs layer. The temperature dependence of 1/19 T 1 shows a hump below T c, however the 1/75 T 1 decreases just below T c, which implies that there are strong in-plane magnetic fluctuations in the CaF layers than in the FeAs layers. This may be caused by the motion of vortices. The absence of the coherence peak suggests unconventional superconductivity in KCa2Fe4As4F2.