Gap Structure of 12442-Type KCa2(Fe1−xCox)4As4F2 (x = 0, 0.1) Revealed by Temperature Dependence of Lower Critical Field.

We report an in-depth investigation on the out-of-plane lower critical field Hc1 of the KCa2(Fe1−x Cox)4As4F2 (12442-type, x = 0, 0.1). The multi-gap feature is revealed by the kink in the temperature-dependent Hc1(T) curve for the two samples with different doping levels. Based on a simplified two-gap model, the magnitudes of the two gaps are determined to be Δ1 = 1.2 meV and Δ2 = 5.0 meV for the sample with x = 0, Δ1 = 0.86 meV and Δ2 = 2.8 meV for that with x = 0.1. With the cobalt doping, the ratio of energy gap to critical transition temperature (Δ/kBTc) remains almost unchanged for the smaller gap and is suppressed by 20% for the larger gap. For the undoped KCa2Fe4As4F2, the obtained gap sizes are generally consistent with the results of angle-resolved photoemission spectroscopy experiments. [ABSTRACT FROM AUTHOR]