Two-gap superconductivity in CaFe0.88Co0.12AsF revealed by temperature dependence of the lower critical field Hcc1 (T)

Abstract

Gap symmetry and structure are crucial issues in understanding the superconducting mechanism of unconventional superconductors. Here, we report an in-depth investigation on the out-of-plane lower critical field Hc1c of fluorine-based 1111 system superconductor CaFe0.88 Co0.12 AsF with Tc = 21 K. A pronounced two-gap feature is revealed by the kink in the temperature-dependent Hc1c (T) curve. The magnitudes of the two gaps are determined to be Δ1 = 0.86 meV and Δ2 = 4.48 meV, which account for 74% and 26% of the total superfluid density, respectively. Our results suggest that the local antiferromagnetic exchange pairing picture is favored compared to the Fermi surface nesting scenario.