Homogeneous coexistence in CaFe1-x Co x AsF and phase segregation in LaFeAsO1-x F x studied via NMR

In iron-based pnictides, one of the interesting topics is homogeneous coexistence or phase segregation at the boundary between antiferromagnetic (AF) and superconducting (SC) phases. We addressed this problem on a microscopic level by means of 75As NMR measurements in LaFeAsO1−x F x (x=0.026) (La1111), and CaFe1−x Co x AsF (x=0.06) (Ca1111) having an intermediate electronic phase diagram between Ba(Fe1−x Co x )2As2 and the La1111 series. NMR spectra for 6 % Co-doped Ca1111 samples were very similar to those for the undoped samples even below T c , suggesting homogeneous coexistence of the AF and SC states. For 2.6 % F-doped La1111 samples, AF and paramagnetic (PM) domains coexist at ambient pressure, and AF and SC domains coexist at 3.0 GPa. The coexistence of domains is explained by considering a SC dome separated from the AF phase in the phase diagram. The homogeneous coexistence support S ±-wave superconductivity, whereas separation of the AF and SC phases gives credence to S ++-wave superconductivity.